Mergers and Ultraluminous IRAS Galaxies

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Ultraluminous Infrared Galaxies

• L_fir > 10¹² L_sun (quasar-like)
• Spectrum peaks at 60-100 microns (hot dust T ~ 50 K)
• Large, concentrated molecular gas masses (few x 10¹⁰ M_sun in inner kpc)
• Energy Source: Starburst or AGN? Not easy to disentangle, probably a combination.
• Optical morphology indicative of galaxy mergers:

from Hibbard & Yun (1999)

[image source here]

The Qualitative Scenario

Step 1: Gas-rich disk galaxies collide.
Step 2: As the galaxies merge, the gas dissipates energy and flows to the center.
Step 3: Central activity (starburst or AGN) ignites.

The Problem of Timescales

Merging timescale:	~ 109 years
Dynamical timescale:	~ 108 years
Starburst timescale:	~ 107 years

But ULIRGs are predominantly in the final stages of merging. Look at the distribution of projected separations  It's not what we expect from a simple model of interaction-induced starburst. Furthermore, under such models, Starbursts should occur early, while the galaxies are widely separated. Gas depletion would result in gas-poor, quiescent mergers.

Something's wrong. What don't we understand?

• dynamics of inflows?
• physics of starbursts/AGN?