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

S

(cm)100 1

non-thermal

free-free

optically-thick free-free

Wood & Churchwell 1989

Compact, “inverted spectrum” sources

Very dense HII regions

Comparison of Radio SEDs(individual clusters)

24.5

25

25.5

26

26.5

27

9 9.5 10 10.5 11

log([ ])GHz

(log L

_[ / / ]erg s Hz

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

continuum of sources

Comparison of Radio SEDs(individual clusters)

24.5

25

25.5

26

26.5

27

9 9.5 10 10.5 11

log([ ])GHz

(log L

[ / / ])erg s Hz

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

• 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

€

M*

Mcloud

≈ 0.5Mcloud

107 Msun

⎛

⎝ ⎜

⎞

⎠ ⎟

12 P

108 kBcm−3K

⎛

⎝ ⎜

⎞

⎠ ⎟

14

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 H92 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

IC 4662 (2Mpc)

Linear Resolution ~ 10 pc

NLyc 20 - 2001049 s-1

Large OB associations

“Young” (<10 Myr) optically selected clusters and ultra-young

radio selected clusters are exclusive

Johnson, Indebetouw, & Pisano 2003

Different wavelengths probe different environments

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)

What can we learn from the near-infrared?

Haro 3

(See also: Hunt, Vanzi, & Thuan, 2001; Plante & Sauvage, 2002)

SBS 0335-052 ultra-low metallicity (Z 1/40 Z)

Color scale: HST NICMOS Pa

Contours: VLA + Pie Town X-band

Color scale: HST ACS F140LP

Contours: VLA + Pie Town X-band

Johnson & Hunt in prep.

NLyc 5,000 1049 s-1

5,000 O7* stars

What can we learn from the mid-infrared?

The radio sources alone account for at least 60% of the mid-IR flux from the entire galaxy

VLA 2 cm contour, Gemini 10m color

(Vacca, Johnson, & Conti 2002)

He 2-10

VLA 2 cm contour, HST V-band color

(Kobulnicky & Johnson 1999)

He2-10 He2-10

New Models: 3D Monte-Carlo Radiation Transfer

(à la Barb Whitney)

Fractal dust structure consistent with the actual ISM

Rin Rout

Fractal Structure

Cocoon Mass (SFE)

Ionizing source(s)

Dust Composition

% Clumpy Dust

Can we use infrared observations to probe the natal environment?

Johnson, Whitney, Indebetouw, & Wood submitted.

Geometric Sequence with Rin increasing

(pseudo-evolutionary sequence)

Near-IRJ, H, K

Spitzer IRAC3.6, (4.5+5.8), 8.0 m

Spitzer MIPS24, 70, 160 m

Example: 90% clumpy, Rout=50pc, SFE=10%

Rin = 5 pcRin = 10 pcRin = 20 pcRin = 30 pcRin = 45 pc

Johnson, Whitney, Indebetouw, & Wood submitted.

Geometric Sequence with Rin increasing

(pseudo-evolutionary sequence)

Near-IRJ, H, K

Spitzer IRAC3.6, (4.5+5.8), 8.0 m

Spitzer MIPS24, 70, 160 m

Example: 90% clumpy, Rout=50pc, SFE=10%

Rin = 5 pcRin = 10 pcRin = 20 pcRin = 30 pcRin = 45 pc

Dependence on Viewing Angle

Near-IRJ, H, K

Spitzer IRAC3.6, (4.5+5.8), 8.0 m

Spitzer MIPS24, 70, 160 m

Indebetouw, Whitney, Johnson, & Wood, ApJ 2006

QuickTime™ and aGIF decompressor

are needed to see this picture.

Variation with Clumpiness alone (averaged over all viewing

angles) Shape of the infrared SED can vary significantly with clumpy fraction

black = smoothred = 99% clumpy

[m]

F

[erg

/s/c

m2 ]

We need to be very careful in our interpretation of IR observations!

DDO 165

NGC1705

NGC1569

NGC7252: 8x107 M_sun

Weidner, Kroupa, & Larsen 2004

Is the formation of super star clusters special?

NGC1705

NGC1569

IC10 IZW18

Extracted from Neff & Ulvestad 2000Whitmore & Zhang 2002

• Linear resolution ~ 100pc• initial luminosity function compatible with a power-law

What is the initial cluster luminosity function?

> -0.1

Thermal radio sources in the Antennae

Resolution: radii ~4pc

HST I-band

Kobulnicky & Johnson 1999, Johnson et al. 2000, Johnson & Kobulnicky 2003, Biswas & Johnson submitted

> -0.1

Thermal radio sources in the He2-10

What is the initial cluster luminosity function?

It appears that in at least some extreme cases cluster formation does not follow a power-law

Probability < 10-

35 from power law

What is going on here?

1) Could be confusion, but this effect should be worse in the Antennae

2) Could be statistics, but similar numbers (12 in Antennae, 7 in He2-10)

3) Could be that for some reason low mass clusters aren’t radio sources, but we see these in other galaxies

4) Could be that dwarf galaxies can isolate a “mode” of star formation

Antennae He 2-10

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)

There is a lot of work to do!

106 M proto cluster at 10 Mpc

JWST

HERSCHELCARMA

ALMASPITZER

SOFIA

?