1. Ultracompact dwarf galaxies - a mixed bag of objects
Michael Hilker (AIfA, University of Bonn, Germany)

2. Overview Introduction
Discovery of compact objects in the Fornax cluster
Physical properties of UCDs
The parameter space of massive hot stellar systems
Theories on the formation of UCDs
Summary
Dr. Michael Hilker (AIfA, University Bonn)

3. Our neighbouring spiral galaxy: Andromeda (M31)
G1 - one of the
most massive
globular clusters
of M31
M31 has
more than
12 satellite
galaxies
and hun-
dreds of
globular
clusters
M32
a compact
elliptical with
apparently no
counterparts,
possesses no
own globular
clusters
NGC 205
a nucleated
dwarf elliptical
with its own
globular
cluster
system
Dr. Michael Hilker (AIfA, University Bonn)

4. A generalised classification scheme of galaxies
mass
star formation activity
M32
UCD GC
Compact galaxy researcher (e.g. MD): What is the smallest compact galaxy?
Globular cluster researcher (e.g. MKP): What is the largest globular cluster?
Dr. Michael Hilker (AIfA, University Bonn)

5. Why have some big star clusters/small compact
galaxies been overlooked for so long?
The small compact elliptical M32 vs. The giant elliptical M87 in Virgo
Both ellipticals are of similar apparent size and brightness. Both images
are 10x10 arcmin. Source: Palomar Digital Sky Survey.
Dr. Michael Hilker (AIfA, University Bonn)

6. The discovery of compact … celebrating the 10-year
objects in Fornax
… celebrating the 10-year
birthday of „UCDs“
Dr. Michael Hilker (AIfA, University Bonn)

7. UCD history Imaging of Fornax fields at the 2.5m DuPont
telescope in Las Campanas 1994
Dwarf galaxies,
cD halo,
central GCS
(Hilker et al.)
Globular
Cluster
Systems
(Kissler-Patig et al.)
1995/96: spectroscopic follow-up of GCs at the NTT (La Silla)
1996: spectroscopic follow-up of dwarfs at the 2.5m (LCO)
Dr. Michael Hilker (AIfA, University Bonn)

8. 1998
UCD history
UCD3
1996
Dr. Michael Hilker (AIfA, University Bonn)

9. UCD history 1999 dEs UCD3 UCD2 Series of three papers:
Hilker et al. 1999
I. A&AS 134, 59
II. A&AS 134, 75
III. A&AS 138, 55
also in
PhD thesis
M. Hilker
(Feb. 1998)
Dr. Michael Hilker (AIfA, University Bonn)

10. UCD history the name „UCD“ was born
2000
Systematic survey by Drinkwater et al.
(2000, PASA 17, 227):
Spectroscopy (2dF) of all sources in
the Fornax cluster down to Bj = 19 mag
(FCSS: Fornax Cluster Spectroscopic
Survey)
5 compact objects found (2 of them in
common with Hilker survey)
Phillipps et al. (2001, ApJ 560, 201):
„Ultracompact Dwarf Galaxies in the
Fornax Cluster“
the name „UCD“ was born
2001
Dr. Michael Hilker (AIfA, University Bonn)

11. Nomenclature
for objects with -13.5<MV<-12.0, 10<rh<30 pc, and 20<0<40 km/s
Pre-”UCD” era: compact dwarf galaxy, compact stellar object, compact object,
compact stellar system, extremely compact dwarf galaxy,
super-massive star cluster, extremely large star cluster, …
Phillipps et al. (2001) : Ultracompact Dwarf Galaxy (UCD)
Kissler-Patig (2005) : Ultra Diffuse Star Cluster (UDSC?)
Hasegan et al. (2005): Dwarf-Globular Transition Object (DGTO)
Note: in late-type spirals/dEs: Nuclear (Star) Cluster (NC)
in starburst/merger galaxies: Super-Star Cluster (SSC)
Young Massive Cluster (YMC)
Merged Star Cluster Complexes (MSCC)
Call for new ideas:
K. Bekki: Very Massive Star Cluster (VMSC)
W.E. Harris: Isolated Central Massive Object (ICMO)
SCGC (single-collapse GC) vs. MCGC (multiple-collapse GC)
“…” ?
Dr. Michael Hilker (AIfA, University Bonn)

12. UCD properties Luminosity: Colour: LSB features: Half-light radius:
Imaging
UCD properties
Spectroscopy
Luminosity:
Colour:
LSB features:
Half-light radius:
Velocity disp.:
Mass:
M/L:
Metallicity:
Age:
-13.5<MV<-11.5
Fornax: red; Virgo: blue
some have LSB envelopes
with 80<Reff<120 pc
10<rh<30 pc
25<0<45 km/s
2-9x107 M
2-5 (Virgo DGTOs 2-9)
Fornax: -0.5 dex
Virgo: dex, [/Fe]=0.3
old (>8-10 Gyr)
deep in surface brightness
high spectral resolution
high spatial resolution
high signal-to-noise
De Propris et al. (2006), Drinkwater et al. (2000, 2003), Evstigneeva et al.
(2005, 2006), Hasegan et al. (2005), Hilker et al. (1999, 2006), Mieske et al.
(2002, 2004, 2006), Phillipps et al. (2001), Richtler et al. (2005), …
Dr. Michael Hilker (AIfA, University Bonn)

13. „The UCD Rush“
Dr. Michael Hilker (AIfA, University Bonn)

14. The ACS Virgo cluster survey -11.8<MV<-10.8 Dwarf-Globular
Transition Objects
(DGTOs)
V-band image
(KPNO 4m)
ACS footprint
-11.8<MV<-10.8
M87
mass-to-light ratio M/LV
half-mass radius rh
Hasegan et al. 2005
(ApJ 627, 203)
Dr. Michael Hilker (AIfA, University Bonn)

15. UCDs in Virgo -13.7<MV<-11.5 Jones et al. (2006)
Dr. Michael Hilker (AIfA, University Bonn)

16. The „all-object“ search goes fainter and wider in Fornax
MV<-10.5
Jones et al. (2006)
Drinkwater et al. (2006)
Firth et al. (see poster)
Dr. Michael Hilker (AIfA, University Bonn)

17. Spectroscopic survey of ~700 globulars around NGC 1399
bright compact
objects: R<19.5
(~ MV<-11.5)
MV<-8.5
Schuberth et al. (2005, 2006)
Richtler et al. (2004, 2005)
Dirsch et al. (2004)
Mieske et al. (2002, 2004)
Drinkwater et al. (2000, 2003)
Hilker et al. (1999)
40x40 armin
Dr. Michael Hilker (AIfA, University Bonn)

18. More discoveries/surveys
Massive star clusters around Cen A
(Harris et al , Martini & Ho 2004, Gomez et al. 2005, Rejkuba et al. 2006)
Non-detection of massive UCDs in galaxy groups
(Evstigneeva et al. 2006, Mieske et al. 2006)
M32 twins in the super-massive galaxy cluster A1689
(Mieske et al. 2004, 2005)
Masses/sizes/colours of dwarf elliptical nuclei
(Geha et al. 2002, 2004, Lotz et al, 2004, VCS team)
Masses/sizes of nuclear clusters in spirals
(Walcher et al. 2005)
Young massive star clusters in spirals/mergers
(Larsen 2001, Maraston et al. 2004, Bastian et al. 2005, …)
Dr. Michael Hilker (AIfA, University Bonn)

19. Filling the parameter space
of hot stellar systems
Dr. Michael Hilker (AIfA, University Bonn)

20. MV, 0, 0, eff, M,
rh, reff, 0, 1, 2, 3
Dr. Michael Hilker (AIfA, University Bonn)

21. The luminosity-size relation
Milky Way
M31/M32/NGC 205
Cen A
Fornax UCDs
Virgo UCDs
Nuclei
A1689
Harris (1996)
McLaughlin & van der Marel (2005)
Barmby et al. (2002)
Ma et al. (2006)
Geha et al. (2003, 2005)
Peng et al. (2005)
Martini & Ho (2004)
Rejkuba et al. (2006, poster)
Mieske et al. (2004, 2005, 2006)
Richtler (2005)
Hasegan et al. (2005)
De Propris et al. (2005)
Evstigneeva et al. (2006)
Hilker et al. (2006) …
Dr. Michael Hilker (AIfA, University Bonn)

22. Luminosity vs. central velocity dispersion
McLaughlin & van der Marel (2005)
Barmby et al. (2002)
Geha et al. (2003, 2005)
Peng et al. (2005)
Martini & Ho (2004)
Rejkuba et al. (2006, poster)
Mieske et al. (2004, 2005, 2006)
Richtler (2005)
Hasegan et al. (2005)
De Propris et al. (2005)
Evstigneeva et al. (2006)
Hilker et al. (2006) …
Dr. Michael Hilker (AIfA, University Bonn)

23. Colour magnitude diagram Milky Way
Cen A
NGC 1399 GCS
Fornax „UCDs“
Virgo „UCDs“
Nuclei
Harris (1996)
Barmby et al. (2002)
Peng et al. (2005)
Kundu et al. (2005)
Mieske et al. (2002, 2004)
Evstigneeva et al. (2006)
Lotz et al. (2004)
Hilker et al. (2006) …
Dr. Michael Hilker (AIfA, University Bonn)

24. Formation scenarios for UCDs
Dr. Michael Hilker (AIfA, University Bonn)

25. “Threshing scenario (remnant nuclei)”
(Bassino et al. 1994, Zinnecker et al. 1988, Bekki et al. 2001, 2003)
vs.
“Massive supercluster complexes”
(Kroupa 1998, Fellhauer & Kroupa 2002, 2005, Kissler-Patig et al. 2006)
“Bright end of the normal GC luminosity function”
(Mieske et al. 2002, 2004, Martini & Ho 2004)
“Genuine compact dwarf galaxies (BCD successors)”
(Richtler et al. 2005, Drinkwater et al. 2005)
Dr. Michael Hilker (AIfA, University Bonn)

26. Simulation of UCD formation from the disruption of dwarf galaxies
orbit
nucleus mass and mass loss
Bekki et al. (2001, 2003)
Dr. Michael Hilker (AIfA, University Bonn)

27. Formation of UCDs by the amalgation of stellar superclusters (SSCs)
Creation of dwarf galaxies by SSCs
first predicted by Kroupa (1998)
Setup of models: super-cluster orbits in
an analytic galactic
potential on an
eccentric orbit
(SUPERBOX)
Fellhauer & Kroupa (2002, 2005)
Dr. Michael Hilker (AIfA, University Bonn)

28. W3: a supermassive young cluster – progenitor of an UCD?
The recent merger NGC 7252
W3: a supermassive young cluster – progenitor of an UCD?
mass: 8 x 107 Mo, age: ~ Myr, MV = –16.2, s0 ~ 45 km/s (Maraston et al. 2004) N E
Dr. Michael Hilker (AIfA, University Bonn)

29. CM relation of compact objects and evolution path of W3
Hilker et al., 2006
(in prep.)
Dr. Michael Hilker (AIfA, University Bonn)

30. Tadpole galaxy Formation of tidal dwarfs (UCDs?)
in supercluster complexes??
Dr. Michael Hilker (AIfA, University Bonn)

31. Formation channels of UCDs: some implications
Hypothesis: UCDs are …
… the most massive GCs of the GC systems in clusters:
- most massive GC mass correlates with host galaxy mass
- formed from most massive molecular clouds (MCs) of host galaxy
- change of formation/collapse physics at critical MC mass
- Fornax: UCDs are connected to metal-rich bulge GC formation
- Virgo: MCs from which UCDs formed were less metal-rich than
in Fornax
… the remnant nuclei of disrupted galaxies:
- strong tidal fields are needed/selective process
- progenitor galaxies are massive dE,Ns or nucleated spirals
- present-day nuclei in Fornax/Virgo are less massive than UCDs
- present-day nuclei in Fornax are more metal-poor than UCDs
- most convincing local candidates: M54, Cen, G1
Dr. Michael Hilker (AIfA, University Bonn)

32. Formation channels of UCDs: some implications
Hypothesis: UCDs are …
… merged super-star clusters from ancient merger events
- there exist present-day examples: Antennae, W3, …
- since UCDs are old, cluster galaxy mergers happened early on
- gas-rich mergers needed (late mergers in clusters are “dry”!)
- Fornax: early mergers already possessed solar metallicity gas
- small numbers: only most massive complexes survived
… the successors of early BCDs
- no external processes needed (mergers, tidal disruption)
- small numbers: BCD formation is rare event
- Fornax: BCDs had metal-rich central starburst
Dr. Michael Hilker (AIfA, University Bonn)

33. old, isolated, massive, compact stellar systems
Some ideas about massive star cluster/UCD formation
mass
Single-collapse GCs Multiple-collapse GCs
(single age and metallicity) (= merged single-collapse GCs)
immediate growth successive
( Myr) growth (> Gyr)
cluster complexes nuclei formation via nuclei growth via
in mergers/BCDs merging of old GCs episodic star for-
(same age/similar in dEs (age/metallicity mation (age/metallicity
metallicity) spread possible) spread probable)
evolution in tidal field over Hubble time
old, isolated, massive, compact stellar systems
Dr. Michael Hilker (AIfA, University Bonn)

34. Next questions to answer
Do UCDs have multiple stellar populations?
look for colour gradients in spatially resolved, deconvolved images
mean ages/metallicities from multi-colour or line index studies
Why do some UCDs have a high M/L? Dark matter?
get spatially resolved velocity dispersion profiles
mass modelling including rotation, anisotropy and tidal effects (see Fellhauer poster)
Is there tidal structure around UCDs?
deep low surface brightness photometry
Do UCDs have cusps or cores? Do they harbour BHs?
high resolution imaging, thorough deconvolution to constrain inner profile
spatially resolved velocity dispersion profiles (see Evstigneeva et al. poster)
What is the kinematics of UCDs within the clusters?
complete spectroscopic survey of bright GCs/UCDs (see Firth et al. poster)
Dr. Michael Hilker (AIfA, University Bonn)

35. Summary ?? ? ? ? brightest GCs, isolated nuclei?
compact elliptical (cE)
ultra-compact dwarfs (UCDs) ? ?? ? ?
nucleated dwarf
elliptical (dE,N)
super-cluster complex
Antennae M(V)=-14 mag
blue compact dwarfs (BCDs)
NGC 1705 M(V)~-16 mag
Dr. Michael Hilker (AIfA, University Bonn)