Download Low radioderived star formation rates in z < 0.5 gammaray burst host

Mon. Not. R. Astron. Soc. 409, L74–L78 (2010)
doi:10.1111/j.1745-3933.2010.00951.x
Low radio-derived star formation rates in z < 0.5 gamma-ray burst host
galaxies
Elizabeth R. Stanway,1 Luke J. M. Davies1 and Andrew J. Levan2
1H
H Wills Physics Laboratory, Tyndall Avenue, Bristol BS8 1TL
of Physics, University of Warwick, Coventry CV4 7AL
2 Department
Accepted 2010 September 10. Received 2010 August 27; in original form 2010 July 6
ABSTRACT
We present 5.5- and 9.0-GHz observations of five gamma-ray burst host galaxies at z <
0.5, taken using the Australia Telescope Compact Array. We determine tight constraints on
the radio continuum flux of four sources (GRB hosts 060218, 060614, 020819 and 990712)
and detect a fifth source, the host of GRB 031203, with a flux density F ν (5.50 GHz) = 216 ±
50 µJy. We discuss the star formation rates of all five sources. Our radio-derived star formation
rates (and upper limits) are largely consistent with those derived from optical observations,
suggesting either that there is little dust-obscured star formation in these sources, or that their
starbursts are too young to have established representative radio continuum emission.
Key words: gamma-ray burst: individual: GRB 031203 – gamma-ray burst: individual:
GRB 060218 – gamma-ray burst: individual: GRB 060614 – gamma-ray burst: individual:
GRB 020819 – gamma-ray burst: individual: GRB 990712 – radio continuum: galaxies.
1 I N T RO D U C T I O N
Long-duration gamma-ray bursts (GRBs) are amongst the most energetic processes in the Universe. They have been identified as a key
tool for locating galaxies in the distant Universe and probing their
environments. GRBs at any redshift select star-forming galaxies
broadly independently of their mass, rather than suffering the bias
towards massive galaxies of flux-limited surveys. However, for this
very reason, the host galaxies of most GRBs are optically faint and
often require Hubble Space Telescope photometry for identification
and study (e.g. Fruchter et al. 2006).
Some tens of GRB host galaxies have now been subjected to detailed study, deriving properties including metallicity, stellar mass
and optical star formation rates (SFRs; Savaglio, Glazebrook &
LeBorgne 2009; Svensson et al. 2010). A smaller overlapping
sample has also been studied with the Spitzer Space Telescope,
further constraining their properties with mid-infrared photometry
(Le Floc’h et al. 2006). However, the subset of such galaxies with
follow-up at longer wavelengths remains remarkably small. While
a few sources have been subject to intense study (e.g. Kohno et al.
2005; Levesque et al. 2010b), only a handful of sources have been
studied at submillimetre wavelengths (Tanvir et al. 2004; Priddey
et al. 2006), and fewer still in radio continuum (Berger et al. 2003;
Michałowski et al. 2009). Multiwavelength observations are essential if the properties of the far-from-uniform host galaxy population
are to be understood.
Some of the most local GRBs are amongst the most difficult to
interpret, with both host-galaxy and afterglow properties of certain
E-mail: [email protected]
sources (e.g. the low-luminosity burst GRB 060218 or the ‘dark
burst’ GRB 020819) atypical of the bulk of the GRB population.
Understanding these nearby bursts, for which detailed data across a
wide range of wavelengths can be obtained, may be key to interpreting the GRBs seen at cosmological distances. One property of GRB
host galaxies directly measurable for nearby sources, but inaccessible in more distant galaxies, is the dust-obscured SFR as measured
by radio continuum flux. New broad-bandwidth correlators, such as
the 4-GHz bandwidth Compact Array Broadband Backend (CABB)
system recently installed at the Australia Telescope Compact Array (ATCA), now allow sensitive limits to be reached on the radio
continuum of even faint sources, directly probing the synchrotron
radiation arising from star formation.
In this Letter, we describe an investigation of the radio fluxes
and inferred SFRs of a sample of low-redshift, z < 0.5, GRB host
galaxies associated with bursts displaying a variety of properties,
and improve previous constraints on the radio-derived SFRs of GRB
host galaxies (e.g. Berger et al. 2003) by an order of magnitude.
We structure the Letter as follows. In Section 2 we describe our
observations and flux constraints on the host galaxies, before converting the radio flux to a SFR in Section 3. In Section 4 we discuss
the interpretation and implications of our results, before presenting
our conclusions in Section 5. We adopt a CDM cosmology with
( , M , h) = (0.7, 0.3, 0.7) throughout.
2 O B S E RVAT I O N S
We obtained continuum measurements of five GRB host galaxies
over the period 2010 January 26–29, using the ATCA in its most
extended 6A configuration. In this configuration, the six antennae
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2010 The Authors. Journal compilation Radio SFRs in GRB host galaxies
L75
Table 1. Observations table. Synthesized beam full width at half-maximum and root mean square
noise in the final image are shown for each source. No host galaxy was detected at 2σ in any of the
3-cm imaging. Fluxes or 2σ limits for an unresolved point source are shown as appropriate for the
GRB host galaxies at 6 cm.
Name
060218
031203
060614
020819
990712
z
Beam3 cm
(arcsec2 )
rms3 cm
(µJy beam−1 )
Beam6 cm
(arcsec2 )
rms6 cm
(µJy beam−1 )
GRB host S6 cm
(µJy)
0.034
0.105
0.125
0.410
0.433
4.2 × 0.5
1.8 × 0.8
0.84 × 0.79
8.1 × 1.6
1.0 × 0.6
14
16
14
46
43
14 × 1.5
5.9 × 2.3
3.1 × 2.4
37 × 1.7
3.5 × 2.0
39
37
33
11
12
<78 (2σ )
216 ± 50
<66 (2σ )
<22 (2σ )
<24 (2σ )
at the ATCA are aligned along an east–west axis and the longest
baselines are 6 km in length. We tuned the new CABB correlator such that one 2-GHz IF was centred at 5500 MHz (6 cm) and
the second at 9000 MHz (3 cm), with full polarization information collected simultaneously at both frequencies. Measurements
of PKS 1934−638, the standard calibrator at the ATCA, were used
for absolute flux calibration. Our observations were associated with
observing programme C2151 (PI: Stanway).
Data were reduced using the dedicated software package MIRIAD
and radio frequency interference carefully flagged on a channel-bychannel basis. Each frequency band had 2048 1-MHz-wide spectral
channels allowing interference to be restricted to a few distinct
channels. This was particularly important at 3 cm where powerful
interference spikes occurred frequently in certain narrow frequency
ranges. Multifrequency synthesis images were constructed from
the 2-GHz bandwidth at each frequency. In each image a number of
additional sources (often NVSS and occasionally 2MASS objects)
were detected and the images were cleaned using a Clark algorithm.
Uniform weighting was used to optimize suppression of sidelobes
in the imaging. In order to probe interesting SFRs across a range of
redshifts, final image depth and synthesized beam size varied from
target to target as shown in Table 1. Several of the GRB hosts observed are northern targets for the ATCA and, as a result, only short
tracks were possible on these sources. Several short exposures were
taken across the available range of hour angles, allowing formation
of the best possible synthesized beam in the imaging data. Despite
that, in the case of GRB 020819B the synthesized beam was highly
extended at a position angle close to zero.
Our results on each of our five target galaxies are summarized in
Table 1 and Fig. 1. Nothing was seen in the 9000 MHz/3 cm images
at the location of any of the GRB host galaxies. We are thus able
to place a non-detection constraint on each source based on the
image rms noise. Of the five GRB hosts, four were also undetected
at 5500 MHz/6 cm, and again we consider 2σ non-detection constraints in the discussion that follows. The fifth galaxy – the host of
GRB 031203/SN 2003lw – is detected as an unresolved point source
at just over 4σ in the imaging, with F ν (5.50 GHz) = 216 ± 50 µJy.
3 RADIO-DERIVED SFRs
Radio continuum flux is a tracer of star formation, correlating well
with far-infrared flux for established stellar populations older than
about 100 Myr (see Bressan, Silva & Granato 2002, for detailed
discussion). Any GRB host galaxy with substantial, ongoing star
formation might be expected to be luminous in the radio, even if
dust obscures star formation indicators in the optical wavebands.
C
C 2010 RAS, MNRAS 409, L74–L78
2010 The Authors. Journal compilation Figure 1. Our constraints on the 6 cm/5500 MHz flux of five z < 0.5 GRB
host galaxies. Contours show the radio-derived star formation rate at a given
flux as a function of redshift (calculated from equation 1) and are labelled
in units of M yr−1 .
We convert our measured radio fluxes and limits into SFRs using
the same prescription as Berger et al. (2003) and Yun & Carilli
(2002):
25fnth ν0α + 0.71 ν0−0.1
Sν (νobs ) =
−6
+ 1.3 × 10
×
ν03
1 − exp[−(ν0 /2000)β ]
exp[(0.048/Td )ν0 ] − 1
(1 + z) SFR
Jy.
dL2
(1)
where SFR is in M yr−1 , the rest-frame frequency ν 0 = (1 +
z)ν obs GHz, dL is the luminosity distance to the source in megaparsecs, α is the radio frequency spectral slope and f nth is a scaling
factor (set equal to unity) to account for normalization of nonthermal synchrotron emission in starburst galaxies (see Yun &
Carilli 2002). Following Berger et al., we set α = −0.6, appropriate for faint radio sources. Using a steeper spectral slope, α =
−0.75, results in SFR estimates 25 per cent higher but does not
significantly affect our conclusions. T d = 58 K and β = 1.35 are,
respectively, the dust temperature and emissivity index (again fixed
to the values of Yun & Carilli 2002, for comparison with previous
studies).
The resultant constraints on the radio-derived SFRs of our target
sources are shown in Table 2 and compared to the SFRs derived
for the same sources from recent analyses of ultraviolet and optical
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E. R. Stanway, L. J. M. Davies and A. J. Levan
study. Svensson et al. estimate a very low SFR (0.44 M yr−1 ) from
fitting the spectral energy distribution, while Levesque et al. calculate SFR = 4.8 M yr−1 from Hα emission. By contrast Savaglio
et al., Margutti et al. (2007) and Prochaska et al. (2004) find higher
values (SFR = 13, 13 and 11 M yr−1 , respectively)
Our radio detection of this source yields SFRRad = 4.8+1.4
−0.9 : rather
low compared to some optical estimates. Our spectral index conGHz
straint (α 9.0
5.5 GHz < 0.8, 2σ ) is too weak to comment on AGN activity.
However, given the findings of Levesque et al. (2010a), it is possible that AGN activity is contributing an appreciable fraction of the
ultraviolet–optical flux and flattening the radio frequency spectral
slope, invalidating the SFR–flux conversion relations used. At z =
0.11, the synthesized beam in our imaging corresponds to a physical
scale of 11.5 × 4.5 kpc and the host galaxy of GRB 031203 appears
as an unresolved point source.
Table 2. Star formation rates and limits derived from our 5500 MHz/6 cm
fluxes using equation (1). For comparison, the star formation rates derived
from optical continuum and emission-line studies by Savaglio et al. (2009),
Svensson et al. (2010) and Levesque et al. (2010a) are shown in columns
SFR1opt , SFR2opt and SFR3opt , respectively. Star formation rates are in units of
M yr−1 , and 2σ limits are shown.
Name
060218
031203
060614
020819
990712
a For
z
SFRRadio
SFR1opt
SFR2opt
SFR3opt
0.034
0.105
0.125
0.410
0.433
<0.18
4.8+1.4
−0.9
<2.1
<8.0
<10.1
0.05
12.7
0.01
6.86
2.39
0.05
0.44
–
14.5
1.07
0.03
4.8
–
10.2a
10.7
the ‘explosion site’ (Levesque et al. 2010b).
4.1.3 The host galaxy of GRB 060614
GRB 060614 was a burst with no associated optical supernova
(Della Valle et al. 2006; Fynbo et al. 2006). Its classification as
a long-duration GRB is still under discussion (see Gehrels et al.
2006; Caito et al. 2009). As such, the source is sometimes excluded
from follow-up studies of long GRBs. However, Savaglio et al.
(2009) adopt a very low SFR of 0.01 M yr−1 for this source based
on its Hα flux, while Fynbo et al. (2006) inferred a similar SFR =
0.014 M yr−1 based on hydrogen and oxygen emission lines. A
‘disguised’ short burst (e.g. Caito et al. 2009 suggest an origin in
the coalescence of binary neutron stars or black holes to explain the
burst duration) may explain the low estimates of ongoing star formation in the host galaxy. Our constraints on this z = 0.125 source
are comparatively weak. We find SFRradio < 210 SFRopt .
Figure 2. A comparison of our radio-inferred star formation rates (shown as
2σ limits or as a point with error bars as appropriate) with those derived from
UV and optical data by other authors. The star formation rate estimates of
Savaglio et al. (2009) are shown by diamonds, those of Svensson et al. (2010)
by triangles and those of Levesque et al. (2010a,b) by squares. GRB 090425
(Michałowski et al. 2009) is included for comparison.
4.1.4 The host galaxy of GRB 020819
GRB 020819 was a dark burst with no associated optical afterglow.
Its host galaxy was identified by Jakobsson et al. (2005) as a type
Scd spiral galaxy at z = 0.41. SFR estimates for this source have
been rather higher than those typical of other low-redshift bursts.
Savaglio et al. find SFR = 6.9 M yr−1 based on rest-frame ultraviolet continuum flux while Svensson et al. determine SFR =
14.5 M yr−1 from SED fitting. Levesque et al. (2010b) took Keck
spectroscopy and made a detailed study of this source determining
a peak SFR of 23.6 M yr−1 in the galaxy nucleus, and a rather
lower 10.2 M yr−1 at the burst location, based on measurements
of Hα line emission. We note that both Levesque et al. (2010b) and
Svensson et al. (2010) assign this source a rather high, supersolar
metallicity [12 + log (O/H) ∼ 9.0] which, together with its nature
as a dark burst, implies that it is atypical of the population. Our
radio observations find a surprisingly tight non-detection constraint
on this source, SFRradio < 8 M yr−1 , comparable with or lower
than most optically derived estimates.
data by Savaglio et al. (2009), Svensson et al. (2010) and Levesque
et al. (2010a,b) in Fig. 2.
4 DISCUSSION
4.1 Notes on individual host galaxies
4.1.1 The host galaxy of GRB 060218
GRB 060218 is associated with supernova SN2006aj and had a
Wolf–Rayet star progenitor implying recent massive star formation
in its host galaxy (Campana et al. 2006; Pian et al. 2006). However,
the host galaxy is extremely faint in the optical and SFR estimates
for this source, based on Hα emission-line flux, have been consistently very low (see Table 2). Wiersema et al. (2007) determined
a similarly low SFR(Hα) = 0.065 M yr−1 . Taking a typical optically derived SFR estimate for this source of 0.05 M yr−1 , our
radio observations constrain the obscured SFR in this source to
SFRradio < 4 SFRopt .
4.1.5 The host galaxy of GRB 990712
GRB 090712 is a pre-Swift burst, originally detected using BeppoSAX. The host galaxy shows two distinct optical components in
what may be an interacting system (Christensen et al. 2004a). Optically derived star formation estimates for this galaxy fall into two
camps. Christensen et al. determined SFRs of 1.3 M yr−1 from
the rest-frame ultraviolet continuum and 2.8 M yr−1 from Hα line
emission. Savaglio et al. and Svensson et al. also determine low
4.1.2 The host galaxy of GRB 031203
The host galaxy of GRB 031203 is the only known long GRB host
with evidence for AGN activity in its optical spectrum (Levesque
et al. 2010a). Estimates of its SFR differ considerably from study to
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2010 The Authors. Journal compilation Radio SFRs in GRB host galaxies
SFRs (2.4 and 1.1 M yr−1 , respectively). However, measurements
of [O II] emission-line flux have yielded higher estimates of ∼10–
11 M yr−1 (Küpcü Yoldaş, Greiner & Perna 2006; Levesque et al.
2010a).
Our upper limit, derived from 6-cm radio flux, is SFRradio <
10 M yr−1 (2σ ). This challenges the higher set of SFR estimates,
and suggest that [O II] line flux may be an unreliable SFR indicator,
perhaps because of the uncertainty introduced by extinction estimates. Adopting 1–2 M yr−1 as a typical optical–SFR estimate,
we determine SFRradio < 5–10 SFRopt .
4.2 Radio properties of GRB host galaxies
Berger et al. (2003) studied 20 GRB host galaxies at 0.7 < z <
4.5, 12 of them at 8.46 GHz with the VLA and four at 1.39 GHz at
the ATCA, with the remainder observed only in the submillimetre.
Although the majority of his sample yielded non-detections, he
estimated that 20 per cent of long GRBs were hosted by luminous
galaxies with SFR > 500 M yr−1 , while a stacking analysis on
the remainder of the sample gave an ensemble average SFR of
100 M yr−1 . Crucially, he found that the radio-/submillimetrederived SFR in these sources was in poor agreement with optically
derived SFRs. If this is true, then the majority of work to date is
underestimating the contribution of optically faint galaxies akin to
GRB hosts to the volume-averaged cosmic star formation history.
However, this finding has been disputed. Unlike submillimetre
galaxies, which appear red in the optical, GRB host galaxies (even
those with submillimetre and radio detections) are typically very
blue (Michałowski et al. 2008). Based on observations at ≤24 µm,
Le Floc’h et al. (2006) have suggested that their sample of GRB
hosts at 0.8 < z < 3.4 is more consistent with typical SFRs of tens of
solar masses per year, similar to their optically derived SFRs, rather
than the 100 M yr−1 suggested by Berger et al. Observations of
long GRB host galaxies at 850 µm using SCUBA and 1.4 mm using
MAMBO have produced similar constraints, with a typical 850 µm
flux density of 0.58 ± 0.36 mJy implying SFRs not exceeding a few
tens of solar masses per year given reasonable assumptions for the
typical redshift (Tanvir et al. 2004; Priddey et al. 2006).
While follow-up studies in the radio have been largely absent
to date, a radio-inferred SFR consistent with estimates based on
infrared, optical and ultraviolet indicators has been found for a lowredshift burst, GRB 090425 at z = 0.009 (shown for comparison in
Fig. 2), by Michałowski et al. (2009) .
No GRB host galaxy in our sample has a SFR exceeding
15 M yr−1 , even allowing for slight uncertainty in the choice of
radio spectral slope α as mentioned above. In at least one example,
GRB 020819, our radio-derived SFR is lower than that already estimated from optical spectroscopy, which suggests that either the
optical measurements are overestimating the SFR or that the radio
flux is yielding an underestimate. Radio continuum flux is relatively insensitive to instantaneous starbursts, taking of the order of
108 yr to become fully representative of ongoing star formation (see
extensive discussion in Bressan et al. 2002). This is comparable
to the ages derived for GRB hosts by some studies (Christensen,
Hjorth & Gorosabel 2004b), although others derive somewhat older
(e.g. Savaglio et al. 2009) or younger (e.g. Levesque et al. 2010a)
ages. If, as theory suggests, GRB progenitors are typically >20 M
stars (e.g. Larsson et al. 2007), then it may not be uncommon for
bursts to occur at the beginning of a starburst, before its continuum
flux becomes well established. As a result, the radio SFRs given in
Table 2 may be underestimating the star formation contribution from
very young starbursts that are detected in the rest-frame ultraviolet.
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None the less, our results suggest that the established, obscured star
formation in these sources cannot exceed the optically determined
SFRs by factors of more than about 5–10 (with a rather weaker constraint on the host of the anomalously sub-luminous GRB 060614,
see Section 4.1).
Given a sample of five galaxies, the absence of an intensely starbursting source, seen in 20 per cent of GRB hosts at radio and
submillimetre wavelengths (e.g. Berger et al. 2003; Smith et al.
2005), is unremarkable. None the less, even excluding the small
sample of submillimetre-luminous host galaxies, an apparent discrepancy exists between the results of this study and the conclusions
of Berger et al. (2003) regarding the typical SFR in a stacked sample
of radio-faint GRB hosts.
The reasons for this are not clear. One possibly may be the presence of high-SFR starbursts lying just below the individual detection limit in Berger et al. (2003). A few intense starbursts each with
an individual SFR of ∼300 M yr−1 may skew the average flux
measured in a radio stack. An alternative explanation may lie in
a difference between the selections applied by the earlier analysis
and that presented here. The GRB hosts in this study were selected
to lie at low redshifts and it is conceivable that evolution exists in
the properties of host galaxies between z < 0.5 and z > 1, with
higher redshift sources more likely to have a mature, dust-shrouded
star-forming population.
In the low-redshift Universe most star formation occurs in the
lower mass, lower SFR systems (e.g. Heavens et al. 2004), i.e.
at high redshifts a greater fraction of the total SFR is in massive
galaxies. So if the GRBs sample all star formation, a higher redshift
survey might naturally be expected to find more obscured examples. Studies of GRB hosts (e.g. Svensson et al. 2010, and others
mentioned above) find no evidence for a redshift evolution in their
optical properties. GRBs appear to occupy similar low-mass lowmetallicity star-forming galaxies at every redshift. However, if the
dust present in more massive, higher redshift systems leads to an
increased fraction of obscured star formation then it is possible that
an evolution in the optical/UV properties of the hosts would not be
apparent, simply because they show only the unobscured tip of the
iceberg.
While our sample is deliberately constrained to the lowest redshift
bursts, the Berger et al. study was unrestricted in redshift, but rather
constrained by the existing sample of known GRB host galaxies at
the time – itself biased towards relatively luminous host galaxies
and afterglows that persisted for a sufficient duration to allow the
bursts to be localized in the pre-Swift era, which may introduce
an unanticipated selection effect. A sizeable fraction of the Berger
et al. (2003) sample (∼1/3) were observed within 18 months of the
initial burst and in some cases less than a year after the GRB. In
these cases the contribution of GRB afterglow flux to the sensitive
observations used by the study may have been underestimated.
However, it is not clear that these effects are sufficient to explain
the inconsistency between the 2003 study and that presented here.
Clearly further observations will be required to explore the redshift
evolution of radio properties in GRB host galaxies and to obtain data
for a statistical sample of these important cosmological probes.
5 CONCLUSIONS
Using the ATCA, we have obtained deep continuum measurements
of five z < 0.5 long GRB host galaxies at 5.5 and 9.0 GHz (6 and
3 cm) in order to constrain the radio-inferred SFRs in these sources.
Our main conclusions can be summarized as follows.
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E. R. Stanway, L. J. M. Davies and A. J. Levan
(i) We detect radio emission from the host of GRB 031203 with
F ν (5.50 GHz) = 216 ± 50 µJy. We do not detect the hosts of GRBs
060218, 060614, 020819 or 990712.
(ii) No GRB host galaxy in our sample has a radio-inferred SFR
exceeding 15 M yr−1 . The limits on SFRs on individual sources
(or calculated value in the case of GRB 031203) are consistent with
their optically derived SFRs, suggesting that there is little dustobscured star formation in these sources.
(iii) This result is consistent with estimates from Spitzer and submillimetre observations and with radio continuum observations of
GRB 980425 by Michałowski et al. (2009), but appears to contradict the high radio-derived SFRs found by Berger et al. (2003). The
reasons for this contradiction are unclear and are likely to remain so
until additional data allow the redshift evolution of GRB host radio
properties to be determined.
(iv) In at least one case, GRB 020819, our non-detection limit
suggests a SFR lower than those derived in the optical. This may
indicate that radio flux underestimates the true SFR due to its insensitivity to the very young starbursts typical of GRB host galaxies.
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AC K N OW L E D G M E N T S
ERS and LJMD gratefully acknowledge support from the UK Science and Technology Facilities Council. Based on data obtained at
the ATCA as part of programme C2151. The Australia Telescope
Compact Array is part of the Australia Telescope which is funded by
the Commonwealth of Australia for operation as a National Facility
managed by CSIRO. We also thank Shari Breen for her assistance
as Duty Astronomer during our observations.
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This paper has been typeset from a TEX/LATEX file prepared by the author.
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2010 The Authors. Journal compilation