Paul's Ultraluminous Infrared Galaxy Pages

Ultraluminous infrared galaxies (ULIRGs) are defined by their characteristic infrared luminosity of more than 10¹² . At these luminosities, ULIRGs (if powered by star formation) are the most spectacular starburst galaxies in the universe, building up an entire stellar population in a short burst. Alternatively, (part of) the luminosity of (some) ULIRGs may be due to a dust-embedded, forming quasar. In either case, as a forming galaxy or as a forming quasar, ULIRGs are of fundamental importance. Morphologically, ULIRGs appear to be major mergers of gas-rich galaxies (as exemplified by NGC4038-4039 or the Antennae), with pronounced concentrations of gas in the centre, where all of infrared luminosity is originating, such as in the nearby ULIRGs NGC6240 and Arp220. Since ULIRGs may be progenitors of elliptical galaxies, they are of great significance for the evolution of galaxies. My main interests are the properties of stars and gas in ULIRGs as probed in the near- and mid-infrared and (sub)millimetre regions (e.g., NGC6240, Arp220 and the Antennae), and the role of ULIRGs in galaxy evolution.

Highlights:

Near-infrared H₂ emission in NGC6240: The near-IR spectra of ULIRGs show strong H₂ vibrational lines while recombination lines such as Brγ are relatively faint. NGC6240 presents a perfect case in point. What is the origin of this strong H₂ emission? Seeing-limited imaging of the H₂ 1-0 S(1) line at 2.12 µm (see image below) revealed that the H₂ emission peaks between the two remnant nuclei. This geometry immediately rules out any excitation mechanism based on stellar processes (UV-excitation, shocks from supernova explosions etc). Instead the H₂ emission reveals the dissipative merging of the interstellar media of the two galaxies, which proceeds faster than the merging (by dynamical friction) of the stellar nuclei. The H₂ emission is generated in generated in slow shocks that radiate away (in spectral lines) the dissipated mechanical energy. High resolution CO imaging confirms this picture.

False-colour image of H₂ 1-0 S(1) emission from NGC6240, with the [FeII] 1.64 µm overlaid in contours. The [FeII] emission arises from young supernova remnants and thus traces the starburst in the stellar nuclei. The H₂ emission is seen to peak between the two nuclei. An inital version of this picture was published in Van der Werf et al (1993). The present version uses new H₂ data obtained in sub-arcsecond seeing with IRAC2 at the ESO/MPG 2.2m telescope at La Silla, Chile.

The structure of the H₂ emitting region is seen more clearly in our NICMOS/HST imaging (below): in addition to concentrations on the 2 nuclei, a distinct peak is seen approximately between the 2 nuclei, in a region without an obvious stellar concentration. This geometry is reminiscent of the less advanced merger NGC4038-4039 (the Antennae). A preliminary analysis of the NICMOS/HST H₂ data of NGC6240 shows that the energy radiated by the shocks, which goes at the expense of the orbital energy of the molecular gas, leads to a gas inflow rate which in the 2 nuclei is sufficient to sustain the observed starburst.

NICMOS/HST images showing the H₂ 1-0 S(1) emission from NGC6240 (right-hand panel) and the underlying continuum (left-hand panel). Note that the H₂ line is not well centred in the fixed NICMOS filter used for this observation, so that the flux of the southern nucleus is overemphasized, and that of the northern nucleus underemphasized with respect to the flux of the concentration between the two nuclei. A preliminary analysis of these results is published in Van der Werf (2000).

ULIRGs at high redshift: While locally, ULIRGs contribute less than a few percent to the integrated energy output of the universe, this is very different at redshifts z~2, where ULIRGs are dominant contributors to the integrated cosmic star formation rate. The study of ULIRGs at high redshift is therefore of fundamental importance. Fortunately, the observability of high-z ULIRGs is helped by strong negative k-correction: when observing in the submillimetre (submm) regime, the cosmological dimming is offset by the fact that the peak of the spectral energy distribution shifts into the observing band. As a result, the redshifted far-infrared radiation from high-z ULIRGs can be observed in the submm regime with modern submm bolometer arrays such as SCUBA on the James Clerk Maxwell Telescope (JCMT), operating at 850 µm and 450 µm simultaneously. A key example is provided by the ULIRG behind the cluster A2218, which is gravitationally lensed into three images, as shown in the image below. Subsequent follow-up investigations using deep optical and near-infrared (near-IR) images identify a faint counterpart to each of the three images, with similar red colours and Hubble Space Telescope morphologies.

This montage shows on the left-hand side a true-colour image of the core of A2218 composed from the HST F450W (blue), HST F814W (green) and WHT/INGRID Ks (red) images. The 850 µm submm image from SCUBA is overlayed as white contours at flux densities of 2.5, 3.3, 5.0, 6.6, 8.3 and 10 mJy/beam. The three images of the multiply imaged submm galaxy are annotated as A, B and C. The yellow line shows the critical line at z=2.515. The right-hand side shows 10"×10" images of the INGRID Ks-band (left column) and HST true-colour image from F450W/F606W/F814W (right column) of the four submm sources in the core of A2218. The contours on the Ks frames show the morphologies of the galaxy in the F814W passband at the resolution of the Ks-band frame. Note how each of the southern submm sources, comprises a near-IR source which is bracketed by two features in the F814W image. The morphological and photometric similarity of these three objects suggests that they are all images of the same background source.

By exploiting a detailed mass model for the cluster lens we estimate a redshift for the source of that z=2.6±0.4. We confirm this estimate using deep optical and near-IR Keck spectroscopy, measuring a redshift of z=2.516.

Principal publications:

Near-infrared line imaging of NGC6240: collision shock and nuclear starburst
Van der Werf, Paul P., Genzel, R., Krabbe, A., Blietz, M., Lutz, D., Drapatz, S., Ward, M.J., Forbes, D.A.
ApJ, 405, 522 (1993)
[ ADS entry ]
H₂ emission as a diagnostic of physical processes in starforming galaxies
Van der Werf, P.P.
in Molecular hydrogen in space
eds. F. Combes & G. Pineau des Forêts
Cambridge University Press, Cambridge, p. 307 (2000)
[ ADS entry | astro-ph preprint ]
A multiply imaged, submillimetre-selected ultraluminous infrared galaxy in a galaxy group at z ~ 2.5
Kneib, J.-P., Van der Werf, P.P., Knudsen, K.K., Smail, I., Blain, A., Frayer, D., Barnard, V., & Ivison, R.
MNRAS, 349, 1211 (2004)
[ ADS entry | MNRAS entry | astro-ph preprint ]

See also:

Starburst galaxies

Galaxy evolution

Interstellar medium of nearby galaxies

ULIRG links