What do we know about the birth of super star clusters? Kelsey Johnson (UVa, NRAO) Collaborators: Indraneil Biswas (UVa) Amy Reines (UVa) Rémy Indebetouw (UVa) Bill Vacca (NASA-Ames) Leslie Hunt (INAF) Barb Whitney (SSI) Chip Kobulicky (UWy) Kenny Wood (St.Andrews)

Why are Super Star Clusters Interesting? Plausibly proto-globular clusters Extreme mode of star formation Formation common in early universe Luminous “simple stellar populations” for probing galaxy evolution Impact on the ISM & IGM How were these incredible clusters formed?

Can we learn from Galactic Star Forming Regions? From Ultracompact HII Regions to Proto Globular Clusters Key Questions: How do the properties of star formation scale between these regimes? How do the cluster properties depend on environment?

Strategy: Look for sources with similar SEDs to Ultracompact HII regions Sn l (cm) 100 1 non-thermal free-free optically-thick free-free Compact, “inverted spectrum” sources Very dense HII regions Wood & Churchwell 1989

Comparison of Radio SEDs (individual clusters) SBS 0335-052 He 2-10 NGC 5253 continuum of sources W49A Johnson et al. in prep, Johnson & Kobulnicky 2003, Mezger et al. 1967, Turner et al. 1998, 2004

Comparison of Radio SEDs (individual clusters) Radii of HII regions < a few pc Electron densities >104 - 106 cm-3  Pressures > 108 kB Ionizing Luminosities > 1052-53 s-1  > 1000s O7-type stars SBS 0335-052 He 2-10 NGC 5253 W49A Johnson et al. in prep, Johnson & Kobulnicky 2003, Mezger et al. 1967, Turner et al. 1998, 2004

What can we learn from radio recombination lines. (e. g What can we learn from radio recombination lines? (e.g. Mohan, Anantharamaiah, & Goss 2001) Densities: ne > 104 cm-3 Radii: r ~ 2-10 pc Ionizing Flux: Nlyc > 1052 Nearly perfect agreement with simple models! Example: prediction for H92a line

What do we (maybe?) know about their lifetimes? *Caveat: The external pressure could be much higher *Caveat: Star formation must be continuous over at least ~10 Myr

Different wavelengths probe different environments IC 4662 (2Mpc) Different wavelengths probe different environments “Young” (<10 Myr) optically selected clusters and ultra-young radio selected clusters are exclusive Linear Resolution ~ 10 pc NLyc  20 - 2001049 s-1  Large OB associations Johnson, Indebetouw, & Pisano 2003

What can we learn from the near-infrared? Haro 3 Radio clusters also have an “infrared excess” Hot dust near the ionizing stars Color scale: HST V-band Contours: VLA X-band Johnson, Indebetouw, Watson, & Kobulnicky 2004 (other examples in: Vanzi & Sauvage 2004, Cresci et al. 2005, Cabanac et al. 2005)

SBS 0335-052 ultra-low metallicity (Z  1/40 Z) NLyc  5,000 1049 s-1  5,000 O7* stars Color scale: HST ACS F140LP Contours: VLA + Pie Town X-band Color scale: HST NICMOS Paa Contours: VLA + Pie Town X-band Johnson & Hunt in prep. (See also: Hunt, Vanzi, & Thuan, 2001; Plante & Sauvage, 2002)

What can we learn from the mid-infrared? VLA 2 cm contour, Gemini 10mm color (Vacca, Johnson, & Conti 2002) He2-10 He 2-10 VLA 2 cm contour, HST V-band color (Kobulnicky & Johnson 1999) The radio sources alone account for at least 60% of the mid-IR flux from the entire galaxy

Can we use infrared observations to probe the natal environment? New Models: 3D Monte-Carlo Radiation Transfer (à la Barb Whitney)  Fractal dust structure consistent with the actual ISM Cocoon Mass (SFE) Ionizing source(s) Fractal Structure % Clumpy Dust Dust Composition Rin Rout

Geometric Sequence with Rin increasing (pseudo-evolutionary sequence) Near-IR J, H, K Spitzer IRAC 3.6, (4.5+5.8), 8.0 mm Spitzer MIPS 24, 70, 160 mm Rin = 30 pc Rin = 45 pc Rin = 20 pc Rin = 5 pc Rin = 10 pc Example: 90% clumpy, Rout=50pc, SFE=10% Johnson, Whitney, Indebetouw, & Wood submitted.

Geometric Sequence with Rin increasing (pseudo-evolutionary sequence) Near-IR J, H, K Spitzer IRAC 3.6, (4.5+5.8), 8.0 mm Spitzer MIPS 24, 70, 160 mm Rin = 30 pc Rin = 45 pc Rin = 20 pc Rin = 5 pc Rin = 10 pc Example: 90% clumpy, Rout=50pc, SFE=10% Johnson, Whitney, Indebetouw, & Wood submitted.

Dependence on Viewing Angle Near-IR J, H, K Spitzer IRAC 3.6, (4.5+5.8), 8.0 mm Spitzer MIPS 24, 70, 160 mm Indebetouw, Whitney, Johnson, & Wood, ApJ 2006

Variation with Clumpiness alone (averaged over all viewing angles) black = smooth red = 99% clumpy l [mm] nFn [erg/s/cm2] Shape of the infrared SED can vary significantly with clumpy fraction  We need to be very careful in our interpretation of IR observations! More escape, less reprocessing

Is the formation of super star clusters special? DDO 165 NGC1705 NGC1569 NGC7252: 8x107 M_sun Weidner, Kroupa, & Larsen 2004 NGC1705 NGC1569 IC10 IZW18

What is the initial cluster luminosity function? Thermal radio sources in the Antennae a > -0.1 Whitmore & Zhang 2002 Extracted from Neff & Ulvestad 2000 Linear resolution ~ 100pc initial luminosity function compatible with a power-law

What is the initial cluster luminosity function? Thermal radio sources in the He2-10 HST I-band a > -0.1 Probability < 10-35 from power law Resolution: radii ~4pc It appears that in at least some extreme cases cluster formation does not follow a power-law Kobulnicky & Johnson 1999, Johnson et al. 2000, Johnson & Kobulnicky 2003, Biswas & Johnson submitted

What is going on here? Antennae He 2-10 Could be confusion, but this effect should be worse in the Antennae Could be statistics, but similar numbers (12 in Antennae, 7 in He2-10) Could be that for some reason low mass clusters aren’t radio sources, but we see these in other galaxies Could be that dwarf galaxies can isolate a “mode” of star formation

Outstanding questions related to Massive Star Formation Are there environmental differences between the formation of small associations and massive clusters? e.g. Environmental requirements? Protostellar interactions? How does the process of star formation vary between small associations and massive clusters? e.g. Star formation efficiency? What is the role of metallicity in super star cluster formation as it relates to globular cluster formation in the early universe? e.g. Cooling, hardness of radiation field?

Looking toward the Future (IR - mm) 106 M proto cluster at 10 Mpc JWST HERSCHEL CARMA ALMA SPITZER SOFIA ? There is a lot of work to do!