Download sub-arcsecond mid-infrared observations

The anatomy of starburst galaxies:
sub-arcsecond mid-infrared observations
Paul van der Werf & Leonie Snijders
Leiden Observatory
Lijiang
August 15, 2005
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
SCUBA 850 µm
(Webb & Van der Werf in preparation)
Sub-arcsecond mid-infrared observations of starburst galaxies
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NGC 4038/4039
NGC 4038/4039 detail
Superstarclusters:
does size matter?
NGC4038/4039
cluster: ≈ 100 pc
Orion:
Orion (M42)
≈ 1.5 pc
30 Doradus
Sub-arcsecond mid-infrared observations of starburst galaxies
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Starformation efficiency
 Starbursts cannot
be simply scaled up.
 More intense starbursts
are also more efficient
with their fuel.
ULIGs:
Milky Way:
Galactic GMCs:
Orion core:
LFIR
M H2
LIR/LCO

SFR/MH2

1
39 L M SFE
4 L M 1
7L M
1
(Gao & Solomon 2001)
LIR  SFR
40 L M 1
Sub-arcsecond mid-infrared observations of starburst galaxies
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Dense vs. diffuse gas: the Antennae
CO J=76
[CI]
SCUBA
850 m
[CI] widespread, CO J=76 isolated!
Sub-arcsecond mid-infrared observations of starburst galaxies
SPIFI/JCMT
(Isaak, Papadopoulos, Van der Werf, Gao in prep.)5
The Antennae with Spitzer/IRAC
(Wang et al., 2004)
Sub-arcsecond mid-infrared observations of starburst galaxies
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Mid-infrared diagnostics





fine-structure lines:
Teff of radiation
field, abundances,
ne
PAH features: UVirradiated 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
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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
Sub-arcsecond mid-infrared observations of starburst galaxies
[NeII] 12.8 m
ESO/VLT VISIR
(Snijders et al., in prep.)
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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
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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
Sub-arcsecond mid-infrared observations of starburst galaxies
[NeII]
[ArIII]
[SIV]
PAH
11.3 m
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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

Sub-arcsecond mid-infrared observations of starburst galaxies
(Morisset et al., 2004)
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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
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The role of dust
Q
ne 4 R 2 c

Ionization parameter U 

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
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Starformation efficiency revisited
ULIGs:
LFIR
M H2
39 L M 1
Milky Way:
4 L M 1
Galactic GMCs:
7 L M 1
Orion core:
Antennae cluster:
 Although the Antennae clusters are
extreme, an extreme starburst is not
simply a collection of 1000 of these.
40 L M 1
3L M 1
 Extreme starbursts are
more efficient with their
fuel, with the entire
molecular ISM forming
stars.
LIR/LCO

SFR/MH2

SFE
Sub-arcsecond mid-infrared observations of starburst galaxies
Antennae
cluster
(Gao & Solomon 2001)
LIR  SFR
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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
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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
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