The earliest phases of Super Star Clusters – a high spatial resolution view Daniel Schaerer (Geneva Observatory, OMP Toulouse) Leticia Martin-Hernandez (Geneva Observatory) Marc Sauvage (CEA Saclay/Paris) Els Peeters (NASA Ames) Motivation: * importance for extragalactic studies * SSC formation / properties Questions Results from mid-IR + radio observations Future studies  Martin-Hernandez et al. 2005, A&A 429, paper submitted

Mid-IR spectra and spectral diagnostics of (UC)HII regions and galaxies ISO spectra: Galactic (UC)HII regions Peeters et al. (2002) Starbusts galaxies and AGN e.g. Sturm et al. (2001)  Fine structure lines (ionised ISM) -> PAH features -> Dust continuum

Mid-IR spectra and spectral diagnostics of (UC)HII regions and galaxies « Global » PAH strength or [NeIII]/[NeII] line ratio (+others) used e.g. as diagnostics of: - starburst versus AGN nature - IMF (upper limit), SF duration Thornley et al. (2000) Genzel et al. (1998)

Mid-IR spectra and spectral diagnostics of (UC)HII regions and galaxies BUT: « Global » PAH strength or [NeIII]/[NeII] line ratio (+ other) potentially flawed by large aperture and related effects! - strong spatial variations of PAH features (destroyed close to ionising sources) - different contributions to low excitation lines (diffuse ISM, HII regions, SSCs) - strong dominance of few/individual SSCs in some cases  Interpretation of « global » large aperture spectra non trivial.  Modeling as single HII region (with « average » ionisation parameter) inadequate

Mid-IR spectra and spectral diagnostics of (UC)HII regions and galaxies ISO SWS: ~14x20’’ apertures  Interpretation of « global » large aperture spectra non trivial.  Modeling as single HII region (with « average » ionisation parameter) inadequate Need for high spatial resolution observations !

Earliest phases of SSC Hidden, ultra-dense, optically thick (radio) clusters now recognised as precursors of « normal » SSCs (Kobulnicky, Johnson, Beck, Turner… later) Open questions: * Stellar content ? * IMF ? Best probes for upper mass cut-off of IMF! * Ages ? lifetimes ?  Most direct diagnostics: fine structure emission lines + photoionisation modeling Kobulnicky & Johnson (1999)

NGC 5253 (prototypical starburst) * Magellanic irregular * Distance~3. 25Mpc (1’’=16pc) * UV-optical: many clusters + diffuse light * (mid)-IR + radio emission dominated by single source!  hidden SSC / supernebula UV (HST) Near-IR (HST) Alonso-Herrero et al. (2003) Near-IR and radio (VLA) Turner & Beck (2004) mid-IR (Keck) Gorjian et al. (2001)

The supernebula in NGC 5253 * Brα/Brγ: mag extinction – A V ~18 mag * Brγ line and free-free flux: ~2000 O7V stars in 0.8 pc radius ~7000 O7V in central 20 pc * Very high resolution radio data: core size ~99 x 39 mas! (1.8x0.7 pc FWHM)  size constraint for modeling (cf. below)! Smaller than core radius of Galactic Globular Clusters * Kinematics (radio rec.lines, Brγ ): v obs ~75 km/s FWHM - Too small to be related to expansion - If virialised motions: v obs ~< v escape (cluster)  gravitationally bound cluster !? Mass ~(5-30)x10 5 Msun! radio (VLA) Keck Turner & Beck (2004), Turner et al. (2003)

TIMMI2 mid-IR observations of the supernebula in NGC 5253 ESO: longslit spectroscopy (1-1.2’’ slit) * Source unresolved with 1.1’’ – negligible slit losses  Bulk of « high »-excitation emission ([ArIII], [SIV] – 27.6, 34.8 eV) observed in large apertures are from supernebula  Only ~20% of [NeII] (21.6 eV) from SSC – rest « diffuse » Martin- Hernandez et al. (2005) flux ratio TIMMI2/ISO aperture 1. 0.

Photoionisation modeling of supernebula in NGC 5253 * Observational constraints: - outer radius from radio [R out =0.8 pc] - electron density from radio continuum: ~5*10 4 cm -3 ( ~UCHII) - composition: ~1/3 solar (optical spectra, also Ne/H of SSC) - Bracketα flux (equivalent to fixing Q 0 - Lyc flux) - mid-IR lines (excitation) from TIMMI2 (supernebula) and ISO (upper limits) - NICMOS/HST: EW(Paschenα)  cluster age ~0-6 Myr * Starburst + photoionisation model (Cloudy+Starburst99) -- varying parameters: - cluster age, IMF (slope: Salpeter+, M up =100, 50, 30 M sun ) - ionisation parameter U= Q 0 /(4πR in 2 εn H c) constrained by ε≤1 (filling factor), i.e. x≤(1-b/R 3 out n 2 e ) 1/3 where x=R in /R out  R in <0.6 pc, and 0.6≤ ε≤1

Photoionisation modeling of supernebula in NGC 5253 – main results Fiducial model predicts TOO high excitation ! Solutions: a) age ~5-6 Myr and standard IMF (M up =100 M sun ) b) low upper mass cut-off (M up <50 M sun ) + young age (<4 Myr) in agreement with lack of SN signatures

Photoionisation modeling of supernebula in NGC 5253 – main results Fiducial model predicts TOO high excitation ! Solutions: a) age ~5-6 Myr and standard IMF (M up =100 M sun ) b) low upper mass cut-off (M up <50 M sun ) + young age (<4 Myr) Other possibilities: - outer radius ~4.5 pc – OK but ~6 x radio size! - steeper IMF – NO - metal enrichment: OK if solar (but 1/3 observed !) - internal dust: opposite effect! NO - matter bounded vs ionisation bounded nebula: opposite effect! - Density gradients: impossible to reconcile all mid-IR lines

Photoionisation modeling of supernebula in NGC 5253 – implications a) age ~5-6 Myr and standard IMF (M up =100 M sun ): « old hidden cluster » - possible to confine region for that long ? self gravity ? (Tan 2004) - but absence of SN puzzling! Have to hide ~100 SN/Myr after >~ 3 Myr - generally embedded SSC phase estimated to ~10-15% of O star lifetime (e.g. Kobulnicky & Johnson 1999) b) low upper mass cut-off (M up <50 M sun ) + young age (<4 Myr): - so far no indication for low M up ! - due to strong gravitational potential ? Other explanations?  Unique object ? General result ??

Other TIMMI2 observations – NGC 3256 NGC 3256, LIRG with double nucleus (D=37Mpc, 1’’=176 pc) - 5’’ separation – mid-IR spectrum of N and S: * resolved + probably multiple SSCs  inappropriate for photoionisation mdeling * PAHs detected in both Radio (Neff et al. 2003) TIMMI2 (Martin-Hernandez et al. 2005)

Other TIMMI2 observations – He 2-10 BCD, WR galaxy (D~9.0 Mpc, 1’’=43.6pc) * 5 compact optically thick ultra-dense HII regions (Kobulnicky & J 1999, Johnson & K 2003) * N-band emission follows radio (Vacca et al. 2002) * mid-IR line emission resolved (>~1.1’’) and extended * PAHs present in regions A and C * radio regions partly multiple (NACO/VLT: Cabanac et al. 2005) TIMMI2 (Martin-Hernandez et al. 2005) Radio + N-band Vacca et al. (2002) V (HST) + N-band Vacca et al. (2002)

Other TIMMI2 observations – II Zw 40 Compact dwarf galaxy (D=9.2Mpc, 1’’=44.6pc) * radio: presence of 1 compact (<1’’) but multiple opt.thick HII region * mid-IR spectrum: slightly resolved Bulk of 12 μm flux (IRAS) from <1.1’’! * ~all [SIV] from compact region  photoionisation model for this region… * No PAH features Radio: Beck et al. (2002) IRAS flux

Other TIMMI2 observations – II Zw 40 Photoionisation model for compact region in II Zw 40 using TIMMI2 and ISO line fluxes + radio size: * too high excitation predicted, as for NGC 5253 supernebula!  same tentative conclusion on IMF and age ! Multiplicity: conclusions likely unchanged

The importance of high spatial resolution * NGC 3256, NGC 5253: 70-80% of [NeII] emission (low excitation) from large aperture (diffuse, non SSC) * Higher excitation lines emitted in more compact regions (as expected…) * 1 compact (<~1.1’’) source containing (likely) all large aperture flux (ISO/SWS): II Zw 40 * similar analysis for PAH features … Martin-Hernandez et al. (2005)

…more/better observations need 8m telescope  VISIR/VLT High spatial resolution study of young starbursts/ hidden SSCs with VISIR and radio. Comparison with larger aperture observations (Spitzer, ISO): - Schaerer, Brandl (Spitzer), Martin-Hernandez, Sauvage, Schmitt (VLA)

Conclusions * mid-IR fine structure line observations provide sole probe of stellar content (IMF, age …) of embedded super star clusters (SSCs) * high spatial resolution observations (and size measurements) crucial for interpretation and photoionisation modeling of embedded SSCs * TIMMI2/ESO observations (+radio) of embedded SSC in NGC 5253: indications for a) “old age” (~5-6 Myr) + standard IMF or b) low upper mass cut-off (Mup<50 Msun) + young age * Possible other such case: SSC in II Zw 40 * More systematic study of embedded SSCs ongoing with VISIR/VLT, Spitzer + radio … * Spectral diagnostics using large aperture observations: mix emission from physically unrelated areas/components (diffuse ISM, SSC …) – BEWARE !