Starburst Galaxies

1. Jodrell Bank Observatory University of Manchester UK Starburst Galaxies A brief introduction to star-forming galaxies, both nearby and at high redshift – with regard to what high resolution radio observationshave revealed Tom Muxlow 8th EVN Symposium, Torun Sept 27 2006 ,

2. What is a starburst galaxy? • High star-formation rate ~ 10 – 100 M๏/year – much higher than normal gas-rich galaxies (eg Milky Way star-formation rate ~ 1 – 5 M๏/year) • Total gas content can be estimated from integrated line profiles • Integrated HI profile  HI Mass • From gas available to fuel the star-formation event  lifetimes • ~few x 109 years for Milky Way – can be maintained for the lifetime of the galaxy • ~108years for starburst galaxy - short compared with the galaxy lifetime •  Implies a ‘burst’ of star-formation

3. NGC 1808 NGC 4038 /4039 What is a starburst galaxy? • Typically they are disturbed galaxies, many involved in mergers • Star-formation sites are marked out by the rare highly-luminous high-mass stars • SED often dominated by strong (F)IR emission

4. Starburst galaxies • Size of starburst region typically ~ kpc • Radio synchrotron emission from plasma (SNR) • Strong FIR emission from UV-heated dust (O stars) • LFIR >1010 Lo (ULIRGS>1012 Lo) • Thermal free-free emission from HII regions Radio FIR

5. Estimating star-formation rate (SFR)SFR indicators are many and varied • Extensively studied - eg Cram et al 1998 ApJ 507 155 • FIR, Radio Continuum, UV, SN etc…. • SFR estimated from FIR and radio are found to be highly correlated over many orders of magnitude • SFR can also be estimated from the numbers of O stars required to ionize the medium and produce thermal free-free continuum emission and forbidden lines (eg [NeII]) • Measuring UV flux directly suffers from extinction problems • No dust extinction problems for thermal free-free emission, but difficult to separate from non-thermal component • SN rate can be used to derive the SFR for stars more massive than 8M๏ x5.5 to convert to SFR over complete stellar mass range (>0.1 M๏) [Salpeter IMF] Both FIR and radio emission determined by the properties of high-mass stars

6. Nearby starburst galaxies • Arp 220 and M82 Arp 220 HST ACS M82 Spitzer

7. Arp220 • Merging ULIRG 77Mpc • HI emission VLA C+D array • Low angular resolution (Hibberd et al NRAO) • Dominated by HI absorption

8. OH mega-masers within eastern disk show a velocity gradient of 320 km s-1 kpc-1 • Radius ~80pc  Enclosed mass ~1.2x107 M๏ • No 108M๏BH… but Chandra finds hard X-rays in western disk Rovilos, Diamond et al (2003) Clements et al 2002 MERLIN - Mundell, Ferruit & Pedlar (2001) • HI absorption studies at high angular resolution probe the merger dynamics • Two counter-rotating disks – original galaxy cores that have survived the initial encounter • Now in final stages of merger Masers are excellent tracers of star-formation See Sessions 7 & 8 this afternoon – and Megan Argo’s poster on OH masers in M82

9. Arp220 Global VLBILonsdale, Lonsdale, Diamond, Conway, Smith, Rovilos, Parra, Thrall… et al • Latest images - New detections + variability Sn + SNR + AGN ? - See Rodrigo Parra’s presentation • Both east and west components show radio Sn and SNR. No obvious AGN seen – but in west component some unusual structures & velocity gradients detected…… MERLIN

10. Starbursts can have embedded AGN • Often the AGN is obscured and may only contribute a small proportion of the total flux density • eg ULIRG Mkn273 – twin merging nuclei Knapen et al 1997 • Flat spectrum radio component detected Contours: MERLIN 5GHz Greyscale: CFHT K-Band HST

11. Mkn273 • Dynamical signature of super-massive object • MERLIN HI absorption (Cole et al 1999) • Rotating disk • M<500pc ~1.4x1010 Mo • VLBA (Carilli & Taylor 2000) • M<220pc ~2x109 Mo • EVN (Bondi et al 2005) • Compact AGN candidate • Could be a compact RSn • Hard X-rays  AGN

12. M82 • One of the nearest (3.2 Mpc) & best studied starburst galaxies • Starburst located in central ~1 kpc • Starburst driven wind (Ha) - Subaru image, X-rays - (Chandra)

13. Triggering the starburst - tidal interaction over the past ~200 x 106 years • Interaction with M81/NGC3077 – M82 in high-speed motion with respect to the M81/NGC3077 group • HI observations Yun et al (1994) M82 +203 km/s -34 km/s M81 +14 km/s NGC3077

14. Previous Starburst Activity ? - evidence from optical clusters • Several ‘super’ starclusters identified ~ 1kpc NE of the centre of M82 • Correspond to 109 year old fossil starburst. Ages derived from evolutionary spectral synthesis models • Last close encounter with M81 (de Grijs et al 2001) N E

15. Current starburst:- has existed for at least 50 x 106 years- heavily obscured optically ~ 20-30 Mv Tidal interaction has channeled large amounts of gas into the central region of M82 inducing a burst of star-formation Current supernova rate ~ 0.1 yr-1 Current star-formation rate (for M > 5 M๏) ~ 2 M๏ yr-1

16. M82 VLA HI absorption(Wills et al 2000) • Also seen in molecular lines (eg CO) • Velocity field shows strong rotation • Detailed fit along major axis suggests an inner bar which is thought to be fuelling the central starburst

17. M82 Supernova remnants • O & B stars are tracers recent star-formation • Typically these become supernovae after ~ 107 years • SNR trace out star-formation sites ~107 years old • 50-60compact sources discovered in M82 • All resolved with MERLIN+VLBI • Most are SNR • Although ~16 are compact HII regions

18. M82 SNR – How old are they? MERLIN resolves all the SNR visible in M82 – derive size distribution Assuming an initial expansion rate ~ 5000km/s  ages ~1000 yr with a SNR appearing every ~ 20-40 yr Cumulative number-size diagram infers that SNR expansion slows with time: D ~ t 0.6 Expansion velocities of ~5-10,000km/s are detected by MERLIN and VLBI MERLIN (MFS) +VLA 1995

19. M81 M82 SNR43.31+592 0.5 pc VLBI Imaging of SNR - in M82 and other nearby starburst galaxies • Measure expansion velocities and deceleration • Probe the nature of the environment – thought to be extremely clumpy • Investigate how the ejecta interact with the environment and eventually move into the Sedov phase of expansion • Do SN in environmental voids produce no observable remnant? In M82, SFR  SN rate of 1 every ~12 years (cf SNR rate of 1 every ~30 years) See review talk by Rob Beswick on SNR-Session 9 See talk by Danielle Fenech on M82 SNR SN43.31+592: Beswick et al 2006 SN1993J: Marcaide et al Bietenholz, Bartel, Rupen et al

20. Structurally-evolving double-lobed structure unlike ring/partial rings seen for all other M82 SNR Expansion velocity(<1800km/s)  age ~ 100 years VLBI Imaging of SNR - 41.95+575An unusual object in M82 !! 41.95+575 has decreased in flux density by 8.5% per year since monitoring began in the 1970s. 100 years at 8.5%/yr  x 3500 !!! In 2000 41.95+575 had a flux density of 24mJy – at birth 100 years ago it would have been ~100 Jy !!!! What sort of violent event is this ?? - GRB afterglow ??

21. Star-formation at High Redshift • Deep galaxy studies indicate that early galaxies merge to form larger systems in a ‘bottom-up’ scenario of galaxy assembly • This implies that galaxy-galaxy interactions were common at early epochs • Such interactions are likely to trigger major star-formation activity. HDF (N) Multi-wavelength studies of a number of fields including deep radio observations involving VLA, ATCA, MERLIN, EVN… have shown that at flux densities <1mJy (1.4GHz) there is new population of faint radio sources that are associated with distant star-forming galaxies Richards, Fomalont, Kellermann, Windhorst, Norris, Garrett, Muxlow…….

22. Deep HDF-N MERLIN + VLA data • Covers ~10 arcmin2 centred on the HDF-N (Lovell telescope beam) • High angular resolution ~0.2-0.5 arcsec • rms ~3.3µJybm-1 - one of the most sensitive 1.4GHz images made • Precis of results: • 92 radio sources with flux densities >40µJy. • Angular sizes of 0.2”–3” • 85% are associated with galaxies brighter than 25th mag • Remaining 15% are optically faint EROs at high redshift (some seen at sub-mm) • Below ~60Jy sources are dominated by starburst systems • Some starbursts show evidence for embedded AGN – See latest VLBI results... VLA: Richards et al 2000 VLBI: Garrett et al 2001 Seungyoup Chi - this meeting MERLIN+VLA: Muxlow et al 2005

23. Contours: Radio linear CI=10µJy/bm Image: Optical colour One of the brighter (~200µJy) starburst galaxies in the NDF-N

24. GOODS NORTH: New ACS & Spitzer data • The historical HDF-N is coincident with the GOODS north field. •  deeper wider field HST ACS images + deep Spitzer images • 8.5’ x 8.5’ MERLIN+VLA radio field centred on HDF-N intersects with 13030 galaxies brighter that 28.3mag in ACS z-band field • Using these new data we can now extend the analysis, to investigate statistically the very weak radio source population below 20µJy. Rob Beswick, Hanna Thrall, Tom Muxlow, Anita Richards, Simon Garrington….

25. 927 23mag galaxies Radio emission from ACS galaxies • Radio flux density within 0.75 arcsecond of all 13030 z-band optical galaxies in the 8.5’ field, binned by magnitude (Note: excluding all brighter radio sources with S1.4>40µJy) Control incorporates a random 7 arcsecond shift Of the ~2700 galaxies brighter than Z=24mag, around 1400 will have radio flux densities of ~4µJy or greater (~8σ for a deep e-MERLIN/EVLA image)

26. Radio source sizes:very weak (sub-20µJy) radio sources Average radio source sizes in each magnitude bin can be derived from flux densities found in annuli over radii of 0.25-2 arcsec For detected systems (brighter than 25mag) average radio source radii are in the range 0.6 – 0.8 arcsec Next generation radio interferometers will need sub-arcsec angular resolution !!

27. ● Starburst Sub-mm Measured redshifts binned by magnitude. We may now derive average luminosities for galaxies brighter than ~24mag Average luminosities for systems with spectroscopic redshifts Only ~1000 of the ~13000 galaxies have published spectroscopic redshifts available (Keck) 377 Spitzer 24µm in 8.5’ field >20µJy 303 (80%) detected in radio (>3σ)  213 with redshifts Beswick et al – work in progress…. 40µJy 4µJy 0.4µJy Starburst Luminosities For those weak sources which we can at present only study statistically... e-MERLIN, EVLA and e-VLBI should image >1000 starburst systems to ~4µJy with perhaps 150-200 at high redshift in a single field. Many thousands of systems with radio flux densities <1µJy will be studied statistically At present…… Can show that the SFR density increases dramatically to z~1 and then flattens… At higher redshifts, the position of the turnover point remains uncertain !! The next few years will be very exciting… SKA and ALMA will ultimately extend this by an additional order of magnitude. With more redshifts, improved SED templates, and extinction-free SFR indicators  solve for cosmic star-formation history – Madau diagram