Star Formation in Star Formation in ClustersClusters

Søren S. LarsenSøren S. Larsen

ESO / ST-ECF, Garching, GermanyESO / ST-ECF, Garching, Germany

HST Observations of HST Observations of ClustersClusters

Progress until ~2000 summarized in Progress until ~2000 summarized in Whitmore 2003, Whitmore 2003, A Decade of HST ScienceA Decade of HST Science

Recent conference Recent conference proceedings/workshops: proceedings/workshops: IAU Symp. 207, “Extragalactic Star IAU Symp. 207, “Extragalactic Star

Clusters”, eds. Grebel, Geisler, Minniti (2001)Clusters”, eds. Grebel, Geisler, Minniti (2001) ESO workshop on “Extragalactic Globular ESO workshop on “Extragalactic Globular

Cluster Systems”, ed. M. Kissler-Patig (2002)Cluster Systems”, ed. M. Kissler-Patig (2002) Cancun Workshop on Young Massive Star Cancun Workshop on Young Massive Star

Clusters, eds. Lamers, Smith, Nota (2003)Clusters, eds. Lamers, Smith, Nota (2003)

HST and Extragalactic HST and Extragalactic Star ClustersStar Clusters

HST is well tailored for observations of HST is well tailored for observations of Extragalactic Star Clusters.Extragalactic Star Clusters. Angular resolutionAngular resolution: At 10 Mpc, 0.1” ~ 5 pc, : At 10 Mpc, 0.1” ~ 5 pc,

similar to typical sizes of star clusters.similar to typical sizes of star clusters. Field sizeField size: ACS covers significant fraction even : ACS covers significant fraction even

of nearby galaxiesof nearby galaxies Spectral rangeSpectral range: Optical and near-UV data : Optical and near-UV data

essential for age-dating young star clustersessential for age-dating young star clusters HST capabilities are HST capabilities are unique unique for the foreseeable for the foreseeable

future (O’Connell 2004, Cancun YMC workshop)future (O’Connell 2004, Cancun YMC workshop) JWST will offer no significant gain in spatial resolution, JWST will offer no significant gain in spatial resolution,

and mostly limited to the IR.and mostly limited to the IR. Ground-based AO: Small field, IR, ÷PSF stabilityGround-based AO: Small field, IR, ÷PSF stability GALEX has UV capability, but low spatial resolutionGALEX has UV capability, but low spatial resolution

The Need for UV The Need for UV photometryphotometry

Even B-V only weakly sensitive to age for young clusters; U-B provides much better leverage.

“S”-sequence (Elson & Fall 1985, ApJ 299, 211; Girardi et al. 1995, A&A 298, 87) reproduces ages of LMC clusters with rms dispersion in log(age)=0.14.

Multiple colors can provide additional constraints on reddening and metallicity (e.g. de Grijs et al. 2003; Anders et al. 2004) but U-band still essential.

107

108 yr

Young Stellar Clusters as of Young Stellar Clusters as of 19881988

Kennicutt & Chu 1988, AJ 95, 720

Young Stellar Clusters as of Young Stellar Clusters as of 20042004

Starburst galaxies NGC 253 Watson et al. 1996, Forbes et al. 2000 NGC 1808 Tacconi-Garman et al. 1996 NGC 3077 Harris et al. 2004, Davidge 2004 NGC 3125 Chandar et al. 2004 NGC 3310 Meurer et al. 1995, de Grijs et al. 2003 NGC 3991 Meurer et al. 1995 NGC 4670 Meurer et al. 1995 NGC 5253 Maiz-Apellaniz 2001, Harris et al. 2004, Turner & Beck 2004, Vanzi & Sauvage 2004 NGC 6745 de Grijs et al. 2003 NGC 7469 Scoville et al. 2000 NGC 7673 Homeier & Gallagher 1999, Homeier et al. 2002 IC 883 Scoville et al. 2000 M 82 O'Connell et al. 1995, de Grijs et al. 2003 Arp 220 Scoville et al. 1998, Scoville et al. 2000 TOL1924-416 Meurer et al. 1995 Zw 049.057 Scoville et al. 2000 VII Zw 031 Scoville et al. 2000 IR 15250+3609 Scoville et al. 2000 IR 17208-0014 Scoville et al. 2000 NGC 520 Kotilainen et al. 2001 NGC 1614 Alonso-Herrero et al. 2001, Kotilainen et al. 2001 NGC 7714 Kotilainen et al. 2001

Nuclear starbursts NGC 4303 Colina & Wada 2000, Colina et al. 2002 NGC 5236 Bohlin et al. 1990, Heap et al. 1993, Harris et al. 2001 NGC 6240 Pasquali et al. 2004, Scoville et al. 2000

Circumnuclear rings NGC 1079 Maoz et al. 1996 NGC 1326 Buta et al. 2000 NGC 1433 Maoz et al. 1996 NGC 1512 Maoz et al. 1996, 2001 NGC 1097 Barth et al. 1995 NGC 2903 Alonso-Herrero et al. 2001 NGC 2997 Maoz et al. 1996 NGC 3310 Elmegreen et al. 2002 NGC 4314 Benedict et al. 1993 NGC 5248 Maoz et al. 1996,2001; Jogee et al. 2002 NGC 6951 Barth et al. 1995 NGC 7552 Meurer et al. 1995

Dwarfs / Irregulars NGC 1140 Hunter et al. 1994 NGC 1156 Larsen & Richtler 1999 NGC 1313 Larsen & Richtler 1999; NGC 1569 Arp & Sandage 1985, O'Connell et al. 1994, de Marchi et al. 1997, Hunter et al. 2000, Maoz et al. 2001, Maiz-Apellaniz 2001, Origlia et al. 2001, Anders et al. 2004, Gilbert & Graham 2003 NGC 1705 Melnick et al. 1985, O'Connell et al. 1994, Billett et al. 2001, Maiz-Apellaniz 2001, Vazquez et al. 2004 NGC 3077 Davidge 2004 NGC 4194 Weistrop et al. 2004 NGC 4214 Billett et al. 2001, Maiz-Apellaniz 2001 NGC 4449 Seitzer & Grebel 1998, Gelatt et al. 2001, Maiz-Apellaniz 2001 ESO-338-IG04 Oestlin et al. 1998 HE 2-10 Conti & Vacca 1994, Johnson et al. 2000, Beck et al. 2001 I Zw 18 Meurer et al. 1995 UGC 7636 Lee et al. 1997 POX 186 Doublier et al. 2000 SBS 0335-052 Vanzi et al. 2000

Mergers / Interacting galaxies NGC 1275 Holtzman et al. 1992, Carlson et al. 1998 NGC 1741 Johnson et al. 1999 NGC 2207 / IC 2163 Elmegreen et al. 2001 NGC 2623 Scoville et al. 2000 NGC 3256 Zepf et al. 1999 NGC 3395/96 Hancock et al. 2003 NGC 3597 Lutz 1991, Carlson et al. 1999, Forbes & Hau 2000 NGC 3690 Meurer et al. 1995 NGC 3921 Schweizer et al. 1996 NGC 4038/39 Whitmore & Schweizer 1995, Whitmore et al. 1999, Mengel et al. 2002 NGC 6052 Holtzman et al. 1996 NGC 6090 Dinshaw et al. 1999, Scoville et al. 2000 NGC 6240 Pasquali et al. 2003 NGC 7252 Whitmore et al. 1993, Miller et al. 1997, Maraston et al. 2004 NGC 7727 Crabtree & Smecker-Hane 1994 II ZW 96 Goldader et al. 1997 Cartwheel Borne et al. 1997 The Mice de Grijs et al. 2003 Tadpole de Grijs et al. 2003 HCG 31 Johnson & Conti 2000 VV 114E/W Scoville et al. 2000 UGC 5101 Scoville et al. 2000 UGC 10214 Tran et al. 2003 IR 10565+2448W Scoville et al. 2000 IR 15206+3342 Arribas & Colina 2002 IR 22491-1808W Scoville et al. 2000 Mrk 273S Scoville et al. 2000 Stephan's Quintet Gallagher et al. 2001 Tidal Tails Knierman et al. 2003

Spiral galaxy disks M 51 Larsen 2000; Bik et al. 2003; Bastian et al. 2004 M 81 Chandar et al. 2001 M101 Bresolin et al. 1996 NGC 2403 Battistini et al. 1984; Drissen et al. 1999; Larsen & Richtler 1997 NGC 2997 Larsen & Richtler 1999 NGC 3621 Larsen & Richtler 1999 NGC 3627 Dolphin & Kennicutt 2002 NGC 5236 Larsen & Richtler 1999 NGC 7793 Larsen & Richtler 1999 NGC 6946 Larsen & Richtler 1999; Elmegreen et al. 2000

Vast majority of studies HST-based. Mostly WFPC2, some WFPC/FOS FOC/STIS/NICMOS/ACS

NGC 3256 - MergerNGC 3256 - Merger

About 1000 “young globular” clusters in 7 7 kpc region with -9 < MB < -15

15%-20% of B-band light in clusters Half-light radii 5-10 pc, only shallow trend

with luminosity (but 1 PC pixel ~ 8 pc) Luminosity function: power-law,

n(L)dLL-1.8dL

(Zepf et al. 1999, AJ 118, 752)

IR-brightest and most gas-rich galaxy in Toomre (1977) list.Distance ~ 37 MpcF450W/F814W composite

M82 starburst - IM82 starburst - I“Infrared plates obtained with the 200-inch telescope show that the central region of M82 contains about a dozen bright knots. Spectroscopic observations suggest that these knots are super star clusters. These super-clusters are up to 100 times brighter than the most luminous star cluster known in the Galaxy” … “It should perhaps be stressed that this nomenclature is not intended to imply that these objects are systems of negative energy”Van den Bergh 1971, A&A 12, 474

WFPC2: APOD Mar 12, 2001 / R. de Grijs

HST/WFPC: Over 100 young clusters with -9.6 < MV < -13.2 (O’Connell et al. 1995, ApJ 446, L1)

M82-B “fossil” starburstM82-B “fossil” starburstIntermediate-age clusters (~1.1 Gyr) with indication of a turn-over in the mass function at log (M/M) ~ 5.2 (de Grijs et al. 2003, MNRAS 340, 197).

Evidence for evolution from power-law to log-normal mass distribution?

Compl.

(Post) starburst dwarf (Post) starburst dwarf NGC1569NGC1569

Arp & Sandage 1985, AJ 90, 24: Spectroscopic evidence for star clusters

Distance only 2.5 pc clusters well resolved with HST

O’Connell et al. 1994, ApJ 433, 65: ‘Super Star Clusters’ resolved with HST/WFPC, MV ≈ -14

Half-light radii 2-3 pc (O’Connell et al. 1994; Hunter et al. 2000, AJ 120, 2383), masses of ~few 105 M; similar to GCs.

SSCs provide 20%-25% of total optical/IR light in central region of the galaxy (Origlia et al. 2001).

Many fainter clusters (Hunter et al. 2000; Anders et al. 2004, MNRAS 347, 17)

Post starburst dwarf- Post starburst dwarf- NGC1569NGC1569 Cluster NGC1569-A has two

components: FOS and STIS spectroscopy +

NICMOS photometry reveals W-R stars in NGC1569-A2 and RSGs in NGC 1569-A1 (Origlia et al. 2001, AJ 122, 815; Maoz et al. 2001, ApJ 554, L139)

“Normal IMF”, suggesting evolution to GC (Ho & Filippenko 1996, Gilbert & Graham 2002)

STIS spectra (Maoz et al. 2001)

NGC1569-A1

NGC1569-A2

Circumnuclear Circumnuclear StarburstsStarbursts

NGC 1512 NGC 5248

F220W/F547M/F160W

F336W/F547M/F658N

F814W/F160W/F187W

CSs occur preferentially in barred galaxies of type S0 - Sc

Significant fraction (30%-50%) of UV light in CSs is emitted by compact sources with radii < 5 pc (Maoz et al. 1996, AJ 111, 2248) and masses up to ~105 M

Most of the visible clusters only mildly reddened (AV < 1 mag) suggesting rapid gas clearing (Maoz et al. 2001, AJ 121, 3048)

Luminosity- and mass functions consistent with power-laws:N(M)dM M-2 dM (note: Buta et al. 2000 find slope of -3.70.1 for NGC 1326)

Young GCs in nucleus and Young GCs in nucleus and diskdisk of M83of M83

Disk: ~150 clusters with -12<MV<-9(Larsen & Richtler 1999; A&A 345, 59) #502: ~100 Myr, 5105 M

#805: ~15 Myr, 4105 M

(Larsen & Richtler 2004; A&A, subm.)

Nucleus (central 300 pc): 45 clusters with masses of 104-105 M; Half-light radii 1 - 4 pc; Ages < 107 years;Mass function consistent with power-law N(M)dM M-2dM

Harris, J., et al. 2001, AJ 122, 3064

Multi-filter, deep WFPC2 data: F336W, F439W, F555W, F656N, F814W

Antennae clusters: Antennae clusters: disruptiondisruption

Lines: Bruzual & Charlot (1996) SSP models for log(M/M) = 6.0, 5.5, 5.0, 4.5 and 4.0

Number of clusters in youngest age bin (<107 yrs) account for total SFR (Fall 2004).

Roughly equal numbers of clusters in equal logarithmic age bins many clusters disrupt. High “infant mortality” (Whitmore 2003).

Cluster disruption timescales may depend on environment (Boutloukos & Lamers 2003, MNRAS 338, 717)

Zhang & Fall 1999, ApJL 527, L81

YMCs: Size-of-sample YMCs: Size-of-sample Effect?Effect?

Log N

MV (

bri g

hte s

t)

Whitmore (2003)

If the cluster luminosity function is a universal power-law then sampling statistics predict brighter clusters in rich cluster systems (e.g. Antennae, other starbursts & mergers).

Whitmore 2003; Larsen 2002, AJ 124, 1393; Billett et al. 2002, 123, 1454

Mass distribution physically more relevant, but harder..

Bound clusters may form more efficiently in high-SFR environments (Meurer et al. 1995, AJ 110, 2665; Larsen & Richtler 2000, A&A 354, 836)

Cluster sizes Cluster sizes

Larsen 2004, A&A 416, 537: Reff M0.100.30 (15 spiral galaxies)

Carlson & Holtzman 2001, PASP 113, 1522

Zepf et al. 1999, AJ 118, 752: r L0.07 (NGC 3256)

Star cluster sizes generally show little dependence on mass

Observations and Observations and implicationsimplications

Most (all?) stars form in clusters, but high Most (all?) stars form in clusters, but high infant mortalityinfant mortality

““Massive” (10Massive” (1055 M M - 10 - 1066 M M) clusters are ) clusters are found in many different environments, found in many different environments, whenever clusters form in large numbers. whenever clusters form in large numbers. Size-of-sample effect?Size-of-sample effect?

Some exceptions e.g. NGC 1569 with few, Some exceptions e.g. NGC 1569 with few, very luminous clusters and “gap” in LF. very luminous clusters and “gap” in LF. Special mechanism? (Gallagher 2004)Special mechanism? (Gallagher 2004)

Cluster sizes nearly independent of mass Cluster sizes nearly independent of mass density increases nearly linearly with mass. density increases nearly linearly with mass. Implications for star formation physics Implications for star formation physics (Ashman & Zepf 2001)?(Ashman & Zepf 2001)?

QuestionsQuestions

Is there a universal (initial) cluster Is there a universal (initial) cluster massmass function? How does MF evolve with age? function? How does MF evolve with age? Does initial power-law MF always evolve Does initial power-law MF always evolve into log-normal shape? into log-normal shape?

What are the properties of star clusters in What are the properties of star clusters in quiescent environments (e.g. Sa/Sb spirals)?quiescent environments (e.g. Sa/Sb spirals)?

How do structural parameters of star How do structural parameters of star clusters depend on mass / age / clusters depend on mass / age / environment etc?environment etc?

All of the above require HST!All of the above require HST!

NGC 7793: ground-based and ACS NGC 7793: ground-based and ACS datadata

Cycle 12 program: 5 nearby spiralsACS F435W/F555W/F814WWFPC2 F336W

OutlookOutlook WFC3 would be an ideal instrument WFC3 would be an ideal instrument

for observations of extragalactic star for observations of extragalactic star clusters: panchromatic coverage, large clusters: panchromatic coverage, large FOV, high resolutionFOV, high resolution

ACS + WFPC2 + NICMOS still ACS + WFPC2 + NICMOS still powerful alternatives, though less powerful alternatives, though less efficientefficient

Cluster sizes/structure: Need for Cluster sizes/structure: Need for accurate PSF modeling, e.g. use jitter accurate PSF modeling, e.g. use jitter info + tinytim if 2-gyro mode.info + tinytim if 2-gyro mode.