Low radio-derived star formation rates in z < 0.5 gamma-ray burst host galaxies

Transcription

1 Mon. Not. R. Astron. Soc. 409, L74 L78 (2010) doi: /j x Low radio-derived star formation rates in z < 0.5 gamma-ray burst host galaxies Elizabeth R. Stanway, 1 LukeJ.M.Davies 1 and Andrew J. Levan 2 1 H H Wills Physics Laboratory, Tyndall Avenue, Bristol BS8 1TL 2 Department of Physics, University of Warwick, Coventry CV4 7AL Accepted 2010 September 10. Received 2010 August 27; in original form 2010 July 6 1 INTRODUCTION 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.r.stanway@bristol.ac.uk 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 , , and ) and detect a fifth source, the host of GRB , 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 gamma-ray burst: individual: GRB gamma-ray burst: individual: GRB gamma-ray burst: individual: GRB gamma-ray burst: individual: GRB radio continuum: galaxies. sources (e.g. the low-luminosity burst GRB or the dark burst GRB ) 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 OBSERVATIONS 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 C 2010 The Authors. Journal compilation C 2010 RAS

2 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 z Beam 3cm rms 3cm Beam 6cm rms 6cm GRB host S 6cm (arcsec 2 ) (µjy beam 1 ) (arcsec 2 ) (µjy beam 1 ) (µjy) <78 (2σ ) ± <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 , 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 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 B 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 /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. 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): { S ν (ν obs ) = 25f nth ν α ν } ν 3 1 exp[ (ν 0 /2000) β ] 0 exp[(0.048/t d )ν 0 ] 1 (1 + z) SFR Jy. (1) dl 2 where SFR is in M yr 1, the rest-frame frequency ν 0 = (1 + z)ν obs GHz, d L 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

3 L76 E. R. Stanway, L. J. M. Davies and A. J. Levan 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 SFR 1 opt, SFR2 opt and SFR3 opt, respectively. Star formation rates are in units of M yr 1,and2σ limits are shown. Name z SFR Radio SFR 1 opt SFR 2 opt SFR 3 opt < < < a < a For the explosion site (Levesque et al. 2010b). 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 (Michałowski et al. 2009) is included for comparison. data by Savaglio et al. (2009), Svensson et al. (2010) and Levesque et al. (2010a,b) in Fig DISCUSSION 4.1 Notes on individual host galaxies The host galaxy of GRB GRB 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) = 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 SFR radio < 4 SFR opt The host galaxy of GRB The host galaxy of GRB 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 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 SFR Rad = :rather low compared to some optical estimates. Our spectral index constraint (α5.5 GHz 9.0 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 kpc and the host galaxy of GRB appears as an unresolved point source The host galaxy of GRB GRB 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 = M yr 1 based on hydrogen and oxygen emission lines. A disguised short burst (e.g. Caito et al 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 = source are comparatively weak. We find SFR radio < 210 SFR opt The host galaxy of GRB GRB 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 = 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, SFR radio < 8M yr 1, comparable with or lower than most optically derived estimates The host galaxy of GRB GRB 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 Radio SFRs in GRB host galaxies L77 SFRs (2.4 and 1.1 M yr 1, respectively). However, measurements of [O II] emission-line flux have yielded higher estimates of M yr 1 (Küpcü Yoldaş, Greiner & Perna 2006; Levesque et al. 2010a). Our upper limit, derived from 6-cm radio flux, is SFR radio < 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 SFR radio < 5 10 SFR opt. 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 at z = (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 , 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 10 8 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. 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 , 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.

5 L78 E. R. Stanway, L. J. M. Davies and A. J. Levan (i) We detect radio emission from the host of GRB with F ν (5.50 GHz) = 216 ± 50 µjy. We do not detect the hosts of GRBs , , or (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 ) 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 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 , 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. ACKNOWLEDGMENTS 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. REFERENCES Berger E. et al., 2003, ApJ, 588, 99 Bressan A., Silva L., Granato G. L., 2002, A&A, 392, 377 Caito L. et al., 2009, A&A, 498, 501 Campana S. et al., 2006, Nat, 442, 1008 Christensen L. et al., 2004a, A&A, 413, 121 Christensen L., Hjorth J., Gorosabel J., 2004b, A&A, 425, 913 Della Valle M. et al., 2006, Nat, 444, 1050 Fruchter A. S. et al., 2006, Nat, 441, 463 Fynbo J. P. U. et al., 2006, Nat, 444, 1047 Gehrels N. et al., 2006, Nat, 444, 1044 Heavens A., Panter B., Jimenez R., Dunlop J., 2004, Nat, 428, 625 Jakobsson P. et al., 2005, ApJ, 629, 45 Kohno K. et al., 2005, PASJ, 57, 147 Küpcü Yoldaş A., Greiner J., Perna R., 2006, A&A, 457, 115 Larsson J., Levan A. J., Davies M. B., Fruchter A. S., 2007, MNRAS, 376, 1285 Le Floc h E. et al., 2006, ApJ, 642, 636 Levesque E. M., Berger E., Kewley L. J., Bagley M. M., 2010a, AJ, 139, 694 Levesque E. M., Kewley L. J., Graham J. F., Fruchter A. S., 2010b, ApJ, 712, L26 Margutti R. et al., 2007, A&A, 474, 815 Michałowski M. J., Hjorth J., Castro Cerón J. M., Watson D., 2008, ApJ, 672, 817 Michałowski M. J. et al., 2009, ApJ, 693, 347 Pian E. et al., 2006, Nat, 442, 1011 Priddey R. S. et al., 2006, MNRAS, 369, 1189 Prochaska J. X. et al., 2004, ApJ, 611, 200 Savaglio S., Glazebrook K., LeBorgne D., 2009, ApJ, 691, 182 Smith I. A. et al., 2005, A&A, 439, 987 Svensson K. M. et al., 2010, MNRAS, 405, 57 Tanvir N. R. et al., 2004, MNRAS, 352, 1073 Wiersema K. et al., 2007, A&A, 464, 529 Yun M. S., Carilli C. L., 2002, ApJ, 568, 88 This paper has been typeset from a TEX/LATEX file prepared by the author.