Interstellar medium in nearby galaxies

Paul P. van der Werf

Principal collaborators: Frank Israel, Padelis Papadopoulos, Edwin Valentijn, Rowin Meijerink, Remo Tilanus, Inti Pelupessy, Vincent Icke

Overview:

Stars form from dusty molecular gas. While the molecular gas is probed usually through the trace species CO, it is composed mostly of H₂. However, being homonuclear, the H₂ molecule has no dipole-allowed transitions. The most direct probe of H₂ is by weak quadrupole transitions, the lowest of which lies at a wavelength of 28 µm, and can thus only be detected from space. The first ever detection of the H₂ 28 µm line in any astronomical object outside the solar system was obtained in the nucleus of the nearby galaxy NGC6946, using the ISO satellite. We subsequently detected H₂ emission at a number of positions in the disk of the edge-on galaxy NGC891. As for dust emission, modern submillimetre (submm) techniques allow high-resolution imaging of the dust emission from galaxies, and our submm study of NGC891 is a good example of this. A further key result is the discovery of a diffuse submm exponential disk in the face-on spiral galaxy M51. We also performed physical modeling of the interplay of star formation and the interstellar medium (ISM) in galaxies, focusing on the active ISM in dwarf galaxies.

Highlights:

First detection of the H₂ 28 µm line - The detection of the lowest H₂ rotational line at a wavelength of 28 µm was one of the principal scientific aims of the Short Wavelength Spectrometer on board the Infrared Space Observatory. We obtained the first ever detection of the H₂ 28 µm line in any astronomical object outside the solar system, in the nucleus of the nearby galaxy NGC6946. We also detected the H₂ line at 17 µm (see spectra below).

Spectra of the H2 0-0 S(0) line at 28 µm (left panel) and the H2 0-0 S(1) line at 17 µm (right panel) from the nucleus of NGC6946, as observed by the Short Wavelength Spectrometer on the Infrared Space Observatory.

The H₂ lines are emitted by a component of warm (~170K) molecular gas that contains only a few percent of the total molecular mass as derived from CO observations. The ¹²CO J=1-0 emission expected from this warm component is 10 to 20% of the total CO signal detected. Hence the H₂/CO conversion factor is affected at no more than the 10 to 20% level by the presence of the warm gas. However, in more luminous objects, where a higher fraction of warm gas is expected, the H₂/CO conversion factor may be strongly affected.

We have also detected the H₂ 28 and 17 µm lines from a position in the disk of NGC6946, 4 kpc East of the nucleus (see spectra below).

Spectra of the H2 0-0 S(0) line at 28 µm (bottom) and the H2 0-0 S(1) line at 17 µm (top) from the disk of NGC6946, at a position 4 kpc East of the nucleus.

H₂ 28 µm emission in the disk of NGC891 - We obtained the first ever detection of the H₂ 28 µm and 17 µm lines in the disk of a normal spiral galaxy: NGC891. The two lines were detected at every observed position along the disk of NGC891, out to a distance of 12 kpc from the nucleus, where even CO is barely detected (see image and spectra below). This was the first detection of extragalactic H₂ emission in regions other than starburst nuclei. The H₂ emisison originates from a warm (T~100K), moderate density (n~200 cm^-3) molecular medium, which can most easily be identified with the warm UV-irradiated envelopes of molecular clouds, where CO is photodissociated. The warm H₂ mass involved is several tens of percent of the cold H₂ mass derived from CO observations.

A true colour image of the edge-on spiral galaxy NGC891 (field of view is 6.7'×6.7'). At 8 positions in the disk, even beyond the field of view of this image, H2 emission was detected. The graphical inset shows the spectra of the H2 lines at 17.0 and 28.2 µm wavelength, detected at the outermost position of the survey, at a distance 12kpc from the nucleus, a position where other observations of lines of molecules, like the CO molecule, are trailing into the noise.

Principal publications:

1. The SWS survey for rotational H₂ lines in late-type galaxies. Observations of the central regions of NGC6946
   Valentijn, Edwin A., Van der Werf, Paul P., De Graauw, Th., & De Jong, T.
   A&A, 315, L145 (1996)
   [ ADS entry | A&A entry | ]
2. First extragalactic direct detection of large scale molecular hydrogen in the disk of NGC891
   Valentijn, E.A., & Van der Werf, Paul P.
   ApJ, 522, L29 (1999)
   [ ADS entry | ApJ entry | press coverage | ]
3. Submillimetre maps of the edge-on galaxy NGC891
   Israel, F.P., Van der Werf, P.P., & Tilanus, R.P.J.
   A&A, 344, L83 (1999)
   [ ADS entry | A&A entry | astro-ph preprint | press coverage | ]
4. 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 ]

See also:

Starburst galaxies

Galactic interstellar medium

Extragalactic interstellar medium links