PAH AND MID-IR
LUMINOSITIES AS MEASURE
OF STAR-FORMATION RATE IN
SPITZER FIRST LOOK SURVEY
GALAXIES
WU HONG
NAOC
Background
polycyclic aromatic hydrocarbons
PAH
The purpose


SFR is the fundamental indicator for
galaxy formation and evolution
Different SFR indicators
different physics, disadvantages
UV Hα Extinction?
FIR
Cirrus?
Submm for high z
radio
AGN contribution
the SFRs, difference of a factor 2

MID-IR indicators
advantages:
low extinction, less circus contribution
Elbaz et al.(2002) relation between LMIR
and LIR
MIR: ISO 6.75μm 15 μm
6.75μm: PAH(polycyclic aromatic
hydrocarbons)
15 μm: VSG continuum(very small grain)
polluted by some PAH features
(12.7 μm) features

MID-IR in SPITZER
8μm: cover the strongest 7.7μm PAH feature
24μm: VSG, less contribution from PAH
expect they can be SFR indicators

TO do THIS: need a sample of star-forming
galaxies to build up the relation.
SAMPLE


Based on the main field of Spitzer FLS
mid-IR images ( IRAC 3.6, 4.5, 5.6, 8.0μm,
MIPS 24 μm) reduced by ourselves
The ovelay region is 3.7 degree2
AUTO magnitudes obtained by Sextractor
SDSS EDR just cover the FLS region
matching the FLS sources with spectra
~400sources: stars, AGNs, galaxies
VLA 1.4GHz (Condon et al. 2003)
Removing stars, qsos, galaxies without line
emissions and narrow line AGNs (BPT diagram)
only keep galaxies with Hαemission (S/N > 5σ)

The sample of star-forming galaxies
91 have Hα and IRAC 4 bands
38 of 91 have 1.4 GHz flux
70 have Hα and MIPS 24 μm
33 of 70 have 1.4 GHz flux
all sources z < 0.2
12 dwarf galaxies with MB > -18 mag
RADIO/Hα/ MIR LUMINOSITIES

1.4GHz luminosities
corrected to luminosities at rest frame
with power law flux ν-α, α=0.8

Hαluminosities
Extinction correction:
Milky Way , Intrinsic from Hα/Hβ
Fibre-aperture correction
rPetro - rfibre

MIR luminosities ( 8 μm and 24 μm)
K-correction, based on M82 template
remove the stellar continuum from 8 μm
based on the 3.6μm flux
=> 8 μm (PAH)
MIR --1.4GHz/Hα RELATIONS
Filled circles:
sample galaxies
Open circles:
dwarf galaxies
Dotted: linear fit
Solid: non-linear
Long-dash:
Linear fit for
radio-quite sources
(Appleton et al. 2005)
Short-dash:
ISO 6.75μm-IR relation
(Elbaz et al. 2002)
Dotted: linear fit
Solid: non-linear
Dot-dash:
linear fit between
ISO6.75μm—Hα for
galactic disks
(Roussel et al. 2002)
STAR FORMATION RATES

Based on MIR/1.4GHz relations

Based on MIR/Hα relations
CONCLUSION AND DISCUSSION



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MIR are well correlated with radio/ Hα
and can be SFR indicators
Even For many low luminosity AGNs, the
24μm emission is dominated by starformation, not by AGNs.
The relation could not be right for ULIRGs
Dwarf galaxies do not follow the PAH-Hα
relation. (low dust-gas ratio and low
metallicity)