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astro-ph new abstracts, Mon, 13 Feb 06 01:00:10 GMT
0602213 -- 0602248 received

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astro-ph/0602213 [abs, ps, pdf, other] :

Title: The Redshift Sensitivities of Dark Energy Surveys
Authors: Fergus Simpson (IoA, Cambridge), Sarah Bridle (UCL)
Comments: 11 pages, 9 figures

Great uncertainty surrounds dark energy, both in terms of its physics, and the choice of methods by which the problem should be addressed. Here we quantify the redshift sensitivities offered by different techniques. We focus on the three methods most adept at constraining w, namely supernovae, cosmic shear, and baryon oscillations. For each we provide insight into the family of w(z) models which are permitted for a particular constraint on either w=w0 or w=w0+wa(1-a). Our results are in the form of "weight functions", which describe the fitted model parameters as a weighted average over the true functional form. For example, we find the recent best-fit from the Supernovae Legacy Survey (w=-1.023) corresponds to the average value of w(z) over the range 0<z<0.4. Whilst there is a strong dependence on the choice of priors, each cosmological probe displays distinctive characteristics in their redshift sensitivities. In the case of proposed future surveys, a SNAP-like supernova survey probes a mean redshift of z ~ 0.3, with baryon oscillations and cosmic shear at z ~ 0.6. If we consider the evolution of w, sensitivities shift to slightly higher redshift. Finally, we find that the weight functions may be expressed as a weighted average of the popular "principal components".

 

astro-ph/0602214 [abs, ps, pdf, other] :

Title: Predicting and analyzing topological correlations in the CMB
Authors: Anastasia Niarchou, Andrew H. Jaffe
Comments: 5 pages, 4 figures. To appear in the proceedings of the 7th astronomy conference of the Hellenic Astronomical Society (8-11 September 2005)

The Cosmic Microwave Background (CMB) power spectrum derived from the first year WMAP data demonstrates an intriguing lack of power at large scales that cannot be accounted for within the framework of the standard cosmological model. We explore the possibility that this anomaly could be explained by waiving the implicit assumption of a simply connected topology, rather than modifying the physics of the standard model. In particular, we assume that the Universe is slightly closed and its spatial section can be described by one of the simplest spherical multi-connected manifolds (the quaternionic, the octahedral, the truncated cube and the Poincare space). We discuss the implications for the CMB in each case.

 

astro-ph/0602215 [abs, ps, pdf, other] :

Title: On a Site of X-ray Emission in AE Aquarii
Authors: N.R. Ikhsanov
Comments: 4 pages, accepted for publication in ApJ Letters

An analysis of recently reported results of XMM-Newton observations of AE Aqr within a hypothesis that the detected X-ray source is located inside the Roche lobe of the white dwarf is presented. I show this hypothesis to be inconsistent with the currently adopted model of mass-transfer in the system. Possible solutions of this problem are briefly discussed.

 

astro-ph/0602216 [abs, ps, pdf, other] :

Title: Long Gamma-Ray Bursts as standard candles
Authors: D. Lazzati (1), G. Ghirlanda (2), G. Ghisellini (2), L. Nava (2), C. Firmani (2), Brian Morsony (1), M. C. Begelman (1) ((1) JILA, Univ. of Colorado, (2) INAF, Oss. Astr. Brera)
Comments: To appear in the proceedings of the 16th Annual October Astrophysics Conference in Maryland "Gamma Ray Bursts in the Swift Era", eds. S. Holt, N. Gehrels & J. Nousek

As soon as it was realized that long GRBs lie at cosmological distances, attempts have been made to use them as cosmological probes. Besides their use as lighthouses, a task that presents mainly the technological challenge of a rapid deep high resolution follow-up, researchers attempted to find the Holy Grail: a way to create a standard candle from GRB observables. We discuss here the attempts and the discovery of the Ghirlanda correlation, to date the best method to standardize the GRB candle. Together with discussing the promises of this method, we will underline the open issues, the required calibrations and how to understand them and keep them under control. Even though GRB cosmology is a field in its infancy, ongoing work and studies will clarify soon if and how GRBs will be able to keep up to the promises.

 

astro-ph/0602217 [abs, ps, pdf, other] :

Title: On the Location of the Snow Line in a Protoplanetary Disk
Authors: M. Lecar (1), M. Podolak (2), D. Sasselov (1), E. Chiang (3) ((1)Harvard-Smithsonian CfA, (2)Tel Aviv Univ., (3)UC Berkeley)
Comments: Accepted for publication in ApJ, 9 pages, 4 figures

In a protoplanetary disk, the inner edge of the region where the temperature falls below the condensation temperature of water is referred to as the 'snow line'. Outside the snow line, water ice increases the surface density of solids by a factor of 4. The mass of the fastest growing planetesimal (the 'isolation mass') scales as the surface density to the 3/2 power. It is thought that ice-enhanced surface densities are required to make the cores of the gas giants (Jupiter and Saturn) before the disk gas dissipates. Observations of the Solar System's asteroid belt suggest that the snow line occurred near 2.7 AU. In this paper we revisit the theoretical determination of the snow line. In a minimum-mass disk characterized by conventional opacities and a mass accretion rate of 10^-8 solar masses per year, the snow line lies at 1.6-1.8 AU, just past the orbit of Mars. The minimum-mass disk, with a mass of 0.02 solar, has a life time of 2 million years with the assumed accretion rate. Moving the snow line past 2.7 AU requires that we increase the disk opacity, accretion rate, and/or disk mass by factors ranging up to an order of magnitude above our assumed baseline values.

 

astro-ph/0602218 [abs, ps, pdf, other] :

Title: The Discovery of a Population of Pre-Main Sequence Stars in N66/NGC346 from Deep HST/ACS Image
Authors: A. Nota, M. Sirianni, E. Sabbi, M. Tosi, M. Clampin, J. Gallagher, M. Meixner, S. Oey, A. Pasquali, L. J. Smith, R. Walterbos, J. Mack
Comments: 8 pages, 4 figures. To be appear on ApJL (accepted)

We report the discovery of a rich population of low mass stars in the young, massive star forming region N66/NGC346 in the Small Magellanic Cloud, from deep V, I and H alpha images taken with the HST/ACS. These stars have likely formed together with the NGC346 cluster, ~3-5 Myr ago. Their magnitude and colors are those of pre-main sequence stars in the mass range 0.6-3 Mo, mostly concentrated in the main cluster, but with secondary subclusters spread over a region across ~45 pc. These subclusters appear to be spatially coincident with previously known knots of molecular gas identified in ground based and ISO observations. We show that N66/NGC346 is a complex region, being shaped by its massive stars, and the observations presented here represent a key step towards the understanding of how star formation occurred and has progressed in this low metallicity environment.

 

astro-ph/0602219 [abs, ps, pdf, other] :

Title: Afterglow Light Curves and Broken Power Laws: A Statistical Study
Authors: Gudlaugur Jóhannesson, Gunnlaugur Björnsson, Einar H. Gudmundsson
Comments: 4 pages, 5 figures, accepted for publication in ApJ Letters

In gamma-ray burst research it is quite common to fit the afterglow light curves with a broken power law to interpret the data. We apply this method to a computer simulated population of afterglows and find systematic differences between the known model parameters of the population and the ones derived from the power law fits. In general, the slope of the electron energy distribution is overestimated from the pre-break light curve slope while being underestimated from the post-break slope. We also find that the jet opening angle derived from the fits is overestimated in narrow jets and underestimated in wider ones. Results from fitting afterglow light curves with broken power laws must therefore be interpreted with caution since the uncertainties in the derived parameters might be larger than estimated from the fit. This may have implications for Hubble diagrams constructed using gamma-ray burst data.

 

astro-ph/0602220 [abs, ps, pdf, other] :

Title: Dynamics and High Energy Emission of the Flaring HST-1 Knot in the M 87 Jet
Authors: L. Stawarz, F. Aharonian, J. Kataoka, M. Ostrowski, A. Siemiginowska, M. Sikora
Comments: 13 pages, 7 figures included. MNRAS submitted

Stimulated by recent observations of a giant radio-to-X-ray synchrotron flare from the HST-1 knot in the M 87 jet, as well as by a detection of a very high energy gamma-ray emission from M 87, we investigated the dynamics and multiwavelength emission of the HST-1 region. We study thermal pressure of the hot interstellar medium in M 87 and argue for a presence of a gaseous condensation in its central parts, linked to the observed central stellar cusp of the elliptical host. Interaction of the jet with its ambient medium characterized by the enhanced in this way thermal pressure is likely to result in formation of a converging reconfinement shock structure in the innermost parts of the M 87 jet. We show that for a realistic set of the outflow parameters, a stationary and flaring part of the HST-1 knot located ~100 pc away from the active center can be associated with the decelerated portion of the jet matter placed downstream of the point where the reconfinement shock reaches the jet axis. We discuss a possible scenario explaining a broad-band brightening of the HST-1 region. We show that assuming a previous epoch of the high central black hole activity resulting in ejection of excess particles and photons down along the jet, one may first expect a high-energy flare of HST-1 due to inverse-Comptonisation of the nuclear radiation, followed after a few years by an increase in its synchrotron continuum. If this is the case, then the recently observed order-of-magnitude increase in the knot luminosity in all spectral bands could be regarded as an unusual echo of the outburst that had happened previously in the highly relativistic active core of the M 87 radio galaxy. We show that very high energy gamma-ray fluxes expected in a framework of the proposed scenario are consistent with the observed ones.

 

astro-ph/0602221 [abs, ps, pdf, other] :

Title: Measuring Magnetic Fields in Ultracool Stars and Brown Dwarfs
Authors: Ansgar Reiners, Gibor Basri
Comments: 36 preprint pages, accepted for publication in ApJ

We present a new method for direct measurement of magnetic fields on ultracool stars and brown dwarfs. It takes advantage of the Wing-Ford band of FeH, which are seen throughout the M and L spectral types. These molecular features are not as blended as other optical molecular bands, are reasonably strong through most of the spectral range, and exhibit a response to magnetic fields which is easier to detect than other magnetic diagnostics, including the usual optical and near-infrared atomic spectral lines that have heretofore been employed. The FeH bands show a systematic growth as the star gets cooler. We do not find any contamination by CrH in the relevant spectral region. We are able to model cool and rapidly-rotating spectra from warmer, slowly-rotating spectra utilizing an interpolation scheme based on optical depth scaling. We show that the FeH features can distinguish between negligible, moderate, and high magnetic fluxes on low-mass dwarfs, with a current accuracy of about one kilogauss. Two different approaches to extracting the information from the spectra are developed and compared. Which one is superior depends on a number of factors. We demostrate the validity of our new procedures by comparing the spectra of three M stars whose magnetic fluxes are already known from atomic line analysis. The low and high field stars are used to produce interpolated moderate-strength spectra which closely resemble the moderate-field star. The assumption of linear behavior for the magnetic effects appears to be reasonable, but until the molecular constants are better understood the method is subject to that assumption, and rather approximate. Nonetheless, it opens a new regime of very low-mass objects to direct confirmation and testing of their magnetic dynamos.

 

astro-ph/0602222 [abs, ps, pdf, other] :

Title: The 2005 outburst of the halo black hole X-ray transient XTE J1118+480
Authors: C. Zurita (1), M.A.P. Torres (2), D.Steeghs (2), P. Rodriguez-Gil (1), T. Munoz-Darias (1), J. Casares (1), T. Shahbaz (1), I.G. Martinez-Pais (1,3), P. Zhao (2), M.R. Garcia (2), A. Piccioni (4), C. Bartolini (4), A. Guarnieri (4), J.S. Bloom (5), C.H. Blake (2), E.E. Falco (2), A.Szentgyorgyi (2), M.Skrutskie (6) ((1) Instituto de Astrofisica de Canarias, (2) Harvard-Smithsonian Center for Astrophysics, (3) Departamento de Astrofisica, Universidad de La Laguna, (4) Dipartimento di Astronomia, Universita di Bologna, (5) Astronomy Department, University of California at Berkeley, (6) Department of Astronomy, University of Virginia)
Comments: Accepted for publication in The Astrophysical Journal

We present optical and infrared monitoring of the 2005 outburst of the halo black hole X-ray transient XTE J1118+480. We measured a total outburst amplitude of ~5.7+-0.1 mag in the R band and ~5 mag in the infrared J, H and K_s bands. The hardness ratio HR2 (5-12 keV/3-5 keV) from the RXTE/ASM data is 1.53+-0.02 at the peak of the outburst indicating a hard spectrum. Both the shape of the light curve and the ratio L_X (1-10 keV)/L_opt resemble the mini-outbursts observed in GRO J0422+32 and XTE J1859+226. During early decline, we find a 0.02-mag amplitude variation consistent with a superhump modulation, like the one observed during the 2000 outburst. Similarly, XTE J1118+480 displayed a double-humped ellipsoidal modulation distorted by a superhump wave when settled into a near-quiescence level, suggesting that the disk expanded to the 3:1 resonance radius after outburst where it remained until early quiescence. The system reached quiescence at R=19.02+-0.03 about three months after the onset of the outburst. The optical rise preceded the X-ray rise by at most 4 days. The spectral energy distributions (SEDs) at the different epochs during outburst are all quasi-power laws with F_nu proportional to nu^alpha increasing toward the blue. At the peak of the outburst we derived alpha=0.49+-0.04 for the optical data alone and alpha=0.1+-0.1 when fitting solely the infrared. This difference between the optical and the infrared SEDs suggests that the infrared is dominated by a different component (a jet?) whereas the optical is presumably showing the disk evolution.

 

astro-ph/0602223 [abs, ps, pdf, other] :

Title: Structural Parameters of Stellar Disks in Edge-on Galaxies from 2MASS Images
Authors: S. Mitronova (SAO Ras), D. Bizyaev (NOAO, Sai)
Comments: 3 pages, 3 figures. Presented as a poster at the 207th AAS Meeting, January 8-12, 2006, Washington DC, USA

We analyze the J, H, and Ks 2MASS images of 140 late-type edge-on galaxies selected from the RFGC catalog (which contains flat galaxies with major-to-minor axis ratio a/b > 7). The NIR scalelengths (h) and scaleheights (z0) of the stellar disks are determined for all selected galaxies. The mean relative ratios of the scaleheights of their stellar disks are 1.00:0.91:0.86 in J:H:Ks bands, respectively. We infer that the scaleheight determined from the Ks-band images is, on average, 13% larger than the extinction-free scaleheight. This difference is much larger if the scaleheights were found from the optical-band images. The relative thickness (z0/h) of the stellar disks correlates well with their deprojected central surface brightness obtained from the 2MASS images. This project was partially supported by grant RFBR 04-02-16518.

 

astro-ph/0602224 [abs, ps, pdf, other] :

Title: Constraints on the Evolution of the Primordial Magnetic Field from the Small Scale CMB Angular Anisotropy
Authors: D. Yamazaki, K. Ichiki, T. Kajino, G. J. Mathews
Comments: 27 pages, 4 figures, submitted to ApJ

Recent observations of the cosmic microwave background (CMB) have extended measured the power spectrum to higher multipoles $l\gtrsim$1000, and there appears to be possible evidence for excess power on small angular scales. The primordial magnetic field (PMF) can strongly affect the CMB power spectrum and the formation of large scale structure. In this paper, we calculate the CMB temperature anisotropies generated by including a power-law magnetic field at the photon last scattering surface (PLSS). We then deduce an upper limit on the primordial magnetic field based upon our theoretical analysis of the power excess on small angular scales. We have taken into account several important effects such as reionization and the modified matter sound speed in the presence of a magnetic field. An upper limit to the field strength of $|B_\lambda|\lesssim$ 4.7 nG at the present scale of 1 Mpc is deduced. This is obtained by comparing the calculated theoretical result including the Sunyaev-Zeldovich (SZ) effect with recent observed data on the small scale CMB anisotropies from the Wilkinson Microwave Anisotropy Probe (WMAP), the Cosmic Background Imager (CBI) and the Arcminute Cosmology Bolometer Array Receiver (ACBAR). We discuss several possible mechanisms for the generation and evolution of the PMF.

 

astro-ph/0602225 [abs, ps, pdf, other] :

Title: A Dimensional Study of Disk Galaxies (An excuse to Talk about the Hubble Sequence)
Authors: X. Hernandez, B. Cervantes-Sodi
Comments: 4 pages, 4 figures, Invited Talk to the 11th Latin-American Regional IAU meeting, Pucon, Chile 2005

We present a highly simplified model of the dynamical structure of a disk galaxy where only two parmeters fully determine the solution, mass and angular momentum. We show through simple physical scalings that once the mass has been fixed, the angular momentum parameter $\lambda$ is expected to regulate such critical galactic disk properties as colour, thickness of the disk and disk to bulge ratio. It is hence expected to be the determinant physical ingeredient resulting in a given Hubble type. A simple analytic estimate of $\lambda$ for an observed system is provided. An explicit comparison of the distribution of several galactic parameters against both Hubble type and $\lambda$ is performed using observed galaxies. Both such distributions exhibit highly similar characteristics for all galactic properties studied, suggesting $\lambda$ as a physically motivated classification parameter for disk galaxies.

 

astro-ph/0602226 [abs, ps, pdf, other] :

Title: Interferometric 890 micron Images of High Redshift Submillimeter Galaxies
Authors: D. Iono (1,2), A. B. Peck (1), A. Pope (3), C. Borys (4), D. Scott (3), D. J. Wilner (1), M. Gurwell (1), P. T. P. Ho (1), M. S. Yun (5), S. Matsushita (6), G. R. Petitpas (1), J. S. Dunlop (7), M. Elvis (1), A. Blain (4), E. Le Floc'h (8) ((1) CfA (2) NAOJ (3) UBC (4) Caltech (5) UMass (6) ASIAA (7) Edinburgh (8) Arizona)
Comments: 11 pages, 2 figures, ApJL accepted

We present high resolution 890 micron images of two 20 mJy submillimeter galaxies, SMMJ123711+622212 and MIPS~J142824.0+352619, obtained using the Submillimeter Array (SMA). Using submillimeter interferometric observations with an angular resolution of 2.5", the coordinates of these high redshift sources are determined with an accuracy of 0.2". The new SMA data on SMMJ123711+622212 reveal an unresolved submm source offset to the east by 0.8" from an optical galaxy found in deep HST images, suggesting either a large galaxy with a dusty central region, or an interacting galaxy system. The SMA image of hyper-luminous (LFIR = 3.2 x 10^{13} Lsun) source MIPS~J142824.0+352619 provides a firm upper limit to the source size of < 1.2. This constraint provides evidence that the foreground lens is only weakly affecting the observed high FIR luminosity.

 

astro-ph/0602227 [abs, ps, pdf, other] :

Title: Millimeter Interferometric Investigations of the Energy Sources of Three Ultraluminous Infrared Galaxies, UGC 5101, Mrk 273, and IRAS 17208-0014, based on HCN to HCO+ Ratios
Authors: Masatoshi Imanishi (1), Kouichiro Nakanishi (2), Kotaro Kohno (3) ((1) NAO Japan, (2) NRO, (3) Univ of Tokyo)
Comments: 15 pages (emulateapj.sty), 8 figures (figures 1-5 resolution reduced), Accepted for publication in Astronomical Journal

We present interferometric observations of three ultraluminous infrared galaxies (ULIRGs; UGC 5101, Mrk 273, and IRAS 17208-0014) in the 3-mm wavelength range, using the Nobeyama Millimeter Array. Both the HCN (J=1-0) and HCO+ (J=1-0) molecular lines were observed simultaneously. HCN emission was clearly detected at the nuclear positions of these ULIRGs, and HCO+ emission was detected at the nuclear positions of UGC 5101 and IRAS 17208-0014. The HCN to HCO+ brightness-temperature ratios toward the nuclei of the three ULIRGs were derived and compared with those of lower luminosity galaxies known to be dominated by active galactic nuclei (AGNs) or starbursts. In UGC 5101 and Mrk 273, where there is evidence for obscured AGNs from previous observations at other wavelengths, we found high HCN/HCO+ ratios (>1.8) that are in the range found for AGN-dominated galaxies. In IRAS 17208-0014, where the presence of a powerful obscured AGN has been unclear, the ratio (1.7) is in between the observed values for starburst- and AGN-dominated galaxies. The high HCN/HCO+ brightness-temperature ratios in UGC 5101 and Mrk 273 could be the consequence of an HCN abundance enhancement, which is expected from chemical effects of the central X-ray emitting AGN on the surrounding dense molecular gas. Our proposed millimeter interferometric method based on HCN/HCO+ ratios may be an effective tool for unveiling elusive buried AGNs at the cores of ULIRGs, especially because of the negligible dust extinction at these wavelengths.

 

astro-ph/0602228 [abs, ps, pdf, other] :

Title: Chandra and Spitzer unveil heavily obscured quasars in the SWIRE/Chandra Survey
Authors: M. Polletta (UCSD), B.J. Wilkes (CfA), B. Siana (SSC/Caltech), C.J. Lonsdale (UCSD, Caltech), R. Kilgard (CfA), H.E. Smith (UCSD), D.-W. Kim (CfA), F. Owen (NRAO), A. Efstathiou (Cyprus), T. Jarrett (IPAC/Caltech), G. Stacey (Cornell), A. Franceschini (Padua U.), M. Rowan-Robinson, T.S.R. Babbedge (IC), S. Berta (Padua U.), F. Fang (SSC/Caltech), D. Farrah (Cornell), E. Gonzalez-Solares (Cambridge), G. Morrison (Univ. of Hawaii), J.A. Surace, D.L. Shupe (SSC/Caltech)
Comments: ApJ accepted (to appear in May 2006 issue, vol. 642, of ApJ) Figures 2, 3, and 14 have been degraded due to space considerations

Using the large multi-wavelength data set in the chandra/SWIRE Survey (0.6 square degrees in the Lockman Hole), we show evidence for the existence of highly obscured (Compton-thick) AGN, estimate a lower limit to their surface density and characterize their multi-wavelength properties. Two independent selection methods based on the X-ray and infrared spectral properties are presented. The two selected samples contain 1) 5 X-ray sources with hard X-ray spectra and column densities $\gtrsim 10^24 cm-2, and 2) 120 infrared sources with red and AGN-dominated infrared spectral energy distributions (SEDs). We estimate a surface density of at least 25 Compton-thick AGN per square degree detected in the infrared in the chandra/SWIRE field of which ~40% show distinct AGN signatures in their optical/near-infrared SEDs, the remainings being dominated by the host-galaxy emission. Only ~33% of all Compton-thick AGN are detected in the X-rays at our depth (F(0.3-8 keV)>10^-15 erg/cm2/s.
We report the discovery of two sources in our sample of Compton-thick AGN, SWIRE_J104409.95+585224.8 (z=2.54) and SWIRE_J104406.30+583954.1 (z=2.43), which are the most luminous Compton-thick AGN at high-z currently known. The properties of these two sources are discussed in detail with an analysis of their spectra, SEDs, luminosities and black-hole masses.

 

astro-ph/0602229 [abs, ps, pdf, other] :

Title: A single-degenerate model for the progenitor of the Type Ia supernova 2002ic
Authors: Zhanwen Han, Philipp Podsiadlowski
Comments: 7 pages, 8 figures, accepted by MNRAS

Supernova 2002ic was an atypical Type Ia supernova (SN Ia) with evidence for substantial amounts of hydrogen associated with the system. Contrary to previous claims, we show that its unusual properties can be understood within the framework of one of the most favoured progenitor models, the so-called supersoft channel. This requires that the donor star was initially relatively massive ($\sim 3\Ms$) and that the system experienced a delayed dynamical instability, leading to a large amount of mass loss from the system in the last few $10^4 $yr before the explosion. This can produce the inferred hydrogen-rich circumstellar environment, most likely with a disc-like geometry. However, to apply these models requires a larger accretion efficiency onto the white dwarf than is assumed in present parameterizations. If this is confirmed, it would most likely increase estimates for the frequency of the single-degenerate channel. Based on population synthesis simulations we estimate that not more than 1 in 100 SNe Ia should belong to this subgroup of SNe Ia.

 

astro-ph/0602230 [abs, ps, pdf, other] :

Title: The ultra-luminous X-ray source in M82: an intermediate mass black hole with a giant companion
Authors: A. Patruno (Univ. Amsterdam), S.F. Portegies Zwart (Univ. Amsterdam), J. Dewi (Univ. Cambridge), C. Hopman (Weizmann)

The starburst galaxy M82 at a distance of 12 million light years, is the host of an unusually bright $2.4-16\times 10^{40}$ erg/s X-ray point source, which is best explained by an accreting black hole $10^{2}$ to $10^4$ times more massive than the Sun. Though the strongest candidate for a so called intermediate mass black hole, the only support stems from the observed luminosity and the 0.05-0.1 Hz quasi periodicity in its signal. Interestingly, the $7-12 $Myr old star cluster MGG-11 which has been associated with the X-ray source is sufficiently dense that an intermediate mass black hole could have been produced in the cluster core via collision runaway. The recently discovered $62.0\pm 2.5$ day periodicity in the X-ray source X-1 further supports the hypothesis that this source is powered by a black hole several hundred times more massive than the Sun. We perform detailed binary evolution simulations with an accreting compact object of $10-5000 \msun $ and find that the X-ray luminosity, the age of the cluster, the observed quasi periodic oscillations and the now observed orbital period are explained best by a black hole of $200-5000 \msun $ that accretes material from a $22-25\msun$ giant companion in a state of Roche-lobe contact. Interestingly such a companion star is consistent with the expectation based on the tidal capture in a young and dense star cluster like MGG-11, making the picture self consistent.

 

astro-ph/0602231 [abs, ps, pdf, other] :

Title: Flux upper limit of gamma-ray emission by GRB050713a from MAGIC Telescope observations
Authors: J. Albert, E. Aliu, H. Anderhub, P. Antoranz, A. Armada, M. Asensio, C. Baixeras, J. A. Barrio, M. Bartelt, H. Bartko, D. Bastieri, R. Bavikadi, W. Bednarek, K. Berger, C. Bigongiari, A. Biland, E. Bisesi, R. K. Bock, T. Bretz, I. Britvitch, M. Camara, A. Chilingarian, S. Ciprini, J. A. Coarasa, S. Commichau, J. L. Contreras, J. Cortina, V. Curtef, V. Danielyan, F. Dazzi, A. De Angelis, R. de los Reyes, B. De Lotto, E. Domingo-Santamaria, D. Dorner, M. Doro, M. Errando, M. Fagiolini, D. Ferenc, E. Fernandez, R. Firpo, J. Flix, M. V. Fonseca, L. Font, N. Galante, M. Garczarczyk, M. Gaug, M. Giller, F. Goebel, D. Hakobyan, M. Hayashida, T. Hengstebeck, D. Hohne, J. Hose, P. Jacon, O. Kalekin, D. Kranich, A. Laille, T. Lenisa, P. Liebing, E. Lindfors, F. Longo, J. Lopez, M. Lopez, E. Lorenz, et al (64 additional authors not shown)
Comments: 14 pages, 4 figures, submitted to ApJ

The long-duration GRB050713a was observed by the MAGIC Telescope, 40 seconds after the burst onset, and followed up for 37 minutes, until twilight. The observation, triggered by a SWIFT alert, covered energies above ~175 GeV. Using standard MAGIC analysis, no evidence for a gamma signal was found. As the redshift of the GRB was not measured directly, the flux upper limit, estimated by MAGIC, is still compatible with the assumption of an unbroken power-law spectrum extending from a few hundred keV to our energy range.

 

astro-ph/0602232 [abs, pdf] :

Title: Solar System Processes Underlying Planetary Formation, Geodynamics, and the Georeactor
Authors: J. Marvin Herndon
Comments: Invited paper for the Special Issue of Earth, Moon and Planets entitled Neutrino Geophysics

Only three processes, operant during the formation of the Solar System, are responsible for the diversity of matter in the Solar System and are directly responsible for planetary internal-structures, including planetocentric nuclear fission reactors, and for dynamical processes, including and especially, geodynamics. These processes are: (i) Low-pressure, low-temperature condensation from solar matter in the remote reaches of the Solar System or in the interstellar medium; (ii) High-pressure, high-temperature condensation from solar matter associated with planetary-formation by raining out from the interiors of giant-gaseous protoplanets, and; (iii) Stripping of the primordial volatile components from the inner portion of the Solar System by super-intense solar wind associated with T-Tauri phase mass-ejections, presumably during the thermonuclear ignition of the Sun. As described herein, these processes lead logically, in a causally related manner, to a coherent vision of planetary formation with profound implications including, but not limited to, (a) Earth formation as a giant gaseous Jupiter-like planet with vast amounts of stored energy of protoplanetary compression in its rock-plus-alloy kernel; (b) Removal of approximately 300 Earth-masses of primordial gases from the Earth, which began Earth's decompression process, making available the stored energy of protoplanetary compression for driving geodynamic processes, which I have described by the new whole-Earth decompression dynamics and which is responsible for emplacing heat at the mantle-crust-interface at the base of the crust through the process I have described, called mantle decompression thermal-tsunami; and, (c)Uranium accumulations at the planetary centers capable of self-sustained nuclear fission chain reactions.

 

astro-ph/0602233 [abs, ps, pdf, other] :

Title: Searching for annihilation radiation from SN 1006 with SPI on INTEGRAL
Authors: E. Kalemci, S. E. Boggs, P.A. Milne, S. P. Reynolds
Comments: 3 pages, 2 figures, accepted for publication in ApJL

Historical Type Ia supernovae are a leading candidate for the source of positrons observed through their diffuse annihilation emission in the Galaxy. However, search for annihilation emission from individual Type Ia supernovae has not been possible before the improved sensitivity of \integral. The total 511 keV annihilation flux from individual SNe Ia, as well as their contribution to the overall diffuse emission, depends critically on the escape fraction of positrons produced in $^{56}$Co decays. Late optical light curves suggest that this fraction may be as high as 5%. We searched for positron annihilation radiation from the historical Type Ia supernova SN 1006 using the SPI instrument on \integral. We did not detect significant 511 keV line emission, with a 3$\sigma$ flux upper limit of 0.59 x 10$^{-4}$ ergs cm^-2 s^-1 for \wsim 1 Msec exposure time, assuming a FWHM of 2.5 keV. This upper limit corresponds to a 7.5% escape fraction, 50% higher than the expected 5% escape scenario, and rules out the possibility that Type Ia supernovae produce all of the positrons in the Galaxy (~ 12% escape fraction), if the mean positron lifetime is less than 10$^{5}$ years. Future observations with \integral will provide stronger limits on the escape fraction of positrons, the mean positron lifetime, and the contribution of Type Ia supernovae to the overall positron content of the Galaxy.

 

astro-ph/0602234 [abs, ps, pdf, other] :

Title: Instabilities of rotating compact stars: a brief overview
Authors: L. Villain
Comments: 19 pages, Proceeding of Cargese School "Astrophysical fluid dynamics" (May 2005) organized by B. Dubrulle and M. Rieutord in honour of J.-P. Zahn and S. Bonazzola

Direct observations of gravitational waves will open in the near future new windows on the Universe. Among the expected sources, instabilities of rotating compact astrophysical objects are waited to be detected with some impatience as this will sign the birth of ``gravitational waves asteroseismology'', a crucial way to improve our knowledge of matter equation of state in conditions that cannot be reproduced in a lab. However, the theoretical work needed to really get informations from to-be-detected signals is still quite large, numerical simulations having become a necessary key ingredient. This article tries to provide a short overview of the main physical topics involved in this field (general relativity, gravitational waves, instabilities of rotating fluids, {\it etc.}), concluding with a brief description of the work that was done in Paris-Meudon Observatory by Silvano Bonazzola and collaborators.

 

astro-ph/0602235 [abs, ps, pdf, other] :

Title: The faint 2005 hard state outburst of Aquila X-1 seen by INTEGRAL and RXTE
Authors: Jerome Rodriguez, Simon E. Shaw, Stephane Corbel
Comments: 4 pages 2 figures, Accepted for publication in A&A as a RESEARCH NOTE

We report on the spectral analysis of RXTE and INTEGRAL data of the 2005 April outburst of the transient Atoll source Aql X-1. Although this outburst is one of the faintest ever detected for this source in the soft X-rays (RXTE/ASM), one of our INTEGRAL observations, taken close to the soft X-ray peak, shows that the source flux was quite high, with a 20-200 keV flux of 2.05 x 10^-9 erg cm^-2 s^-1. On this occasion we detect the source up to 150 keV for the first time. We compare and discuss the similarity of the source behavior with that of black hole transients especially XTE J1550-564.

 

astro-ph/0602236 [abs, ps, pdf, other] :

Title: GRB 050505: A high redshift burst discovered by Swift
Authors: C. P. Hurkett, J. P. Osborne, K.L. Page, E. Rol, M.R. Goad, P.T. O'Brien, A. Beardmore, O. Godet, D. N. Burrows, N. R. Tanvir, A. Levan, B. Zhang, D. Malesani, J. E. Hill, J. A. Kennea, R. Chapman, V. La Parola, M. Perri, P. Romano, R. Smith, N. Gehrels
Comments: 9 pages, 6 figures. Accepted by MNRAS

We report the discovery and subsequent multi-wavelength afterglow behaviour of the high redshift (z = 4.27) Gamma Ray Burst GRB 050505. This burst is the third most distant burst, measured by spectroscopic redshift, discovered after GRB 000131 (z = 4.50) and GRB 050904 (z = 6.29). GRB 050505 is a long GRB with a multipeaked gamma-ray light curve, with a duration of T_90 = 63+/-2 s and an inferred isotropic release in gamma-rays of ~4.44 x 10^53 ergs in the 1-10^4 keV rest frame energy range. The Swift X-Ray Telescope followed the afterglow for 14 days, detecting two breaks in the light curve at 7.4(+/-1.5) ks and 58.0 (+9.9/-15.4) ks after the burst trigger. The power law decay slopes before, between and after these breaks were 0.25 (+0.16/-0.17), 1.17 (+0.08/-0.09) and 1.97 (+0.27/-0.28) respectively. The light curve can also be fit with a `smoothly broken' power law model with a break observed at ~ T+18.5 ks, with decay slopes of ~0.4 and ~1.8 before and after the break respectively. The X-ray afterglow shows no spectral variation over the course of the Swift observations, being well fit with a single power law of photon index ~1.90. This behaviour is expected for the cessation of continued energisation of the ISM shock followed by a break caused by a jet, either uniform or structured. Neither break is consistent with a cooling break. The spectral energy distribution indeed shows the cooling frequency to be below the X-ray but above optical frequencies. The optical -- X-ray spectrum also shows that there is significant X-ray absorption in excess of that due to our Galaxy but very little optical/UV extinction, with E(B-V) ~0.10 for a SMC-like extinction curve.

 

astro-ph/0602237 [abs, ps, pdf, other] :

Title: Testing the inverse-Compton catastrophe scenario in the intra-day variable blazar S5 0716+71. I. Simultaneous broadband observations during November 2003
Authors: L. Ostorero, S. J. Wagner, J. Gracia, E. Ferrero, T. P. Krichbaum, S. Britzen, A. Witzel, K. Nilsson, M. Villata, et al
Comments: 15 pages, 3 EPS figures, 3 tables, to appear in A&A

Some intra-day variable, compact extra-galactic radio sources show brightness temperatures severely exceeding 10^{12} K, the limit set by catastrophic inverse-Compton (IC) cooling in sources of incoherent synchrotron radiation. The violation of the IC limit, possible under non-stationary conditions, would lead to IC avalanches in the soft-gamma-ray energy band during transient periods. For the first time, broadband signatures of possible IC catastrophes were searched for in S5 0716+71. A multifrequency observing campaign targetting S5 0716+71 was carried out in November 2003 under the framework of the European Network for the Investigation of Galactic nuclei through Multifrequency Analysis (ENIGMA) together with a campaign by the Whole Earth Blazar Telescope (WEBT), involving a pointing by the soft-gamma-ray satellite INTEGRAL, optical, near-infrared, sub-millimeter, millimeter, radio, and Very Long Baseline Array (VLBA) monitoring. S5 0716+71 was very bright at radio frequencies and in a rather faint optical state during the INTEGRAL pointing; significant inter-day and low intra-day variability was recorded in the radio regime, while typical fast variability features were observed in the optical band. No correlation was found between the radio and optical emission. The source was not detected by INTEGRAL, neither by the X-ray monitor JEM-X nor by the gamma-ray imager ISGRI, but upper limits to the source emission in the 3-200 keV energy band were estimated. A brightness temperature Tb>2.1x10^{14} K was inferred from the radio variability, but no corresponding signatures of IC avalanches were recorded at higher energies. The absence of IC-catastrophe signatures provides either a lower limit delta>8 to the Doppler factor affecting the radio emission or strong constraints for modelling of the Compton catastrophes in S5 0716+71.

 

astro-ph/0602238 [abs, ps, pdf, other] :

Title: A Shack-Hartmann wavefront sensor projected onto the sky with reduced focal anisoplanatism
Authors: T. Butterley, G. D. Love, R. W. Wilson, R. M. Myers, T. J. Morris
Comments: 8 pages, 5 figures, accepted for publication in MNRAS

A method for producing a laser guide star wavefront sensor for adaptive optics with reduced focal anisoplanatism is presented. A theoretical analysis and numerical simulations have been carried out and the results are presented. The technique, named SPLASH (Sky-Projected Laser Array Shack-Hartmann), is shown to suffer considerably less from focal anisoplanatism than a conventional laser guide star system. The method is potentially suitable for large telescope apertures (~8m), and possibly for extremely large telescopes.

 

astro-ph/0602239 [abs, ps, pdf, other] :

Title: The emission spectrum of the strong Fe II emitter BAL Seyfert 1 galaxy IRAS 07598+6508
Authors: M.-P. Veron, M. Joly, P. Veron, T. Boroson, S. Lipari, P. Ogle
Comments: 15 pages, 4 figures, accepted by A&A

The narrow-line Seyfert 1 galaxy IRAS 07598+6508 is known to be a stong Fe II emitter. The analysis of several high S/N ratio spectra shows that its spectrum is dominated by a relatively narrow "broad line" region (1 780 km s$^{-1}$ FWHM) emitting not only Fe II, but also Ti II and Cr II lines. Although we were unable to find a completely satisfactory physical model, we got the best agreement with the observations with collisional rather than radiative models, with a high density (n=10$^{15}$ cm$^{-3}$), a high column density (N$_{H}$=10$^{25}$ cm$^{-2}$) and a microturbulence of 100 km s$^{-1}$. This BLR is qualitatively similar to the one observed in I Zw 1. We have not found traces in IRAS 07598+6508 of the narrow line regions found in I Zw 1.

 

astro-ph/0602240 [abs, ps, pdf, other] :

Title: The Millennium Galaxy Catalogue: morphological classification and bimodality in the colour-concentration plane
Authors: Simon P. Driver, Paul D. Allen, Alister W. Graham, Ewan Cameron, Jochen Liske, Simon C. Ellis, Nicholas J.G. Cross, Roberto De Propris, Steven Phillipps, Warrick J. Couch
Comments: Accepted for publication in MNRAS, 23 pages, 17 figures. Comments welcome. MGC website is at: this http URL

Using 10 095 galaxies (B < 20 mag) from the Millennium Galaxy Catalogue, we derive B-band luminosity distributions and selected bivariate brightness distributions for the galaxy population. All subdivisions extract highly correlated sub-sets of the galaxy population which consistently point towards two overlapping distributions. A clear bimodality in the observed distribution is seen in both the rest-(u-r) colour and log(n) distributions. The rest-(u-r) colour bimodality becomes more pronounced when using the core colour as opposed to global colour. The two populations are extremely well separated in the colour-log(n) plane. Using our sample of 3 314 (B < 19 mag) eyeball classified galaxies, we show that the bulge-dominated, early-type galaxies populate one peak and the bulge-less, late-type galaxies occupy the second. The early- and mid-type spirals sprawl across and between the peaks. This constitutes extremely strong evidence that the fundamental way to divide the luminous galaxy population is into bulges and discs and that the galaxy bimodality reflects the two component nature of galaxies and not two distinct galaxy classes. We argue that these two-components require two independent formation mechanisms/processes and advocate early bulge formation through initial collapse and ongoing disc formation through splashback, infall and merging/accretion. We calculate the B-band luminosity-densities and stellar-mass densities within each subdivision and estimate that the z ~ 0 stellar mass content in spheroids, bulges and discs is 35 +/- 2 per cent, 18 +/- 7 and 47 +/- 7 per cent respectively. [Abridged]

 

astro-ph/0602241 [abs, ps, pdf, other] :

Title: Size and Orientation of the `Z' in ZRGs
Authors: Christian Zier
Comments: 2 pages, to appear in the proceedings of ESO Astrophysics Symposia: Groups of Galaxies in the Nearby Universe, Santiago de Chile, Dec. 2005

Some X-shaped radio galaxies (XRGs) show a Z-symmetric morphology in the less luminous secondary lobes. Our geometrical arguments strongly support a merger of two galaxies as mechanism for the formation of these sources (ZRG). They also strengthen the conjecture that a jet is aligned with the spin of the BH at its base and that the jet flips into the direction of the orbital angular momentum of the pre-merger binary black hole (BHB). We could also restrict the distance where the pre-merger jet is bent into Z-shape by the inspiralling galaxy to the range of 30-100 kpc. One of three possible orientations of the jet relative to our line of sight is more likely than the others and allows us to deduce the direction of the spin of the merged BH. The existence of XRGs and ZRGs proves that the binary has merged, contrary to previous speculations that after a merger of two galaxies the decay of the BHB stalls due to loss cone depletion. In ZRGs the black holes probably merge on timescales of some 10^8 yr after the bending of the jet in a distance of about 50 kpc. Thus, in a way, the bending starts a stop watch for the rest of the merger.

 

astro-ph/0602242 [abs, ps, pdf, other] :

Title: Evidence that Quasars and Related Active Galaxies are Good Radio Standard Candles and that they are Likely to be a Lot Closer than their Redshifts Imply
Authors: M. B. Bell
Comments: 28 pages with 29 figures, submitted to the Astrophysical Journal

For many years some astronomers have continued to argue, using redshift periodicities and quasar-galaxy associations, that quasars may be closer than their redshifts imply. Here, for the first time using raw radio data, I re-examine this question and find new evidence that supports this argument. Using VLBA flux densities and angular motions in jets, I show that the central engine of quasars and BL Lac objects appears to be a good radio standard candle. Using this information, relative distances are calculated and absolute radio distances are then obtained by referencing to a source whose true distance has been obtained using Cepheid variables. The results reveal that in this model most of the strong radio sources found in early surveys are nearer than 100 Mpc. (abridged)

 

astro-ph/0602243 [abs, ps, pdf, other] :

Title: Non-thermal Cosmic Backgrounds and prospects for future high-energy observations of blazars
Authors: P. Giommi, S. Colafrancesco
Comments: 11 pages, 6 figures, Invited talk at the meeting "Gamma Wave 2005", Bonifacio, September 2005. To be published in "Experimental Astronomy"

We discuss the contribution of the blazar population to the extragalactic background radiation across the electromagnetic (e.m.) spectrum with particular reference to the microwave, hard-X-ray and gamma-ray bands. Our estimates are based on a recently derived blazar radio LogN-LogS that was built by combining several radio and multi-frequency surveys. We show that blazar emission integrated over cosmic time gives rise to a considerable broad-band non-thermal cosmic background that dominates the extragalactic brightness in the high-energy part of the e.m. spectrum. We also estimate the number of blazars that are expected to be detected by future planned or hypothetical missions operating in the X-ray and gamma-ray energy bands.

 

astro-ph/0602244 [abs, ps, pdf, other] :

Title: Pro-Am Collaboration and the AAVSO
Authors: Arne A. Henden
Comments: 15 pages, 10 figures, conference proceeding. NOTE: figures are very low resolution due to astro-ph limitations. Please go to this http URL for a higher resolution version
Journal-ref: 2006, ASPC 349, 165

Professionals need to be aware that there is a valuable resource available and waiting to be used - the amateur astronomy community. We give some examples of how pro-am collaborations have worked in the past, indicate the advantages and disadvantages of such collaborations, and suggest methods by which a professional can find and work effectively with amateur astronomers.

 

astro-ph/0602245 [abs, ps, pdf, other] :

Title: Testing Accretion Disk Theory in Black Hole X-ray Binaries
Authors: Shane W. Davis, Chris Done, Omer M. Blaes
Comments: 14 pages, emulateapj, submitted to ApJ

We present results from spectral modeling of three black hole X-ray binaries: LMC X-3, GRO J1655-40, and XTE J1550-564. Using a sample of disk dominated observations, we fit the data with a range of spectral models that includes a simple multitemperature blackbody (DISKBB), a relativistic accretion disk model based on color-corrected blackbodies (KERRBB), and a relativistic model based on non-LTE atmosphere models within an \alpha prescription (BHSPEC). BHSPEC provides the best fit for a BeppoSAX observation of LMC X-3, which has the broadest energy coverage of our sample. It also provides the best fit for multiple epochs of Rossi X-ray Timing Explorer (RXTE) data in this source, except at the very highest luminosity (L/L_{Edd} \gtrsim 0.7), where additional physics must be coming into play. BHSPEC is also the best-fit model for multi-epoch RXTE observations of GRO J1655-40 and XTE J1550-564, although the best-fit inclination of the inner disk differs from the binary inclination. All our fits prefer \alpha=0.01 to \alpha=0.1, in apparent disagreement with the large stresses inferred from the rapid rise times observed in outbursts of these two sources. In all three sources our fits imply moderate black hole spins (a/M ~ 0.1 - 0.8), or lower if there is a significant stress at the last stable orbit.

 

astro-ph/0602246 [abs, ps, pdf, other] :

Title: Infrared Dark Clouds: precursors to star clusters
Authors: J. M. Rathborne, J. M. Jackson, R. Simon
Comments: Accepted for publication in ApJ. 19 pages. 10 figures

Infrared Dark Clouds (IRDCs) are dense molecular clouds seen as extinction features against the bright mid-infrared Galactic background. Millimeter continuum maps toward 38 IRDCs reveal extended cold dust emission to be associated with each of the IRDCs. IRDCs range in morphology from filamentary to compact and have masses of 120 to 16,000 Msun, with a median mass of ~940 Msun. Each IRDC contains at least one compact (<=0.5 pc) dust core and most show multiple cores. We find 140 cold millimeter cores unassociated with MSX 8um emission. The core masses range from 10 to 2,100 Msun, with a median mass of ~120 Msun. The slope of the IRDC core mass spectrum (alpha ~ 2.1 +/- 0.4) is similar to that of the stellar IMF. Assuming that each core will form a single star, the majority of the cores will form OB stars. IRDC cores have similar sizes, masses, and densities as hot cores associated with individual, young high-mass stars, but they are much colder. We therefore suggest that IRDC represent an earlier evolutionary phase in high-mass star formation. In addition, because IRDCs contain many compact cores, and have the same sizes and masses as molecular clumps associated with young clusters, we suggest that IRDCs are the cold precursors to star clusters. Indeed, an estimate of the star formation rate within molecular clumps with similar properties to IRDCs (~2 Msun/yr) is comparable to the global star formation rate in the Galaxy, supporting the idea that all stars may form in such clumps.

 

astro-ph/0602247 [abs, ps, pdf, other] :

Title: Turbulent Velocity Fields in SPH--simulated Galaxy Clusters
Authors: F. Vazza, G. Tormen, R. Cassano, G. Brunetti, K. Dolag
Comments: 5 pages, 5 figures, submitted to MNRAS Letters

We present a study of the turbulent velocity fields in the Intra Cluster Medium of a sample of 21 galaxy clusters simulated by the SPH--code Gadget2, using a new numerical scheme where the artificial viscosity is suppressed outside shocks. The turbulent motions in the ICM of our simulated clusters are detected with a novel method devised to better disentangle laminar bulk motions from chaotic ones. We focus on the scaling law between the turbulent energy content of the gas particles and the total mass, and find that the energy in the form of turbulence scales approximatively with the thermal energy of clusters. We follow the evolution with time of the scaling laws and discuss the physical origin of the observed trends. The simulated data are in agreement with independent semi--analytical calculations, and the combination between the two methods allows to constrain the scaling law over more than two decades in cluster mass.

 

astro-ph/0602248 [abs, ps, pdf, other] :

Title: The DEEP2 Galaxy Redshift Survey: Mean Ages and Metallicities of Red Field Galaxies at z ~ 0.9 from Stacked Keck/DEIMOS Spectra
Authors: Ricardo P. Schiavon, S. M. Faber, Nicholas Konidaris, Genevieve Graves, Christopher N.A. Willmer, Benjamin J. Weiner, Alison L. Coil, Michael C. Cooper, Marc Davis, Justin Harker, David C. Koo, Jeffrey A. Newman, Renbin Yan
Comments: Submitted to The Astrophysical Journal Letters

As part of the DEEP2 galaxy redshift survey, we analyze absorption line strengths in stacked Keck/DEIMOS spectra of red field galaxies with weak to no emission lines, at redshifts 0.7 <= z <= 1. Comparison with models of stellar population synthesis shows that red galaxies at z ~ 0.9 have mean luminosity-weighted ages of the order of only 1 Gyr and at least solar metallicities. This result cannot be reconciled with a scenario where all stars evolved passively after forming at very high z. Rather, a significant fraction of stars can be no more than 1 Gyr old, which means that star formation continued to at least z ~ 1.2. Furthermore, a comparison of these distant galaxies with a local SDSS sample, using stellar populations synthesis models, shows that the drop in the equivalent width of Hdelta from z ~ 0.9 to 0.1 is less than predicted by passively evolving models. This admits of two interpretations: either each individual galaxy experiences continuing low-level star formation, or the red-sequence galaxy population from z ~ 0.9 to 0.1 is continually being added to by new galaxies with younger stars.

 

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