The anatomy of starburst galaxies: sub-arcsecond mid-infrared observations

1. The anatomy of starburst galaxies:sub-arcsecond mid-infrared observations Lijiang August 15, 2005

2. SCUBA 850 µm Understanding starforming galaxies • Can starbursts be scaled up? • Clustered vs. extended star formation, dense vs. diffuse gas • Use of “direct” diagnostics: hot dust continuum, PAH emission, ionic lines (Webb & Van der Werf in preparation) Sub-arcsecond mid-infrared observations of starburst galaxies

3. NGC 4038/4039 detail NGC 4038/4039 Orion (M42) 30 Doradus Superstarclusters:does size matter? NGC4038/4039 cluster: ≈ 100 pc Orion: ≈ 1.5 pc Sub-arcsecond mid-infrared observations of starburst galaxies

4. LIR/LCO  SFR/MH2  SFE (Gao & Solomon 2001) LIR SFR Starformation efficiency • Starbursts cannot • be simply scaled up. • More intense starbursts are also more efficient with their fuel. Sub-arcsecond mid-infrared observations of starburst galaxies

5. SCUBA 850 m Dense vs. diffuse gas: the Antennae CO J=76 [CI] [CI] widespread, CO J=76 isolated! SPIFI/JCMT (Isaak, Papadopoulos, Van der Werf, Gao in prep.) Sub-arcsecond mid-infrared observations of starburst galaxies

6. The Antennae with Spitzer/IRAC (Wang et al., 2004) Sub-arcsecond mid-infrared observations of starburst galaxies

7. Mid-infrared diagnostics • fine-structure lines: Teff of radiation field, abundances, ne • PAH features: UV-irradiated dust • hot dust continuum • H2 lines: warm molecular gas • silicate features: foreground absorption Antennae Eastern cluster Spitzer/IRS 5 slit) (courtesy B. Brandl) Ground-based N-band Sub-arcsecond mid-infrared observations of starburst galaxies

8. The Antennae: 12 m at 0.3 resolution • In [NeII] 12.8m, the compact obscured cluster appears to be double (separation 0.5) • Largest component is resolved with D ≈ 50 pc • Dust continuum shows only 1 (extended) object contours: dust continuum [NeII] 12.8 m ESO/VLT VISIR (Snijders et al., in prep.) Sub-arcsecond mid-infrared observations of starburst galaxies

9. Comparison with stellar light Ks-band (2.2 m) shows no evidence of substructure or a 2nd component: strong and variable obscuration Ks-band, seeing 0.4 ESO/VLT ISAAC (Mengel et al., 2002) Sub-arcsecond mid-infrared observations of starburst galaxies

10. VISIR N-band spectra of the Antennae clusters • Continuum is compact: ≈ 50% of Spitzer/IRS continuum (5 slit) detected in 0.6 VISIR slit • PAH emission is extended: very low equivalent widths in VISIR slit • Line ratios [NeIII]/[NeII] and [SIV]/[NeII] increase in smaller slits: sample compact high excitation regions [NeII] [SIV] [ArIII] PAH 11.3 m Sub-arcsecond mid-infrared observations of starburst galaxies

11. Photoionization analysis • Assuming abundances, ionic lines with different ionization potentials probe the Teff of the ionizing radiation field • Complication: theoretical O-star spectra differ widely in EUV • Also: density (pressure) dependence because of different critical densities (Morisset et al., 2004) Sub-arcsecond mid-infrared observations of starburst galaxies

12. Diagnostic line ratios (Eastern cluster) Density estimates: • lower limit from radio continuum (D=70 pc homogeneous sphere): ne=360 cm–3 • near-IR [FeIII] line ratios: ne=3000–10000 cm–3 Te ≈ 104 K  P/k ≈ 3·107–108 K cm–3 Teff ≈ 46000 K O3 stars (assuming Hillier & Miller 1998, Pauldrach et al., 2001 O-star spectra) age ≈ 1 Myr (Dopita et al., in prep.) Sub-arcsecond mid-infrared observations of starburst galaxies

13. The role of dust • Ionization parameter • For solar abundances, log U > –2.0 implies substantial (>50%) absorption of UV-photons by dust in stead of hydrogen (Dopita et al., 2003). • In this case, log U > –2.0 if R < 20 pc: very likely • Confirmed by observed LFIR/LBr • Dust-dominated HII regions  diagnostics like EW(Br) for age problematic • Calculate number of O3-stars from IR-luminosity: 1000 O3 stars Sub-arcsecond mid-infrared observations of starburst galaxies

14. LIR/LCO  SFR/MH2  SFE (Gao & Solomon 2001) LIR SFR Starformation efficiency revisited • Although the Antennae clusters are extreme, an extreme starburst is not simply a collection of 1000 of these. Antennae cluster • Extreme starbursts are more efficient with their fuel, with the entire molecular ISM forming stars. Sub-arcsecond mid-infrared observations of starburst galaxies

15. Under pressure: extreme starbursts • Star formation in the Antennae appears to occur in a two-phase medium, with the star formation occurring in the dense phase. • In extreme starbursts such as ULIGs, the dense phase is dominant (or the diffuse phase may be completely absent). Pressure effect? • Confirming observation 1: CO 6–5/[CI] in Mrk 231 is high, comparable to the eastern cluster in the Antennae (Isaak, Papadopoulos & Van der Werf, in preparation). • Confirming observation 2: in ULIGs, recombination lines are always very faint compared to far-IR flux density; LFIR/LBr is high.  star formation in ULIGs is dominated by compact HII regions.  dense phase is dominant (diffuse phase absent?). Extreme starbursts are characterized by high pressures! Sub-arcsecond mid-infrared observations of starburst galaxies

16. Conclusions • Hot dust continuum traces principally very recent star formation and is therefore a poor tracer of global star formation. • PAH emission is a better tracer of global star formation but its use as a quantitative diagnostic is not yet established. • Mid-infrared line ratios depend strongly on aperture, and hence need high spatial resolution. • Extreme and compact starburst regions such as the Antennae obscured superstarclusters are dust-dominated. Likely, this is generally true in extreme starbursts such as ULIGs. • At the young inferred ages, a -function starburst is probably a poor model; also, spatial substructure in the superstarcluster must be taken into account. • Extreme starbursts cannot be constructed by adding up smaller starbursts; extreme starbursts are characterized by high pressures. Sub-arcsecond mid-infrared observations of starburst galaxies