Star-formation, AGN and Ultra-luminous infrared galaxies

An update on mergers from Alfredo:

Star-formation, AGN and Ultra-luminous infrared galaxies (ULIRGs)

Looking at our galaxies in the infrared allows us to discover the overall star-formation rate of the system. Yet if we are interested in the fine details of what really fuel the energetic output of our mergers we need to have a closer look to the light we get from them.

Using a emission lines comparison technique called BPT (after Baldwin & Phillips & Terlevich 1981) we can tell what is really going on at the core of our galaxies.

TV News says we don’t see any significant difference between the general IRAS-detected and the Luminous infrared galaxies. However, when we look at the most luminous infrared galaxies, the ULIRGs the fraction of AGN rises dramatically. This result is important because it conforms to many other studies on the role of AGN in ULIRGs.

We also explore the timescale for a LIRG to become a ULIRG. This is obviously an imperfect analysis for various reasons: firstly not all LIRGs turn into ULIRGs, secondly we use Newtonian mechanics in our calculations and lastly our constraints are quite approximate.

Gyr = gigayear = a billion years

We assume that the Infrared luminosity peaks when the two galaxies coalesce, so we discard all the post-merger LIRGs. Another requirement is that the total mass of the progenitor system is less than the mass of the ULIRGs. We calculate the distance between the two cores and we used a typical group velocity of 400km/s. Under those conditions we find a timescale of 50 million years.

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