1. how they might form and evolve
Properties of Nuclear Star Clusters Torsten Böker, European Space Agency ESTEC, Noordwijk, Netherlands
Outline:
what they are
what we know
how they might form and evolve
why we should care
in collaboration with
C.J. Walcher, E. Schinnerer, R. van der Marel, H.-W. Rix, L.C. Ho, J.C. Shields
Astronomy at High Angular Resolution - Bad Honnef, 23 April 2008

2. What is a Nuclear Star Cluster?
Late-type Spiral Galaxy NGC 300

3. N.B.: “pure” disks do exist!
Bland-Hawthorn et al. (2005)
NGC 300 disk is exponential out to ~10 Rd and has no bulge whatsoever!
about 20% of galaxies in the local universe look like this
“pure” disks are a challenge for models of structure formation - how did they form, and how did they avoid bulge formation in mergers?
(see e.g. Koda, Milosavljevic, & Shapiro 2008)

4. What is not a Nuclear Star Cluster?
(a.k.a. “Stellar Nucleus”)
light excess
Mention this and that
light deficit
Working Definition:
A “Nuclear Star Cluster” is a compact,
yet extended, source that is located at
the galaxy center and adds light
in excess of the disk/bulge profile.
(Ferrarese et al. 2006)

5. Nuclear Clusters are common!
WFPC2, F814W
Late-type Spirals (Sm-Scd)
ACS, F475W
VCS early types (S0-E)
Detection Frequency is generally high:
~ 50% of Sc - Sa (Carollo et al.)
~ 75% of Sm - Scd (Böker et al.)
~ 70% of Virgo S0, E, dS0, dE (Cote et al.)
lower for brightest gE and faintest dE
nucleation rare in dSph and dIrr
probably lower limits because of
- low SB contrast in bright bulges
- extinction in starburst nuclei
- ill-defined photocenter
 NC formation a generic process in most galaxies!

6. Sizes and Luminosities
Data Model Residual
- typical size reff ~ 2-5 pc
- typical luminosity ~ L
(correlates with galaxy luminosity)
nucl. cl.
MW glob. cl.
MW GCs
NCs

7. NCs are similar in all galaxy types!
(dwarf) spheroidals
Scd-Sm
Cote et al. 2005
Böker et al. 2002
from: ACSVCS
(Cote et al. 2005)

8. The color of nuclear clusters
[Carollo et al. 2001]
[Rossa et al. 2006]
Broad range of colors  range of age, Z, AV, …
colors consistent with stellar populations  non-thermal nuclei rare
spectra (see later)  most variation due to age
blue (i.e. young) NCs are absent in early-type galaxies!

9. Spectroscopy I: Stellar Populations and Cluster Ages
log(age)
7.0
Similar studies needed for early-type galaxies!
10.0
Ang
HST/STIS (Rossa et al. 2006)
NCs in late-type spirals
- have multiple generations of stars
- very often contain a young population (< 1 Gyr)
nearly always contain an old population (many Gyrs)
→ SF continues (in discrete “delta bursts”) long after cluster is born

10. Spectroscopy II: Dynamical Mass
need high spectral resolution → 8m telescope
need correct templates → check for presence of young stars!
log(age)
7.3
9.4
VLT/UVES (Walcher et al. 2005/2006)
13 km/s <  < 34 km/s  typical NC mass is a few million M

11. Comparison to other stellar systems
NC E/Bulges GC NC
E/Bulges
surface brightness vs. size
surface brightness vs. mass
 NCs are “normal”stellar clusters, and not small bulges!

12. Dependence on Hubble Type
- NCs in early-type spirals tend to be older and more massive:
Log(age)
Log(mass)
(Rossa et al. 2006)
NCs in early-type spirals are made from a higher number of “delta bursts”
they either started the process earlier, or
they are more efficient in funneling gas to the nucleus
(i.e. they have a higher duty cycle)

13. OK, but how did they get there?
- homogeneity of NC properties across Hubble sequence implies a quite generic formation mechanism. There are many possible scenarios:
credit: M. Milosavljevic

14. Migration and Merging of stellar clusters
1 kpc
M33 with HST/ACS
(Corbelli & Walterbos 2007)
(Milosavljevic & Agarwal 2008)

15. But…..
…unlikely to work for “quiescent” galaxies with low SFR!!

16. Gas transport towards the nucleus…
e.g. molecular gas in NGC 6946 at 0.35” with PdBI
CO(1-0)
CO(2-1)
(Schinnerer et al. 2006, 2007)

17. … due to stellar bars! Data Model
Intensity
- central clump (Ø ~ 25 pc) contains molecular gas mass of 1.6·107 M
- total SFR (H, mm-cont., & “hot cores”)
over the past 10 Myr is ca. 0.1 M/yr
→ about 106 M in stars have formed
Velocity
Dispersion
no NC can be identified in NGC 6946
→ maybe NC formation still happens
in the present-day universe....
(Schinnerer et al. 2006, 2007)

18. Edge-on gas “disks” around NCs
V-I color maps
(Seth et al. 2006)
10 pc

19. Connection to Black Holes
NCs and SMBHs follow same scaling relations: MCMO~0.002·Mgal
(Wehner & Harris 2006, Ferrarese et al. 2006, Rossa et al. 2006)
---> are Nuclear Clusters “failed” Black Holes?

20. NCs and BHs can co-exist
The NC in NGC 4395 hosts a
low-luminosity AGN
(Filippenko & Sargent 1989)
BH mass is between 104 and 105 M
(Filippenko & Ho 2003)
Another case: NGC 1042 (and then there’s the Milky Way…)
(Shields et al., ApJ, in press)

21. Are (some) globular clusters left-over nuclei?
Lee et al. 1999
Omega Cen has complex star formation history (multiple stellar populations)
is the most massive and most flattened GC in the Milky Way
has a central black hole of 4·104 M (Noyola, Gebhardt, & Bergmann 2008)
--> it is generally viewed as the remnant nucleus of an accreted dwarf galaxy
But the Milky Way accreted more than one dwarf galaxy…..

22. Where are all the other “left-over” nuclei?
Omega Cen is not as unique as thought:
NGC 2808, Piotto et al. 2007
NGC 6441, Caloi & d’Antona 2007
What if GCs form as NCs in disk galaxies of the early universe? This could explain
the presence of multiple stellar populations in GCs
the “universal” mass fraction of GC systems (~0.2% of host galaxy mass, McLaughlin 1999)
“Quick-look” analysis shows that this is not implausible (see Böker 2008, ApJL)

23. Summary nuclear star clusters are a common feature of galaxies
their typical luminosities, sizes, masses, and
stellar populations are now fairly well characterized
evidence for in-situ growth via repetitive starburst episodes
gas accretion can be observed in some nearby cases
interesting connections to
supermassive BHs
globular clusters (are they remnant nuclei?)