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 star-formation regions is hidden by dust

• Radio emission due to– Complexes of dense H II regions energized by

Super Star Clusters• Estimate ionizing fluxmassive 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 mJythermalsource

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

7mm VLA, 25 pc (2 arcsec) square (Turner & Beck 2004)

From KJ

SBS 0335-052 (Johnson et al., in prep)53 Mpc, ultra-low metallicity (Z 1/40 Z)

Massive proto-cluster detected in mid-IR: Av > 15 - 30 AND similar embedded stellar mass Hunt, Vanzi, & Thuan (2001) Plante & Sauvage (2002)

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!Yikes!

See also: radio observations of Hunt, Dyer, Thuan, & Ulvestad (2004)

From KJ

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

• N Lyc ~ 700 - 2500 1049 s-1 (700 - 2500 O7* stars)

Stellar Masses 105-6 M

• HII masses < 5% stellar masses

• Radii of HII regions < 4 pc

• Electron densities >104 - 106 cm-3

Pressures > 108 kB

What do we think we know about the embedded HII regions? Example: He2-10

Johnson & Kobulnicky 2003

(1.3cm on 3.6cm)

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

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

J. Hibbard

HST WFPC2(Alonso-Herrero et al. 2000)

Tidal tails in Arp 299Stars: blue; HI: contours

30 kpc

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 6cmBlue: HST 250nm Green: HST 814nmArp 299

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?

April 2002 Feb. 2003 13cm 3.6cm

3 pc

Neff, Ulvestad, & Teng 2004

• More VLBA + GBT imaging

• 2.3, 8.4 GHz in 2003Dec, 2004Jun

• Total of 15 SNe at 2 and 8 GHz

Inside Source B1, 2nd 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