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

QuestionsResults from mid-IR + radio observations Future studies Martin-Hernandez et al. 2005, A&A 429, 449 + 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 AGNe.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 bylarge 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 re

solution observations !

Earliest phases of SSC

Hidden, ultra-dense, optically thick (radio) clusters now recognised as precursors of « normal » SSCs(Kobulnicky, Johnson, Beck, Turner… 1999 + 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: 0.8-2.0 mag extinction – AV~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 ): vobs ~75 km/s FWHM

- Too small to be related to expansion - If virialised motions: vobs ~< vescape(cluster)

gravitationally bound cluster !? Mass ~(5-30)x105 Msun!

radio (VLA) Keck Turner & Beck (2004), Turner et al. (2003)

TIMMI2 mid-IR observations of the supernebula in NGC 5253ESO: TIMMI2@3.6m 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 [Rout =0.8 pc]

- electron density from radio continuum: ~5*104 cm-3 (~UCHII) - composition: ~1/3 solar (optical spectra, also Ne/H of SSC) - Bracketα flux (equivalent to fixing Q0 - 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+, Mup=100, 50, 30 Msun)

- ionisation parameter U= Q0/(4πRin2εnHc) constrained by

ε≤1 (filling factor), i.e. x≤(1-b/R3outn2

e)1/3 where x=Rin/Rout

Rin<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 (Mup=100 Msun)

b) low upper mass cut-off (Mup<50 Msun) +

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 (Mup=100 Msun)

b) low upper mass cut-off (Mup<50 Msun) +

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 (Mup=100 Msun):« 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 (Mup<50 Msun) + young age (<4 Myr):- so far no indication for low Mup!- due to strong gravitational potential ? Other explanations?

Unique object ? General result ??

Other TIMMI2 observations – NGC 3256

NGC 3256, LIRG with doublenucleus (D=37Mpc, 1’’=176 pc)- 5’’ separation –mid-IR spectrum of N and S:* resolved + probably multiple SSCs inappropriatefor 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-bandVacca et al. (2002)

V (HST) + N-bandVacca 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 andISO 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 SSCswith VISIR and radio. Comparison with larger apertureobservations (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 !