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Science Spotlight from this week's observations

Proposal ID = 12613
Principle Investigator =  Knud Jahnke, Max-Planck-Institut fur 
	  Astronomie, Heidelberg	
Title = "Are major galaxy mergers a significant mechanism to trigger 
      massive black hole growth at z=2?"
Time = Apr 29, 2012 05:42:05 - 06:28:11
Target = SDSSJ145645.53+110142.6 
Instrument = WFC3/IR

Background:

It is believed that massive black holes reside in the centers of most, if not all, galaxies. Some of these black holes are the sites of Active Galactic Nuclei (AGN), or quasars, resulting from the accretion of gaseous material falling onto the black hole. Why aren't all massive black holes the sites of quasars? A popular model is that the gas has been disturbed from its typical circular orbit around the galaxy by a major merger, i.e., the collision with a roughly equal mass galaxy. This interaction changes the trajectory of the gas and stars. Some of this material falls in toward the center of the galaxy and acts to "feed the monster" black hole, producing the quasar. The problem is that there is only limited observational evidence to support this model; the frequency of quasars in merging galaxies is not clearly higher than in normal galaxies. The proposers are using the WFC3 to look for a correlation at higher redshift, closer to the peak of star formation around z~2. This is an example of a snapshot proposal, where a long list of potential targets are provided but only a fraction are actually observed. This provides more flexibility when scheduling the observations, hence providing a more efficient use of the observing time.

Paraphrasing from the abstract:

We propose to test the common and plausible, but empirically unsupported assumption, that black hole accretion in luminous quasars is triggered by major, gas-rich mergers. We have already shown with the aid of ACS imaging that at recent times, since z~1, there is no evidence for triggering of nuclear activity being dominated by merging. Crucially, with the arrival of WFC3, such a test has now also become possible at the peak of black hole growth (at z~2), where the mode of accretion may well be different and, indeed, related to merger events. We will obtain WFC3/IR SNAPshot observations of ~45 high-mass quasars at z~2 with luminosities that dominate the luminosity function and, hence, black hole growth. The inferred major merger fraction will then be compared to the major merger fraction in a sample of 150 galaxies (from the CANDLES project) without active nucleus but that are similar in mass and redshift, and analyzed in a fully analogous manner. From the sample sizes and the demonstrated ability to identify major mergers in WFC3 imaging, it follows that we will determine at the 95% confidence level whether luminous quasars have an increased merger fraction by at least a factor two and, hence, whether at z~2 major mergers play a significant role in black hole accretion.


You can find most of this information and more from the following webpage: http://www.stsci.edu/hst/ by entering "12613" in the Prop. ID box.