Transcript Document

THE FAR-INFRARED
FIR = IRAS region (60-100 micron)
(1 micron = 1A/10^4)
Log λ Lλ (10^30 ergs/s)
TIR = 8-1000 micron
0.1
1
10
100
1000
Lambda (micron)
Silva et al. 1998
THE FAR-INFRARED
Part of the luminosity of a galaxy is absorbed by interstellar dust and reemitted in the IR (10-300 micron)
Log λ Lλ (10^30 ergs/s)
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)
Lambda (micron)
0.1
1
10
100
1000
Lambda (micron)
Silva et al. 1998
THE FAR-INFRARED
Two contributions to the FIR emission:
a)
young stars in starforming regions (warm, λ ~ 60 micron)
b)
an “infrared cirrus” component (cooler, λ>100 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
THE SFR-FIR CALIBRATION
“One” calibration based on spectrophotometric models and found :
a)
Assuming the dust reradiates all the bolometric luminosity (!)
(Optically thick case)
b)
For starbursts (constant SFR) of ages < 10^8 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…
Indicators of ongoing star-formation activity - Timescales
Emission lines
< 3 x 107 yrs
UV-continuum emission
it depends…
FIR emission
< a few 10^7 (but…it
depends on the dominant population of stars heating the dust)
Radio emission
as FIR (?)
The FIR luminosity correlates with
other SFR tracers such as the UV
continuum and Halpha luminosities.
FIR flux
LATE-TYPE STARFORMING GALAXIES
Halpha flux
MIR EMISSION AS A SFR INDICATOR
Log λ Lλ (10^30 ergs/s)
Near-IR
J,H,K bands
12000,16000,22000 A =
1.2, 1.6, 2.2 micron
Mid-IR
Far-IR
0.1
1
10
100
Lambda (micron)
1000
6-20 micron
>25 micron (60-100)
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 CC and C-H vibrational bands in hydrocarbons (large, carbonrich 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
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…
FROM MIR TO FIR
….much better correlated than with the B band (Chary & Elbaz 2001)
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)
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 < z < 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).
Negative k-correction for sub-mm sources
k ( z)  (1  z)
 F (  )S (  )d 
 F (  1 z )S (  )d 
K-correction is the dimming
due to the (1+z) 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
THE FIR-RADIO CORRELATION
Log L1.49Ghz
Van der Kruit
1971, 1973
Log LFIR
Condon ARAA 1992
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)
SNae
Thermal component (free-free emission from ionized
hydrogen in HII regions)
O, B stars
Condon ARAA 1992
THE FIR-RADIO CORRELATION
is still surprising
α ~ 0.8
Non-thermal
Thermal
α ~ 0.1
Due to difference in spectral shape, the relative contribution varies with
frequency. At <5Ghz (1.4Ghz commonly used), non-thermal conponent
dominates (90%) in luminous galaxies
Condon ARAA 1992
Indicators of ongoing star-formation activity - Timescales
Emission lines
< 3 x 107 yrs
UV-continuum emission
it depends…
FIR emission
< a few 10^7 (but…)
Radio emission
as FIR (?)
(Could be higher: relativistic electrons have lifetimes ≤ 10^8 yr)
1) SFR = 0.9 X 10-41 L(Hα) E(Hα) ergs/s
primaria
2) SFR = 2.0 X 10-41 L([OII]) E(Hα) ergs/s
secondaria
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)
6) SUBMILLIMITRICO COME FIR
secondaria
7)
secondaria
8)
secondaria
erg/s
Hopkins 2004
SFR (Msun yr-1 Mpc-3)
1+z
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 < 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).