Annual Report 2011 Max Planck Institute for Astronomy

32 II. Highlights
These are galaxies in the special phase during which
their black holes grow by swallowing surrounding matter
and in the process emit enormous amounts of electromagnetic
radiation. Characteristic emission lines allow
the measurement of the black hole mass using a standard
method which relates the width of these emission lines to
the mass of the central black hole (Fig.II.3.1).
However, for these galaxies it is the galaxy’s mass
itself that is the challenge: At such distances, standard
methods of estimating a galaxy’s mass become exceedingly
uncertain or fail altogether. Now we have
for the first time succeeded in directly and simultaneously
“weighing” both a galaxy and its central
black hole at such a great distance using a sophisticated
and novel method. The galaxy studied is SDSS
J090543.56+043347.3, selected from the quasar sample
of the Sloan Digital Sky Survey and lying at a redshift
z 1.3, corresponding to a distance of 8.8 billion lightyears
from us.
The key idea behind dynamical galaxy masses is the
following: Stars and gas in a galaxy orbit its centre. The
different orbital speeds of the gas clouds are a direct function
of the galaxy’s mass distribution. Determine orbital
speeds and you can determine the galaxy’s total mass.
This task was very challenging. Mainly, we had to overcome
two problems: For one, at such a great distance, the
angular size of SDSS J090543.56+043347.3 amounts to
about one arc-second – the apparent size of an ordinary
DVD viewed from a distance of 25 kilometres. In order
to obtain the dynamical mass of the galaxy from the motion
of the galaxy’s gas clouds at different regions across
the galaxy had to be resolved. This was only possible using
the unique combination of the SiNfONi integral field
spectrograph at the Very Large Telescope (VLT) belonging
to the European Southern Observatory (eSO) on Cerro
Paranal in northern Chile. SiNfONi was coupled with an
adaptive optics (AO) system with the ParSec laser guide
star which was co-developed by MPIA, to strongly reduce
the atmospheric distortion of celestial images. SiNfONi is
able to produce a spectrum for each of 1600 pixels over a
3 3 arc-second field while the AO system increased the
spatial resolution from about 1 arc-second to the 0.35 arcseconds
required for this project.
The second difficulty is the fact that SDSS
J090543.56+043347.3 is a quasar, whose central region
emits intense light, several times brighter than the emission
from the underlying galaxy – which poses an
obvious problem. For the measurement of the gas velocity
field, first the extremely intense nuclear light around
the black hole had to be separated from the light emitted
by the moving gas clouds in the rest of the galaxy. Only
after this process, the analysis and modelling of the velocity
structure of the galaxy can become possible, resulting
in the derived dynamical mass inside the central
5.25 1.05 kiloparsecs of the galaxy of MDYN 2.05 +1.68
–0.74
1011 solar masses. The data and analysis involved in
this are shown in Figures II.3.2 and II.3.3.
Combining this result with the mass value of the gal-
axy’s central black hole of M BH,Hα 2.83 +1.93
–1.13 10 8 solar
masses, which we measured from the nuclear Hα
line in the same dataset, we were able to compute the
ratio of black hole to dynamical bulge mass of the galaxy.
As it turns out, this value is nearly the same as that
which would be expected for a present-day galaxy (Fig.
II.3.4). Apparently, nothing major has changed between
now and then: At least out to this distance, 9 billion
years into the past, the correlation between galaxies
and their black holes appears to be very close to
their modern-day counterparts.
When we estimate the expected star formation and
black hole growth in SDSS J090543.56+043347.3 we
find that both black hole and stellar mass are not expected
to grow by more than 50 % between z 1.3
and today – apparently, over this long period of time
stellar and black hole mass do not change very much.
However, what will still happen is that the orbits of the
stars in SDSS J090543.56+043347.3 will be reshuffled
by collisions with smaller companion galaxies from orbits
in the stellar disk to the stellar bulge. This is supported
by the fact that we see indications for a substantial
gaseous disk component in the galaxy but expect
that by z 0 it will be largely a spheroidal galaxy without
many disk stars.
We have now started to expand this novel analysis
to a larger set of 15 further galaxies using a total of 80
hours of SiNfONi time; 30 hours of guaranteed time invested
by MPIA have already been observed, 50 fur-
Fig. II.3.4: Local z 0 scaling relations (symbols and regression
line) for black hole vs. dynamical host galaxy mass (Häring
& Rix 2004, ApJ, 604, L89 with modified values by Sani et
al. 2011, MNRAS, 413, 1479). Overplotted is our estimate of
the dynamical galaxy mass of SDSS J090543.56+043347.3 vs.
the black hole mass, once for an old literature value based on
the MgII line (blue star) and vs. our new, improved black hole
mass based on Ha. There is no discernable offset from the
z 0 relation.
Black Hole Mass [M ]
10 10
10 9
10 8
10 7
108 106 Credit: Katherine J. Inskip
109 10 10 10
Dynamical Bulge Mass [M ]
11 1012 1013