Proceedings of the
Workshop
"
The Magellanic Clouds and Other Dwarf Galaxies"
of the
Bonn/Bochum-Graduiertenkolleg
The Center of the Fornax Cluster:
Dwarf Galaxies, cD Halo and Globular Clusters
1Sternwarte der Universität Bonn,
Auf dem Hügel 71, D-53121 Bonn, Germany
2Departamento de
Astronomía y Astrofísica,
P. Universidad Católica, Santiago, Chile
3UCO/Lick Observatory,
University of California, Santa Cruz, USA
Received 12th March 1998
1. Introduction
The centers of galaxy clusters are the sites of the highest galaxy density
in the universe. Among the various types dwarf ellipticals (dE) are the most
strongly clustered galaxies in high-density environments.
Several striking characteristics are seen in the center region of clusters:
(1) most central galaxies possess extraordinarily rich globular cluster systems;
(2) often a cD galaxy exists; (3) different
types of dwarf galaxies have different clustering properties and in some cases
the faint end of the dwarf galaxy luminosity seems to depend on the
cluster-centric distance; (4) often an X-ray halo, which constrains the
(dark) matter distribution, exists. The question arises, whether these
properties have some interrelation.
We want to focus our attention on the properties of the
dwarf galaxy population of the compact and evolved Fornax galaxy cluster.
The study is based on photometric and spectroscopic observations of
galaxies in the central region of the Fornax cluster.
Moreover, the question is addressed, whether a scenario in which dwarf
galaxies have been accreted and dissolved in the cluster center can explain the
observed properties of the extraordinarily rich globular cluster system and the
extended cD halo around the central galaxy NGC 1399.
2. Photometry & Spectroscopy
About 870 galaxies were identified in selected CCD fields (covering
0.17 square degrees) down to a limiting magnitude of
V≅22 mag.
Their photometric properties, such as total V magnitude, (V-I)
color, peak and effective surface brightness, and effective radius of these
galaxies as well as the profile fit parameters (exponential and/or de
Vaucouleur laws) of the surface brightness profiles of a subsample are
compiled in a catalog (Hilker et al. 1998a).
In spite of the high surface density of galaxies, we found only 4
new Fornax members (including 2 compact objects, see below) beyond the
Fornax Cluster Catalog (FCC, Ferguson 1989).
Our limiting surface brightness of about
24 mag arsec-2, similar to the FCC, prevents us from
detecting the equivalent of the faint Local Group dwarf spheroidals in Fornax.
However, among the high surface brightness
objects compact dwarfs might be hidden, as shown by 2 newly
discovered nucleus-like Fornax members.
A significant excess of faint galaxies around NGC 1399 is caused by the
superposition of a background galaxy cluster on the center of the
Fornax cluster. The average radial velocity of 19 members of the background
cluster is vhelio = 33 700 km s-1
(z = 0.11).
Our spectroscopic sample contains 94 galaxies brighter than about
Vtot = 20 mag (Hilker et al. 1998b).
8 objects were identified as Fornax members due to their radial velocities.
5 of them are already listed as dE,Ns in the FCC.
Among the 3 ``new'' Fornax members, there are 2 that do not follow
the µ-V relation of dEs, but are very compact and have very
high surface brightness.
The measured line indices (especially Mg2, Hβ, and iron) of the brighter
of the compact objects (MV = -13.4 mag) point to a
solar metallicity, whereas the fainter compact object
(MV = -12.2 mag) as well as the dE,Ns has line indices
that are similar to those of old metal-poor globular clusters (GCs).
However, it is impossible to identify clearly the compact objects either as
very bright GCs, isolated nuclei of dE,Ns, or even compact ellipticals.
2.1. NGC 1427A
The Magellanic type irregular NGC 1427A is an outstanding object in the
center region of the Fornax cluster. It is about 3 magnitudes brighter than
other dIs in the cluster.
NGC 1427A is comparable in size, luminosity and color to the LMC.
A half-ring of OB associations and H II regions in the
south-west and a ``blob'' in the north indicate recent star formation.
We found about 34 cluster candidates which are uniformly distributed over
the galaxy. Their color as well as their luminosity distribution is comparable
to the young and intermediate-age cluster population of the LMC
(ages <2-4 Gyr).
Two possible explanations for the recent star formation in NGC 1427A are
the triggering of star formation by the passage of NGC 1427A through the
gas associated with the Fornax cluster or by the collision with another dwarf
galaxy. More details are given in Hilker et al. (1997).
3. The dwarf galaxy infall scenario
We compared the slopes and extensions of the surface density profiles of
the GCs and galaxies, the surface brightness profile of the galaxy light,
and the particle (number) density profile of the X-ray gas with each other.
The similarity of the slopes of blue (= metal-poor) GCs and
cD halo points to a common dynamical status which is also supported
by similar velocity dispersions.
In contrast, the slope of the dwarf galaxy distribution is significantly
flatter in the center of the cluster.
Further, the flat slope of the faint end of the dwarf luminosity function
suggests that many dwarf galaxies might have been accreted in the center
region of the cluster and the disrupted material was distributed in the
flat extended envelope.
NGC 1399 possesses around 5800 GCs, about 10 times more than the other
ellipticals in the Fornax cluster (e.g. Kissler-Patig et al. 1997).
About 5000 GCs can be assigned to the cD halo under the assumption that
the bulge has a ``normal'' specific frequency of 4.
With view on the efficient formation of GCs in merging events and starburst
galaxies (e.g. Meurer et al. 1995; Whitmore & Schweizer 1995), the
specific frequency of the cD halo is compatible with the GCs formed in
infalling gas-rich dwarf galaxies.
A detailed treatment of this topic, considering also other processes like
stripping and capture of rich GCSs of gas-poor dwarfs and the survival
of nuclei of dissolved dwarf ellipticals, can be found in Hilker (1998,
Chapter 5).
References
- Ferguson H.C., 1989, AJ 98, 367
- Hilker M., 1998, PhD
thesis, University of Bonn
- Hilker M., Bomans D.J., Kissler-Patig M., Infante L., 1997,
A&A 327, 562
- Hilker M., Kissler-Patig M., Richtler T., Infante L., Quintana H., 1998a,
A&AS, in press [astro-ph/9807143]
- Hilker M., Infante L., Vieira G., Kissler-Patig M., Richtler T., 1998b,
A&AS, in press [astro-ph/9807144]
- Kissler-Patig M., Kohle S., Hilker M., Richtler T., Infante L., Qintana H.,
1997, A&A 319, 470 [astro-ph/9608082]
- Meurer G.R., Heckman T.M., Leitherer C., Kinney A., Robert C., Garnett D.R.,
1995, AJ 110, 2665
- Whitmore B.C., Schweizer F., 1995, AJ 109, 960
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| First version: | 02nd | July, | 1998
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| Last update: | 29th | September, | 1998
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Jochen M. Braun &
Tom Richtler
(E-Mail: jbraun|richtler@astro.uni-bonn.de)