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  Current Research Highlight :: August 2007 all highlights

Connecting star formation and black hole growth in local galaxies.

One of the big puzzles in astrophysics today is the cause of the observed correlation between the mass of black holes and the mass of the galaxy bulges in which they live. Evidently black holes and galaxy bulges must have had a common growth history, but what is this history and why do they share it? Scientists at Max Planck Institute for Astrophysics (MPA) have used the Sloan Digital Sky Survey to investigate the recent star formation history of galaxy bulges in the local Universe, and compare with their current black hole growth rates.

Fig. 1: A statistical analysis of the spectra of galaxy bulges have provided astronomers with new parameters which quantify the recent star formation history of galaxies. The different regions of the diagram are inhabited by galaxies with different star formation histories as labelled. Here the regions of the diagram have been colour coded according to the rate of accretion of the black hole at the center of the galaxy. Red is strong accretion and purple is weak accretion. The black points indicate the position of galaxies with the highest black hole accretion rates, which noticeably do not all lie in the starburst/post-starburst region of the diagram.

Fig. 2: Sloan Digital Sky Survey images of a sample of the galaxies identified by the scientists as having experienced a recent strong burst of star formation in their central bulge. Notice the streams of stars around the main galaxy, dust lanes, and lopsidedness of the galaxy in some images: signs of a recent gravitational disruption probably through an encounter with another galaxy. The images are 1' square, and the circle in the top left shows the size over which the spectra are taken, and therefore the size in which the star formation histories are derived.

It is now believed that at the center of every massive galaxy in the local Universe resides a supermassive Black Hole. Observationally it has been determined that this Black Hole makes up about 0.1% of the bulge mass, regardless of the size of the galaxy (linkPfeil.gif Research Highlight April 2005). There are many theories to explain this correlation, from the rapid build up of all the black hole and bulge mass early on in the life of the galaxy, to a gradual build up of both through slow accretion of gas into the bulge of the galaxy tto fuel both star formation and subsequently the black hole (linkPfeil.gif Research Highlight February 2005).

The observational indication of a growing black hole comes in the form of high excitation lines emitted from ions in the gas surrounding the black hole (seyferts and liners), or scattered light directly from the accretion disk feeding the black hole (quasar). These objects are called Active Galactic Nuclei (AGN). In particular, in the optical wavelength range, the [OIII] emission line gives a good estimate of the black hole growth rate (linkPfeil.gif Research Highlight July 2004). In order to combine this information on the black hole growth with the recent star formation history of galaxies, Vivienne Wild, Guinevere Kauffmann and Tim Heckman (Johns Hopkins University, USA) developed a new method based on a principal component analysis (PCA) of the galaxy spectra. By combining information from all the pixels in the galaxy spectrum, this method allows useful information to be derived from spectra of much lower quality than with previous methods.

Using their new method, the scientists determined the recent star formation history of ~33,000 low redshift bulge-dominated galaxies from the Sloan Digital Sky Survey (SDSS). Because these galaxies are near by, the spectra obtained for them only cover their centers, and thus the recent star formation history derived for them applies only to the bulge of the galaxy. The scientists found that the galaxy bulges could be classified into three main groups: (1) Those that have no sign of any recent star formation (55%); (2) Those that have undergone a recent burst of star formation, where the mass of stars formed in the burst is greater than ~1% of the bulge mass (4%); (3) Those that have some ongoing star formation, but no signs of a strong recent or ongoing starburst (39%). They also found that although almost half of the AGN are found in bulges in the 1st class, with no sign of recent associated star formation, the black holes in these bulges are growing only extremely slowly. In fact the majority of black hole growth in the local Universe (>60%) is coming from black holes hosted by bulges in the 3rd class - with some ongoing star formation, but no evidence for spectacular recent or ongoing starbursts. This contradicts recent theories that the mechanism responsible for concurrent black hole and bulge mass growth is through massive mergers of galaxies and rapid build up of both.

However, the 4% of bulges which were identified as having undergone a recent strong burst of star formation, do show signs of a recent disturbance - tidal tails, excess "lopsidedness" in their light distribution and the older bursts are hosted by compacter galaxies. These bulges are more than twice as likely to host black holes which are accreting very strongly than those galaxies with ordinary levels of star formation.


Vivienne Wild

Publication

Vivienne Wild, Guinevere Kauffmann, Tim Heckman, Stephane Charlot, Gerard Lemson, Jarle Brinchmann, Tim Reichard, Anna Pasquali,
"Bursty Stellar populations and obscured AGN in galaxy bulges",
Mon. Not. R. Astron. Soc., accepted, (2007) linkPfeilExtern.gifarXiv0706.3113


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