ultra-compact dwarf galaxies: dark matter dominated systems? or simple stellar clusters?

1st California Postdoc Symposium, UC Santa Cruz, August 21 & 22 2008.

Arna Karick

UC Davis/IGPP, LLNL

Friends of UCDs:- Michael Gregg (UC Davis/LLNL)- Mike West (ESO Chile)- Michael Drinkwater, Peter Firth & Katya Evstigneeva (UQ)- Michael Hilker (ESO Garching)- Kenji Bekki (UNSW)- Bryn Jones (Queen Mary, London)- Steve Phillipps (Bristol)

Ultra-Compact Dwarf GalaxiesDark Matter Dominated Systems? or Simple Stellar Clusters?

The UCD story in 30 minutes or less…Over the past decade, a new class of compact stellar systems

have been discovered in galaxy clusters….

First Discoveries

Fornax Cluster Spectroscopic Survey

Follow-up Observations: HST/STIS and VLT/UVES & Keck/ESI

The UCD Rush

A few words on nomenclature

Summary of General Properties

Recent and Ongoing Investigations

1995: West et al.Galaxy clusters contain NIGC ∝ Mcl

Distribution follows the cluster mass distribution

1996: Minniti et al.

Small survey of NGC1399 globular clustersObject near NGC1404 is identified as a verycompact dwarf galaxy: V=18.5, (V– I) = I.48

1999: Hilker et al.Imaging of Fornax Cluster galaxies.Two compact objects are spectroscopicallyconfirmed to be cluster members. These

are later re-discovered in the FCSS.

“..can be explained by a a very bright GC or a compact elliptical like M32. Another explanation may be that these represent the nuclei of dwarf elliptical galaxies.”

- SE region (30’ across) of the Fornax Cluster.

(at d=20 Mpc, 30’~183 kpc)

- NGC1399 (central cluster galaxy): vrad =1425 kms-1

- brightest compact object is near NGC1404

(vrad=1947 kms-1) and has vrad=1591 ± 36 kms-1.

Image: CTIO 4m MOSAIC (g’,r’,i’), Karick, Gregg & Drinkwater

The First Discoveries

Image: ~3x2 degree B,V,I mosaic, taken with the CTIO Curtis Schmidt Telescope (Karick, Gregg & Drinkwater )

Fornax Cluster Spectroscopic Survey1999 – 2001: FCSS: Drinkwater et al. 2000, Phillips et al. 2001

Galactic stars

Fornax

all-object spectroscopicsurvey of ~14,000

resolved and unresolvedobjects (selected fromthe APM catalogue) in

four Fornax Fields.

~340 Fornax galaxies

+5 new cluster membersidentified within a two

degree region, centeredon NGC1399.

QSOs

Galaxies


V=1491 kms-1

V=1312 kms-1

V=1591 kms-1

V=1920 kms-1

V=1355 kms-1

Luminosities: −13.5 < MV < −12.3 mag[NGC1399 GCs: −11.5 < MV < −6 mag]

Velocities: 1312 kms-1 < vrad <1920 kms-1

Sizes: < 100pc(complete at these luminosities)

~150 kpc

Early studies predicted GC populations toextend further out (Forbes et. Al 1997)

~650 spectroscopically confirmed GCs(~450 Dirsch et al. 2004, +200 Schubeth et al.)

HST/STIS imaging of UCDsHST/STIS imaging of 5 UCDs and a comparison Fornax Cluster dE,N, FCC303(Drinkwater et al. 2003)

UCD profiles are well fitted by King and de Vaucouleurs R1/4 law profiles.

Effective radii range from, re=10–22 pc.

UCD3, the most luminous UCD, required

a 60 pc scale length exponential halo.

FCC303 (dE,N) has an 8 pc core and a

300 pc halo. Most compact Virgo dwarfs

have scale lengths of 160pc.

FCC303 UCD1 UCD2

UCD3 UCD4 UCD5

Galactic GCs have typical effective radii of re= 3–5 pc.

Largest M31 GCs have M/L ~ 1– 2 M

King Model Mass estimates: UCDs: 1 – 5 x 107 M,

FCC303 (nuc): 1.4 x 107 M,

most massive GCs: 4 x 106 M

→ UCDs have M/L ~ 2 – 4 M

VLT/UVES & Keck/ESI SpectroscopyVLT/UVES and Keck/ESI Spectroscopy of 5 UCDs and a comparison dE,N, FCC303(Drinkwater et al. 2003, Hilker et al. 2006 & Evstigneeva et al. 2007)

Velocity dispersions range from, σ = 24–37 km s-1

considerably higher than Galactic GCs (~5-10 kms-1).UCDs lie well off the GC: L∝σ1.7 relation in a previously

unoccupied region, but on an extrapolationof the elliptical galaxy: L∝σ4 Faber-Jackson relation.

Galaxies? (DM) or Stellar Clusters? (no DM?)

GALAXY THRESHING (Bekki et al. 2001, 2003)

The location of the UCDs and dE,Ns supports the “galaxy

threshing” model. A normal dE,N halo, accounting for

~98% of the dE,Ns luminosity is tidally stripped, reducing

the total luminosity by a factor of 100 but barely changing

the central velocity dispersion. Over time this disruption contributes diffuse stellar material to the ICM. The surviving

nuclei are dispersed into intracluster space or added to

the envelopes of brighter galaxies where they canmasquerade as stars. Evstigneeva et al. 2006

The Fornax Cluster20 Mpc away, z ~ 0.0043~340 “likely” members in the FCC 186 morphologically classified as dwarf galaxies (Ferguson 1989)

Velocity dispersion: giants 308 ± 30 kms-1

dwarfs 429 ± 41 kms-1 (infalling?)

Evidence for cluster substructure: Chandra: Scharf et al. (2004) FLAIR: Drinkwater, Colless & Gregg (2001) 6dF: Karick et al. (2008, sub)

(radial) infall region: galaxies are unlikely to be interacting gas rich dwarfs dominate (Waugh, PhD 2006) high SFR in dwarf galaxies (Drinkwater et al. 1999)

Central core: hot gas and high densities dEs and Es dominate galaxies likely to interact

Top: optical image and SW subcluster from the Flair analysis.Bottom: soft emission (0.3-1.5 keV)

The UCD Rush

First simulations of “galaxy threshing” (Bekki et al. 2000, 2001)

Fornax Compact Object Survey: ~50 “compact objects” (Mieske et al. 2002,04) → First metallicity measurements, [Fe/H]avg ~ –0.5 dex

Follow-up 2dF spectroscopy of Fornax: 60 more UCDs (Gregg et al. 2008) → Further investigation of the stripping hypothesis.

Deep u,g,r,i,z imaging of Fornax UCDs (Karick, PhD 2005) → Color analysis and candidate selection for follow-up spectroscopy

Follow-up VLT/FLAMES spectroscopy: 30 more UCDs (Firth et al. 2007) : 150 UCDs/IGCs (Bergond et al. 2007)

Keck/LRIS and Magellan/IMACS spectra of 90 Fornax UCDs,GCs and dE,Ns → Significant vairations in stellar population (Gregg & Karick, in prep)

10 UCDs discovered in Abell 1689 appear to be stripped dwarf galaxies. → two objects have properties similar to M32.

Massive GCs found in CenA but no UCDs (Martini et al. 2004)

6 “Dwarf-Globular Transition Objects” (DGTOs) discovered near M87 (ACSVCS) → highest M/L ~ 6 – 9 M, cannot be explained by SSP (Hasegan et al. 2005)

9 UCDs discovered in Virgo from 2dF observations (Jones et al. 2006)

Survey of 6 nearby galaxy groups - no UCDs (Evstigneeva et al. 2007)

‘99 – ‘01

‘02 – ‘06

The UCD Rush


Fornax Compact Object Survey: ~50 “compact objects” (Mieske et al. 2002,04) → First metallicity measurements, [Fe/H]est ~ –0.5 dex




Keck/LRIS and Magellan/IMACS spectra of 90 Fornax UCDs,GCs and dE,Ns → Significant vairations in stellar population (Gregg & Karick, in prep)






‘99 – ‘01

‘02 – ‘06

The UCD Rush


Fornax Compact Object Survey: ~50 “compact objects” (Mieske et al. 2002,04) → First metallicity measurements, [Fe/H]avg ~ –0.5 dex




Keck/LRIS and Magellan/IMACS spectra of 90 Fornax UCDs, GCs and dE,Ns → Significant vairations in stellar population (Gregg & Karick, in prep)






‘99 – ‘01

‘02 – ‘06


Gregg et al.2008

60’

Fornax Cluster

Fornax

9 objects widely distributed around M87 withluminosities (-13.7 < Mv < -11.5) similar to theFornax objects.

(Jones et al. 2006)

6 objects (+13 candidates) close to M87.Three have M/L ratios of 6-9 M whichcannot be explained by SSP models.

(Hasegan et al. 2006)

4 candidate IGCs near M87 from HST/ACSimaging. Fainter than Fornax UCDs andmetal-poor.

(Williams et al. 2006)

Another 6 UCDs between M87 and M49

(Firth et al. 2008)

Virgo Cluster

A few words on nomeclature..

With each new investigation comes a new acronym:…not to be confused with Ultra-faint, Ultra-handsome Dwarf Galaxies

Ultra-Compact Dwarf Galaxies (UCDs)

Ultra-Diffuse Star Clusters (UDSs)

Compact Cluster Objects (CCOs)

Compact Cluster Systems (CCSs)

Old Massive Clusters (OMCs)

Compact Stellar Systems (CSS)

Dwarf-Globular Transition Objects (DGTOs)

“…UCDs or PYFA’s”

-- E. Werhner, CEDGSC, Munich July 2008

Massive Compact Objects (MCOs)

“… definition of MCOs includes a number of objects that

are considered as UCDs in other works..”

(Dabringhausen et al 2008)

M31 (Andromeda) Globular Clusters

M31: - 1012 M roughly 80% mass of the MW- roughly 460 GCs

Back seat bandits:

Mayall-II/G1 :- massive (10 x 106 M) x2 mass of ωCen- metallicity spread (several generations of stars) core of a tidally stripped dwarf galaxy?- large ellipticity

Mayall II is named after Nicholas U. Mayall and O.J. Eggen in 1953. Sidney van den Bergh named it G1 in 1977.

Milky Way Globular Clustersderived from the Latin word globulus - a small sphere

- roughly 160 globulars (GCs) in the Milky Way - 90% lie within 100,000 light-years (0.5r stellar halo)- single age (old ~13 Gyr), single (low) met populations- (all stars form at the same time)- Very little or no HI gas and dust - (stars + stellar winds drive out gas)- M/L ratios ≈ 1 (unlike galaxies - NO DARK MATTER)- typical effective radii of re = 3-5 pc

Back seat bandits:

Omega Centauri (wCen) - old (12 Gyr), massive (5 x 106 M) - metallicity spread (several generations of stars)- core of a tidally stripped dwarf galaxy?- contains a 4 x 105 M BH (Gemini observations)

M15 : massive and most dense GC (core collapse), probably contains a central black hole.

General Properties

“UCDs - a mixed bag of objects” Hillker et al. 2006

Unusually luminous globular clusters.

(Hilker et al. 1999; Mieske et al. 2002)

Extremely luminous star clusters formed from

young clusters during galaxy mergers. (Fellhauer & Kroupa 2002, Maraston et al. 2004)

Remnant nuclei of dE,Ns or low luminosity

spirals which have been tidally disrupted or “threshed” during passages close to the central cluster galaxy, NGC1399. (Bekki et al. 2001, 2003, Goerdt et al 2008)

Extreme cases of known galaxies. (e.g. dE,Ns with very faint halos)

Highly compact galaxies formed in the early Universe in their own DM halos?

(Blanchard et al. 1992, Tegmark et al. 1997)

-smoothness of GCLF support this scenario.

- break in the L vs. Reff relation? collpase physics

- is there an upper mass limit for normal GCs?

- is there a turnover point on the L ∝ σ 1/4 relation?

- UCDs once resided in DM halos which were stripped

- IGCs should be more concentrated than dEs

- requires low σ, radial orbits and very compact cores

- ωCen, M22, G1?

- final UCD may contain dark matter but is not DM

dominated

- no dark matter?

Formation Scenarios

- would have dark matter

…back seat bandits

UCD1 has strong Mg, Hb absorption as well as a strong CN feature, similar to NGC1399 -- larger more metal-rich system?

Two UCDs/IGCs are X-ray point sources in the Chandra Fornax Survey.L=2 x108 ergs-1 (Scharf et al. 2005)Consistent with a NS LMXRB

Preliminary analysis of IMACS spectroscopy by Marla Geha showed variation in line indices.

CTIO imaging (Karick etal 2008)and ACSimaging (Puzia &Hilker2008,in prep) showssome red clusters are

extended.

Stellar Populations of UCDsFornax Virgo

Lick/IDS metallicites; Mgb, <Fe>, Hβ gives [α/Fe]. Also CaT measurements Virgo UCDs are older (>10Gyr) on average than dE,N nuclei and have lower metallicities,[Z/H] of -1.35 to -0.5 dex Fornax UCDs are more metal-rich, [Fe/H] ~ -0.9 to -0.5 ± 0.5 dex Similar to GC results in M49 and M87 (Cohen et al. 1998, 2003) In Virgo the threshing scenario seems less plausible.

Hasegan et al. (2005) found similar objects in Virgo - DGTOs Metallicities: –1.5 < [Fe/H] < –0.7 dex

Velocity dispersions: 20 — 30 kms-1 and M/L ratios: 6 - 9 M

The largest GCs in M31 have M/L ~ 1 — 2 M

VLT/UVES Spectroscopy :

Four Fornax UCDs + dE,N FCC303 (Hilker et al. 2006)

Velocity dispersions of UCDs: σ = 22.9 ± 2.6 — 29.2 ± 1.5 kms-1

Velocity dispersion of FCC303: σ = 27.1 ± 3.1 kms-1

Dynamical masses of UCDs: Mmod = 1.8 — 3.73 x 107 M [UCD3: 9.45 x 107 M] Dynamical M/L ratios: M/L = 3.15 — 4.99 (consitent with SSP M/L)

Dynamical Masses and M/L ratios

But high M/L ratioscan be explainedby tidal “heating”.

Fellhauer & Kroupa(2006)

High M/L ratio’s CAN be reduced by

adopting various stellar populations

models with differing IMFs

Dynamical Masses and M/L ratios

An extreme IMF as anexplanation for high M/L

ratios in UCDs?

The CO index as a tracer ofbottom heavy IMF

(Mieske & Kroupa 2008)

Salpeter IMF vs.Kroupa IMF ?

(Dabringhausen et al 2008)

….Relaxation time seems to be important

UV-metallicity properties of FornaxUCDs/IGCs

The FUV is sensitive to the excess UV emission produced by EXTREME horizontalbranch stars (EBH). This “UV-excess” is observed in many early-type galaxies,ωCen and some M87 GCs.

What are the UV — optical properties of UCDs in Fornax?

&

Do Fornax UCDs exhibit a UV — Mg2 or [Fe/H] relation?

Work with Michael Gregg, Soo-Chang Rey & Marla Geha (preliminary results)

GALEX Detections:

UV detections:

detect sources using SExtractor &compared to GALEX cataloguephotometry.

g,r,i,z UCD photometry:

Used more accurate coordinatesfrom our CTIO g,r,I,z imaging tomatch to GALEX detections (< 4”radii)

100 cataloguedUCDs/IGCs within the

GALEX field

GALEX NUV (1344-1786Å ) &FUV (1771-2831Å) archivalimages: 1.2 deg FOV with 1.5”pixels (~4.5” FWHM)

100 UCDs/IGCs/GCs

need to be careful!

Optical Photometry

accurate ACS/HRC V & I (F606W & F814W) photometry of 12 UCDs (Evstigneeva et al. 2008)

CTIO 4m-Blanco MOSAIC imaging of the core of Fornax g,r, i,z photometry of UCDs/GCs (Karick et al. 2008, PhD thesis)

UV — optical properties of UCDs ...preliminary results

Fornax UCDs (red points) appear to

be similar to the UV-bright clusters

of M87 (black points).

→ He - enrichment?

→ enhanced mass loss of old stars?

→ something else?

g,r,i,z & V, I converted to HST STMAG

GALEX (FUV - V) to HST (FUV-V)STMAG

similar UV-optical analysis by

Mieske et al. 2008 (in press)Sohn et al. (2006)

Sohn et al. (2006)

IMACS spectroscopy of UCDs/GCs…preliminary results

Medium resolution (1.3Å), high S/N (~50) spectroscopy of:32 Fornax UCDs and 61 GCs to measure line strength indices.Observations taken by Marla Geha.

Mean [Fe/H] derived from variousabsorption lines (Mg2, Fe etc.)using calibrations from Puzia et al. (2002)

In progress.. (Gregg &Karick 2008)-- Flux calibrate data-- can measure the strength of the D4000 break to get age estimates-- determine [α/Fe] ratios.-- compare to UV pop models.(may also affect our Fe/H values)

UV — Mg2 properties of UCDs ...preliminary results

Sohn et al. (2006)

Further investigation..UV-excess vs. UCD mass + metallicity?UV-Sloan photometry -intermediate age populations (with Soo-Chang Rey)

ConclusionsThere is a lot more work to do!

Stay tuned for more exciting results.

Acknowledgements

Thanks UCSC for organising this conference

Some of the work reported here was done under theauspices of the U.S. Department of Energy by

Lawrence Livermore National Laboratory in part underContract W-7405-Eng-48 and in part under Contract

DE-AC52-07NA27344.

ultra-compact dwarf galaxies: dark matter dominated systems? or simple stellar clusters?

Science

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new cluster members

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