(perpetually) UNDER CONSTRUCTION

Luminous Infrared Galaxy observations with BIMA

Robert Gruendl, Yu Gao, K.Y. Lo, Chorng-Yuan Hwang, Siow-Wang Lee, Ting-Hui Lee, and Wei-Hao Wang

Introduction

Luminous infrared (IR) galaxies (LIRGs) are so-called because they emit most of their bolometric luminosity in the far-IR (up to 90%). LIRGs are the dominant class of galaxies in the local universe at these high luminosities (a few times 10^11 solar luminosities) and most LIRGs are found in interacting/merging galaxy systems that are rich in molecular gas. The two most widely considered explanations for the high IR luminosity are:

that starbursts (concentrated bursts of star formation) are occurring in the molecular gas reservoir that has been organized by the interaction between the host galaxies. The newly formed stars then heat the surrounding dust which re-radiates the energy in the IR.
that an AGN (Active Galactic Nucleus) ``monster'' which may be either pre-existing or have been recently formed and is now being fueled by the inflow of molecular material be driven in to the galaxy center by the interaction.

However, it is not clear whether starbursts, an AGN, both, or neither dominate most of the IR luminosity. In either case, numerical simulations of galaxy-galaxy interactions predict that a large fraction of the gaseous material will be driven inward where presumably the conditions for star formation are more likely. These predictions have not been systematically investigated through observations.

Previous CO imaging studies have concentrated on relatively more advanced merger systems (e.g., Scoville et al. 1991) in which the interstellar medium (ISM) has already been highly disrupted by the galaxy-galaxy interactions and the starbursts. A study of the molecular gas properties at various phases of the merging process in LIRGs would help identify the key physical processes involved. In order to isolate the conditions in the ISM leading to starbursts, we have started a program to study a sample of LIRGs chosen to represent different phases of the interacting/merging process, using the newly expanded Berkeley-Illinois-Maryland Association (BIMA) millimeter array (Welch et al. 1996). The goal is to sample statistically the evolution of physical conditions of the molecular material in LIRGs as compared with the properties of the IR emission along a merger sequence. The sample as a whole will provide insight into the galaxy-galaxy interactions, evolution and starbursts which take place when galaxies merge.

A Merger Sequence

One of the differences between this study of LIRGs and past efforts is that we are attempting to systematically probe the evolution of the interstellar medium by observing many IR bright galaxy systems that appear to be at different stages of an interaction -- a merger sequence. The systems we are currently observing were chosen to be IR bright (and therefore presumably gas rich) and with previous single-dish CO observations to insure that they were detectable with BIMA. Second the projected separation between the nuclei was considered in an attempt to chose galaxy pairs at varying stages of interacting. The distances to these systems vary by more than a factor of two (65-165 Mpc) which highlights the scarcity of nearby systems and the difficulty in choosing a uniform yet observable set of objects.

The current sample of galaxies observed can be divided into four rough categories:

Early Mergers
These systems show little or no disruption due to the interaction and are separated by distances roughly the same as their optical size. So far we have observations of three systems which might fit into this category.
Arp 302, NGC 6670, UGC 2369, Arp 293, UGC 8335, and IRAS 14394+5332
Intermediate Mergers
The nuclei of the intermediate stage mergers are still clearly separated but now the optical disks also overlap. Furthermore, all these systems show an obvious sign that they are being perturbed. (In each case below the galaxies show tidal tails which have been drawn out by the passage of the two galaxies.)
Mrk 848, Arp 55, Arp 244 ("the Antennae"), and NGC 7592.
Advanced Mergers
In the advanced stages of a merger the two galaxies will coalesce into a single object. While all the objects shown here have obvious signs that they are being disrupted, the nuclei are beginning to merge/overlap.
NGC 5256, NGC 6090 and NGC 1614
Isolated systems:
In our search for candidates to be observed we also found two systems that are apparently not engaged in an interaction with another galaxy yet had IR luminosities that are comparable to the interacting systems in our sample.
NGC 3110 and NGC 3147

The BIMA observations

The BIMA array is ideally suited to study these LIRGs given its large primary beam and wide spectral bandwidth. (Only a single pointing is necessary to observe these objects with the exception of Arp 244.) The observations presented here were all made with the 9-element BIMA array in the H or C configurations in 1996. Some of these LIRGs are currently being imaged with higher resolution ~2'' in the B-array and plans are underway to observe more systems in the future.

(perpetually) UNDER CONSTRUCTION

Luminous Infrared Galaxy observations with BIMA

Robert Gruendl, Yu Gao, K.Y. Lo, Chorng-Yuan Hwang, Siow-Wang Lee, Ting-Hui Lee, and Wei-Hao Wang

Introduction

A Merger Sequence

Early Mergers

Intermediate Mergers

Advanced Mergers

Isolated systems:

The BIMA observations

Some Miscellaneous References that We Have Pulled Together