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 ( 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 ( 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 ( 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) arXiv0706.3113