Transcript PowerPoint

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, 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 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
Genzel
et al.
(1998)
Thornley et al. (2000)
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
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
Near-IR and radio (VLA)
Turner & Beck (2004)
UV
(HST)
mid-IR (Keck)
Gorjian et al. (2001)
Near-IR (HST)
Alonso-Herrero et al. (2003)
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 5253
ESO: [email protected] 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 »
flux ratio
TIMMI2/ISO
aperture
1.
0.
MartinHernandez
et al.
(2005)
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/R3outn2e)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
TIMMI2
(Martin-Hernandez et al. 2005)
Radio (Neff et al. 2003)
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
Other TIMMI2 observations – He 2-10
BCD, WR galaxy (D~9.0 Mpc, 1’’=43.6pc)
Radio + N-band
Vacca et al. (2002)
* 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)
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
IRAS flux
Radio: Beck et al. (2002)
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 !