der Paul van der Werf & Leonie Snijders Leiden Observatory The anatomy of starburst galaxies: sub-arcsecond mid-infrared observations Lijiang August 15, 2005

Sub-arcsecond mid-infrared observations of starburst galaxies2 Understanding starforming galaxies (Webb & Van der Werf in preparation) SCUBA 850 µm  Can starbursts be scaled up?  Clustered vs. extended star formation, formation, dense vs. diffuse gas dense vs. diffuse gas  Use of “direct” diagnostics: hot dust continuum, hot dust continuum, PAH emission, ionic lines PAH emission, ionic lines

Sub-arcsecond mid-infrared observations of starburst galaxies3 NGC 4038/4039 Orion (M42) 30 Doradus NGC 4038/4039 detail Superstarclusters: does size matter? NGC4038/4039 cluster: ≈ 100 pc Orion: ≈ 1.5 pc

Sub-arcsecond mid-infrared observations of starburst galaxies4 Starformation efficiency  Starbursts cannot be simply scaled up. be simply scaled up.  More intense starbursts are also more efficient are also more efficient with their fuel. with their fuel. (Gao & Solomon 2001) L IR  SFR L IR / L CO  SFR/ M H 2 SFE

Sub-arcsecond mid-infrared observations of starburst galaxies5SCUBA 850  m Dense vs. diffuse gas: the Antennae SPIFI/JCMT (Isaak, Papadopoulos, Van der Werf, Gao in prep. ) [C I ] widespread, CO J =7  6 isolated! [C I ] CO J =7  6

Sub-arcsecond mid-infrared observations of starburst galaxies6 The Antennae with Spitzer/IRAC (Wang et al., 2004)

Sub-arcsecond mid-infrared observations of starburst galaxies7 Mid-infrared diagnostics  fine-structure lines: T eff of radiation field, abundances, n e  PAH features: UV- irradiated dust  hot dust continuum  H 2 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 galaxies8 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 [NeII] 12.8  m ESO/VLT VISIR (Snijders et al., in prep.) contours: dust continuum

Sub-arcsecond mid-infrared observations of starburst galaxies9 Comparison with stellar light Ks-band (2.2  m) shows no evidence of substructure or a 2 nd component: strong and variable obscuration Ks-band, seeing 0.4  ESO/VLT ISAAC (Mengel et al., 2002)

Sub-arcsecond mid-infrared observations of starburst galaxies10 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 [ArIII] [SIV] [NeII] PAH 11.3  m

Sub-arcsecond mid-infrared observations of starburst galaxies11 Photoionization analysis  Assuming abundances, ionic lines with different ionization potentials probe the T eff 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 galaxies12 Diagnostic line ratios (Eastern cluster) Density estimates:  lower limit from radio continuum ( D =70 pc homogeneous sphere): n e =360 cm –3  near-IR [FeIII] line ratios: n e =3000– cm –3 T e ≈ 10 4 K  P/k ≈ 3 · 10 7 –10 8 K cm –3  T eff ≈ 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 galaxies13 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 –2.0 if R < 20 pc: very likely  Confirmed by observed L FIR / L Br   Dust-dominated HII regions  diagnostics like EW(Br  ) for age problematic  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 galaxies14 Starformation efficiency revisited  Extreme starbursts are more efficient with their fuel, with the entire molecular ISM forming stars. (Gao & Solomon 2001) L IR  SFR L IR / L CO  SFR/ M H 2 SFE Antennae cluster  Although the Antennae clusters are extreme, an extreme starburst is not simply a collection of 1000 of these.

Sub-arcsecond mid-infrared observations of starburst galaxies15 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; L FIR / L Br  is high.  star formation in ULIGs is dominated by compact HII regions.  dense phase is dominant (diffuse phase absent?).  star formation in ULIGs is dominated by compact HII regions.  dense phase is dominant (diffuse phase absent?). Extreme starbursts are characterized by high pressures! Extreme starbursts are characterized by high pressures!

Sub-arcsecond mid-infrared observations of starburst galaxies16 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 galaxies17 Conclusions  Excellent complementarity: IRAM/JCMT  CHAMP+  HIFI (also IRS/Spitzer)

Sub-arcsecond mid-infrared observations of starburst galaxies18 Conclusions  Excellent complementarity: IRAM/JCMT  CHAMP+  HIFI (also IRS/Spitzer)

Sub-arcsecond mid-infrared observations of starburst galaxies19 Conclusions  Excellent complementarity: IRAM/JCMT  CHAMP+  HIFI (also IRS/Spitzer)