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Star Formation in Clusters. Søren S. Larsen ESO / ST-ECF, Garching, Germany. HST Observations of Clusters. Progress until ~2000 summarized in Whitmore 2003, A Decade of HST Science Recent conference proceedings/workshops:

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Star formation in clusters

Star Formation in Clusters

Søren S. Larsen

ESO / ST-ECF, Garching, Germany


Hst observations of clusters
HST Observations of Clusters

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

  • Recent conference proceedings/workshops:

    • IAU Symp. 207, “Extragalactic Star Clusters”, eds. Grebel, Geisler, Minniti (2001)

    • ESO workshop on “Extragalactic Globular Cluster Systems”, ed. M. Kissler-Patig (2002)

    • Cancun Workshop on Young Massive Star Clusters, eds. Lamers, Smith, Nota (2003)


Hst and extragalactic star clusters
HST and Extragalactic Star Clusters

  • HST is well tailored for observations of Extragalactic Star Clusters.

    • Angular resolution: At 10 Mpc, 0.1” ~ 5 pc, similar to typical sizes of star clusters.

    • Field size: ACS covers significant fraction even of nearby galaxies

    • Spectral range: Optical and near-UV data essential for age-dating young star clusters

  • HST capabilities are unique for the foreseeable future (O’Connell 2004, Cancun YMC workshop)

    • JWST will offer no significant gain in spatial resolution, and mostly limited to the IR.

    • Ground-based AO: Small field, IR, ÷PSF stability

    • GALEX has UV capability, but low spatial resolution


The need for uv photometry
The Need for UV photometry

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.

107

108 yr

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.


Young stellar clusters as of 1988

Kennicutt & Chu 1988, AJ 95, 720

Young Stellar Clusters as of 1988


Young stellar clusters as of 2004

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

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

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

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

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

Young Stellar Clusters as of 2004


Ngc 3256 merger
NGC 3256 - Merger

IR-brightest and most gas-rich galaxy in Toomre (1977) list.

Distance ~ 37 Mpc

F450W/F814W composite

  • 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)dLL-1.8dL

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


M82 starburst i
M82 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

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

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


M82 b fossil starburst

Compl.

M82-B “fossil” starburst

Intermediate-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?


Post starburst dwarf ngc1569
(Post) starburst dwarf NGC1569

  • 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 ngc15691
Post starburst dwarf- NGC1569

NGC1569-A1

  • 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)

NGC1569-A2

STIS spectra (Maoz et al. 2001)


Circumnuclear starbursts
Circumnuclear Starbursts

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 -3.70.1 for NGC 1326)

NGC 1512 NGC 5248

F220W/F547M/F160W

F336W/F547M/F658N

F814W/F160W/F187W


Young gcs in nucleus and disk of m83
Young GCs in nucleus and disk of M83

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

Disk: ~150 clusters with -12<MV<-9

(Larsen & Richtler 1999; A&A 345, 59)

#502: ~100 Myr, 5105M

#805: ~15 Myr, 4105M

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



Antennae clusters disruption
Antennae clusters: disruption F814W

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 effect
YMCs: Size-of-sample Effect? F814W

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..

MV (brightest)

Log N

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

Whitmore (2003)


Cluster sizes
Cluster sizes F814W

Star cluster sizes generally show little dependence on mass

Larsen 2004, A&A 416, 537:

Reff  M0.100.30 (15 spiral galaxies)

Zepf et al. 1999, AJ 118, 752:

r  L0.07 (NGC 3256)

Carlson & Holtzman 2001, PASP 113, 1522


Observations and implications
Observations and implications F814W

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

  • “Massive” (105 M - 106M) clusters are found in many different environments, whenever clusters form in large numbers. Size-of-sample effect?

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

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


Questions
Questions F814W

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

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

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

  • All of the above require HST!


Ngc 7793 ground based and acs data
NGC 7793: ground-based and ACS data F814W

  • Cycle 12 program:

  • 5 nearby spirals

  • ACS F435W/F555W/F814W

  • WFPC2 F336W


Outlook
Outlook F814W

  • WFC3 would be an ideal instrument for observations of extragalactic star clusters: panchromatic coverage, large FOV, high resolution

  • ACS + WFPC2 + NICMOS still powerful alternatives, though less efficient

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


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