1. 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)

2. 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?

3. 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?

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

5. Comparison of Radio SEDs
(individual clusters)
SBS
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

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

7. 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

8. 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

9. 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  1049 s-1
 Large OB associations
Johnson, Indebetouw, & Pisano 2003

10. 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)

11. 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)

12. 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

13. 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

14. Geometric Sequence with Rin increasing (pseudo-evolutionary sequence)
Near-IR
J, H, K
Spitzer IRAC
3.6, ( ), 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.

15. Geometric Sequence with Rin increasing (pseudo-evolutionary sequence)
Near-IR
J, H, K
Spitzer IRAC
3.6, ( ), 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.

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

17. 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

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

19. 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

20. What is the initial cluster luminosity function?
Thermal radio sources in the He2-10
HST I-band
a > -0.1
Probability < 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

21. 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

22. 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?

23. 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!