Transcript Probing the Birth of Super Star Clusters

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
Compact, “inverted spectrum” sources
Very dense HII regions
non-thermal
Sn
free-free
optically-thick free-free
100
Wood & Churchwell 1989
l (cm)
1
Comparison of Radio SEDs
(individual clusters)
27
SBS 0335-052
log(L[erg/s/Hz]
26.5
He 2-10
NGC 5253
26
continuum of sources
25.5
25
W49A
24.5
9
9.5
10
10.5
11
log(n[GHz])
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
27
SBS 0335-052
log(Ln[erg/s/Hz])
26.5
He 2-10
• Electron densities >104 - 106 cm-3
NGC 5253
26

25.5
25
• Ionizing Luminosities > 1052-53 s-1
W49A

24.5
9
9.5
10
10.5
Pressures > 108 kB
11
> 1000s O7-type stars
log(n[GHz])
1
 M
 2

M*
P
 0.5 7 cloud   8
3 
M cloud
 10 M sun   10 k B cm K 
1
4

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. 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
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)
Color scale: HST ACS F140LP
Color scale: HST NICMOS Paa
Contours: VLA + Pie Town X-band
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?
He2-10
He 2-10
He2-10
VLA 2 cm contour, HST V-band color
VLA 2 cm contour, Gemini 10mm color
(Kobulnicky & Johnson 1999)
(Vacca, Johnson, & Conti 2002)
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
Ionizing source(s)
Fractal Structure
% Clumpy Dust
Cocoon Mass (SFE)
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
RRinin==10
5 pc
20
30
45
pc
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
RRinin==10
5 pc
20
30
45
pc
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
QuickTime™ and a
GIF decompressor
are needed to see this picture.
Indebetouw, Whitney, Johnson, & Wood, ApJ 2006
Variation with Clumpiness alone
(averaged over all viewing angles)
 We need to be
very careful in our
interpretation of IR
observations!
nFn [erg/s/cm2]
Shape of the infrared
SED can vary
significantly with
clumpy fraction
black = smooth
red = 99% clumpy
l [mm]
Is the formation of super star clusters special?
NGC7252:
8x107 M_sun
NGC1569
NGC1705
NGC1569
NGC1705
DDO 165
Weidner, Kroupa, & Larsen 2004
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
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
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)
SOFIA
CARMA
HERSCHEL
SPITZER
ALMA
JWST
106 M proto cluster at 10 Mpc
There is a lot of work to do!
?