..

4J1l- Jv-.L ~ .d~~ b~ ~ MtV?0

~ aJ~~ ~b6d ~ ~

o~~U/J ~7~ il ~ ~7e Or> ~~

d~ ~: ~

oe!O~ ~~oPV 2

~s~

(~;.. ~ ,,-~~d~~~ .... )

- ------ r

Sec. 8.6] QSO chemistry and the growth of galactic bulges I71

12 ~ ...

11'0'" rn ~

rn rn 10

'" a III 9'0 ..c:: I

! .:.: 8 CJ

:c'" ~ 7 .3

6

NGC 4486B.

0 ,

•",.

••• I·,'

,

• ·1".. ~" , , ..'- .,e'''~ ••

eO

,

• 0

8 9 10 11 12 13 log bulge stellar mass (solar)

Figure 8.5. The relation between the masses of black holes in galactic nuclei and the stellar mass in the surrounding bulge, derived from kinematic measures with the Hubble Space Telescope. Both are plotted logarithmically, with solar masses as the implied unit. The dashed line marks a constant fraction 0.5% of the bulge mass, which is a fair representation over three orders of magnitude here. The outlier is NGC 4486B, a companion to M87 in the Virgo Cluster which shows evidence that many of its stars have been tidally stripped during encounters with M87 itself. The three open symbols represent upper limits in black-hole mass. (The data were taken from Magorrian et al., Astronomical Journal, 115, 2285, 1998.)

suggests something special about their surroundings: that there is a close link between early star formation in deep potential wells and the occurrence of a powerful AGN. It has been possible to use dynamical evidence to measure the masses of the central unseen objects in a variety of galaxies, especially using the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope. The demographics of these central masses in galaxies today suggests that the link between these masses and their surroundings extends quite generally to the formation of galactic spheroids. For a wide range of galaxies, active and inactive, there is a good correlation (Figure 8.5); often called the Magorrian relation between the stellar mass in the spheroidal bulge component and the mass of the central object (presumably a supermassive black hole), such that the central object has about 1/200 the mass of the stars. This is most naturally explained if growth of the central object is a byproduct of evolution in the spheroid, since if it came first there is no obvious way to regulate the amount of surrounding mate~ial condensed into stars. In contrast, the growth of a central point mass in a stellar system can be self-regulating, and will saturate in a non-interacting system once all stars on "death orbits" have been disrupted. Scattering of additional stars into the region of phase space that is vulnerable to such deep encounters is very slow, so the central black hole will stop rapid growth once its mass becomes dynamic­ally significant in the inner regions.

It is important that this mass relation holds for such a wide variety of present-day galaxies, spiral bulges as well as ellipticals from M32 to M87. This makes it difficult to