Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.stsci.edu/~marel/abstracts/psdir/carneg02laine.ps Äàòà èçìåíåíèÿ: Mon Mar 31 00:35:29 2003 Äàòà èíäåêñèðîâàíèÿ: Tue Feb 5 04:14:39 2013 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: galaxy cluster

Carnegie Observatories Astrophysics Series, Vol. 1:
Coevolution of Black Holes and Galaxies, 2003
ed. L. C. Ho (Pasadena: Carnegie Observatories,
http://www.ociw.edu/ociw/symposia/series/symposium1/proceedings.html)
Central Surface Brightness Profiles
in Brightest Cluster Galaxies
S. LAINE 1;2 , R. P. VAN DER MAREL 2 , T. R. LAUER 3 , M. POSTMAN 2 ,
C. P. O'DEA 2 , and F. N. OWEN 4
(1) STScI
(2) SIRTF Science Center
(3) NOAO (4) NRAO
Abstract
We present results from an HST WFPC2 I­band imaging study of the centers of 81 brightest
cluster galaxies (BCGs). The images show a rich variety of morphological features. We were
able to derive surface brightness profiles for 60 galaxies. Of those, 88% have well­resolved
cores. The remaining twelve percent have ``power­law'' profiles. Therefore, the central
structure of even the brightest galaxies shows a bimodality. A correlation between the power­
law BCGs and the so­called ff parameter, related to the slope of the surface brightness profile
at a large radius, suggests a scenario in which elliptical galaxies evolve from power­law
profiles to core profiles through accretion and merging. This is consistent with theoretical
scenarios which invoke the formation of massive black hole binaries during merger events.
1.1 Introduction
By their very definition, brightest cluster galaxies (BCGs) are highly luminous el­
liptical galaxies. Their high luminosities and their central location in galaxy clusters suggest
that the processes which shape the centers of giant ellipticals should be most easily observ­
able in these systems. Due to their high luminosities, and the strong correlation between
galaxy luminosity and central black hole mass (Kormendy & Gebhardt 2001), BCGs are
expected to harbor the most massive black holes. Due to their central location in galaxy
clusters, we expect them to cannibalize other cluster galaxies even at the current epoch. The
great homogeneity in the global properties and in the environment of the BCGs gives us
perhaps the best opportunity to understand the central structures found in elliptical galaxies.
Therefore, we have chosen to study a large sample of BCGs to learn more about the physical
processes that shape the centers of bright elliptical galaxies. Further details of this work can
be found in Laine et al. (2003).
1.2 Sample
Our final sample is a subsample of the Lauer & Postman (1994) and Postman &
Lauer (1995) sample of 119 galaxies. BCGs were picked randomly from this sample for
HST snapshot observations. In all, 75 galaxies were observed. One of these turned out to be
a foreground galaxy. We added seven BCGs from the HST archive. The observations were
taken with the F814W filter, which is close to the I­band.
1.3 Results
We have discovered the following features in our images:
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S. Laine et al.
Fig. 1.1. Examples of double nuclei. BCGs of Abells 347 and 3526.
ffl Multiple nuclei. 40% of our sample BCGs have a second nucleus within the area imaged
by the PC chip. In two cases, we have detected double nuclei (see Fig. 1.1). However, a
large fraction of the secondary nuclei may be foreground or background projections.
ffl Dust features. We have found dust features in 38% of our sample. These features include
nuclear dust disks, dust filaments, dust rings, dust spirals and patchy dust (see Fig. 1.2).
ffl Nuclear stellar disks. We found two BCGs that have an edge­on disk morphology in the
nucleus, revealed by their high ellipticity.
ffl Central point sources. We identified ten BCGs with bright point sources near the cen­
ter. Such point sources are likely to be related to AGN­like nuclear activity, and were
discarded from the fits to the surface brightness profiles.
ffl Central surface brightness depressions. Six galaxies in the sample have surface brightness
profiles that decrease towards the center (see Fig. 1.3; the sixth galaxy is the BCG of
Abell 347, shown in Fig. 1.1).
We fitted the surface brightness profiles, derived from deconvolved images, with the
Nuker law.
I(r) = I 0 (r=r b ) -fl (1 + [r=r b ] Ü ) fl-fi
Ü (1.1)
We allowed for both positive and negative fl values. The fits were generally performed
over the radial range from 0.02 arcsec (i.e., the central pixel) to 10 arcsec from the galaxy
center. We define the quantity \Gamma 0:05 , which is the power­law slope -d logI=d logr at 0.05
arcsecs from the galaxy center. This is the last reliable radius outside the HST resolution
limit in the deconvolved profiles, and it offers the best view of the cusp slope as the radius
approaches zero. Examples of the fits are shown in Fig. 1.4, and some of the resulting
parameters are in Table 1.1.
Out of the 60 galaxies for which we have fitted surface brightness profiles, 52 have
\Gamma 0:05 Ÿ 0:3 and r b – 0:15 arcsecs. These are core galaxies with well­resolved cores. An­
other six galaxies have \Gamma 0:05 – 0:5. These are power­law galaxies. The remaining galaxy is
an intermediate type galaxy. We also estimated upper limits to the radius of any true break
in the power­law galaxies.
We found that the power­law galaxies exist only among BCGs that are less bright than
M V = -22:6. Figure 1.5 shows the break radius r b for the BCGs, in physical units, vs. the
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Fig. 1.2. Examples of the different dust features.
Fig. 1.3. Examples of galaxies with central light depressions.
absolute galaxy magnitude M V . As reported previously, there is a correlation between r b
and M V in the sense that lower­luminosity galaxies have smaller break radii. The only other
significant correlation that we found is between the central slope and the parameter ff, which
measures the logarithmic slope of the metric luminosity at 10 kpc.
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Table 1.1. Nuker fit parameters. \Gamma 0:05 is the power­law slope at 0.05 arcsec, `` is a core
profile, n is a power­law profile and ``i'' is an intermediate profile. Upper limits for the
break radius r b (in arcsec) are shown in parenthesis.
ABELL r b \Gamma 0:05 Profile ABELL r b \Gamma 0:05 Profile
76 0.28 ­0.03 `` 2040 0.39 0.19 ``
119 0.80 0.06 `` 2147 2.21 0.18 ``
168 0.06 0.19 `` 2162 1.04 0.02 ``
189 0.65 (!0.05) 0.85 n 2197 0.78 0.02 ``
193 0.39 0.23 `` 2247 1.99 (!0.03) 0.85 n
194 0.70 0.10 `` 2572 0.18 0.21 ``
195 0.49 0.10 `` 2589 0.25 0.05 ``
260 0.75 ­0.01 `` 2877 0.98 0.01 ``
261 0.98 (!0.05) 0.76 n 3144 0.39 0.11 ``
295 0.63 0.13 `` 3193 0.26 0.20 ``
347 0.39 ­0.03 `` 3376 1.95 0.05 ``
376 0.68 0.20 `` 3395 0.37 0.04 ``
397 1.06 0.07 `` 3526 1.40 0.10 ``
419 0.47 (!0.1) 0.60 n 3528 0.61 0.18 ``
496 0.73 0.10 `` 3532 0.43 0.18 ``
533 0.31 0.10 `` 3554 0.52 0.04 ``
548 0.40 0.16 `` 3556 0.47 0.10 ``
634 0.23 ­0.04 `` 3558 1.73 0.05 ``
779 1.04 0.03 `` 3562 1.15 0.03 ``
912 0.21 (!0.08) 0.55 n 3564 0.24 0.20 ``
999 0.62 0.05 `` 3570 0.26 0.17 ``
1016 0.23 0.25 `` 3571 1.15 0.02 ``
1142 0.19 0.12 `` 3574 0.76 ­0.02 ``
1177 0.57 0.14 `` 3656 0.99 0.09 ``
1228 0.27 (!0.08) 0.60 n 3677 0.27 0.13 ``
1314 1.19 0.17 `` 3716 0.44 ­0.02 ``
1367 0.93 0.12 `` 3736 0.92 0.13 ``
1631 0.20 0.12 `` 3742 0.22 0.11 ``
1656 2.10 0.03 `` 3747 0.18 0.05 ``
1983 0.10 0.44 i 4038 0.33 0.17 ``
1.4 Discussion
Our results support scenarios in which elliptical galaxies evolve from power­law
profiles to core profiles through accretion and merging. As two galaxies merge, the forma­
tion and coalescence of a binary black hole system creates a core­type surface brightness
profile. More massive galaxies have undergone more accretion events, and this causes core­
type profiles to be more common amongst the most luminous galaxies. If there is no central
massive black hole, any dense nucleus cannibalized may settle into the core of the luminous
galaxy without being disrupted, creating a central power­law profile.
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Fig. 1.4. Examples of Nuker­law fits to our surface brightness profiles. The break
radius is shown with an arrow.
Fig. 1.5. The total luminosity vs. the break radius in pc. A best fitting line to the
combined BCG + Faber et al. (1997) sample is also shown.
1.5 Conclusions
ffl Among 60 BCGs, 88% have ``core­type'' profiles, and the rest have either ``power­law'' or
intermediate­type profiles.
ffl We suggest that BCGs with low luminosities and low ff values have had the least pronounced
accretion histories.
ffl We interpret our results as evidence that elliptical galaxies evolve from power­law to core
profiles through accretion and merging. Such an evolution is naturally explained in theo­
retical studies where the merging progenitors both have central black holes.
References
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Kormendy, J., & Gebhardt, K. 2001, in the 20th Texas Symposium on Relativistic Astrophysics, ed. J. C. Wheeler
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Laine, S., van der Marel, R. P., Lauer, T. R., Postman, M., O'Dea, C. P., & Owen, F. N. 2003, AJ, AJ, 125, 478
Lauer, T. R., & Postman, M. 1994, ApJ, 425, 418
Postman, M., & Lauer, T. R. 1995, ApJ, 440, 28
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