A&A 460, 67-81 (2006)
DOI: 10.1051/0004-6361:20065629
H. Hirashita1 - L. K. Hunt2
1 - Center for Computational Sciences, University of Tsukuba,
Tsukuba, Ibaraki 305-8577, Japan
2 -
INAF - Istituto di Radioastronomia-Sezione Firenze, Largo E. Fermi 5,
50125 Firenze, Italy
Received 18 May 2006 / Accepted 30 August 2006
Abstract
We investigate the radio spectral energy distributions
(SEDs) of young star-forming galaxies and how
they evolve
with time. The duration and luminosity of the nonthermal
radio emission from supernova remnants (SNRs) are
constrained by
using the observational radio SEDs of SBS 0335-052 and I Zw 18, which
are the two lowest-metallicity blue compact dwarf galaxies
in the nearby universe.
The typical radio "fluence'' for SNRs in SBS 0335-052, that is the
radio energy emitted
per SNR over its radiative lifetime, is estimated
to be
6-
at 5 GHz.
On the other hand, the radio fluence in I Zw 18 is
1-
at 5 GHz.
We discuss the origin of this variation and propose
scaling relations between synchrotron luminosity and gas
density. We have also predicted the time dependence of the radio
spectral index and of the spectrum itself, for both
the "active'' (SBS 0335-052) and "passive'' (I Zw 18) cases.
These models enable us to roughly age date and classify
radio spectra of star-forming galaxies into active/passive
classes. Implications for high-z galaxy evolution are
also discussed.
Key words: galaxies: dwarf - galaxies: evolution - galaxies: ISM - ISM: supernova remnants - radio continuum: galaxies
Radio emission from galaxies is known to be connected with star formation activity. Two radiative processes are responsible: thermal free-free radiation from ionized gas in H II regions and synchrotron radiation from relativistic electrons spiraling in magnetic fields (Condon 1992). The former originates from ionized gas around massive stars, while the latter comes from supernova remnants (SNRs) whose progenitors are massive stars. Since massive stars have short lifetimes, both thermal and nonthermal emission should trace the current star formation rate (SFR).
Nevertheless, the physical basis of the radio emission is
not fully established.
The observed radio flux in luminous evolved starbursts
and normal spiral galaxies is dominated by nonthermal emission,
but not from the short timescale radiation of discrete SNRs;
more than 90% of it is
probably due to the diffusion of cosmic ray electrons
in the galaxy disk (Condon 1992) over timescales of
107-108 yr (Helou & Bicay 1993).
Indeed, if the theoretically estimated adiabatic
timescale (e.g., Woltjer 1972)
and the observationally obtained (surface brightness vs. diameter) relation for discrete SNRs
(e.g., Clark & Caswell 1976) are used to derive
the nonthermal emission in galaxies
(e.g., Biermann 1976; Ulvestad 1982),
the emission is underpredicted by a factor of 10 or so.
In very young galaxies with ages 10 Myr, however, the
cosmic-ray diffusion mechanisms have not yet had a chance to
dominate the radio emission. Nonthermal synchrotron radiation
from discrete SNRs would be expected
to dominate over the diffuse component.
Moreover, in galaxies with starbursts younger than
3 Myr,
there should be very little synchrotron emission since the SNRs have not
yet exploded. Indeed, some
galaxies with a deficit of nonthermal emission are observed
(Roussel et al. 2003) and appear to be due to starbursts
observed within a few Myr of their onset, after a long period
(
100 Myr) of quiescence.
In this paper, we investigate theoretically the time evolution of the radio spectral energy distribution (SED) in young star-forming galaxies. In principle, this is a straightforward process because we know that the radio SED can be determined by the star formation history, for which we have already developed the formalism (Hirashita et al. 2002; Hirashita & Hunt 2004). In practice, it is a difficult exercise because of the unknown nature of the time evolution of the nonthermal radio component. Hence, we adopt nearby blue compact dwarf galaxies (BCDs) as an observational sample to constrain our theoretical model of radio emission. Most BCDs have a young age of current star formation (although most of them had previous starburst episodes), and a low metallicity (e.g., Izotov & Thuan 2004). This implies that they are relatively unevolved chemically and may provide a reasonable template to investigate star formation properties of young galaxies. The BCDs as a class have radio spectra with different properties than those of evolved luminous star-forming galaxies (Klein et al. 1991); flatter radio spectra of BCDs indicate that they have a lower fraction of nonthermal emission than more luminous systems. Hence, they may lack the diffuse emission that characterizes the radio spectrum of larger disk galaxies, and thus are ideal targets for constraining our models.
Two classes of star-formation activity in BCDs have recently emerged
observationally, as proposed by
Hunt et al. (2003a) and
Hunt & Hirashita (2006, in preparation).
They argue that the star-formation modes in the two most
metal-poor galaxies, SBS 0335-052 and I Zw 18,
are very different,
in spite of their similar metallicities
(
and
,
respectively;
Skillman & Kennicutt 1993;
Izotov et al. 1999). The major star-forming
region of SBS 0335-052 is compact and dense (radius
pc, number density
cm-3;
Dale et al. 2001; Izotov & Thuan 1999).
Moreover, SBS 0335-052 hosts several super star clusters (SSCs),
detectable H2emission lines in the near-infrared (NIR)
(Vanzi et al. 2000), a large dust extinction
(
mag; Thuan et al. 1999;
Hunt et al. 2001;
Plante & Sauvage 2002), and high dust temperature
(Hunt et al. 2001; Dale et al. 2001;
Takeuchi et al. 2003). On the contrary, the
star-forming regions in I Zw 18 are diffuse
(
pc,
cm-3), and contain
no SSCs. NIR H2 emission has not been detected (Hunt et al.,
private communication), and the dust extinction
is moderate (
mag; Cannon et al. 2002). We
consider a region with such properties as "passive'' following
Hunt et al. (2003a), while a SBS 0335-052-like
star-forming property is considered "active''.
The similar metallicities of SBS 0335-052 (active) and
I Zw 18 (passive) imply that
the chemical abundance is not a primary factor in determining
the star-forming properties. We argue that the compactness of
star-forming regions, which affect gas density,
gas dynamics, and so on, is important in the dichotomy
of active and passive modes.
In this paper, we extend this active-passive classification into the radio regime by focusing on SBS 0335-052 and I Zw 18 as representatives of metal-poor BCDs, and also of two star-forming modes in BCDs: "active'' and "passive''. This paper is organized as follows. First, in Sect. 2 we explain the model that describes the evolution of radio SEDs. Some basic results are given in Sect. 3, where observational data of SBS 0335-052 and I Zw 18 are compared with the results for active and passive modes. The constraints obtained here are compared with theoretical descriptions of the physical processes of synchrotron radiation in Sect. 4. In Sect. 5 we generalize the results of our models with predictions for the time evolution of radio SEDs distinguishing between active and passive modes, and discuss implications for high-redshift star formation. Finally we give our conclusions in Sect. 6.
The radio emission from galaxies is interpreted to be composed of two components (Condon 1992): thermal free-free (bremsstrahlung) emission and nonthermal synchrotron radiation. The former requires treatment of ionized regions around star-forming regions and the latter is related to SNRs. Our models include these processes as described below. We approximate the dominant star-forming region in a BCD to a single zone with spherical symmetry.
The free-free radiation in ionized regions is responsible
for the flat thermal component of radio SEDs. The
luminosity of the thermal component is proportional to
the number of ionizing photons (with energy larger
than 13.6 eV) emitted per unit time,
.
In order to calculate
,
we need the stellar
ionizing photon luminosity and the stellar lifetime.
We use the fitting formulae of Schaerer (2002)
for the stellar lifetime
(denoted as
in Schaerer 2002) and the number of ionizing
photons emitted per unit time Q(m) (see Table 6 of
Schaerer 2002). Those quantities are
calculated as functions of stellar
mass at the zero age main sequence, m.
We adopt Z=0 (zero metallicity) for the stellar
properties because we treat a metal-poor phase.
If we adopt the solar metallicity instead,
is
2 times smaller
(Schaerer 2002).
Considering those two extreme metallicities, we consider
that the uncertainty caused by the stellar metallicity
is within a factor of 2.
The evolution of
as a function of time
t is calculated by
![]() |
(1) |
The number of ionizing photons can be related to the
thermal radio luminosity at the frequency ,
as (Hunt et al. 2004,
hereafter H04;
valid for
)
![]() |
(2) |
A part of the radio emission is self-absorbed by the ionized gas.
We estimate the optical depth of the free-free
radiation,
as
![]() |
(6) |
Nonthermal radio emission from galaxies originates from
SNRs. Since we consider young star-forming regions,
we only
treat Type II supernovae (SNe II), whose progenitors have
short lifetimes.
The rate of SNe II as a function of time,
,
is given by
In order to constrain the radio energy emitted by a SNR
over its entire lifetime - the "fluence'' or
time-luminosity integral - we treat the duration
(
)
and the luminosity at a frequency
(
)
as parameters to be determined
from observational constraints. Then, the nonthermal
luminosity is written as
After accounting for the absorption, the nonthermal
radio component emitted by the star-forming region
is estimated as
![]() |
(11) |
Stars form as a result of the gravitational collapse of a gas
cloud. Therefore, it is physically reasonable to relate the
SFR with the free-fall (or dynamical) timescale of
gas (Elmegreen 2000).
We consider a star-forming region with an initial number
density of neutral hydrogen
.
The free-fall time,
,
is estimated as
The SFR, ,
basically scales with the gas mass
divided by the free-fall time:
The initial hydrogen number density can be related to
the gas mass in the star-forming region as
![]() |
(19) |
In order to treat the range of evolution variations from deeply
embedded H II regions to normal H II regions,
it is crucial to include pressure-driven expansion of H II regions.
To focus on the radio SED, we adopt a simple analytical
approximation, which divides
the evolution of an H II region into two
stages: the first stage is the growth of ionizing front
due to the increase of ionizing photons, and the second
is the pressure-driven expansion of ionized gas. The
expansion speed of ionizing front in the first stage
is simply estimated by the increasing rate of the
Strömgren radius. The Strömgren radius
under the initial density is defined by the
following relation:
![]() |
(21) |
Once
is satisfied,
pressure-driven expansion is treated. Since the density
evolves, we define the Strömgren radius
under the current density:
![]() |
(22) |
![]() |
(25) |
![]() |
(26) |
The dynamical expansion is treated as long as
.
When the SFR
declines significantly,
begins to
decrease. Thus,
can be
realized at a certain time. When
,
is replaced with
and
is fixed;
that is, we finish treating the dynamical expansion.
Here we use observations to constrain
some of the physical parameters in
our models. In particular, the radiative energy -time-luminosity
integral- of
the nonthermal synchrotron component is the most
important parameter to be obtained in this paper.
Two representative metal-poor BCDs, SBS 0335-052 and I Zw 18 (Hirashita & Hunt 2004), are used here.
In our previous works (Hirashita et al. 2002;
Hirashita & Hunt 2004), SBS 0335-052 has been
used as a proxy for a genuinely young galaxy in which
the mass of underlying old population is negligible
(Vanzi et al. 2000).
The same may be true for I Zw 18 in which the mass fraction
due to an underlying evolved stellar population (2 Gyr)
is not more than
20% (Hunt et al. 2003b).
We adopt 54.3 Mpc for the distance of SBS 0335-052 (Thuan et al. 1997)
and 12.6 Mpc for I Zw 18 (Östlin 2000).
We calculate the time dependence of the radio SED, ,
by
summing the thermal and nonthermal components:
![]() |
(27) |
We adopt
pc and
(i.e.,
;
yr). We selected these
values after surveying the full range of
parameters so that the
observable quantities (SFR,
,
and
EM) are consistent with observations at the
age estimated by Vanzi et al. (2000).
The time evolution of the relevant
quantities is shown below to verify whether those
values indeed reproduce the observations.
The radius of the star-forming region is also consistent with
Takeuchi et al. (2005), who
reproduce the observational FIR SED of
SBS 0335-052.
![]() |
Figure 1:
Time evolution of basic quantities
concerning radio emission,
with the solid and dashed lines corresponding to
the continuous and burst SFR, respectively.
The initial conditions are selected to
be the "active'' mode, consistent with
the observations of SBS 0335-052 at ![]() ![]() ![]() ![]() ![]() ![]() |
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Table 1: Models of radio SEDs.
First, we evaluate the continuous SFR. The time
evolution of ,
,
EM, and SFR
is shown in Figs. 1a,b,c, and d,
respectively. The SFR averaged for 5 Myr
is
0.8
with the
total stellar mass formed of
.
The SFR of stars more massive than 5
(
),
which is less affected by the assumed IMF, is
estimated to be 0.14
yr-1, which is
consistent with the estimate by H04
(
-0.15
yr-1).
At t=5 Myr,
,
which is
also compatible with the observational results
(Izotov et al. 1999; H04).
The radius of the ionized region
monotonically
increases until t=3.7 Myr because of the
pressure-driven expansion and the increase of
.
At t=3.7 Myr, the entire
star-forming region is ionized. Until then, the
increasing behavior of
is approximated by
as derived in
Appendix A. In
Fig. 1a, we show the slope of
(dotted line), which
is roughly consistent with the calculated
result (solid line). The initial
evolution of density is also explained by the
analytical relation derived in Appendix as shown
in Fig. 1b
(
). As a result, the
emission measure decreases as
as shown in
Fig. 1c.
![]() |
Figure 2:
Radio SEDs at t=5 Myr calculated by
a) model A, b) model B, c) model E, d) model F,
e) model G, and f) model H for the same initial
condition as Fig. 1. For
model E, we adopt the best
fit parameter of
![]() ![]() ![]() |
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If we adopt the burst SFR (s=5.1; Eq. (18)),
we obtain the time evolutions as depicted in
Fig. 1. As mentioned above,
value of s is
determined to ensure that the total stellar mass
formed is the same between the burst and continuous
modes. Since the star
formation stops on a short timescale, the driving
force of the dynamical expansion decreases more rapidly
than in the case of the continuous SFR. Thus, the
maximum radius of the ionized region is smaller
and the density is kept higher. This high density
makes the EM higher in the burst SFR, and the EM
derived observationally by H04
(
pc cm-6) is more
consistent with the burst SFR than with the
continuous SFR at
Myr.
Although the burst scenario is more consistent with
the observations of SBS 0335-052, in what follows we examine the
two cases as extremes for the SFR.
We show the radio SEDs calculated by various models
listed in Table 1, and examine whether or not the
model predictions agree with observational data.
The radio data are shown in Fig. 2:
the squares are from H04, and the cross is from
Dale et al. (2001). The SED at t=5 Myr is
presented in Fig. 2,
where the dotted and dashed lines represent the
nonthermal and thermal components, respectively.
The sum of those two components is shown by the
solid line in each panel. The panel (a) shows the
result of model A, in which we adopt the
"standard'' model for the nonthermal radio emission.
In this case, the thermal emission dominates the
radio SED. The model prediction is inconsistent with
the observational SED in the following two aspects:
(i) the theoretical flux is systematically smaller
than the observed one; (ii) the flat SED, dominated
by the free-free component, is inconsistent with the
observational SED at GHz,
which requires a prominent nonthermal contribution.
Indeed H04 show that the nonthermal fraction of
the radio luminosity at 5 GHz is
0.7.
If we adopt model B with a burst SFR instead of model A, the
thermal component decreases, because the mean
age of the stars is older. In this case also,
however, the nonthermal radiation
of model B falls well below the data presented in Fig. 2.
As long as we adopt the "standard''
nonthermal component (i.e., models A-D), we cannot
explain the prominent nonthermal contribution.
Therefore, we exclude
models A-D, and adopt the parameterized model for
the nonthermal component (i.e., models E-H).
In the parameterized model of the nonthermal
component,
is treated
as a parameter to be determined from observations
(Eq. (10)). Hence, we estimate
by using the observational
data of SBS 0335-052 taken by H04 and
Dale et al. (2001). We adopt the same
initial conditions and parameters as above, and assume
an age of t=5 Myr; we then
search for the value of
in models E-H which minimizes
for the six data points.
The best-fit
are listed in
Table 2 with
and
(reduced
;
the number of
freedom is five), and the
best-fit SEDs for models E-H are shown in
Figs. 2c-f, respectively.
Table 2: Best fit parameters for the nonthermal component (SBS 0335-052).
The burst models require higher
.
In the burst models, all the stars form at
the beginning, so that the mean stellar
age is older. Thus, the thermal emission of
the burst models is fainter than that of the
continuous star formation models.
This is why we require a higher
nonthermal time-luminosity integral of a SN II in the burst
SFR models (models F and H) than in the continuous SFR models
(E and G) to explain the observed radio emission.
With the nonthermal component comparable to or larger than
the thermal component, the points at 4.86, 8.46, and
22.5 GHz are well reproduced. The spectral slope of
the nonthermal component is also consistent with
the observations. Thus nonthermal radio
emission comparable to or stronger than the
thermal radio emission is required
even in this young (<5 Myr) galaxy.
In general, the fits are satisfactory for
models E-H except for the point at
14.9 GHz.
The fractions of nonthermal
emission are 0.44, 0.79, 0.37, and 0.79 at
5 GHz in models E, F, G, and H, respectively.
The fractions in models F and H are more consistent with
the value of 0.7 at 5 GHz inferred from the spectral
decomposition by H04.
The nonthermal feature of the radio spectrum
is inconsistent with an age of t<3.5 Myr, because of the
time required for the onset of SNe. On the other hand,
as long as we adopt an age between 4 Myr and 7 Myr,
the difference in the derived
is within a factor of
2
(i.e., within the uncertainty among models E-H.)
Thus, we conclude that
derived
here is robust against the age uncertainty.
![]() |
Figure 3: Same as Fig. 1 but for the I Zw 18 "passive'' model. |
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The nonthermal radiative energy emitted at GHz
over the entire lifetime of a SNR is predicted to be
5.8-
W Hz-1 yr
(Table 2). This is more than
an order of magnitude larger than the value used in the
"standard'' model, in which the nonthermal
radio energy emitted by a SNR is estimated based on
the observed
(surface luminosity vs. radius) relation and the theoretical adiabatic lifetime.
That assumption leads
to the total radio energy per unit frequency
as (assuming
)
![]() |
(29) |
![]() |
(31) |
We now apply our model to another "template'' of nearby young BCDs, I Zw 18, whose nebular oxygen abundance is low (12+log(O/H)=7.2, Skillman & Kennicutt 1993). The star-forming region of I Zw 18 is more diffuse than that of SBS 0335-052 (e.g., Hunt et al. 2003a). Thus, the different gas density of I Zw 18 from SBS 0335-052 may provide us with independent information on the density dependence of radio emission.
For the initial density and the radius, we assume
and
,
consistently with optical spectra and images
(Skillman & Kennicutt 1993; Hunt et al. 2003a;
Hirashita & Hunt 2004).
The resulting gas mass is
with
yr; the gas mass is comparable to the
observationally determined values by
van Zee et al. (1998) (
for their H I-A component)
and Lequeux & Viallefond (1980)
(
for their component 1).
We adopt an age of 10-15 Myr for the dominant burst in I Zw 18.
This is consistent with the results of our model of infrared
dust emission
(Hirashita & Hunt 2004), which gives an
age of 10-15 Myr and consistent with the observed metallicity and
dust abundance.
Takeuchi et al. (2003) show that
the dust mass derived by Cannon et al. (2002)
is consistent with an age of 10-30 Myr, which is also
consistent with the observational upper limits of
infrared flux. Recchi et al. (2004)
(and references therein)
argue that stars older than 0.5-1 Gyr,
if any, do not produce a significant
contribution to the metal budget of
I Zw 18, but intermediate
age stars with an age of a few hundred Myr
may be required from studies of the chemical
abundances. An analysis of the color-magnitude
diagram by Aloisi et al. (1999)
also suggests the existence of intermediate-age
populations. Hunt et al. (2003b)
derive an age of <500 Myr for the
oldest stellar population (but see
Östlin 2000) and 15 Myr
for the youngest one in the main body.
First, we evaluate a continuous SFR.
The time evolution of ,
,
EM, and SFR is shown in
Figs. 3a-d.
In the first 1.4 Myr,
the expansion speed of the ionizing front is faster
than the sound speed because of small gas density.
Thus, the dynamical response of the system begins
later compared to SBS 0335-052. Moreover, because of the
low density,
the emission measure remains smaller than in SBS 0335-052.
The star formation
stops at t=7.9 Myr because the entire star-forming
region is ionized. The total stellar mass formed is
.
In the burst SFR, the SFR is constant up to
Myr, a little shorter than the
above duration (7.9 Myr). Thus, the expansion of
the ionized region stops earlier and
remains higher. However, the decline of
occurs earlier because of the shorter duration,
and the resulting EM is roughly similar to that
for the continuous SFR.
As a result, there is little difference in
the evolution of emission measure between the
continuous SFR and the burst SFR in a low-density regime.
We also calculate radio SEDs with various models listed
in Table 1. As mentioned above, we adopt an
age of 10-20 Myr.
Because the (continuous mode of) star formation stops at
t=7.9 Myr, the radio emission
decreases between t=10 and 20 Myr. At 10 Myr, the
thermal component is too high to be consistent with the
observations reported by
Hunt et al. (2005).
The age should be older than 12 Myr to be consistent
with the data. At 15 Myr, the thermal
component is well below the observations.
Thus, we examine the two ages,
t=12 Myr and 15 Myr, as high and low limiting luminosities for
the thermal component.
The emission measures at t=12 Myr and 15 Myr are
pc cm-6 and
pc cm-6, respectively.
Those are larger than the EM estimated by
Hunt et al. (2005)
(
pc cm-6), but smaller
than their extreme case
(
pc cm-6).
If we adopt an age of
20 Myr, the emission
measure becomes too small to be consistent with
observations.
Now we examine the radio SED at t=12 Myr, but
consider only the p-models for the nonthermal
component, since the s-models are rejected after the
investigation of SBS 0335-052. In fact, the s-models are
also unacceptable for I Zw 18.
As in Sect. 3.1,
the best-fit nonthermal time-integrated luminosity (fluence)
per SN II at 5 GHz (
)
is
sought by minimizing
for the six data
points adopted from Hunt et al. (2005) and
references therein. The data are shown in
Figs. 4 and 5:
the lower three data points are from
the VLA (Hunt et al. 2005), while the upper
three are from single-dish radio telescopes (Klein et al. 1991).
Although the discrepancy between those two data sets may
arise from the sensitivity for the diffuse component,
the discrepancy is only
40% at
GHz.
Thus, the following conclusions are not changed by the
uncertainty in the diffuse component.
![]() |
Figure 4:
Radio SED at t=12 Myr calculated for
I Zw 18 by using models E, F, G, and H
(panels a), b), c), and
d), respectively).
The dotted and dashed lines
represent the thermal and nonthermal components,
respectively, and the solid line shows the sum of
those two components.
The squares are observational data taken from
Hunt et al. (2005) and
references therein. The reduced ![]() ![]() |
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![]() |
Figure 5: Same as Fig. 4 but for t=15 Myr. |
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We include all the six data points to obtain the best-fit values
for
,
which are shown in
Table 3 for t=12 Myr, and
in Table 4 for t=15 Myr. Applying
the values reported in those tables, we examine the
radio SEDs. The results are shown in
Figs. 4 and 5 for
t=12 and 15 Myr, respectively. Those figures confirm
that the nonthermal luminosity derived from the
parameters in Tables 3 and
4 is consistent with the
data points. Moreover, the spectral index
at
GHz is reproduced by introducing a
significant contribution from the nonthermal
component.
Table 3: Best fit parameters for the nonthermal component (I Zw 18 at t=12 Myr).
Table 4: Best fit parameters for the nonthermal component (I Zw 18 at t=15 Myr).
The spectral decomposition by Hunt et al. (2005)
gives a nonthermal fraction of 0.59 at 5 GHz, corresponding
to a thermal flux of 0.54 mJy.
This is approximately realized for models F and H
shown in Panels b) and
d) of Fig. 4, and for models E
and G shown in Panels
a) and c) of Fig. 5.
However, other solutions with a thermal component
of 0.3-1 mJy are also permitted.
The younger age of 12 Myr gives
0.20
for the averaged SFR,
while 15 Myr gives 0.16
.
The massive SFR is estimated as
SFR
and 0.029
yr-1,
respectively. Those values are 1.5-2 times larger
than the estimate of Hunt et al. (2005).
If we assume a smaller SFR according to Hunt et al. (2005),
required to
fit the observational SED becomes 1.5-2 times smaller.
Cannon et al. (2005) also provide
fluxes of I Zw 18 at GHz and
GHz; their
fluxes are lower than the values given by Hunt
et al. (2005), but it is possible that
the discrepancy comes from the sensitivity to the
diffuse component. However, they also derive a spectral
index that indicates a prominent nonthermal contribution.
Consequently, our conclusion that a strong contribution from
the nonthermal component is required should be robust
(and
derived by us should be relatively secure).
We have derived values of
for
I Zw 18 between
W Hz-1 yr and
W Hz-1 yr at 5 GHz.
Since the radio spectrum is dominated by the
nonthermal component already at 15 Myr,
those derived values should be robust for older ages,
because the thermal component steeply declines thereafter.
On the other hand, if we adopt an age smaller
than 10 Myr, the fraction of the thermal component
is too large to be consistent with observations.
Thus, even though there is uncertainty in the age of
I Zw 18, we can safely conclude that
of
I Zw 18 is two to five times smaller than that of SBS 0335-052.
The typical densities of ionized
regions are
103 cm-3 and
102 cm-3 in SBS 0335-052 and I Zw 18,
respectively.
We propose in Sect. 4 a scaling relation
.
Since
W Hz-1 yr has been obtained for SBS 0335-052,
the above density scaling relation predicts that
W Hz-1 yr. The above values derived for I Zw 18 are in this range.
For I Zw 18, we obtain the following value for the
nonthermal radiative energy over the entire
lifetime of a SNR:
![]() |
(32) |
![]() |
(33) |
Through comparison with observations, we have obtained the radio energy radiated by the nonthermal synchrotron component. In this section, we relate the observationally-derived values to the physical quantities governing the synchrotron radiation.
The energy spectrum of energetic
electrons radiating synchrotron radiation is
assumed to follow a power law as
![]() |
(36) |
![]() |
(37) |
The spectral index observed in individual SNRs is
roughly
(Clark & Caswell 1976); i.e.,
.
Since the frequency of the power-law synchrotron
radiation ranges at least from MHz to GHz, we can
assume that
.
In any case, because of the logarithmic dependence,
the results are not sensitive to the assumed
.
The total nonthermal radio luminosity of a region
can be calculated by integrating
over all
the volume V as
![]() |
(38) |
![]() |
(40) |
![]() |
(41) |
In Sect. 3.1 for SBS 0335-052 we derived the
nonthermal radio energy emitted at 5 GHz per SN as
1022W Hz-1 yr. Comparing this value with the theoretical
expression given in Eq. (42), we obtain
,
where
we have assumed
erg and
.
Either
G or
or both.
We first investigate the possibility that the
magnetic fields could be amplified by turbulence
generated from SNe as proposed by Balsara et al. (2004).
We define the ratio of thermal energy to magnetic energy,
,
where
n is the particle number density,
is
the Boltzmann constant, and T is the gas
temperature. In SBS 0335-052, we can assume
and
K (Sect. 3.1).
Then, the magnetic field strength becomes
G.
The magnetic field could be amplified in the
star-forming region until the magnetic pressure
becomes comparable to the thermal pressure
(otherwise, the magnetic
fields would not be confined within the region).
Thus,
would be a reasonable condition.
If we conservatively assume that
,
we obtain the magnetic field strength in SBS 0335-052 as
100
G.
The production efficiency of energetic electrons
is not well known, but it could be assumed to be
of an order of 1% or more (e.g.,
Zirakashvili & Völk 2006).
Here we conservatively assume
.
With the quantities
estimated in this paragraph, we predict
yr.
This timescale can be interpreted in two ways. One is
the radiative lifetime of a SNR. The adiabatic lifetime of a
SNR is estimated as
yr
if we assume
cm-3(Eq. (9)). This is much shorter
than the above
.
However, the SNR
continues to expand after the cooling due to the
momentum conservation. If we adopt the expansion law
of Shull (1980), it finally reaches the
sound speed of the ionized medium and the shock
disappears after
105 yr. Therefore, the
production of energetic electrons in
shocked gas may
continue on a timescale of
105 yr, which is
roughly consistent with
estimated
above.
From the expansion law, we can derive
.
The other interpretation of
is that
it reflects the lifetime of an energetic electron.
The energy loss of energetic electrons should be
considered at frequencies larger than
estimated as
We investigate both cases for
.
The first is
based on the radiative lifetime of a SNR, i.e.,
yr, and the
second on the lifetime of energetic
electrons, i.e.,
given
in Eq. (44). By using
Eq. (42), these scaling relations
result in the following expression for
:
It is generally difficult to observe the magnetic
field strength in BCDs. If we use (Sect. 4.2), we can express the
magnetic field strength in terms of the gas density as
,
where we have used
K. A similar observational scaling
(
)
was observationally found by
Niklas & Beck (1997).
Inserting this expression into
the first case in Eq. (45),
we obtain a scaling relation as
W Hz-1 yr. If we assume that
does not change among
galaxies and that
scales only with density, we obtain
,
where
is the value
obtained for SBS 0335-052 as reported in
Table 2.
In the second case in Eq. (45),
there is no dependence on magnetic fields and gas
density, if
does not
depend on those quantities. Thus, we also consider
the case where
is independent
of n.
Finally we propose yet another scaling of the magnetic field
strength, assuming that the magnetic field is
amplified by SNe, as in the turbulence-driven amplification
mechanism proposed by Balsara et al. (2004).
Since the SN rate is proportional to the SFR, the energy density
of the magnetic fields could scale with the SFR
density (
).
Equation (15) implies that the
SFR density scales as n3/2 (this scaling also
expresses a Schmidt law; Schmidt 1959).
Thus, we obtain
.
With this scaling relation, and normalizing it to
SBS 0335-052, we obtain
for the first
case in Eq. (45).
Since the energy density of energetic electrons is
proportional to the SN rate, this scaling indicates
that the magnetic energy density scales with
the energy density of energetic electrons.
In summary, the possible scaling of
is summarized by the following three cases:
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Figure 6: Time evolution of an "active'' radio SED. The same parameters as used to model SBS 0335-052 are adopted as a representative case for the "active'' mode. Panels a), b), c), and d) represent the SEDs at 3.5, 5, 10, and 15 Myr, respectively. The dotted and dashed lines represent the thermal and nonthermal components, respectively, and the solid line shows the sum of those two components. |
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The typical number density of I Zw 18 is
(Sect. 3.2).
Comparing Table 2 (model for SBS 0335-052) with
Tables 3 and 4 (model
for I Zw 18 at 12 and 15 Myr, respectively), we find that the
best model estimate for
,
where the subscript "IZw'' indicates the value for I Zw 18.
Scaling a would imply no density dependence, so that we would
expect
;
this is highly inconsistent with our models.
If, instead, we use Scaling b (
),
we would derive
,
still a bit larger than our model predictions.
Scaling c (
)
would give
,
which is the most consistent with
our results, although Scaling b cannot be rejected with certainty.
Interestingly, the nonthermal
luminosity observed in discrete SNRs in ten external galaxies
appears to be positively correlated with density
(Hunt & Reynolds 2006),
in a way that is consistent with our results.
If the time-luminosity integral varies as n0.7,
as in the seemingly most likely dependence, then
the observed correlation of
implies
that the radiative lifetime of a remnant should vary
as n-0.6.
The slope of the
lnt - n relation is rather uncertain
(Hunt & Reynolds 2006), but cannot be less
than unity.
With
,
the remnant lifetime would vary
as n-0.3.
These values encompass the expansion lifetime dependence
and the adiabatic
one
.
Cannon & Skillman (2004) propose the radio
spectral index as an age indicator.
The radio spectral index is defined by fitting the
radio spectrum with a functional form of
.
In their paper, the observational
spectral index is determined with the data at
,
4.9, and 8.5 GHz.
They argue that
with ages
1 Myr
because of the free-free absorption. Then the optically-thin thermal
spectral index (
)
appears
within a typical lifetime of H II regions
(
10 Myr) and finally the spectrum steepens because
of the increasing contribution from a synchrotron component.
However, we have shown in Sects. 3.1 and 3.2 that the gas density is also important for the shape of the radio spectrum. The dense and compact star-forming region in SBS 0335-052 shows a strong burst of star-formation with strong free-free absorption. We have dubbed this characteristic "active''. On the other hand, the diffuse star-forming region in I Zw 18 has more mild star-formation with much less absorption. We have called this mode "passive''. Since the radio free-free absorption affects the radio spectral index, we can distinguish these two classes of star formation, "active'' and "passive'', through the radio spectral index. This also means that the age is not the only factor that determines the spectral index; the gas density is also important in its determination.
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Figure 7: Same as Fig. 6 but for the "passive'' mode, with the I Zw 18 model as a representative case. |
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Here we examine the time evolution of radio spectra, adopting
the same parameters as used for SBS 0335-052 and I Zw 18, considering them
as representative cases for "active'' and "passive'',
respectively. Model G is adopted here, but the qualitative
behavior of the radio spectral index is not much affected
by the specific details of the adopted models.
We use
estimated for model G
in Tables 2 and
3 for the active and passive cases,
respectively. In Figs. 6 and
7, we present the radio SEDs calculated
by the models for SBS 0335-052 and I Zw 18, respectively.
The snapshots at t=3.5 Myr (soon after the first explosion
of SNe at
3 Myr), 5 Myr, 10 Myr, and 15 Myr are
shown. We observe that the free-free absorption around
GHz is prominent in the active case even at
Myr.
On the contrary, there is little absorption even at
Myr in the passive case. This indicates that
the absorption feature is not a simple indicator of
age as suggested by Cannon & Skillman (2004).
The density of the star-forming region is also important for
the spectral index around
GHz.
By comparing Figs. 6 and
7, we also see that the nonthermal
component becomes comparable to the thermal luminosity
earlier in the active case than in the passive case.
This mainly comes from the difference in time-integrated
radio luminosity of a SN (
).
Thus, we suggest that a nonthermal-dominated radio SED
is observed more often in active BCDs than in passive
BCDs. This is true if
correlates with gas density.
Observations compiled by Hunt & Reynolds (2006)
support this view.
Finally we show the time evolution of radio spectral index
.
The spectral index defined at
and
is calculated by the following
equation:
![]() |
(47) |
Another important consideration is that the radio spectrum
can be flat (i.e.,
)
even when the contribution from the nonthermal
component is significant. Indeed, at t=5 Myr, the
nonthermal and thermal
components are comparable in the
active mode (Fig. 6), and the
spectral slope at high frequency indicates the presence
of the nonthermal contribution. However, if we define
the spectral index by using
GHz and 5 GHz,
the spectral index becomes
0. This spectral
index could be misinterpreted as thermal. Thus,
in order to avoid such a misinterpretation, it is
important to derive a spectral index at
GHz. Our results are also applicable to high-z samples
(e.g., Oyabu et al. 2005) in the future as shown below.
![]() |
Figure 8:
Time evolution of the radio spectral
index defined at 1.5 GHz and 5 GHz
[
![]() |
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High-z primeval galaxies tend to have a high
density; they are formed and evolve in deep gravitational
potentials (e.g., Padmanabhan 1993).
Therefore, the gas in such primeval objects may
have high pressure, and the star-forming regions
may be expected to mimic the active mode
(Hirashita & Hunt 2004).
Indeed, the number density of gas in high-z galaxies
is estimated as 103 cm-3(Norman & Spaans 1997), and is similar
to that of SBS 0335-052, typical of the active class.
Therefore, the
nonthermal component could dominate
on short timescales after the onset of star formation
while the radio emission is optically thick at
GHz. The dense environment could also aid
amplification of magnetic fields as proposed by the
scaling relations in Sect. 4.3.
This paper also has implications for the production of high-energy electrons. It would be reasonable to consider that the production occurs on an expansion timescale SNRs, since the shocks associated with SNRs can be responsible for the acceleration of electrons (e.g., Axford 1981). The lifetime of nonthermal radiation per SN is determined by the minimum of two timescales: the lifetime of the shock and the synchrotron loss timescale.
In order to investigate the time evolution of the
radio SEDs of metal-poor star-forming galaxies, we have
constructed a radio SED model by
treating the thermal and nonthermal components
consistently with the star formation history.
In particular, the
duration and luminosity of the
nonthermal radio emission from supernova remnants
(SNRs) has been constrained by using the observational radio SEDs
of SBS 0335-052 and I Zw 18, both of which are reasonable proxies for
young galaxies
in the nearby universe. In SBS 0335-052, the typical radio energy emitted
per SNR over its lifetime is estimated to be
.
In
I Zw 18, another representative of metal-poor star-forming
galaxies, we find that
erg.
Both estimates are significantly larger than
previous estimates based on the
relation.
These values of the two "template'' galaxies can be
simultaneously explained by a simple density scaling
relation of
,
indicating that
the magnetic pressure scales with the gas pressure,
or that the magnetic fields are amplified
by SNe, or both.
We have also predicted the time dependence of the radio
spectral index and of the radio SED itself, for both
the active (SBS 0335-052) and passive (I Zw 18) cases.
These models enable us to roughly age date and classify
radio spectra of star-forming galaxies into active/passive classes.
Since the radio emission around GHz can be
affected by the free-free absorption especially in the
active class, the spectral index defined at
GHz should be used to estimate the contribution
from the nonthermal component. On the other hand,
the free-free absorption feature around
GHz
could be used to select the active class.
Acknowledgements
We thank the anonymous referee for useful comments which improved this paper considerably. We are grateful to T. Yoshida, D. Urosevic, Y. Kato, T. Kamae, and H. Kamaya for stimulating discussions on supernovae, high energy phenomena, and magnetic fields. H.H. has been supported by the University of Tsukuba Research Initiative and by Grants-in-Aid for Scientific Research of the Ministry of Education, Culture, Sports, Science and Technology (Nos. 18026002 and 18740097). This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration (NASA). We have relied upon NASA's Astrophysics Data System Abstract Service (ADS).
We derive some approximate analytic solutions for
expanding ionized regions based on
Eqs. (23) and (24).
By combining these two equations,
we obtain
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(A.4) |