THE FAR-INFRARED

FIR = IRAS region (60-100 micron)

TIR = 8-1000 micron (1 micron = 1A/10^4)

Silva et al. 1998

0.1 1 10 100 1000

Lambda (micron)

Log

λ L λ

(1

0^30

erg

s/s)

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)

Silva et al. 1998

0.1 1 10 100 1000

Lambda (micron)

Lambda (micron)

Log

λ L λ

(1

0^30

erg

s/s)

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 (?)

LATE-TYPE STARFORMING GALAXIES

The FIR luminosity correlates with other SFR tracers such as the UV continuum and Halpha luminosities.

FIR

flu

x

Halpha flux

MIR EMISSION AS A SFR INDICATOR

0.1 1 10 100 1000

Lambda (micron)

Log

λ L λ

(1

0^30

erg

s/s) Near-IR J,H,K bands

12000,16000,22000 A =

1.2, 1.6, 2.2 micron

Mid-IR 6-20 micron

Far-IR >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 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

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

Blain et al (2002) Phys. Rept, 369,111

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!

k(z) (1 z)F ()S()d

F ( 1z )S()d

THE FIR-RADIO CORRELATION

Condon ARAA 1992

Van der Kruit 1971, 1973

Log LFIR

Log

L1

.49G

hz

THE FIR-RADIO CORRELATION

Condon ARAA 1992

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

THE FIR-RADIO CORRELATION

Condon ARAA 1992

Non-thermal

Thermal

is still surprising

α ~ 0.8

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

α ~ 0.1

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)

2) SFR = 2.0 X 10-41 L([OII]) E(Hα) ergs/s

3) SFR = 1.4 X 10-28 Lnu ergs/s/Hz (L dust-corrected)

1) SFR = 0.9 X 10-41 L(Hα) E(Hα) ergs/s

4) SFR = 4.5 X 10-44 LFIR (ergs/s)

(Solar luminosities)

6) SUBMILLIMITRICO COME FIR

5)

7)

8)

erg/s

primaria

primaria

primaria

secondaria

secondaria

secondaria

secondaria

secondaria

1 + z

SF

R (

Msu

n y

r-1 M

pc-3

)

Hopkins 2004

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).