Issue |
A&A
Volume 498, Number 3, May II 2009
|
|
---|---|---|
Page(s) | 705 - 710 | |
Section | Cosmology (including clusters of galaxies) | |
DOI | https://doi.org/10.1051/0004-6361/200811248 | |
Published online | 19 March 2009 |
A search for massive ultra-compact dwarf galaxies in the Centaurus galaxy cluster
(Research Note)
S. Mieske1 - M. Hilker2 - I. Misgeld2,3 - A. Jordán4,5 - L. Infante4 - M. Kissler-Patig2
1 - European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
2 -
European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
3 -
Argelander Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
4 -
Departamento de Astronomía y Astrofísica, Pontificia
Universidad Católica de Chile, Casilla 306, Santiago 22, Chile
5 -
Harvard-Smithsonian Center for Astrophysics,
60 Garden St., Cambridge, MA 02138, USA
Received 29 October 2008 / Accepted 27 January 2009
Abstract
Context. In a previous paper, we initiated a search for ultra-compact dwarf galaxies (UCDs) in the Centaurus galaxy cluster, resulting in the discovery of 27 compact objects with
-12.2<MV<-10.9 mag. Our overall survey completeness was 15-20% within 120 kpc projected clustercentric distance.
Aims. To constrain the luminosity distribution of the brightest UCDs in Centaurus in a more accurate way, we continue our search by improving substantially our survey completeness specifically for brightnesses of MV<-12 mag (V0<21.3 mag).
Methods. Using VIMOS at the VLT, we obtained low resolution spectra of 400 compact objects with
19.3<V0<21.3 mag (
-14<MV<-12 mag at the Centaurus distance) in the central 25' of the Centaurus cluster, which corresponds to a projected radius of 150 kpc. Our survey had complete area coverage within a radius of
kpc.
Results. For 94% of the sources included in the masks, we measure successfully a redshift. Because of incompleteness in the slit assignment, our final completeness in the area surveyed is 52%. Among our targets, we find three new UCDs in the magnitude range
-12.2<MV<-12 mag, hence at the faint limit of our survey. For one of these targets HST WFPC2 imaging is available, yielding a size estimate of
pc. At 95% confidence, we can reject the hypothesis that in the area surveyed there are more than 2 massive UCDs with MV<-12.2 mag and
pc. Hence our survey confirms the extreme rarity of massive UCDs. We find that the radial distributions of Centaurus and Fornax UCDs with respect to their host clusters' centres agree within the 2
level.
Key words: galaxies: clusters: individual: Centaurus - galaxies: dwarf - galaxies: fundamental parameters - galaxies: nuclei - galaxies: star clusters
1 Introduction
A new class of compact stellar systems called ``ultra-compact dwarf galaxies'' (UCDs, Phillipps et al. 2001) has been established (Hilker et al. 1999; Drinkwater et al. 2000, 2003; Hasegan et al. 2005; Jones et al. 2006; Mieske et al. 2007; Firth et al. 2007; Misgeld et al. 2008). UCDs are characterised by typical luminosities of -13.5<MV<-11.0 mag, half-light radii of

To broaden the environmental baseline of UCD research, we have embarked on UCD searches in the Centaurus galaxy cluster (Mieske et al. 2007) and the Hydra I galaxy cluster (Misgeld et al. 2008), based on data obtained with VIMOS at the VLT (program 076.B-0293). From the UCD search in Centaurus, we have found 27 compact objects with radial velocities consistent with them being members of Centaurus, covering an absolute magnitude range -12.2 <MV< -10.9 mag. Their distribution in magnitude and space was found to be consistent with that of the GC population. We found no very luminous UCDs with -13.5 < MV< -12.2 mag as found in the Virgo and Fornax cluster (Jones et al. 2006; Chilingarian & Mamon 2008; Drinkwater et al. 2000), which may be due to the moderate overall completeness of 15-20% within 120 kpc: the survey covered about 50-60% of the total area within 120 kpc, within which only 30% of the photometrically selected sources were observed, because of too high candidate density (see Mieske et al. 2007; and Fig. 2).
In this Research Note, we report on the second part of our search for
UCDs in Centaurus with VIMOS (program 380.B-0207), in which we have attempted to constrain more accurately their luminosity distribution at the bright
end. We increase the area coverage and focus on the bright luminosity
regime MV<-12 mag. Our medium-term strategy is to derive dynamical
mass estimates for UCDs beyond the Fornax and Virgo clusters to
investigate in depth whether average M/L ratios of UCDs systematically
vary with environment. Technically, ground-based medium-to-high
resolution spectroscopy (
)
is possible only for objects
with
mag (e.g., Mieske et al. 2008a; Chilingarian
et al. 2008), which translates to a feasibility limit of
mag at the distance of the Centaurus cluster
(
mag, Mieske et al. 2005). This is
another driver for focusing on the bright luminosity regime MV<-12 mag.
2 The data
The data for this publication were obtained in service mode with the VIsible MultiObject Spectrograph VIMOS (Le Fevre et al. 2003) mounted on UT3 Melipal at the VLT (programme 380.B-0207). VIMOS allows simultaneous observing of 4 quadrants, each of dimension

2.1 Candidate selection
The candidates for our search for bright UCDs were selected from the VIMOS pre-imaging in the V and R filters which were taken under clear conditions. Prior to applying any selection, we matched the detections in V,R to the catalog of well calibrated FORS photometry (Mieske et al. 2005) of the central Centaurus cluster in V and I, whose areas overlap with the VIMOS pre-imaging. From this matching we were able to verify that the V-band VIMOS zeropoints available from the ESO QC web pages![[*]](/icons/foot_motif.png)
For dereddening the apparent magnitudes we used data from Schlegel et al. (1998). To select sources as compact object candidates, we defined three criteria regarding size, colour, and luminosity:
- 1.
- unresolved in the VIMOS pre-imaging date (as judged by the
SExtractor star-galaxy separator, Bertin & Arnouts 1996).
At the distance of the Centaurus cluster (45 Mpc, Mieske et al. 2005), the typical PSF FWHM of 0.85'' corresponds to
190 pc. In Fig. 1, we present SExtractor star-galaxy separator values measured for simulated seeing convolved UCD images that had been superimposed on our VIMOS pre-images. The simulations were based on the structural parameters of UCD3 (Hilker et al. 2007), assuming a Centaurus cluster distance of 45 Mpc (Mieske et al. 2005), and a seeing of 0.85'' (
190 pc). UCD3 itself has
pc. To simulate UCDs of a range of sizes, we scaled the surface brightness profile of UCD3 to smaller/larger radii. The plot shows that the SExtractor star classifier value flips from ``star'' to ``galaxy'' for
pc. Our size selection criterion hence encompasses all known UCDs, except the two most massive ones, each of which have
pc (Evstigneeva et al. 2008). This corresponds to
95% of all known UCDs, and
85% of known UCDs with MV<-12 mag
;
- 2.
- with de-reddened colours of
0.42<(V-R)0<0.9 mag. This (V-R)0range corresponds to a (V-I)0 range from 0.65 to 1.50 mag (see
Fig. 3), which is the colour range typically covered by GCs
(e.g. Gebhardt & Kissler-Patig 1999; Larsen et al. 2001; Kundu & Whitmore 2001). This also
includes the colours of UCDs discovered in the first part of our
search (Mieske et al. 2007). Figure 3 shows the
directly measured (V-R)0 colours, and the corresponding (V-I)0scale. This scale is derived from matching the V and R preimaging
photometry of unresolved sources with V, I photometry from spatially
overlapping FORS data (see above; Mieske et al. 2005). The
derived scaling in apparent magnitude space is
with an rms of 0.11 mag; - 3.
- with dereddened apparent magnitudes 19.3<V0<21.3 mag ( -14<MV<-12 mag). The faint magnitude cut is more than a magnitude brighter than in the first part of our search (Mieske et al. 2007).
![]() |
Figure 1:
This plot illustrates the upper limit to UCD effective
radius that our candidate selection criterion for unresolved sources
corresponds to. The y-axis shows the SExtractor star-classifier
value (Bertin & Arnouts 1996), which is defined to be 1
for an unresolved source (``star''), and 0 for a resolved source
(``galaxy''). The SExtractor star-classifier value flips from
``star'' to ``galaxy'' for
|
Open with DEXTER |
![]() |
Figure 2:
Map of the central Centaurus cluster. The relative
coordinates are with respect to NGC 4696, the central galaxy of the
Cen30 subcluster. The dotted squares indicate the VIMOS pointing
observed in P76 (Mieske et al. 2007). Note that the
VIMOS field-of-view consists of four quadrants. The small (red)
dots indicate the UCDs detected in that survey, covering a magnitude
range
-12.2<MV<-10.8 mag. For the new run in P80 which is
reported upon in this paper, we re-observed the dotted pointings
(see text) with two masks per quadrant, added two more pointings
(solid squares), and focused only on MV<-12 mag. The large (blue)
dots indicate the three UCDs found in this new run (see
Sect. 3). The dashed circle indicates a projected
clustercentric distance of |
Open with DEXTER |
2.2 Spectroscopic observations
Within our 16 masks (4 pointings
We used the medium resolution MR grism with the order-sorting filter GG475. This covers the wavelength range from 4800 to 10 000 Å at a
dispersion of 2.5 Å per pixel. The average seeing for the
spectroscopic observations was around 0.8'', at a slit width of 1.0''. With a pixel scale of 0.2'', the instrumental resolution
(FWHM) is 10-12 Å, corresponding to a velocity resolution of 600 km s-1. For each pointing, the total exposure time was
2100 s. Arc-lamp exposures for wavelength calibration were
attached to each science exposure.
2.3 Data reduction
For the data reduction from 2D raw spectra to wavelength calibrated 1D spectra, we used the recipe vmmosobsstare provided by the ESO VIMOS pipeline![[*]](/icons/foot_motif.png)
The radial velocity measurements of the calibrated 1D spectra were
performed by cross-correlation using the IRAF task fxcor (Tonry &
Davis 1979) in the RV package. As a template in the
cross-correlation, we used a synthetic spectrum created to resemble a
typical early-type galaxy (Quintana et al. 1996). This
template has proven to be the most reliable for this type of radial
velocity surveys (e.g., Mieske et al. 2004; Misgeld et al. 2008). For a measurement to be accepted as reliable, we
ensured that the cross-correlation confidence value R was higher
than 5.5. We then repeated our execution of fxcor for those spectra
for which R<5.5 was achieved in the first execution, and accepted
measurements for those that showed clearly identifiable
cross-correlation peaks (see Fig. 5 for an example). For
more than 90% of our observed sources, we could reliably measure a
redshift (see Fig. 3). The radial velocity measurement errors
were of the order 50-100 km s-1. As a cluster membership criterion, we
required that
km s-1, excluding both foreground
stars and background galaxies.
3 Results
Figure 6 shows a radial velocity histogram of the 389 sources with measured redshifts. Of these 389 sources, 380 are foreground stars, and six objects are background galaxies with
![[*]](/icons/foot_motif.png)



![]() |
Figure 3:
Map ( left) and CMD ( right) of the objects observed in the
VIMOS P80 run. Dots indicate all objects to which a slit was
assigned. Small (black) circles indicate foreground stars. Large
(red) open circles indicate background sources (
|
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![]() |
Figure 4:
Completeness of our survey. Left panel: plotted are
all photometrically selected objects (small dots) and successfully
observed objects (small hexagons). Note the difference to
Fig. 3, where only objects included in the masks are
plotted. The circle indicates a projected radius of |
Open with DEXTER |
Table 1:
Properties of the 3 massive UCDs detected in our survey,
ordered by magnitude. Errors are given in parentheses. ``CCOS'' in the
object identifier stands for Centaurus Compact Object Survey, see also
Mieske et al. (2007). The last column gives a 2upper limit to the half-light radius in pc estimated from HST WFPC2
archival imaging.
We note that we discovered no UCD in the bright luminosity
regime
mag, within which also only
a few UCDs have been found in Fornax and Virgo. This confirms the
rarity of these extreme objects. Given our overall survey
completeness of
50% and assuming a Poisson distribution for UCD
number counts, we can exclude at a 95% confidence level the existence of more
than two UCDs with MV<-12.2 mag and
pc,
within 120 kpc of NGC 4696.
![]() |
Figure 5: Four example spectra of our VIMOS dataset, with the corresponding cross-correlation results for radial velocity measurement indicated on top. Y-axis units are flux in ADU for the bottom plots, and cross-correlation height h for the top plots. Objects from upper left to bottom right: a foreground star with cross-correlation confidence level R>5.5; one of the three Centaurus cluster members (CCOS J192.129-41.401), also with R>5.5; a foreground star with R=5.0, whose radial velocity measurement was accepted; a source for which no radial velocity could be measured. |
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![]() |
Figure 6: Left panel: radial velocity histogram of the observed sources. The background regime (>6000 km s-1) is excluded. The three Centaurus cluster members are clearly distinguishable. Right panel: histogram of the confidence level R achieved in the radial velocity measurement via cross-correlation with a template spectrum. The solid histogram indicates the sources with reliable measurement, the dotted histogram indicates sources without an acceptable cross-correlation result. |
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In spite of the lack of such very bright UCDs, there is now a total of
eight confirmed UCDs with MV<-12 mag in Centaurus (this paper,
Mieske et al. 2007), comparable to the numbers in
Virgo/Fornax (Jones et al. 2006; Firth et al. 2007). Seven of those eight sources belong to the main
cluster Cen30. Given our completeness of 50% within the central
120 kpc of Cen30, we can constrain the true number of UCDs with
MV<-12 mag in that area to be
.
How does this compare to
the number of GCs expected by extrapolating a Gaussian globular
cluster luminosity function to MV<-12 mag? For the central Cen30
galaxy NGC 4696, we would expect a total of
14 000 GCs, adopting
a specific frequency of 7.3 (Mieske et al. 2005) and an
absolute magnitude of MV=-23.2 mag (Misgeld et al. 2009)
for NGC 4696. Since we observed within a radius of 120 kpc, we assume
that we include most of the GC systems (e.g., Rhode &
Zepf 2001, showed that the GC system of NGC 4472, the most
luminous early-type galaxy in Virgo, extends to
80 kpc). Adopting an absolute turnover magnitude of MV=-7.4 mag (Kundu
& Whitmore 2001) and a GCLF width of 1.35 mag (Jordán
et al. 2006, 2007), the expected number of GCs
with MV<-12 mag is about 5. Although being on the low side, this
remains within the 2
margin of uncertainty of the estimated
number of UCDs. We note that the expected number of GCs drops below 0.5 only for MV<-12.8 mag. From a purely statistical point of view,
only the very brightest UCD luminosities (
mag) are
thus unaccounted for by a Gaussian GCLF.
![]() |
Figure 7: Positions of confirmed UCDs in the Fornax (open red circles; Mieske et al. 2008b) and Centaurus clusters (filled blue circles; this paper and Mieske et al. 2007), relative to the central galaxies and normalised to the r500 radius of the respective clusters. UCDs are selected as compact stellar systems with MV<-11 mag. The dashed circle indicates the coverage of the search for bright UCDs in Centaurus presented in this paper. The dotted square indicates the central quadrant of the P76 Centaurus survey (Mieske et al. 2007), within which the survey completeness was twice as high as outside (see also Fig. 3). |
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With the database of Centaurus UCDs at hand, it is worthwhile to
compare their spatial distribution to those in Fornax. In
Fig. 7, we show the projected distance of these two UCD
populations (with UCDs defined as compact stellar systems with
MV<-11 mag) relative to the central galaxies of Centaurus and
Fornax. The Fornax UCD database is the same as used in Mieske et al. (2008b). The distances are normalised to the r500radii of either cluster, for which Reiprich & Boehringer
(2002) measured
r500=840 kpc for Fornax, and
r500=1.14 Mpc for Centaurus. Within the radius of 120 kpc
surveyed for this publication, the Centaurus UCD population is
slightly more clustered than the Fornax UCD population, at the 96.5%
confidence level according to a KS-test. However, this may be at least
partially because all Centaurus UCDs with MV>-12mag were discovered in our P76 survey, which had a more complete area
coverage for r<50 kpc than for r>50 kpc (see Fig. 2). When
restricted to r<50 kpc in Centaurus and a corresponding r<35kpc in Fornax, the cumulative radial distribution of both samples is
indistinguishable. When considering only UCDs with MV<-12 mag -
for which the spatial survey coverage is comparable between Fornax and
Centaurus - we also find indistinguishable distributions according to
a KS-test. We can therefore state that within 2,
Fornax and
Centaurus UCDs have the same radial distribution, when scaled to the
respective r500 radius of their host clusters.
There are a number of ongoing investigations of the UCD luminosity function towards very bright luminosities (MV<-12 mag) in a range of environments, (e.g., the present work, Wehner & Harris 2008; and Misgeld et al. 2008). It will be interesting to investigate how the luminosities and masses of the most massive UCDs correlate with the properties of their host environments (Hilker et al. 2009, in preparation), as previously studied in an analogous fashion for globular cluster systems (Whitmore 2003; Larsen 2002) and systems of young massive clusters (e.g. Weidner et al. 2004).
References
- Bertin, E., & Arnouts, S. 1996, A&AS, 117, 393 [NASA ADS] [CrossRef] [EDP Sciences] (In the text)
- Chilingarian, I., & Mamon, G. 2008, MNRAS, 385L, 83 [NASA ADS] (In the text)
- Drinkwater, M. J., Jones, J. B., Gregg, M. D., & Phillipps, S. 2000, PASA, 17, 227 [NASA ADS] (In the text)
- Drinkwater, M. J., Gregg, M. D., Hilker, M., et al. 2003, Nature, 423, 519 [NASA ADS] [CrossRef] (In the text)
- Evstigneeva, E. A., Gregg, M. D., Drinkwater, M. J., & Hilker, M. 2007, AJ, 133, 1722 [NASA ADS] [CrossRef] (In the text)
- Fellhauer, M., & Kroupa, P. 2002, MNRAS, 330, 642 [NASA ADS] [CrossRef]
- Fellhauer, M., & Kroupa, P. 2005, MNRAS, 359, 223 [NASA ADS] [CrossRef]
- Firth, P., Drinkwater, M. J., Evstigneeva, E. A., et al. 2007, MNRAS, 382, 1342 [NASA ADS] (In the text)
- Gebhardt, K., & Kissler-Patig, M. 1999, AJ, 118, 1526 [NASA ADS] [CrossRef] (In the text)
- Hasegan, M., Jordán, A., Côté, P., et al. (VCS team) 2005, ApJ, 627, 203 (In the text)
- Hilker, M., Infante, L., Vieira, G., Kissler-Patig, M., & Richtler, T. 1999, A&AS, 134, 75 [NASA ADS] [CrossRef] [EDP Sciences] (In the text)
- Hilker, M., Baumgardt, H., Infante, L., et al. 2007, A&A, 463, 119 [NASA ADS] [CrossRef] [EDP Sciences] (In the text)
- Jones, J. B., Drinkwater, M. J., Jurek, R., et al. 2006, AJ, 131, 312 [NASA ADS] [CrossRef] (In the text)
- Jordán, A., Blakeslee, J. P., Peng, E. W., et al. 2004, ApJS, 154, 509 [NASA ADS] [CrossRef] (In the text)
- Jordán, A., Côté, P., Blakeslee, J. P., et al. 2005, ApJ, 634, 1002 [NASA ADS] [CrossRef] (In the text)
- Jordán, A., McLaughlin, D. E., Côté, P., et al. 2006, ApJ, 651, L25 [NASA ADS] [CrossRef] (In the text)
- Jordán, A., McLaughlin, D. E., Côté, P., et al. 2007, ApJS, 171, 101 [NASA ADS] [CrossRef] (In the text)
- Kundu, A., & Whitmore, B. 2001, AJ, 121, 2950 [NASA ADS] [CrossRef] (In the text)
- Larsen, S. S. 2002, AJ, 124, 1393 [NASA ADS] [CrossRef] (In the text)
- Larsen, S. S., Brodie, J. P., Huchra, J. P., Forbes, D. A., & Grillmair, C. J. 2001, AJ, 121, 2974 [NASA ADS] [CrossRef] (In the text)
- Le Fevre, O., Saisse, M., Mancini, D., et al. 2003, SPIE, 4841, 1670 [NASA ADS] (In the text)
- Mieske, S., Hilker, M., & Infante, L. 2004, A&A, 418, 445 [NASA ADS] [CrossRef] [EDP Sciences] (In the text)
- Mieske, S., Hilker, M., & Infante, L. 2005, A&A, 438, 103 [NASA ADS] [CrossRef] [EDP Sciences] (In the text)
- Mieske, S., Hilker, M., Jordán, A., Infante, L., & Kissler-Patig, M. 2007, A&A, 472, 111 [NASA ADS] [CrossRef] [EDP Sciences] (In the text)
- Mieske, S., Hilker, M., Jordán, A., et al. 2008a, A&A, 487, 921 [NASA ADS] [CrossRef] [EDP Sciences] (In the text)
- Mieske, S., Hilker, M., Bomans, D. J., et al. 2008b, A&A, 489, 1023 [NASA ADS] [CrossRef] [EDP Sciences] (In the text)
- Misgeld, I., Mieske, S., & Hilker, M. 2008, A&A, 486, 697 [NASA ADS] [CrossRef] [EDP Sciences] (In the text)
- Misgeld, I., Hilker, M., & Mieske, S. 2009, A&A, submitted (In the text)
- Phillipps, S., Drinkwater, M. J., Gregg, M. D., & Jones, J. B. 2001, ApJ, 560, 201 [NASA ADS] [CrossRef] (In the text)
- Quintana, H., Ramirez, A., & Way, M. J. 1996, AJ, 111, 603 [NASA ADS] [CrossRef] (In the text)
- Reiprich, T., & Böhringer, H. 2002, ApJ, 567, 716 [NASA ADS] [CrossRef] (In the text)
- Rejkuba, M., Dubath, P., Minniti, D., & Meylan, G. 2007, A&A, 469, 147 [NASA ADS] [CrossRef] [EDP Sciences] (In the text)
- Rhode, K. L., & Zepf, S. E. 2001, AJ, 121, 210 [NASA ADS] [CrossRef] (In the text)
- Schlegel, D. J., Finkbeiner, D. P., & Davis, M. 1998, ApJ, 500, 525 [NASA ADS] [CrossRef] (In the text)
- Thomas, P. A., Drinkwater, M. J., & Evstigneeva, E. 2008, MNRAS, 389, 102 [NASA ADS] [CrossRef] (In the text)
- Tonry, J., & Davis, M. 1979, AJ, 84, 1511 [NASA ADS] [CrossRef] (In the text)
- Wehner, E., & Harris, W. 2008, ApJL, 668, 35 [CrossRef] (In the text)
- Weidner, C., Kroupa, P., & Larsen, S. S. 2004, MNRAS, 350, 1503 [NASA ADS] [CrossRef] (In the text)
- Whitmore, B. 2003, in Extragalactic Globular Cluster Systems: Proceedings of the ESO Workshop Held in Garching, Germany, 27-30 August 2002, ESO Astrophys. Symp., ed. M. Kissler-Patig (Springer-Verlag), 336 (In the text)
Footnotes
- ... cluster
- Based on observations obtained in service mode at the VLT (programme 080.B-0307).
- ...
pages
- http://www.eso.org/observing/dfo/quality/index_vimos.html
- ...<-12 mag
- A complementary observing campaign targeting canonical dwarf galaxies - including resolved UCD candidates - in Centaurus has been approved for ESO observing period P83.
- ...
pipeline
- http://www.eso.org/projects/dfs/dfs-shared/web/vimos/vimos-pipe-recipes.html
- ...
- The list of foreground stars and background galaxies including their coordinates is available upon request from smieske@eso.org
All Tables
Table 1:
Properties of the 3 massive UCDs detected in our survey,
ordered by magnitude. Errors are given in parentheses. ``CCOS'' in the
object identifier stands for Centaurus Compact Object Survey, see also
Mieske et al. (2007). The last column gives a 2upper limit to the half-light radius in pc estimated from HST WFPC2
archival imaging.
All Figures
![]() |
Figure 1:
This plot illustrates the upper limit to UCD effective
radius that our candidate selection criterion for unresolved sources
corresponds to. The y-axis shows the SExtractor star-classifier
value (Bertin & Arnouts 1996), which is defined to be 1
for an unresolved source (``star''), and 0 for a resolved source
(``galaxy''). The SExtractor star-classifier value flips from
``star'' to ``galaxy'' for
|
Open with DEXTER | |
In the text |
![]() |
Figure 2:
Map of the central Centaurus cluster. The relative
coordinates are with respect to NGC 4696, the central galaxy of the
Cen30 subcluster. The dotted squares indicate the VIMOS pointing
observed in P76 (Mieske et al. 2007). Note that the
VIMOS field-of-view consists of four quadrants. The small (red)
dots indicate the UCDs detected in that survey, covering a magnitude
range
-12.2<MV<-10.8 mag. For the new run in P80 which is
reported upon in this paper, we re-observed the dotted pointings
(see text) with two masks per quadrant, added two more pointings
(solid squares), and focused only on MV<-12 mag. The large (blue)
dots indicate the three UCDs found in this new run (see
Sect. 3). The dashed circle indicates a projected
clustercentric distance of |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Map ( left) and CMD ( right) of the objects observed in the
VIMOS P80 run. Dots indicate all objects to which a slit was
assigned. Small (black) circles indicate foreground stars. Large
(red) open circles indicate background sources (
|
Open with DEXTER | |
In the text |
![]() |
Figure 4:
Completeness of our survey. Left panel: plotted are
all photometrically selected objects (small dots) and successfully
observed objects (small hexagons). Note the difference to
Fig. 3, where only objects included in the masks are
plotted. The circle indicates a projected radius of |
Open with DEXTER | |
In the text |
![]() |
Figure 5: Four example spectra of our VIMOS dataset, with the corresponding cross-correlation results for radial velocity measurement indicated on top. Y-axis units are flux in ADU for the bottom plots, and cross-correlation height h for the top plots. Objects from upper left to bottom right: a foreground star with cross-correlation confidence level R>5.5; one of the three Centaurus cluster members (CCOS J192.129-41.401), also with R>5.5; a foreground star with R=5.0, whose radial velocity measurement was accepted; a source for which no radial velocity could be measured. |
Open with DEXTER | |
In the text |
![]() |
Figure 6: Left panel: radial velocity histogram of the observed sources. The background regime (>6000 km s-1) is excluded. The three Centaurus cluster members are clearly distinguishable. Right panel: histogram of the confidence level R achieved in the radial velocity measurement via cross-correlation with a template spectrum. The solid histogram indicates the sources with reliable measurement, the dotted histogram indicates sources without an acceptable cross-correlation result. |
Open with DEXTER | |
In the text |
![]() |
Figure 7: Positions of confirmed UCDs in the Fornax (open red circles; Mieske et al. 2008b) and Centaurus clusters (filled blue circles; this paper and Mieske et al. 2007), relative to the central galaxies and normalised to the r500 radius of the respective clusters. UCDs are selected as compact stellar systems with MV<-11 mag. The dashed circle indicates the coverage of the search for bright UCDs in Centaurus presented in this paper. The dotted square indicates the central quadrant of the P76 Centaurus survey (Mieske et al. 2007), within which the survey completeness was twice as high as outside (see also Fig. 3). |
Open with DEXTER | |
In the text |
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