The Interstellar Medium of Dwarf Galaxies

In this project we study the properties of the ISM in galaxies that have low masses. With their shallow potentials, they lack the large-scale density waves or stellar bars which in massive galaxies permanently stir up the gas. In dwarf galaxies, star formation is patchy and hence allows to study the properties of the ISM under strongly varying conditions. Such investigations may help to understand and properly interpret observations of galaxies at large distances or redshifts. While galaxies in the early universe are arguably similar to low-mass galaxies in the local universe, observations of high-z galaxies will not be carried out in any detail for a long time to come. Since stars form out of molecular material, we are primarily interested in the distribution and properties of the molecular gas, best traced by the carbon monoxide. In regions with intense UV radiation fields, the CO molecule is dissociated on the surfaces of clouds, and atomic carbon can be traced in its ionized form. Observations of CO, [CI] and [CII] therefore deliver the best picture of the state of the ISM in the vicinity of star-forming regions, which are also known as photon-dominated regions (PDRs). Another important issue is the (in)famous conversion factor X_CO = N(H₂)/I_CO, which permits to estimate the abundance of molecular gas consisting mainly of H₂. In spite of the debate, its value for the interpretation of high-z galaxies is beyond any doubt. Studying it in environments of varying physical conditions (metallicity, radiation field) is therefore a prime task of Galactic and extragalactic mm astronomy.

We therefore focus our studies on dwarf galaxies with strong to intense star formation, such as for instance NGC 1569, a prototypical object. One may consider its centre as a huge PDR region, heated by the intense starburst. The images below show the Hα (left) and CO (right) emission. The CO emission, mapped with the IRAM 30-m telescope on Pico Veleta, encompasses the innermost brightest portion of NGC 1569. The blow-up contains a map obtained with the IRAM interferometer on Plateau de Bure.

Since a number of powerful telescopes are being deployed in the Atacama desert in the Chilean Andes, we had started a project to investigate galaxies in the southern hemisphere. A prime candidate is, of course, the Large Magellanic Cloud. Its proximity and nearly face-on view makes it an excellent nearby lab to scrutinize the ISM – apart from also being a galaxy. A mapping programme of the CO (1→0) line in southern galaxies is underway, using MOPRA with its 30" angular resolution at 115 GHz. Jorge Pineda was working on the LMC observations as part of his Ph.D. peroject. These show a spectacular distribution of molecular clouds south of 30 Doradus at 8 pc spatial resolution! This was done in collaboration with Jürgen Ott and others. Below we show the situation in the LMC. The image on the left is the CO(1→0) emission mapped by Fukui et al. (1999) with the previous NANTEN telescope. On the right we have juxtaposed the prominent ridge of molecular gas south of 30 Dor as mapped at different angular resolutions, with an 8.8' beam by Cohen et al. (1988) using the Columbia 1.2-m telescope (CTIO), with a 2.6' beam by Fukui et al. (1999) using NANTEN, and with a 33" beam (smoothed to 40" here) by Pineda et al. (in prep.) using MOPRA.

We map the PDR regions of southern galaxies in the CO(4→3) line and in the carbon fine structure, or [CI] lines, ³P₁→³P₀ at 490 GHz and ³P₂→³P₁ at 810 GHz, using the newly deployed NANTEN2 telescope. These observations will provide a beam that nicely matches the measurements with MOPRA. The project will be conducted in collaboration with Yasuo Fukui and Norikazu Mizuno (University of Nagoya), and Carsten Kramer and Jürgen Stutzki (University of Cologne). Summaries of our project goals may be found in two posters, one devoted dwarf galaxies, one dealing with the LMC.

Our research had been financially supported by the DFG in the framework of SFB494 “Die Entwicklung der interstellaren Materie: Terahertz-Spektroskopie in Weltall und Labor”.