Compact Extragalactic Star Formation: Peering Through the Dust at Centimeter Wavelengths

1. Jim Ulvestad NRAO 7 Sept. 2004 Compact Extragalactic Star Formation: Peering Through the Dust at Centimeter Wavelengths

2. 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

3. 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?

4. 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

5. What Can Radio Emission Reveal about Extragalactic Starbursts? • Optical radiation from youngest star-formation 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

6. M82 (Kronberg et al. 1985; Muxlow et al. 1994) NGC 253 (Ulvestad & Antonucci 1997) Nearby Starbursts 25 pc 8 mJy thermal source

7. 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

8. NGC 5253 (~4 Mpc) 7mm VLA, 25 pc (2 arcsec) square (Turner & Beck 2004) Linear Resolution ~ 2-4 pc NLyc  7 1052 s-1  Super star clusters From KJ

9. 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) Yikes! Massive proto-cluster detected in mid-IR: Av > 15 - 30 AND similar embedded stellar mass Hunt, Vanzi, & Thuan (2001) Plante & Sauvage (2002) See also: radio observations of Hunt, Dyer, Thuan, & Ulvestad (2004) From KJ

10. WFPC2, with CO overlay (Whitmore et al. 1999; Wilson et al. 2000) VLA 5 GHz image (Neff & Ulvestad 2000) Needs EVLA sensitivity and resolution Nearest Merger—The “Antennae” 5 mJy 30,000 O7-equivalent stars

11. SSC and Related Radio Sources

12. 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 • Pressures > 108 kB • N Lyc ~700 - 2500  1049 s-1 (700 - 2500 O7* stars) • Stellar Masses  105-6 M • HII masses < 5% stellar masses From KJ Johnson & Kobulnicky 2003

13. 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

14. 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 • 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 30 kpc J. Hibbard HST WFPC2 (Alonso-Herrero et al. 2000)

15. Arp 299 Radio Emission • No radio emission at optical SN positions • Four Strong Radio Peaks • A and B: galaxy nuclei • C and C’: overlap region • Alonso-Herrero et al. IR/opt. (2000) • 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: VLA 6cm Blue: HST 250nm Green: HST 814nm Arp 299

16. 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 super-star clusters? 3 pc Neff, Ulvestad, & Teng 2004 April 2002 Feb. 2003 13cm 3.6cm

17. More VLBA + GBT imaging • 2.3, 8.4 GHz in 2003Dec, 2004Jun • Total of 15 SNe at 2 and 8 GHz

18. Inside Source B1, 2nd Nucleus

19. 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

20. 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