Interstellar Medium and Star Formation in Galaxies

My research in this field devides into the following related lines:

We are interested in this material outside the parent galaxies. With carbon monoxide (CO) observations we try to find out what happens to the molecular gas during galaxy interactions. Is the molecular gas torn out of the parent galaxies and how is the mechanism for it? Are collisions between molecular gas clouds important? Is molecular gas forming from HI outside galaxies? How is star formation (SF) happening in this material?
Our CO observations have shown that molecular gas is amazingly ubiquitous in the expelled gas in interactions. We have found large amounts of molecular gas in the intragroup medium of Stephan's Quintet (Lisenfeld et al. 2002) and in the bridges of head-on collisional systems (Braine et al. 2003, 2004).

Tidal Dwarf Galaxies (TDGs)  are  small galaxies forming from material ejected from the disks of spiral galaxies through collisions.TDGs are an exciting topic since they are the only galaxies currently forming in the local universe. They allow therefore to study processes -- galaxy formation and evolution and the onset of SF -- similar to what occurred in the early universe but in local objects that can be observed at a high sensitivity and resolution. Many questions are still open with respect to their origin and future. Its is e.g. neither clear whether they are gravitationally bound objects nor for how long they will survive.  Another open question is how common the origin in a tidal interaction is for dwarf galaxies in general.

We have been interested in the molecular gas and SF in these objects and have tried to address questions as: How is molecular gas (traced by CO) related to the SF regions (traced by Halpha emission)? How does SF proceed in TDGs? We approaching these questions mainly by CO observations with the IRAM 30m telescope and PdB interferometer of different TDGs.
The observations have shown that the molecular gas is generally closely associated with Halpha and that the CO line width is consistent with a low DM content of TDGs (Braine et al. 2000, 2001).

Related papers of our group:

• Braine J.,  Lisenfeld, U., Duc, P.-A., Leon, S., 2000, Nature, 403, 867, Formation of molecular gas in the tidal debris of violent galaxy-galaxy interactions
• Braine, J., Duc, P.-A., Lisenfeld, U., Charmandaris, V., Vallejo, O., Leon, S., Brinks E.,  2001, A&A, 378, 51, Abundant molecular gas in Tidal Dwarf Galaxies: On-going galaxy formation
• Braine, J.,,Davoust, E., Zhu, M., Lisenfeld, U., Motch, C., Seaquist, E. R., 2003, A&A, 408, L13, A molecular gas bridge between the Taffy galaxies
• Braine, J., Lisenfeld, U., Duc, P.-A., Brinks, E., Charmandaris, V., Leon, S., 2004, A&A, in press, Colliding molecular clouds in head-on galaxy collisions
  
• Lisenfeld, U.,  Braine, J., Duc, P.-A., Leon, S., Charmandaris, V., Brinks, E., 2002, A&A, 394, 823, Abundant molecular gas in the interegalactic medium of Stephan's Quintet
• Lisenfeld, U.; Braine, J.; Vallejo, O.; Duc, P.-A.; Leon, S.; Brinks, E.; Charmandaris, V., 2002, Modes of Star Formation and the Origin of Field Populations, ASP Conference Proceedings, Vol. 285. Edited by Eva K. Grebel and Walfgang Brandner. ISBN: 1-58381-128-1. San Francisco: Astronomical Society of the Pacific, 2002., p.406, Star Formation in Tidal Dwarf Galaxies

A mayor and long-lasting debate in astronomy involves that relative  roles of ``nature'' and ``nurture'' in galaxy formation and evolution. It is now well known that certain types of galaxy activity, as, for instance, the SF activity, are strongly affected, or even driven, by the  influence of nearby galaxies.  What is so far lacking is a well-defined reference sample of isolated galaxies in order to assess the frequency and amplitude of the external influences.

In collaboration with a group of scientists, mostly from the  Instituto de Astrofísica de Andalucía, we are producing such a reference sample, consisting of about 760 isolated galaxies taken from the sample of Karachentseva (1973, Comm. Spec. Ap. Obs, USSR 8, 1; 1980, Sov. Stro. 24, 665). This project is called AMIGA (Analysis of the interestellar Medium of Isolated GAlaxies) and more information (as well as, at a later stage, the data for the sample) can be found at http://www.iaa.es/AMIGA.html). The database will consist of:

• blue luminosity as a tracer of visible light, stellar content and, in a   rough way, the mass,
•  the FIR emission where most of the flux from newly formed stars is   re-radiated
• atomic gas, as a fundamental ingredient of the interstellar medium (ISM) and a very sensible   tracer of interaction,
• molecular gas (traced by CO) , as the raw material for SF
• Halpha luminosity, a good tracer for recent SF in places where  the extinction is not high and, 
• radiocontinuum emission as a useful tracer, that is not affected by  extinction, for the current SF rate and nuclear activity. 

This database will allow us to perform a statistical study of the properties of the ISM (cold/warm/hot gas and dust)  and SF of isolated galaxies.  These data will provide a basis to quantitatively evaluate the properties of interacting galaxies, which is of special interest  in view of the large amount of data that will be generated  during the next decade for high redshift galaxies -- many of which are interacting -- with new, sensitive  instruments (e.g. ALMA, GTC). This project has started one year ago and a large fraction of the data has already been observed or collected from the literature.

Related papers of our group can be found at http://www.iaa.es/AMIGA.html/papers.html

Between the ISM and massive SF in galaxies exists a tight relation. Dense gas is the fuel for SF and  therefore  the observations of molecular gas allows to study the conditions enabling SF.  Massive stars, on the other hand, react back to the ISM by their ionizing emission and their energy input through supernova explosions. Radio observations help in different ways to study these processes:

- In the millimeter range, observations of the molecular gas (above all CO) are the basis to investigate the conditions for the formation of stars. Observations of the dust emission give us complementary information about the dense interstellar medium being the site of SF. The dust emission at mm wavelength is particularly interesting because it traces cold dust which represents the bulk of the dust mass. Dust emission is furthermore a good tracer of the SFR which is not affected by extinction.

- The radio emission in the cm wavelength range traces mainly synchrotron emission of CR electrons. The observations of this emission in edge-on  galaxies allows to study radio halos which are most likely driven by to the energy input of supernovae. With observations of the molecular gas and dust in the millimeter range and CREs in the cm radio range we are trying to approach the following questions: