the mir template spectrum of star-forming galaxies a sings perspective jd smith
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The MIR Template Spectrum of Star-Forming Galaxies
A SINGS Perspective
JD Smith
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Do we need the IR to understand star formation?
Question Assumed Rhetorical(yes)
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SINGS
A Spitzer infrared imaging and spectroscopic survey of 75 nearby galaxies (<~30Mpc).
Spans broad range of physical and star-formation properties.
Copious ancillary and complementary data: UV, U, V, B, R, I, H, Pa, J, H, K, sub-mm, CO 1.3mm, 18cm, 21cm, 22cm
SFRI’s, dust, PAH formation, metals, …See Kennicutt et al., 2003 (PASP)
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Outline
PAHs, masters of the mid-infrared.Why PAHs should matter to you.State of the templates.The variable PAH template:
Why should it vary?SINGS PhotometrySINGS Spectroscopy
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PAHs
Large planar aromatic molecules (~100 Carbons, 10Å diameter)
Vibrational (C-C, C-H) emission bands at 3.3, 6.2, 7.7, 8.6, 11.3μm,…(UIB,AFE,AIB,…)
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PAH Environments & Mechanisms
Origin: carbon star outflows, grain-grain collisions in shocks, “shattering” of carbonaceous dust, …
Emission: vibrational transitions responsible for MIR bands excited by wide range of radiation fields, including UV-deficient starlight from M supergiants!
Destruction: photo-ionization in hard UV radiation fields, sputtering in shocks, coagulation…
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PAHs in the High-z context
Up to 30% FIR luminosity in PAH features (10% typical).
Strongest PAH features shift through MIPS 24um band at z=0.4—3.
Photometric redshifts at z=1-2 (especially without optical counterparts).
Interpretation of 15, 16, 22 & 24μm faint object counts in deep survey fields — GOODS, FLS, Lockman, etc.
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Appetizer: A New 17µm PAH
Smith et. al, 2004
L17.1µm =0.5L11.3µm
SINGS Galaxy NGC 7331
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Photometric Redshifts
LeFloch et al., 2004
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Devriendt, 1999
Templates Used (and re-used)
Sources of PAH templates used to date:Theoretical/Lab PAH band models.Few bright ISO galaxies (e.g. Arp220)Average ISO Key Project “normal” galaxy
spectrum.Chary & Elbaz, 2001Dale et al., 2001
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ISO’s Key Project Template
Lu et al, 2003 ISOPhot 2.5—11.6μm 25”×25” aperture
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Papovich et al., 2004
Deep 24μm counts
Fitting the Number Counts
Slight Deviations in MIR PAH shape
Large shifts in inferred dust
properties, SFRs, and luminosities!
Lagache et al., 2004
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Is the PAH Spectrum Constant?
Typical Argument:
1.PAH molecules are photo-excited by single photons to T>100K.
2.Insensitive to radiation field.3.Uniform IR emission spectrum.
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Is the PAH Spectrum Constant?
PAH Formation, Excitation & Destruction mechanisms very poorly understood.
Large range in mixture of PAH molecular weights: CNHM
Neutral vs Charged…Strong feature variations with
metallicity:Z H/C 11.3μm strong.
―BUT―
Low-Z starbursts: Engelbracht, 2005
Z/5
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SINGS Photometry
24μm is an excellent tracer of ionizing photons (Pa-), and SFR.
8μm variable, not good SF tracer: Under(over)-luminous in high (low) ionized radiation fields.
UV tracing evolved, 200-300Myr populations (lower SFRD=lower SN feedback)
Calzetti et al., 2005
M51
Using 8μm as PAH proxy (with 24μm, Pa-
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SINGS Photometry
Full Spatially Resolved SEDs
Dale et al., 2005
M81
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SINGS Photometry
Global SINGS Infrared SEDs
Dale et al., 2005
Low-Z: 8μm deficient
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The SINGS Template Approach
Spectrally map areas centered on nucleus and extra-nuclear (HII, etc.) regions.
Native spatial sampling of 20-200pc.Average over physically matched
spatial regions (e.g. 1kpc, ¼kpc, etc.).Quantify variations both within and
among galaxies in our sample.Ultimate goal: tie PAH variations
consistently into FIR photometric SED models.
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8.0µm
IRS Spectral Mapping
5—15μm
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8.0µm
IRS Spectral Mapping
15—38μm
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Factors of ~2 variations in PAH band relative strengths and equivalent widths!
M51, inner few kpc
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Variations among galaxies
SINGS First delivery:
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SummaryAccurate understanding of PAH spectrum
crucial to modeling high-redshift sources.SINGS offers a unique spatially resolved
spectral dataset ideal for studying variations in the PAH emission spectrum.
SINGS Photometry hinting at strong spatial variability of 8 PAH.
Large variations of PAH strength ratios and equivalent widths (factor of 2x) on sub-kpc scales in many galaxies.
Averaging over few kpc smooths variations, but significant global differences remain.
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M51
Galex 1500ÅIRAC 3.6µmIRAC 8.0µm
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SINGS Sample
Large range of luminosity, Z/Z, morphological type, SFR, gas/star content, IR activity, ...
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SINGS IRS Spectral Mapping
14-37µm
5-8µm7-15µm
10-20µm
19-37µm
All IRS Modules
Uniform mapping strategy: maximize useful overlap.
Build up 3D spectral cubes.
1.5—5” spatial resolution: 10s—100s pc