1. Introduction

Radio – FIR Spectral Energy
Distribution of Young Starbursts

Typical radio spectral energy distribution (SED) of galaxies
Condon (1992)

Hiroyuki Hirashita1 and L. K. Hunt2

M82(1University of Tsukuba, Japan; 2Firenze, Italy)Radio

Abstract: We investigate the radio spectral energy distributions (SEDs) of young starforming galaxies. The duration and luminosity of the nonthermal radio emission fromsupernova remnants (SNRs) are constrained by using the observed radio SEDs of SBS
0335–052 and I Zw 18, which are the two lowest-metallicity blue compact dwarf galaxies inthe nearby universe. The typical radio energy emitted per SNR over its radiative lifetime inSBS 0335–052 is estimated to be ~ 6–22×1022 W Hz–1 yr at 5 GHz. On the other hand, theradio energy per SNR in I Zw 18 is ~ 1–3×1022 W Hz–1 yr at 5 GHz. We discuss the originof this variation and propose scaling relations between synchrotron luminosity and gasdensity. These models enable us to roughly age date and classify radio spectra of starforming galaxies into active (e.g., SBS0335–052)/passive (e.g., I Zw 18) classes.
1

dustsynchrotron
free-free

Related to star
Free-free: H II regions
formation activity
Synchrotron: Supernova Remnants of galaxies
2Density-Dependent Σ – t Relation

Synchrotron Component
Radio synchrotron emission from supernova remnants(SNRs) can be estimated as(supernova rate) × (radio energy per SNR)
↑Estimated from the Σ (radio surface brightness) – t (age)relation

Arbutina & Urošević (2005)
Supernovae in high-density environments tend tohave high radio surface brightness.
Roughly representativeof dense regions
Roughly representativeof thin regions

Recently, Arbutina et al. (2004) have found that theΣ – t relation depends on density.
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2. Radio SED Model

Aim of This Work

We assume a spherical star-forming region aconstant density.

To reexamine the radio SED of galaxies,
especially the nonthermal part originating fromsupernovae: e.g., dependence on the ambient ISM
densityParameters
Gas Mass: Mgas
Number density: nYoung galaxies are favorable for an observational
sample. ← The diffuse radio synchrotron component
has not yet been established, and the synchrotron
component comes from supernovae.

Star formation rate (SFR)〜 εMgas / tff

We use metal-poor blue compact dwarf galaxies
5(BCDs) to constrain the radio energy of SNRs.

Evolution of Ionized Region (freefree radiation source)

Mgas
rSF
nwith tff ~ 1/(Gρ)1/2
6Synchrotron Radiation
Luminosity = l(SN)τγ
 l(SN): Radio luminosity per supernova
remnant (SNR) τ: Lifetime of radio radiation of SNR
 γ: supernova rate (← given by SFR) Size is determined by the Strömgrenradius.
 Dynamical expansion due to the pressureexcess is also included (Spitzer 1978).
 Star formation is stopped if the entire
region is ionized.

l(SN) and τ are constrained observationally.

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SBS 0335–052

3. Comparison with BCDs

Age < 5 MyrMetallicity
1/41 Zsun

blue compact dwarf galaxies (BCDs)
Typical age is young. ⇒ easy to constrain the radio
emission from Type II supernovae, since the diffuse
radio emission has not been established.
Low metallicity (<~1/10 Zsun)
Lower complexity than giant galaxies ← One onezone model is easy to apply.

300 pc9ResultComparison with
SBS 0335-052 at age~ 5 Myr
Total

ThermalLarge free-freeabsorption⇒ characteristicof dense/compact(“active”) classD = 53 Mpc

Vanzi et al. (2000)

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Estimate of Physical Quantities
Theoretical prediction
 l(SN) τ ~ 4.4 × 1027 erg/s/Hz・yr× (Ee/1048 erg) (B/10 µG)1.5 (τ/105 yr)
Observationally derived value by us l(SN) τ ~ 1 – 5 × 1030 erg/s/Hz・yr (for SBS 0335)

Nonthermall(SN) τ = (1 – 5) × 1023 W yr / Hz @ 1 GHz
11B = 100 µG ← 1/10 of the gas pressure
Ee = 1049 erg ← 1% of ESN contributes to the productionof energetic electrons
τ = 105 yr ← duration of SN shock123

I Zw 18

D = 12.6 Mpc
I Zw 18 (Brown et al. 2002)

Application to the Far-Infrared –Radio Correlation
Evolution of radio luminosity ← Model in this poster
Evolution of FIR luminosity ← Hirashita & Hunt (2004)
TheoryObservation

300 pcSmall free-free absorption⇒ characteristic ofdiffuse (“passive”) classn ~ 100 cm–3

Best-fit l(SN) τ isconsistent withscaling B 2∝n 13

Dust enrichmentoccurs on atimescale of dust
enrichment bysupernovae.

The correlation
is establishedBCD sample: Hopkins et al. (2002)
on < ~ 5 Myr.
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4. Summary(1) The radio SEDs of representative low-metallicity
BCDs are dominated by synchrotron radiation.
 Magnetic fields have already been amplified.
 Energetic electrons have been accelerated.
(2) The FIR – radio correlation is also explained bya. Radio emission from supernova remnants, andb. FIR emission from dust produced bysupervnovae.
← The correlation is established in 3 – 5 Myr.
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