THE FAR-INFRARED

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THE FAR-INFRARED
FIR IRAS region (60-100 micron) TIR
8-1000 micron
(1 micron 1A/104)
Log ? L? (1030 ergs/s)
0.1 1 10 100 1000 Lambda
(micron)
Silva et al. 1998
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THE FAR-INFRARED
Part of the luminosity of a galaxy is absorbed by
interstellar dust and re-emitted in the IR
(10-300 micron) The most heavily extincted part
of the stellar continuum is the UV therefore
the FIR emission can be a sensitive tracer of
young stellar populations (and current SF)
Log ? L? (1030 ergs/s)
Lambda (micron)
0.1 1 10 100 1000 Lambda
(micron)
Silva et al. 1998
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THE FAR-INFRARED

• Two contributions to the FIR emission
• young stars in starforming regions (warm, ? 60
  micron)
• an infrared cirrus component (cooler, ?gt100
  micron), associated with more extended dust
  heated by the interstellar radiation field

Whenever young stars dominate the UV-visible
emission and dust opacity is high then a)
dominates and the FIR is a good indicator of
SFR This is the case in Luminous and
Ultraluminous Infrared Galaxies, and mostly works
also in late-type starforming galaxies In at
least some of the early-type galaxies the FIR
emission is due to older stars or AGNs, therefore
in these the FIR emission is not a good tracer of
SF
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THE SFR-FIR CALIBRATION

• One calibration based on spectrophotometric
  models and found
• Assuming the dust reradiates all the bolometric
  luminosity (!) (Optically thick case)
• For starbursts (constant SFR) of ages lt 108 yrs
• SFR(solar masses/yr) 4.5 X 10-44 LFIR (ergs/s)
• where LFIR is the luminosity integrated over
  8-1000 micron
• 
  (Kennicutt 1998)
• Most of other published calibrations within 30.
• In quiescent starforming galaxies, the
  contribution from older stars will tend to lower
  the coefficient above.

Keeping in mind that no calibration applies to
all galaxy types and SFHs
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Indicators of ongoing star-formation activity -
Timescales Emission lines
lt 3 x 107 yrs UV-continuum emission
it depends FIR emission
lt a few 107 (butit depends on
the dominant population of stars heating the
dust) Radio
emission as FIR (?)
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LATE-TYPE STARFORMING GALAXIES
The FIR luminosity correlates with other SFR
tracers such as the UV continuum and Halpha
luminosities.
FIR flux
Halpha flux
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MIR EMISSION AS A SFR INDICATOR
Near-IR J,H,K bands
12000,16000,22000 A 1.2, 1.6, 2.2
micron Mid-IR 6-20 micron Far-IR
gt25 micron (60-100)
Log ? L? (1030 ergs/s)
0.1 1 10 100 1000 Lambda
(micron)
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MIR EMISSION AS A SFR INDICATOR

• In principle, complex relation between MIR
  emission and SFR
• continuum emission by warm small dust grains
  heated by young stars or an AGN
• unidentified infrared bands (UIBs a family of
  features at 3.3, 6.2, 7.7, 8.6, 11.3, 12.7
  micron) thought to result from C-C and C-H
  vibrational bands in hydrocarbons (large,
  carbon-rich molecules as polycyclic aromatic
  hydrocarbins, or PAHs?)
• continuum emission from the photosphere of
  evolved stars
• emission lines from the ionized interstellar gas

e.g. Genzel Cesarsky ARAA 2000
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FROM MIR TO FIR
Empirical relation between MIR(typically
15micron) and FIR luminosities Chary Elbaz
2001 strong correlations between luminosity at
12 and 15micron and total IR luminosity
(8-1000micron)
As it is done for calibrating OII vs Halpha
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FROM MIR TO FIR
.much better correlated than with the B band
(Chary Elbaz 2001)
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FROM MIR TO FIR ANOTHER METHOD
Infrared (8-1000micron) luminosities are
interpolated between the MIR and the radio fluxes
using best-fitting templates of various
starbursts/starforming galaxies and AGNs. (e.g.
Flores et al. 1999)
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SUBMILLIMITER OBSERVATIONS
Sampling the IR emission with 850micron fluxes
(e.g. Hughes et al. 1998) Negative K-corrections
the flux density of a galaxy at 800micron with
fixed intrinsic luminosity is expected to be
roughly constant at all redshifts 1 lt z lt 10
While the Lyman break technique prefentially
selects UV-bright starbursts, the submillimiter
emission best identifies IR luminous starbursts.
The approaches are complementary (debated
relation between the two populations).
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Negative k-correction for sub-mm sources
K-correction is the dimming due to the (1z)
shifting of the wavelength band (and its width)
for a filter with response S(?) In the
Rayleigh-Jeans tail of the dust blackbody
spectrum, galaxies get brighter as they are
redshifted to greater distance!
Blain et al (2002) Phys. Rept, 369,111
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THE FIR-RADIO CORRELATION
Van der Kruit 1971, 1973
Log L1.49Ghz
Log LFIR
Condon ARAA 1992
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THE FIR-RADIO CORRELATION
is surprising !!

For FIR warm and cirrus contribution
Radio emission originates from complex and poorly
understood physics of cosmic-ray generation and
energy transfer Non-thermal component
(synchrotron emission of relativistic electrons
spiraling in a galaxy magnetic field) Thermal
component (free-free emission from ionized
hydrogen in HII regions)
SNae
O, B stars
Condon ARAA 1992
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THE FIR-RADIO CORRELATION
is still surprising

a 0.8
Non-thermal
a 0.1
Thermal
Due to difference in spectral shape, the relative
contribution varies with frequency. At lt5Ghz
(1.4Ghz commonly used), non-thermal conponent
dominates (90) in luminous galaxies
Condon ARAA 1992
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Indicators of ongoing star-formation activity -
Timescales Emission lines
lt 3 x 107 yrs UV-continuum emission
it depends FIR emission
lt a few 107 (but)
Radio emission
as FIR (?) (Could be higher
relativistic electrons have lifetimes 108 yr)
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primaria
1) SFR 0.9 X 10-41 L(Ha) E(Ha) ergs/s
secondaria
2) SFR 2.0 X 10-41 L(OII) E(Ha) ergs/s
3) SFR 1.4 X 10-28 Lnu ergs/s/Hz (L
dust-corrected)
primaria
4) SFR 4.5 X 10-44 LFIR (ergs/s)
primaria
5)
secondaria
(Solar luminosities)
secondaria
6) SUBMILLIMITRICO COME FIR
secondaria
7)
8)
secondaria
erg/s
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1 z
Hopkins 2004
SFR (Msun yr-1 Mpc-3)
Evolution of SFR density with redshift, using a
common obscuration correction where necessary.
The points are color-coded by rest-frame
wavelength as follows Blue UV green O II
red H   and H   pink X-ray, FIR,
submillimeter, and radio. The solid line shows
the evolving 1.4 GHz LF derived by Haarsma et al.
(2000). The dot-dashed line shows the
least-squares fit to all the z lt 1 data points,
log(  ) 3.10 log(1 z) - 1.80. The dotted
lines show pure luminosity evolution for the
Condon (1989) 1.4 GHz LF, at rates of Q 2.5
(lower dotted line) and Q 4.1 (upper dotted
line). The dashed line shows the "fossil" record
from Local Group galaxies (Hopkins et al. 2001b).
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