Transcript Compact Extragalactic Star Formation

Compact Extragalactic Star
Formation: Peering Through the
Dust at Centimeter Wavelengths
Jim Ulvestad
NRAO
7 Sept. 2004
Collaborators
• Susan Neff
• Stacy Teng
• Thanks to Kelsey
Johnson for supplying
several viewgraphs!
Outline
– Context
– Starburst radio
emission
– Super Star Clusters
– Supernovae
• Arp 299 (NGC 3690)
case study
Some Global Questions
• What are the properties of the youngest
massive star clusters? How do they evolve
to become globular clusters today? What is
the luminosity function of SSCs, and the
mass function of their star formation?
• Is optical/IR modeling of star formation in
SSCs consistent with radio observations?
• How do supernovae evolve in dense
environments?
Formation of Stars & Star Clusters
• The first few stages
are not visible
optically and in
near-IR
• Even SSCs may be
hidden in dust
From Kelsey Johnson
What Can Radio Emission Reveal
about Extragalactic Starbursts?
• Optical radiation from youngest starformation regions is hidden by dust
• Radio emission due to
– Complexes of dense H II regions energized by
Super Star Clusters
• Estimate ionizing fluxmassive star population
– Individual supernova remnants or young SNe
• Estimate supernova rate & evolution
– Overlapping supernova remnants
Nearby Starbursts
• M82 (Kronberg et al.
1985; Muxlow et al. 1994)
• NGC 253 (Ulvestad &
Antonucci 1997)
25 pc
8 mJy
thermal
source
Results from M82, NGC 253
• Little or no source variability
• Steep spectrum sources resolve into SNRs
• Flat-spectrum sources typically H II
complexes energized by hot stars
N(UV)/s = 1051 (D/2.5 Mpc)2 (S5 GHz/1 mJy)
– 1049 photons/s = 1 O7 star
– Strongest NGC 253 thermal source is ~8 mJy
• 750 O7-equivalent stars in a few parsecs
NGC 5253 (~4 Mpc)
Linear Resolution ~ 2-4 pc
NLyc  7 1052 s-1
 Super star clusters
From KJ
7mm VLA, 25
pc (2 arcsec)
square (Turner
& Beck 2004)
SBS 0335-052 (Johnson et al., in prep)
53 Mpc, ultra-low metallicity (Z  1/40 Z)
VLA 1.3cm contours, HST I-band color
Spectral Index: 1.3cm & 3.6cm
Linear Resolution ~ 75pc
NLyc  12,000 1049 s-1
 Super star cluster(s)
Massive proto-cluster
detected in mid-IR: Av > 15 - 30
AND similar embedded stellar mass
Hunt, Vanzi, & Thuan (2001)
Plante & Sauvage (2002)
From KJ
Yikes!
See also: radio observations of
Hunt, Dyer, Thuan, & Ulvestad
(2004)
Nearest Merger—The “Antennae”
• WFPC2, with CO
overlay (Whitmore et al.
1999; Wilson et al. 2000)
• VLA 5 GHz image (Neff
& Ulvestad 2000)
– Needs EVLA sensitivity
and resolution
5 mJy 30,000 O7-equivalent stars
SSC and Related Radio Sources
What do we think we know about the embedded HII regions?
Example: He2-10
(1.3cm on 3.6cm)
• Radii of HII regions < 4 pc
• Electron densities >104 - 106 cm-3
•N
Pressures > 108 kB
Lyc
~ 700 - 2500  1049 s-1
(700 - 2500 O7* stars)
Stellar Masses  105-6 M
• HII masses < 5% stellar masses
Johnson & Kobulnicky 2003
From KJ
Global Lessons from Radio SSCs
• Most recent star formation regions are
bright in mid-IR and radio
– Radio SSC diameters are a few parsecs
– Tens to thousands of O7-equivalent stars
• Recombination linewidths and sizes
indicate some SSCs are bound, depending
on stellar mass function (e.g., Turner et al.
2003)
– Otherwise, overpressure would cause the SSCs
expansion in ~106 yr or less
Tidal tails in Arp 299
Stars: blue; HI: contours
Arp 299 History
• At least one previous interaction
– ~700 million years ago
– HI and stellar tidal tails,
~ 150 kpc in extent
30 kpc
• Near beginning of current
interaction
– Two disks still clearly identifiable
– Nuclear separation ~3.5 kpc
– Disks interacting and distorted
• Burst of star formation 6-8
million years ago (at the
beginning of current pass)
– Should be seeing supernovae
J. Hibbard
HST WFPC2
(AlonsoHerrero et
al. 2000)
Arp 299
Radio Emission
• No radio emission at optical
positions
• Four Strong Radio Peaks
SN
– A and B: galaxy nuclei
– C and C’: overlap region
• Alonso-Herrero et al. IR/opt. (2000) Arp 299
– Assume starbursts Gaussian in time, 5
Myr wide, peak 5 Myr after start
– A: 7 Myr post-peak, 0.6 SN/yr
• 700 million solar masses in young stars
• 140 solar masses/yr in star formation
– B1: 5 Myr post-peak, 0.1 SN/yr
– C & C’: 4 Myr post-peak, 0.05 SN/yr
Red:
Blue:
Green:
VLA 6cm
HST 250nm
HST 814nm
Arp 299
Inside Source A
A “nest” of four
young SNe, within
100 pc
and
A young supernova,
only 2 pc from one
of the other sources
Tracing superstar clusters?
3 pc
Neff,
Ulvestad,
& Teng
2004
April 2002
13cm
Feb. 2003
3.6cm
• More
VLBA +
GBT
imaging
• 2.3, 8.4
GHz in
2003Dec,
2004Jun
• Total of 15
SNe at 2
and 8 GHz
Inside Source B1,
nd
2
Nucleus
Beginning of a Luminosity Function
• Arp 299-A SN rate
is reputed to be
about 6 times M82.
– Accounting for
incompleteness,
looks okay within a
factor of two
Arp 299 Summary
• IR/optical SN rate looks about right
• Youngest source still has not broken out, and is
not detected at 2.3 GHz
– No other strongly inverted sources, so any other very
young supernovae are 10 times less powerful (or don’t
exist)
– No obvious variability in other sources
• First detection of young supernovae in the 2nd
galaxy nucleus
• No radio supernovae seen in other star formation
regions