michael d. gregg et al- a large population of ultra-compact dwarf galaxies in the fornax cluster

8/3/2019 Michael D. Gregg et al- A large population of ultra-compact dwarf galaxies in the Fornax Cluster

1/20

Draft 4, 04/24/06

A large population of ultra-compact dwarf galaxies in the Fornax

Cluster

Michael D. Gregg, Arna M. Karick

Physics Dept., U.C. Davis, and IGPP, Lawrence Livermore National Laboratory, L-413,

Livermore, CA 94550, USA

gregg,[email protected]

Michael J. Drinkwater, Ekaterina Evstigneeva, Russell Jurek

Department of Physics, University of Queensland, QLD 4023, Australia

mjd,katya,[email protected]

Steven Phillipps

Department of Physics, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, United

Kingdom

[email protected]

Terry Bridges

Department of Physics Queens University Kingston, Ontario, Canada K7L 3N6

[email protected]

J. Bryn Jones

Astronomy Unit, School of Mathematical Sciences, Queen Mary University of London, Mile

End Road, London E1 4NS, United Kingdom

[email protected]

and

Kenji Bekki, Warrick J. Couch

University of New South Wales


2/20

2

bekki,[email protected]

ABSTRACT

The previously cataloged ultra-compact dwarf (UCD) galaxies in the Fornax

and Virgo clusters all have 17.5 < bJ < 20. Using the 2dF spectrograph on theAnglo-Australian Telescope, we have carried out a search for fainter UCDs in the

Fornax Cluster. In the magnitude interval 19.5 < bJ < 21.5, we have found 56

additional compact cluster members within a projected radius of 0.9 (320 kpc) of

the cluster center. There is luminosity and spatial overlap with objects classified

as globular clusters belonging to the central cluster galaxy NGC 1399. The UCDs

are a dynamically distinct population, however, from both the globular clusters

and the nucleated dwarf ellipticals in Fornax. Correcting for incompleteness, we

estimate that there are 70 10 UCDs to bJ < 21.5 in Fornax, and hence the

UCDs outnumber other galaxy types in the central region of the Fornax Cluster.

Subject headings: galaxies: clusters, clusters: globular

1. Introduction

Galaxy transformation processes in dense environments should leave rich clusters littered

with the remains of disrupted galaxies (Bassino et al. 1994; West et al. 1995; Bekki et al.

2001; Moore 2004). Observational evidence which might verify these processes has remained

scant, limited to detections of diffuse light trails and intra-cluster stars (Gregg & West 1998;Adami et al. 2005; Durrell et al. 2002; Ford et al. 2002; Feldmeier et al. 2004). A new

class of extremely compact galaxy has recently been discovered in the Fornax and Virgo

clusters; though a magnitude smaller in physical size than conventional dwarf galaxies, they

are relatively easy to detect in nearby clusters. Hilker et al. (1999) found two compact objects

in a spectroscopic survey of the Fornax cluster center, and our Fornax Cluster Spectroscopic

Survey (FCSS, Drinkwater et al. 2000b; see also Phillipps et al. 2001, Drinkwater et al. 2003,

Jones et al. 2006) identified six cluster members which were either unresolved or marginally

resolved in ground-based imaging, including the two Hilker et al. objects. These objects have

13 MB 11, sizes 100 pc, and do not resemble any known type of galaxy (Phillipps

et al. 2001), so were named ultra-compact dwarf (UCD) galaxies. High-resolution HubbleSpace Telescope imaging and VLT echelle spectroscopy have since established that UCDs are

a new type of low-luminosity compact galaxy distinct from both globular star clusters and

all known types of dwarf galaxy (Drinkwater et al. 2003). In the Virgo cluster, Hasegan et al.


3/20

3

(2005) and Jones et al. (2006) have identified 15 20 objects which are morphologically

indistinguishable from the Fornax UCDs, suggesting that such objects are ubiquitous in

clusters.

Explanations for the origin and nature of ultra-compact dwarfs include unusually lu-

minous globular clusters (e.g. Hilker et al. 1999; Drinkwater et al. 2000a; Phillipps et al.

2001; Mieske et al. 2002), evolved extremely luminous star clusters formed in galaxy interac-tions (Fellhauer & Kroupa 2002; Maraston et al. 2004), and low luminosity analogs of M32

(Drinkwater et al. 2000a). Some theories have argued that highly compact galaxies might

have formed in the early Universe (Blanchard, Valls-Gabaud & Mamon 1992; Tegmark et

al. 1997), and UCDs may yet prove to be such objects.

A favored hypothesis for the formation of UCDs, however, is that they are the remnant

nuclei of dwarf elliptical galaxies that have been tidally disrupted during passages close to the

central cluster galaxy; we refer to this process as galaxy threshing (Bekki et al. 2001, 2003).

Recently, a deeper survey of the central region of the Fornax Cluster found 54 new globular

cluster-like objects within 20

of NGC1399 down to V = 21.0 mag (to MB 9.8 mag)(Mieske et al. 2004). They conclude that the brighter (V < 20) objects are consistent with

UCDs formed by the threshing process but that most of the fainter objects are genuine

globular clusters. The UCDs may also be the remnant nuclei of or giant star clusters from

late type galaxies destroyed by the cluster potential, as has been suggested for the giant

globular G1 in Andromeda (Meylan et al. 2001). Whatever their nature, these compact

objects are an important constituent of galaxy clusters, and determining their origin and

evolution will help in understanding the formation of galaxy clusters.

We present the results of observations which extend the search for fainter UCDs in

the Fornax Cluster over the much larger field accessible to 2dF. Based on the luminosityfunction of dE,N nuclei in the Virgo Cluster, searching 1.5 magnitudes deeper for UCDs in

the Fornax Cluster should approximately triple the original sample of 6 UCDs. In Section 3

we present our surprising result that 56 new compact objects were found, many more than

expected. Preliminary results of these observations were presented in ?; here we present

further observations and discuss the properties of the UCDs in detail, and compare them to

objects classified as globular clusters. We adopt a distance of 20 Mpc to the Fornax Cluster

(Drinkwater et al. 2001).


4/20

4

2. Observations

The original FCSS observations in the central field of the Fornax Cluster produced 6

UCDs in the range 16.5 < bJ < 20. Here we define UCDs as objects which were classified

as stellar (unresolved) in the photographic APM catalog but were found to have redshifts

consistent with membership of the Fornax Cluster (600 < cz < 2500km/s); see Drinkwater

et al. (2000b). Allowing for incompleteness, we might expect one more UCD in this magni-

tude range (Drinkwater et al. 2000a; Jones et al. 2006). If UCDs arise by galaxy threshing

of nucleated dwarf elliptical (dE,N), then the UCD luminosity distribution should follow that

of the dE,N nuclei (Binggeli & Cameron 1991), and extension of our search 1.5 magnitudes

fainter (to bJ 21.5) than the original discovery observations should triple the UCD sample

size to 20.

In 2003 October and 2004 November we made new 2dF observations in Fornax to test

this prediction. As in our search of the Virgo cluster, we restricted the observations to a

limited color range (bJ r < 1.7) and also to slightly less than the whole 2dF field (radius

< 0.9), selecting targets from the APM Catalog of bJ and r photographic survey plates. Inthe extension to fainter magnitudes for this current work we became limited by the depth

of the r plate data which reaches only r < 20.4 mag. For objects that were not detected

on the r plate we therefore did not apply a color selection, but observed all objects with

18 < bJ < 21.5; for these objects, we have only an upper limit on their bJ r colors, but all

are bluer than bJ r = 1.7 (Fig. 1).

The 2dF observations (Table 1) and reductions were carried out in the standard fashion

as outlined in Drinkwater et al. (2000b), except that longer exposure times were used to

reach the fainter magnitude limits. In 4 nights we observed 2500 unresolved stellar targets

of which 56 proved to be cluster members. Combined with our first sample of 6 UCDs, thisbrings the total of UCDs in Fornax to 62 (Table 2). Their color, magnitude, and spatial

distributions are shown in Fig. 1 and Fig. 2. Contained in our full sample of 62 UCDs,

there are 29 objects previously listed by Mieske et al. (2004) in their study of the central 20

region. The sample we present here extends almost to the full 1 radius of the 2dF system.

In Table 2, we identify objects already found in the lists of Mieske et al. (2002) and

Dirsch et al. (2004). There are now at least three distinct naming conventions for UCDs in

Fornax; with the large numbers now being found throughout the cluster (and we anticipate

also in Virgo), we suggest that UCDs be designated by their J2000 coordinates to avoid

confusion as much as possible.

Our final samples were 96% complete (defined as the fraction of the input targets with

well-measured redshifts) in the magnitude range 16 < bJ < 20.5 (21 UCDs found), 82%


5/20

5

complete in the magnitude range 20.5 < bJ < 21 (22 UCDs found), and 36% complete in the

fainter range 21 < bJ < 21.5 (19 UCDs found). The completeness limits are also indicated

in Fig. 1. Correcting for completeness we therefore expect the total number of UCDs in this

region (radius


6/20

6

that it is unquestionably dominated by ordinary globulars, even though the brightest objects

in it may be UCDs indeed, as noted above, many objects overlap with our sample, but are

too few in number to make a difference in the statistical analysis below. Since all objects

in our sample of 62 UCDs have been selected in a uniform manner, for the sake of analysis,

we will consider all of them to be UCDs, even though some of the fainter objects, especially

close to the center of NGC1399, may be ordinary globulars.

We compare the velocity distributions of the UCDs to both the NGC 1399 globular

clusters and the Fornax population of nucleated dwarf elliptical (dE,N) galaxies in the middle

panel of Fig. 4. To explore the trends with cluster position, we have formed running means

of the velocities and dispersions within each of the three samples (Fig. 5). The UCD and

dE sample means have been computed for subsamples of 11 objects as a function of cluster

radial position; the subsample size for the noisier velocity data but much larger GC sample

is 41 objects.

In the radius range 6 < R < 10, the GCs have a mean velocity of 1432kms1 ,

almost exactly that of NGC1399 (1415 km s1

), while the 10 UCDs in this interval have amuch larger mean velocity of 1679 km s1 , indicating that these UCDs are not part of the

dynamical system of NGC 1399. A Students t-test shows that these velocity means differ

with 98% significance. The UCDs and dE population velocities merge seamlessly, confirming

that the UCDs as a group belong to the cluster potential and not to NGC 1399 alone. There

are 19 UCDs within 6 of the center of NGC 1399; their mean velocity is 1417 km s1 , so

these objects are probably truly associated with the NGC 1399 dynamical system, as can be

expected from their proximity.

The running means of the velocity dispersions also reveal some interesting differences

(bottom panel, Fig. 4). The UCDs have a lower overall velocity dispersion (26220kms) thaneither the dE,N galaxies within 60 (41763kms1 ) or the GCs (33411kms1 ), at greater

than 99.5% significance level (F-test). This indicates that the UCDs form a more relaxed

or lower-energy population, which, if they started life as dE,N galaxies, would facilitate

threshing to convert them to UCDs: dEs with low energies relative to the cluster will be

most affected and more quickly stripped of their halos, while objects with high energies will

minimize their time spent near the cluster center, increasing the likelihood that they will

remain as dEs over the life of the cluster.

To test that the velocity dispersion differences are not spurious and result from sys-

tematics between the 2dF UCD fiber spectroscopy versus the long slit data for the GCs,we compared the measured velocities for the 25 objects within 20 of the Fornax center

(Fig. 5). The scatter is consistent with the typical errors of 100kms1 , though there are

four or five outliers. The 2dF spectra of these five are reasonably good and we have con-


7/20

7

fidence in the derived velocities. There is no systematic trend with cluster position of 2dF

and literature velocity differences for the UCDs, except possibly over a restricted interval

< 8. Using the fitted line shown, a velocity correction can be derived to apply to the 2dF

results; this, however, produces an even lower dispersion for the UCDs in this interval. We

conclude that the differences in the velocity means and, especially, velocity dispersions for

the three samples are real. The only selection criterion which separates the GCs and UCDs,

however, is apparent magnitude, so the dynamical difference strongly suggest that the UCDs

form a distinct population of objects and are not merely the bright end of the GC luminosity

function.

Even though the GC surveys are not complete to the large radii over which we have

searched for UCD candidates, it is instructive to compare the radial distributions of the

different populations. In Fig. 6 we show the cumulative radial distributions of the populations

within the 54 radius of the complete UCD sample. Setting aside the GCs, this plot again

emphasizes that the UCDs are much more centrally concentrated than the normal dwarf

galaxies. A two-sample Kolmogorov-Smirnov test shows that the UCD and dE,N galaxy

radial distributions differ at the 99.9% confidence level. This is not unexpected in the

threshing model for formation of UCDs because the objects on lower energy orbits would

be preferentially threshed; in fact, the dearth of dE galaxies relative to UCDs in the central

region of Fornax could be interpreted as support for tidal destruction of dwarf galaxy halos.

4. Discussion

Our observations have extended the Fornax Cluster Spectrographic Survey results to

search for fainter ultra-compact dwarfs in the central 1.

8 diameter region of the FornaxCluster, revealing an unexpectedly large number of compact objects spread throughout in-

tracluster space. The size of our sample allows us to make statistical comparisons of this

population with globular clusters associated with the central galaxy and the general pop-

ulation of nucleated dwarf galaxies. Although there is some overlap in our sample with

cataloged globular clusters, we show that the UCDs are dynamically distinct from both the

NGC 1399 globular clusters and the Fornax dE,N galaxies, having higher mean velocity and

a lower velocity dispersion. The UCDs are much more widely distributed across Fornax than

the GCs. The spatial distribution of UCDs is significantly different from that of existing

nucleated dwarf galaxies in the same part of the cluster: the UCDs are much more centrally

concentrated, as would be expected from the significantly smaller velocity dispersion.

These results lead us to revise our original hypothesis for the formation of UCDs, that

they represent a small subset of the current distribution of cluster dE,N galaxies which have


8/20

8

have been tidally disrupted by the cluster potential (galaxy threshing) to leave just the

nuclei remaining. In the galaxy threshing process the remaining nuclei keep the same orbits

as the parent galaxies, so would have the same radial distributions and velocity dispersion:

there is no significant loss of energy that would allow them to fall into closer orbits around

the central galaxy. Only a subset of dwarf galaxies, however, are on orbits that will lead

to threshing, so a difference in velocity dispersion between the objects threshed and those

not (yet) threshed is expected. In a future paper, we will model this in detail to ascertain

whether the observed distribution and velocities of UCDs in Fornax are consistent with

threshing of an initial population of dE,N galaxies. The results presented here suggest that

if UCDs are created by threshing dE,Ns, then a substantial fraction of UCDs must be from

a parent population which formed on closer, low energy orbits around the cluster center,

objects predetermined to become UCDs.

In future work we plan further high-resolution spectroscopic observations to distinguish

UCDs and globular clusters using their internal dynamics. It is vital to determine if the

intra-cluster space between the giant galaxies is occupied by a population of faint compact

objects: true intra-cluster globular clusters perhaps (West et al. 1995). We also plan to

use cosmological simulations of cluster formation to determine if there might have been

populations of dwarf galaxies formed on low-energy orbits more readily susceptible to galaxy

threshing, facilitating the formation of UCDs.

We are grateful to the Anglo-Australian Observatory for continued support of our ob-

serving sessions with the 2dF spectrograph and, in particular, for allowing the final observa-

tions to be performed in service mode at short notice. We thank Peter Thomas for helpful

discussions about the dynamical simulations. This work has been supported by a Discov-

ery Project grant from the Australian Research Council and grant No. 0407445 from theNational Science Foundation. Part of the work reported here was done at the Institute of

Geophysics and Planetary Physics, under the auspices of the U.S. Department of Energy by

Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48.

REFERENCES

Adami, C., Slezak, E., Durret, F., Conselice, C. J., Cuillandre, J. C., Gallagher, J. S.,

Mazure, A., Pello, R., Picat, J. P., & Ulmer, M. P. 2005, A&A, 429, 39

Bassino, L. P., Muzzio, J. C., & Rabolli, M. 1994, ApJ, 431, 634

Bekki, K., Couch, W. J., & Drinkwater, M. J. 2001, ApJ, 552, L105


9/20

9

Bekki, K., Couch, W. J., Drinkwater, M. J., & Shioya, Y. 2003, MNRAS, 344, 399

Binggeli, B. & Cameron, L. M. 1991, A&A, 252, 27

. 1993, A&AS, 98, 297

Burkert, A. 1995, ApJ, 447, L25

Dirsch, B., Richtler, T., Geisler, D., Gebhardt, K., Hilker, M., Alonso, M. V., Forte, J. C.,

Grebel, E. K., Infante, L., Larsen, S., Minniti, D., & Rejkuba, M. 2004, AJ, 127, 2114

Drinkwater, M. J., Gregg, M. D., & Colless, M. 2001, ApJ, 548, L139

Drinkwater, M. J., Gregg, M. D., Hilker, M., Bekki, K., Couch, W. J., Ferguson, H. C.,

Jones, J. B., & Phillipps, S. 2003, Nature, 423, 519

Drinkwater, M. J., Jones, J. B., Gregg, M. D., & Phillipps, S. 2000a, Publications of the

Astronomical Society of Australia, 17, 227

Drinkwater, M. J., Phillipps, S., Jones, J. B., Gregg, M. D., Deady, J. H., Davies, J. I.,

Parker, Q. A., Sadler, E. M., & Smith, R. M. 2000b, A&A, 355, 900

Durrell, P. R., Ciardullo, R., Feldmeier, J. J., Jacoby, G. H., & Sigurdsson, S. 2002, ApJ,

570, 119

Feldmeier, J. J., Ciardullo, R., Jacoby, G. H., & Durrell, P. R. 2004, ApJ, 615, 196

Fellhauer, M. & Kroupa, P. 2002, MNRAS, 330, 642

Ford, H., Peng, E., & Freeman, K. 2002, in ASP Conf. Ser. 273: The Dynamics, Structure& History of Galaxies: A Workshop in Honour of Professor Ken Freeman, 41+

Ghigna, S., Moore, B., Governato, F., Lake, G., Quinn, T., & Stadel, J. 1998, MNRAS, 300,

146

Gregg, M. D. & West, M. J. 1998, Nature, 396, 549

Hasegan, M., Jordan, A., Cote, P., Djorgovski, S. G., McLaughlin, D. E., Blakeslee, J. P.,

Mei, S., West, M. J., Peng, E. W., Ferrarese, L., Milosavljevic, M., Tonry, J. L., &

Merritt, D. 2005, ApJ, 627, 203

Hilker, M., Infante, L., Vieira, G., Kissler-Patig, M., & Richtler, T. 1999, A&AS, 134, 75


10/20

10

Ikebe, Y., Ezawa, H., Fukazawa, Y., Hirayama, M., Izhisaki, Y., Kikuchi, K., Kubo, H.,

Makishima, K., Matsushita, K., Ohashi, T., Takahashi, T., & Tamura, T. 1996,

Nature, 379, 427

Jones, J. B., Drinkwater, M. J., Jurek, R., Phillipps, S., Gregg, M. D., Bekki, K., Couch,

W. J., Karick, A., Parker, Q. A., & Smith, R. M. 2006, AJ, 131, 312

Lotz, J. M., Telford, R., Ferguson, H. C., Miller, B. W., Stiavelli, M., & Mack, J. 2001, ApJ,

552, 572

Maraston, C., Bastian, N., Saglia, R. P., Kissler-Patig, M., Schweizer, F., & Goudfrooij, P.

2004, A&A, 416, 467

Mieske, S., Hilker, M., & Infante, L. 2002, A&A, 383, 823

. 2004, A&A, 418, 445

Moore, B. 2004, in Clusters of Galaxies: Probes of Cosmological Structure and GalaxyEvolution, 296+

Navarro, J. F., Frenk, C. S., & White, S. D. M. 1996, ApJ, 462, 563

Phillipps, S., Drinkwater, M. J., Gregg, M. D., & Jones, J. B. 2001, ApJ, 560, 201

Salucci, P. & Burkert, A. 2000, ApJ, 537, L9

West, M. J., Cote, P., Jones, C., Forman, W., & Marzke, R. O. 1995, ApJ, 453, L77

This preprint was prepared with the AAS LATEX macros v5.0.


11/20

11

Table 1: Observations.

Date Set exp(h) seeing (arcsec)2003 October 21 15 2.0 2.0

2003 October 21 16 2.0 1.7

2003 October 22 14 1.5 1.7

2003 October 22 17 4.5 2.3

2003 October 23 18 2.5 1.5

2003 October 23 19 4.0 1.5

2004 November 12 20 4.0 1.4


12/20

12

Fig. 1. Color-magnitude distribution of confirmed stars (dots) and UCDs (triangles) in

our survey field. The bJ magnitudes are plotted against bJ r color. The sample was

selected by limiting bJ magnitude and the completeness limits (fraction of targets with

measured redshifts) are shown for the three bJ ranges indicated by horizontal dotted lines.

The r < 20.4 detection limit of the r plate is shown by the angled dotted line: UCDssatisfying our bJ limits but not detected on the r plate are plotted here as open triangles

with arbitrary bJ r = 0 colors. All the UCDs, including these, satisfy our color selection

bJ r < 1.7 shown by the vertical dashed line.


13/20

13

Fig. 2. Distribution of all 62 UCDs overlaid on a two-degree wide bJ Digitized Sky Surveyimage of the center of the Fornax Cluster. The circle indicates the 0.9 degree radius within

which the UCD search is complete.


14/20

14

Fig. 3. Luminosities of UCDs (this paper) compared to N1399 GCs (Dirsch et al. 2004)

and Virgo dE,N nuclei (Binggeli & Cameron 1991).


15/20

15

Fig. 4. Comparison of the UCD, GC, and dE populations as a function of cluster position.

The upper panel shows that the three samples are relatively separate in magnitude, though

there is a little overlap between the GCs and UCDs. In the bottom two panels, running means

of the velocity and dispersion for the UCDs (solid) compared to those for globular clusters

(dotted) (Dirsch et al. 2004) and nucleated dwarf elliptical galaxies (dashed) (Drinkwater

et al. 2001) reveals differences in dynamical properties. Although the UCDs and dE,N

samples form a continuous distribution in velocity space, there appears to be a real difference

in the mean velocities of the UCDs and GCs in the 6 10 radius interval. The velocity

dispersions of the three classes differ markedly as a function of cluster radius.


16/20

16

Fig. 5. Comparison of the 2dF UCD velocities with literature values for 25 objects within

21 of the cluster center. There is no systematic trend with either velocity or with cluster

position, except possibly for the 18 objects inside 8. The dotted line is a fit to these objects,

excluding the three outliers. If this trend is applied to the 2dF UCD velocities, the UCD

sample dispersion drops to even lower values in this radial interval.


17/20

17

Fig. 6. Cumulative radial distributions of UCDs compared to globular clusters (Dirsch

et al. 2004) and nucleated dwarf elliptical galaxies (Drinkwater et al. 2001).


18/20

18

Table 2. Ultra-compact dwarfs in Fornax

N R bJ vrad vrad Notes

(2000) mag mag km s1 km s1

1 03 34 51.51 35 44 02.8 20.3 21.0 1368.5 67.5

2 03 35 12.34 35 12 59.4 0.0 20.8 1446.2 49.4

3 03 36 22.28 35 36 34.3 19.7 20.4 1461.6 75.9

4 03 36 26.72 35 22 01.6 19.1 20.1 1498.8 116.8

5 03 36 27.74 35 14 13.9 18.9 20.2 1296.9 45.3

6 03 36 28.70 34 56 30.7 0.0 21.1 1657.7 65.2

7 03 36 34.36 35 19 32.5 19.9 21.0 1816.8 103.8

8 03 36 47.65 35 29 36.9 19.9 20.9 1445.9 88.9

9 03 36 47.74 35 48 34.1 20.3 20.9 1340.4 90.5

10 03 36 51.68 35 30 38.9 20.2 21.3 1374.9 45.7

11 03 37 03.30 35 38 04.6 18.6 19.9 1491.4 38.7 UCD 1, 2-2031

12 03 37 24.91 35 36 09.7 19.6 20.3 1496.1 55.013 03 37 27.61 35 30 12.6 19.4 20.5 1828.2 77.2

14 03 37 38.29 35 20 20.6 20.4 21.3 2225.6 87.2

15 03 37 41.85 35 41 22.8 20.2 21.3 1175.2 60.8 2-078

16 03 37 43.06 35 22 11.9 20.2 20.7 1146.4 86.3

17 03 37 43.56 35 15 09.6 20.4 18.9 1640.5 99.0 4-2028

18 03 37 43.60 35 22 51.8 19.2 20.1 1326.1 82.2

19 03 37 45.13 35 29 01.6 19.9 20.9 1640.7 62.9

20 03 38 05.08 35 24 09.6 18.4 19.4 1211.8 31.8 UCD 6, 2-2143

21 03 38 06.33 35 28 58.8 18.7 115.1 1311.8 57.1 UCD 2, 2-2111, 91:93

22 03 38 06.53 35 23 04.0 19.6 20.5 1510.3 64.3 2-2153

23 03 38 09.27 35 35 07.0 20.2 21.4 1805.5 57.5

24 03 38 10.39 35 24 06.1 18.9 20.1 1549.3 64.0 81:4725 03 38 10.78 35 25 46.0 19.4 20.5 1764.1 55.2 2-2134

26 03 38 12.02 35 39 57.2 19.7 21.2 1307.2 62.7 2-073

27 03 38 14.25 35 26 43.8 20.3 21.5 1376.6 95.6

28 03 38 16.54 35 26 19.7 19.7 20.6 1124.7 100.5 0-2024

29 03 38 16.70 35 20 23.1 19.5 20.5 1552.5 87.6 80:115

30 03 38 17.61 35 33 02.8 19.8 20.8 1505.4 92.3 89:22

31 03 38 18.48 35 27 39.8 20.3 21.3 1332.1 62.9 0-2062, 89:107

32 03 38 21.73 35 26 16.5 0.0 21.0 1402.5 75.5 80:12

33 03 38 21.84 35 25 13.8 0.0 20.5 1410.8 170.4 80:30

34 03 38 23.27 35 20 00.8 19.8 20.7 1370.3 63.5 0-2066

35 03 38 23.78 35 13 49.5 19.3 20.2 1992.5 199.1 3-2027

36 03 38 25.08 35 29 25.3 20.2 21.1 1157.5 63.9 2-2106

37 03 38 25.56 35 37 42.8 19.6 20.5 1697.9 52.3 1-2024


19/20

19

Table 2Continued

N R bJ vrad vrad Notes(2000) mag mag km s1 km s1

38 03 38 26.76 35 30 07.7 20.1 21.1 1474.9 72.2 0-2069

39 03 38 28.83 35 28 47.1 0.0 21.1 1460.0 76.5

40 03 38 29.04 35 22 56.5 19.5 20.5 1720.4 73.8 0-2031

41 03 38 29.07 35 25 00.3 0.0 20.7 1491.0 72.6 78:117

42 03 38 36.86 35 28 09.5 20.2 20.8 1364.7 55.6

43 03 38 36.99 35 25 44.2 0.0 21.0 1322.1 87.4

44 03 38 39.37 35 27 05.8 20.4 21.4 1644.4 135.2 1-058

45 03 38 40.23 35 27 03.1 0.0 20.8 1230.3 111.9

46 03 38 41.98 35 33 13.4 19.2 19.8 2079.7 147.7 1-021

47 03 38 43.14 35 28 01.5 20.2 21.0 1574.1 116.648 03 38 45.81 35 34 27.4 20.1 20.6 1844.5 86.6

49 03 38 47.49 35 37 13.5 20.0 21.1 1893.3 68.3

50 03 38 50.73 35 33 48.3 20.4 20.4 1886.5 105.1

51 03 38 54.10 35 33 33.6 17.0 17.7 1590.7 35.8 UCD 3, 1-2053

52 03 39 17.72 35 25 30.2 19.9 20.8 1021.7 46.1 1-060

53 03 39 20.56 35 19 14.6 18.9 20.2 1420.2 63.9 3-2004

54 03 39 34.78 35 53 44.2 20.0 20.7 1528.0 73.5

55 03 39 35.95 35 28 24.5 18.4 18.8 1920.0 39.9 UCD 4, 1-2083

56 03 39 37.21 35 15 21.7 20.0 20.9 1800.2 92.5 3-2019

57 03 39 43.56 35 26 59.5 19.6 20.0 1447.5 101.2

58 03 39 52.58 35 04 24.1 19.0 19.7 1355.0 72.3 UCD 5

59 03 40 19.94 35 15 29.8 20.1 21.1 1649.5 73.5

60 03 40 24.98 35 06 37.6 19.8 20.5 1432.5 59.1

61 03 40 37.11 34 58 40.0 20.1 21.3 1811.1 159.3

62 03 41 35.88 35 54 57.8 19.8 21.0 1628.8 56.5

Note. Photometry is from the APM digitized sky survey database. Original 6

UCDs from Phillipps et al. (2001) and Jones et al. (2006) are identified. Also identified

are objects in common with Mieske et al. (2002), Mieske et al. (2004) (hyphenated) or

Dirsch et al. (2004) (colons).


20/20

20

Table 3: Velocity distributions.

Sample N v v GC t, F-test dE,N t, F-test

( k ms1 ) ( km s1 ) v, v v, vGlobular clusters (all) 442 144616 33411

Globular clusters (< 6) 331 145116 33711

Globular clusters (> 6

10

) 112 1432

16 324

11UCDs (all) 62 152735 24625 93.2 99.6 99.98

UCDs (< 6) 19 141740 17929 99.7

UCDs (6 10) 10 167940 21729 98.0 80.8

dE,Ns 48 150257 39641

dE,Ns < 60 23 142387 41763

Note. The final 2 columns show the results of Students t-tests or F-tests that the UCD mean velocity

v or velocity dispersions v of the samples in each row are different from the corresponding GC and dE,Nsamples. The percentage values indicate the statistical significance that the differences seen are real. No

data indicates that the significance is low. For comparison, the velocity of NGC 1399 is 1415 58kms1 and

the mean cluster galaxy velocity is 1493 36kms1 with v = 374 26kms1 . There is a known difference

between the giant (v = 308 30kms1 ) and dwarf (v = 429 41kms

1 ) galaxy velocity dispersions

(Drinkwater et al. 2001).

michael d. gregg et al- a large population of ultra-compact dwarf galaxies in the fornax cluster

Documents