The afterglow and host galaxy of GRB 090205: evidence for a Ly-alpha emitter at z=4.65
P. D'Avanzo, M. Perri, D. Fugazza, R. Salvaterra, G. Chincarini, R. Margutti, X. F. Wu, C. C. Thoene, A. Fernandez-Soto, T. N. Ukwatta, D. N. Burrows, N. Gehrels, P. Meszaros, K. Toma, B. Zhang, S. Covino, S. Campana, V. D'Elia, M. Della Valle, S. Piranomonte
aa r X i v : . [ a s t r o - ph . H E ] J un Astronomy&Astrophysicsmanuscript no. GRB090205˙AA˙accepted c (cid:13)
ESO 2018November 13, 2018
The afterglow and host galaxy of GRB 090205: evidence for a Ly − α emitter at z = . ⋆ P. D’Avanzo , M. Perri , D. Fugazza , R. Salvaterra , G. Chincarini , , R. Margutti , , X. F. Wu , , , C. C. Th¨one , A.Fern´andez-Soto , T. N. Ukwatta , , D. N. Burrows , N. Gehrels , P. Meszaros , , , K. Toma , , B. Zhang , S.Covino , S. Campana , V. D’Elia , , M. Della Valle , , and S. Piranomonte INAF-Osservatorio Astronomico di Brera, via Bianchi 46, I-23807, Merate, Italy. ASI Science Data Center, via Galileo Galilei, I-00044, Frascati, Italy. Universit`a degli Studi dell’Insubria, Dipartimento di Fisica e Matematica, via Valleggio 11, I-22100, Como, Italy. Universit`a degli Studi di Milano-Bicocca, Dipartimento di Fisica, piazza delle Scienze 3, I-20126, Milano, Italy. Department of Astronomy and Astrophysics, Pensylvania State University, 525 Davey Lab, University Park, PA 16802, USA. Center for Particle Astrophysics, Pensylvania State University. Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China. Instituto de F´ısica de Cantabria (CSIC-UC), 39005, Santander, Spain. The George Washington University, Washington, D.C., 20052, USA. NASA / Goddard Space Flight Center, Greenbelt, MD 20771, USA. Department of Physics, Pensylvania State University. Department of Physics and Astronomy, University of Nevada, Las Vegas, NV 89154, USA. INAF-Osservatorio Astronomico di Roma, via di Frascati 33, I-00040, Monteporzio Catone (Roma), Italy. INAF-Osservatorio Astronomico di Capodimonte, salita Moiariello 16, I-80131 Napoli, Italy. International Center for Relativistic Astrophysics, piazza della Repubblica 10, I-65122, Pescara, Italy.Received; accepted
ABSTRACT
Aims.
Gamma-ray bursts have been proved to be detectable up to distances much larger than any other astrophysical object, providingthe most e ff ective way, complementary to ordinary surveys, to study the high redshift universe. To this end, we present here the resultsof an observational campaign devoted to the study of the high − z GRB 090205.
Methods.
We carried out optical / NIR spectroscopy and imaging of GRB 090205 with the ESO-VLT starting from hours after theevent up to several days later to detect the host galaxy. We compared the results obtained from our optical / NIR observations with theavailable
Swift high-energy data of this burst.
Results.
Our observational campaign led to the detection of the optical afterglow and host galaxy of GRB 090205 and to the firstmeasure of its redshift, z = .
65. Similar to other, recent high- z GRBs, GRB 090205 has a short duration in the rest-frame with T , rf = . ∼
150 Myr. Moreover, the metallicity of Z > . Z ⊙ derived from the GRB afterglow spectrum is among the highest derived fromGRB afterglow measurement at high- z , suggesting that the burst occured in a rather enriched envirorment. Finally, a detailed analysisof the afterglow spectrum shows the existence of a line corresponding to Lyman- α emission at the redshift of the burst. GRB 090205is thus hosted in a typical Lyman- α emitter (LAE) at z = .
65. This makes the GRB 090205 host the farthest GRB host galaxy,spectroscopically confirmed, detected to date.
Key words. gamma ray: bursts - gamma ray: individual GRB090205
1. Introduction
Gamma-Ray Bursts (GRBs) are powerful flashes of high-energyphotons occuring at an average rate of a few per day through-out the Universe. Thanks to their optical brightness that typi-cally overshines the luminosity of their host galaxy, they are de-tectable up to extremely high redshift, as clearly shown by the re-cent detection of GRB 090423 at z ∼ . Send o ff print requests to : P. D’Avanzo, [email protected] ⋆ The results reported in this paper are based on observations car-ried out at ESO telescopes under programmes Id 082.A-031 and 283.D-5033. reionization epoch. Indeed, GRBs can be used to identify high- z galaxies and study their metal and dust content through the iden-tification of metal absorption lines in their optical afterglow.Two classes of GRBs, short and long, have been identifiedon the bases of their observed duration (shorter or longer than ∼ Swift satellite has questioned thissimple scheme calling for a classification invoking multiple ob-servational criteria (see Zhang et al. 2009). To this end, promptemission properties like the isotropic gamma-ray energy release( E γ, iso ) and the peak energy ( E p ) seem to provide a promis-ing tool for GRB’s classification, as shown by the E p , i − E γ, iso correlation (Amati et al. 2008) and its derivations (see, e.g.,Lv et al. 2010 and references therein). While it is widely be-
1. D’Avanzo et al.: The afterglow and host galaxy of GRB 090205: evidence for a Ly − α emitter at z = . lieved that the majority of long GRBs originate from the col-lapse of massive stars, the nature of the progenitors of shortones is still unclear, though likely linked to the merger of twocompact objects. Long GRBs are typically found to be hosted inlow-mass, blue galaxies with high specific star formation rates(SSFR), whereas short GRBs are generally hosted in more het-erogeneous types of galaxies, at least some with lower SSFR(see e.g. Fruchter et al. 2006; Berger 2009; Savaglio et al.2009;Fong et al. 2010).In this paper we report the detection of GRB 090205 at z = .
65 and the study of the properties of its host galaxy, a youngstarburst. The paper is organized as follows. In Section 2, wereport the detection of GRB 090205 by
Swift (Section 2.1) andthe discovery and study of its optical afterglow and of its hostgalaxy (Section 2.2). The discussion about the nature of the burstis given in Section 3.1, whereas the interpretation of its afterglowis reported in Section 3.2. In Section 3.3, we discuss the natureof the host galaxy of GRB 090502 and finally, we summarizebriefly our main conclusions in Section 4.The standard cosmological parameters ( h = . Ω m = . Ω Λ = .
73) have been assumed and magnitudes are givenin the AB system. All errors are at the 90% confidence level,unless stated otherwise.
2. Observations
GRB 090205 triggered
Swift -BAT (Perri et al. 2009) on Feb.2009, 5 th at 23:03:14 UT (hereafter, T ). The mask-weightedlight curve shows a single peak starting at T − T + T +
100 s (Fig. 1). Theduration of the prompt emission is T = . ± . T − . T + .
6s in the 15-150 keV band can be fit by a simple power-law modelwith photon index
Γ = . ± .
23. Alternatively, an equallygood fit can be obtained by a cut-o ff power-law model with pho-ton index a = . ± . E p = ± ∼
30 keV is also found from the rela-tion between E p and Γ obtained by Sakamoto et al. (2009). Thefluence in the 15–150 keV band is F γ = (1 . ± . × − ergcm − and the 1-s peak photon flux measured from T + .
09 s inthe 15-150 keV band is P = . ± . − s − (Cummingset al. 2009). Following the method described in Ukwatta et al.(2010), we performed the spectral lag analysis of the BAT datafrom T −
20 s to T +
20 s in four energy bands (12–25 keV,25–50 keV, 50–100 keV, 100–350 keV) with a time bin of 1024ms. All lags are consistent with zero, but with relatively largeuncertainties, given the faintness of the prompt emission.
Swift -XRT began to observe the field of GRB 090205 at ∼
89 s af-ter the trigger, identifying a fading uncatalogued X-ray sourcelocated at the UVOT-enhanced position RA (J2000): 14h 43m38.69s and Dec (J2000): -27d 51 ′ ′′ with an uncertainty of1.8 ′′ (radius, 90% confidence, Evans et al. 2009). Swift -UVOTbegan settled observations of the field of GRB 090205 92 s af-ter the BAT trigger, but no source was identified at the enhancedSwift XRT position. The burst was declared a “burst of interest”by Gehrels & Perri (2009).
The XRT data were processed with the XRTDAS software pack-age (v.2.5.0) developed at the ASI Science Data Center (ASDC)and distributed by HEASARC within the HEASOFT package
Fig. 1.
Four channels and combined BAT mask-weighted lightcurve of GRB 090205. Bin size is 1024 ms.(v. 6.7). Event files were calibrated and cleaned with standardfiltering criteria with the xrtpipeline task using the latest cal-ibration files available in the Swift CALDB. The X-ray lightcurve (Fig. 2) shows a complex behavior. At ∼ T + ∼ T + ∼ T +
20 ks, respectively. A fit with a dou-ble broken power-law ( F ( t ) ∝ t − α ) gives indices α = . + . − . for t < T + t b , , α = − . + . − . for T + t b , < t < T + t b , ,and α = . + . − . for t > T + t b , (excluding the flaring activ-ity), where t b , = + − s and t b , = + − s. We performeda time resolved spectral analysis of the X-ray afterglow dur-ing the first Swift orbit (spanning from T + T +
2. D’Avanzo et al.: The afterglow and host galaxy of GRB 090205: evidence for a Ly − α emitter at z = .
100 1000 10 − − . . C oun t R a t e ( . − . k e V ) ( s − ) time since BAT trigger (s) Fig. 2.
X-ray light curve in count rate. The solid line showsthe best power-law fit obtained excluding the flaring activity(marked by open circles) present at ∼ T + ∼ T + − = . × − erg cm − s − . Errors are at 68% c.l.ks) in three di ff erent time intervals: (i) t < < < t < Γ X ∼ . Γ X , = . ± . , Γ X , = . ± . , Γ X , = . ± . N H ∼ × cm − , i.e. no spectral evolution isobserved during the first orbit, although the data are compatiblewith a gradual softening of the spectrum. No evidence of intrin-sic absorption at the redshift of the burst is found. The 2(3)- σ upper limit is N H , z < . . × cm − . These limits are ob-tained using the entire XRT dataset (i.e. using T +
100 s – T + T + T + Γ X = . ± .
21 andGalactic N H ∼ × cm − , while for the second time interval(from T + T + Γ X = . ± .
35. We note, however, thatduring this flaring activity the photon index values are consistent(at the 90% CL) with the values measured at earlier times.
A complete log of all our optical / NIR ground based observationsis reported in Tab. 1.
We observed the field of GRB 090205 with the ESO-VLTin imaging mode starting about 7.1 hours after the burst.Observations were carried out in R and I − band with the FORS1camera. Within the enhanced X-ray position, we identifieda source at the following coordinates: RA(J2000) =
14h 43m38.70s and Dec(J2000) = -27d 51 ′ ′′ with an uncertainty of0.3 ′′ (D’Avanzo et al. 2009a). The source is detected in bothbands with R AB = . ± .
04 and I AB = . ± .
02, thered color suggesting a high redshift object. All values given hereare not corrected for Galactic extinction of E ( B − V ) = . AB m ag time from burst (d) Fig. 3.
Optical R − ( dots ) and I − band ( squares ) afterglow lightcurve. The solid line shows the best power-law fit. Magnitudeare in AB system and are not corrected for Galactic extinction.Errors are at 68% c.l.the object is very well detected in the i ′ -band, marginally in the r ′ -band and not in the g ′ -band. Interpreting the non detection inthe g ′ -band and the large r ′ − i ′ color as due to Lyman- α absorp-tion in the GRB host, a photometric redshift of 4 . ± . ∼ .
34 days after the trigger in the R − band (D’Avanzoet al. 2009b). The source is detected with R AB = . ± . R and, possibly, in the I − band light curves (see Sect 2.2.3).Assuming a power-law decay F ( t ) = F + kt − α , the decay indexis α opt = . + . − . , steeper (but consistent within the errors) thanthe decay index derived from the X-ray light curve at the sameepochs. A plot of our R − and I − band observations is shown inFig. 3. We observed the source with the ESO VLT about 9.0 hours afterthe burst with the FORS1 camera in spectroscopic mode. Wetook a 20 min spectrum with the 300V grism (11 Å FWHM)using a slit with 1 ′′ width. We covered wavelength range 4000-10000 Å with a resolution of R =
440 (Th¨one et al. 2009; Fugazzaet al. 2009). The spectrum was reduced with standard tasks inIRAF, combined and flux calibrated using observations of thestandard star EG274 taken on Feb. 7, 2009.The most prominent feature in the spectrum is the DampedLyman Alpha system (DLA) at 6873 Å in the observer frame.Furthermore, we detect Ly − β and Ly − γ in absorption as well asthe Lyman break. Redwards of the DLA, we detect a range ofabsorption lines from the host galaxy, SII, SiII, CII, SiIV andCIV (see Table 2). We also detect the fine structure transition ofSiII* λ = . ± . ∼ − α
3. D’Avanzo et al.: The afterglow and host galaxy of GRB 090205: evidence for a Ly − α emitter at z = . Table 1.
VLT observation log for GRB 090205.
Mean time Exposure time Time since GRB Seeing Instrument Magnitude Filter(UT) (s) (days) ( ′′ )2009 Feb 06.25042 2 ×
180 00.28984 0.7 VLT / FORS1 22 . ± . R ×
180 00.29693 0.7 VLT / FORS1 20 . ± . I ×
180 01.34443 1.0 VLT / FORS1 24 . ± . R ×
180 20.33671 0.9 VLT / FORS1 26 . ± . R ×
120 33.40199 0.7 VLT / FORS1 − I ×
180 46.26294 0.7 VLT / FORS1 25 . ± . I × ×
18 50.46076 0.8 VLT / ISAAC > . J × ×
64 185.06036 0.9 VLT / HAWKI > . J × ×
65 185.56745 0.8 VLT / HAWKI > . Ks × ×
65 186.06452 0.6 VLT / HAWKI > . H × / FORS1 − V + GG Notes.
Magnitudes are in the AB system and are not corrected for Galactic absorption. Errors and upper limits are given at 1 σ and 3 σ confidencelevel respectively. emission at 1215 Å. We will discuss it more in detail in the nextsection.We fitted the red wing of the DLA using the MIDAS fitlymanpackage. The resulting column density was determined to be logN H / cm − = ± H / cm − = ≥ − < / cm − = / cm − = / cm − = ff ected by depletion onto dust.Using the solar abundances reported in Asplund et al. (2009),we find the metallicity in the host along the line of sight to be[ M / H ] > − .
57 or Z > . Z ⊙ . We continued to monitor the field of GRB 090205 at late times tofurther study the GRB host. We obtained an image in the R − bandwith the FORS1 camera ∼ . R AB ∼ . ± . R − band light curve, that we inter-pret as due to the host galaxy (Fig. 3). Further I − band monitor-ing, carried out ∼ . reveals an object with I AB = . ± .
1. Comparing this detection with the previous one As reported in Tab. 1, another epoch of I − band imaging was takenat t − T ∼
33 d but, erroneously, no dithering was performed amongdi ff erent frames. The resulting image was thus highly a ff ected by fring-ing (see http: // / sci / facilities / paranal / instruments / fors / ) andit was not possible to obtain reliable measures of photometry. Table 2.
Measured wavelengths, derived redshift and equivalentwidths (EWs) of detected absorption lines λ obs λ rest id z EW rest log N[Å] [Å] [Å] [cm − ]5495 972.54 Ly γ — — —5791 1025.72 Ly β — — —6870 1215.67 Ly − α — 98.5 20.73 ± > ± blended —— 1260.53 FeII — blended —7149.40 1264.74 SiII* 4.6529 0.42 ± > ± > ± ± blended —7873.45 1393.76 SiIV 4.6490 2.07 ± ± ± ± Notes.
The EW for the blended systems include the contributions fromall transitions in the blended line. For blended systems, the redshift isnot mentioned due to the large uncertainty. We only give lower limitson the column density for mildly saturated lines which we define hereas EW < obtained in the R − band, the resulting unabsorbed R − I color isconsistent with that of the afterglow. This suggests a flattening ofthe light curve in the I − band too, in agreement with the hypoth-esis that we detected the host galaxy of GRB 090205. We alsocarried out deep, late-time ( t − T ∼
180 d) NIR observationsof the field of GRB 090205 with VLT / HAWK-I in
JHK − bands.The host is not detected in any of the observed bands up to alimiting AB magnitude of J > . H > . K s > . σ c.l.). The results are reported in Tab. 1 and in Figs. 6,7.As already mentioned, the afterglow spectrum shows anemission line at ∼ α absorption,corresponding to Ly- α emission at the same redshift of the GRB.In order to check the reliability of the line detection, and to ex-clude the possibility that it is due to some atmospheric emis-sion or absorption contaminating feature, we performed a de-tailed analysis of the 2-D spectrum (see Fig. 5). At the wave-length corresponding to the Ly − α line emission we measure2101 ±
51 counts (sky + object). The counts corresponding only
4. D’Avanzo et al.: The afterglow and host galaxy of GRB 090205: evidence for a Ly − α emitter at z = . Fig. 4.
VLT / FORS1 spectrum of the GRB 090205 afterglow at z = . ± . ±
21, so that the object counts are 265 ±
55 (68%c.l.). The corresponding signal-to-noise ratio is 5.2. Anotherstriking evidence we obtain from the 2-D spectrum is the mea-sure of a spatial displacement of 1 . ± . . ′′ ± . ′′ ) from the centroid of the afterglow continuum traceand the “spot” corresponding to the Ly − α emission (see Fig. 5).Doing precise astrometry on our afterglow and host galaxy im-ages obtained with FORS1, we measure the same o ff set betweenthe afterglow and the host galaxy positions (0 . ′′ ± . ′′ , cor-responding to a physical o ff set of about 3 kpc), thus makingstronger the hypothesis that this emission line is really due toLy − α from the host galaxy. Using the flux-calibrated afterglowspectrum we derive a flux of 1 . × − erg s − cm − . This fluxtransforms into a Ly − α luminosity of 4 . × erg s − . Wenote that this value is in the range of luminosities observed forthe other GRB-LAE hosts , i.e. 1 − × erg s − (Jakobssonet al. 2005).
3. Discussion
One interesting aspect of this burst is that, similarly to other,high- z GRBs (e.g. GRB 080913 at z = .
7, Greiner et al. 2009;GRB 090423 at z = .
2, Salvaterra et al. 2009, Tanvir et al.2009), it shows a short duration in the emitter rest frame, T , r f ∼ . z = .
65, the isotropic gamma- GRB 971214 ( z = . z = . z = . z = . z = . z = . ray energy release in the redshifted 15-150 keV band is E γ, iso = . ± . × erg and the intrinsic peak energy is E p , i = ±
85 keV. These values make GRB 090205 consistent withthe observed E p , i − E γ, iso correlation (Amati et al. 2008), that isknown to be followed only by long GRBs (see also Piranomonteet al. 2008) and proposed as an indicator of GRBs with a massivestellar collapse origin (Type II GRBs; Zhang et al. 2009). Indeed,the E p , i − E γ, iso correlation has been used recently to supportthe long classification of a few rest-frame short duration burstssuch as GRB 090423 (Salvaterra et al. 2009) and GRB 090426(Antonelli et al. 2009). A Type II classification for GRB 090205is also supported by applying the classification method reportedin Lv et al. (2010), being ǫ = E γ, iso , / E p , z , / ∼ .
26, where E γ, iso , = E γ, iso / erg and E p , z , = E p (1 + z ) / keV. Thisvalue puts GRB 090205 in the high- ǫ regime, which is related tolong (Type II) GRBs.In conclusion, even if a massive stellar collapse origin forGRB 090205 may appear puzzling (although not unheard of) inlight of its rest frame short duration, the prompt emission proper-ties of this GRB favors for a Type II classification. Furthermore,we note that, while the existence at high- z of a population ofbursts originating from the merging of double compact objects isexpected on theoretical ground (Belczynski et al. 2010), their de-tection would imply a very flat luminosity function for the shortburst population, in contrast with a recent analysis of BATSEand Swift data (Salvaterra et al. 2008).
As shown in Sect. 2.1, the X-ray afterglow evolution can be di-vided into three stages. The closure relation for the first stage is α − . β = . + . − . , which can not constrain any model dueto the large scatter of the error bars. The closure relation for the Despite their short rest-frame duration, the high − z GRB 080913and GRB 090423 were classified as Type II bursts (Zhang et al. 2009).5. D’Avanzo et al.: The afterglow and host galaxy of GRB 090205: evidence for a Ly − α emitter at z = . Fig. 5.
Detailed analysis of the Ly − α emission line detection.The top panel shows the blow up of the region centered onthe Ly − α emission at 6873.45Å (dashed line), corresponding to1215.67Å in the emitter rest frame. The central panel shows thecorresponding signal-to-noise ratio. The bottom panel reportsthe counts from the sky (blue line) and for the sky + object (redline). The position of the Ly − α emission in the 2-D spectrum(shown in both panels), is marked by a circle and has an o ff setwith respect to the afterglow continuum (corresponding to 0 . ′′ ;see Sec. 2.2.3 for details).third stage is α − . β = − . ± .
31, quite consistent with thetheoretical expectation of α − . β = − .
5, where α = (3 p − / β = p /
2. Therefore, for t > s, the inferred power law in-dex of electron energy distribution shaped by shock accelerationis p = . + . − . . Therefore, the simplest forward shock modelfor the first and third stages corresponds to the X-ray band beingabove the cooling and typical frequencies of synchrotron radia-tion, i.e., ν x > max ( ν m , ν c ).The second stage shows a rise of the X-ray flux with time.Interpreting it as due to the emergence of the Synchrotron-Self-Compton component in the X-ray band, there should be signif-icant spectral hardening around the transition time t ∼
500 s,which is contrary to what we observed. Alternatively, the risemay be due to the continuous energy injection ( L inj ∝ t − q ) fromlate time central engine activities (e.g., Dai & Lu 1998) or re-freshed shock (Rees & Meszaros 1992) by a late time ejectawith varying Lorentz factors within the ejecta ( M ( > Γ ) ∝ Γ − s ).Since the X-ray spectral index in the rising phase is steep, thecharacteristic frequencies ν c and ν m should be below the X-ray band. According to Table 2 of Zhang et al. (2006), weuse the relation of α = ( q − / + ( q + β / q = + α − β ) / (1 + β ) = − . + . − . , where α = − . + . − . and β = . ± .
24. For the matter-dominated injection model, s = (10 − q ) / (2 + q ) = . + . − . for the ISM case and s = / q − = − . + . − . for the wind case. Since s < (cid:16) (cid:17) − q ∼ . + . − . .The X-ray afterglow clearly shows the presence of late-time temporal variability (4 ks < t <
20 ks). The variable af-terglow is characterised by a flux contrast ∆ F / F ∼
3, where ∆ F is the flux enhancement due to the possible flares and F isthe flux level of the underlying continuum. This together withthe upper limit on the variability ratio ∆ t / t < .
3, places theGRB 090205 possible X-ray flares at the boundary between den-sity fluctuations produced by many regions viewed o ff -axis andrefreshed shocks (see Ioka et al. 2005; Chincarini et al. 2007,their Fig. 15). However, the low statistics prevents us fromdrawing quantitative conclusions on both the temporal (seeChincarini et al., 2010 for an updated analysis on 113 GRB X-ray flares) and spectral behaviour of this possible flaring activity(Falcone et al. 2007).The Galactic extinction corrected I -band flux density at t ∼
25 ks is ∼ . µ Jy. At this time, the 0.3 - 10 keV count rate is ∼ . × − counts s − , corresponding to a flux of 2 . × − erg cm − s − . Assuming the late-time X-ray spectral index β = .
07, the X-ray flux density at ν X = Hz is ∼ . × − µ Jy.So the near infrared to X-ray overall spectral index at t ∼
25 ksis β NIR − X ∼ .
0, suggesting that the optical / NIR and the X-rayemission are from the same origin.In conclusion, the X-ray and optical afterglow can be ex-plained within the standard forward shock model with ν c , ν m <ν opt and ν X . The early rebrightening in the X-ray afterglow canbe interpreted as due to the energy injection into the forwardshock by the central engine. We note that more complex mod-elling of the rebrightening phase (i.e. two-component models)are not strictly required by the data. Our photometric campaign carried out with VLT / FORS, ISAACand HAWK-I (see Sect. 2.2.3) allows us to detect the GRB hostgalaxy in the R and I band and to put strong upper limits on thecontinuum in the NIR bands (J, H, and K). The observed mag-nitude and limits are reported in Table 1 and shown in Figs. 6,7.The blue color, ( I − K ) AB < .
1, argues for a starburst galaxy,whereas ellipticals, Sab, Scd type of galaxy are discarded (seeFig. 6). We therefore model the photometric data with a familyof synthetic starburst SEDs computed from the outputs of theStarburst99 code (Leitherer et al. 1999; Vazquez & Leitherer2005). We adopt a Salpeter initial mass function in the massrange 0.1-100 M ⊙ and a metallicity of Z = . Z ⊙ consistentlywith the metallicity obtained from the GRB afterglow spectrum.Di ff erent ages of the stellar population are considered and thesynthetic SEDs are normalized to reproduce the observed mag-nitude in the I-band. The absorption due to the intergalacticmedium shortwards the Ly − α has been modelled as in Salvaterra& Ferrara (2003, see Section 2.2). The theoretical SEDs areshown in Fig. 7 from top to bottom with stellar ages of 500, 100,50, 10 Myr, respectively. We find that the upper limits in the NIRbands provide a strong limit to the age of the stellar population.In order not to exceed the J and K band upper limits, the stellarpopulation should be younger than τ <
150 Myr. In this case, thecorresponding stellar mass is M ⋆ < × M ⊙ , in agreementwith average mass of long GRB host galaxies (10 . − . M ⊙ Savaglio et al. 2009). We neglect here the possible presence of
6. D’Avanzo et al.: The afterglow and host galaxy of GRB 090205: evidence for a Ly − α emitter at z = . dust inside the host galaxy. However, we note that dust extinc-tion would result in a reddening of the host SED, strengtheningour limits on the stellar age and mass.As described in Sect. 2.2.3, we found evidence that the hostgalaxy of GRB 090205 is a Ly − α emitter. The Ly − α emis-sion line lies at z = . ± . ∆ z = . ± . ∆ v = ±
153 km s − ). This is inline with the results obtained through spectroscopic studies per-formed on large samples of Lyman break galaxies (LBG) at z ∼ ff sets between the Ly − α emission and in-terstellar absorption line redshifts of the order of ∼
600 km s − ,with large dispersion ( ∼ −
500 km s − ; see e.g. Adelberger etal. 2003; Shapley et al. 2003; Bielby et al. 2010). Such kinemat-ics is usually interpreted as due to large-scale outflows caused bysupernova–driven wind, resulting from intense star formation,that blueshift absorption lines from the interstellar gas. At theredshift of the burst, the Ly − α luminosity is 4 . × erg s − .This value lies in the range of luminosities of other LAEs iden-tified by dedicated surveys at z ∼ . z ∼ . − α luminosity function ofthese objects, measuring L ⋆ = + − × erg s − . Similarly,Wang et al. (2009) find L ⋆ = . ± . × erg s − for a sampleof 110 LAEs detected in the Large Area Lyman Alpha (LALA)survey. Our findings suggest thus that this burst exploded intoa 0.6-0.7 L ⋆ LAE at that redshift. Transforming the luminosityin a star-formation rate using the formula from Kennicutt (1998)for H α and assuming a factor of 8 between H α and Ly − α , wederive a SFR of 4.2 M ⊙ yr − which is among the typical valuesfound for other Ly- α emitters hosting GRBs (see, e.g. Jakobssonet al. 2005) and typical galaxies hosting GRBs (Savaglio et al.2009). However the above values should be interpreted as lowerlimits. During the acquisition of the spectrum, the slit was cen-tered on the afterglow position, so that we lost part (about 50%)of the Ly − α flux coming from the host galaxy, due to the 0 . ′′ o ff set we discussed in Sec. 2.2.3.Our analysis of the GRB afterglow spectrum provides alsoa lower limit on the galaxy metallicity, Z ≥ . Z ⊙ . Giventhe limit on the stellar mass obtained above, this metallicity isconsistent with the mass-metallicity measured for Lyman BreakGalaxies at z = − z GRBs. Comparing the metallicityof GRB 090205 to those determined for other GRBs at variousredshifts, we find little or no evolution with redshift, in contrastwith what found for the QSO selected DLA population (Fynboet al. 2006; Savaglio et al. 2009).
4. Conclusions
We report the detection and study of GRB 090205 at z = . z GRBs, GRB 090205 has a shortduration in the rest-frame with T , r f = . ν c , ν m < ν opt and ν X , where the early rebrightening Fig. 6.
Top panel: available data and expected magnitude of theGRB 090205 host for di ff erent galaxy types. The SEDs are nor-malized to reproduce the I band measure. Open circles (filledtriangles) represent the data (upper limits), whereas the horizon-tal lines show the expected AB apparent magnitude of the objectin the VRI JHK bands for di ff erent host galaxy types: magentais for Elliptical, red for Sab, yellow for Scd, green for Irregular,cyan and blue for starburst galaxies. Vertical lines mark the po-sition of Lyman limit, Ly β and Ly − α (from left to right), respec-tively. Bottom panels: expected ( I − J ), ( I − H ) and ( I − K ) colorsfor the di ff erent galaxy types. Observational limit on the colorsare plotted with triangles.in the X-ray afterglow can be interpreted as due to the energyinjection into the forward shock by the central engine.Finally, we report the detection of the host galaxy ofGRB 090205, which is found to be a typical LAE at z = . τ <
150 Myr) stellar population, further supporting the longclassification for this GRB. The obtained mass and SFR are inline with typical values of GRB host galaxies, while the metallic-ity derived from the GRB afterglow spectrum is among the high-est derived from GRB afterglow measurement at high- z , suggest-ing that the burst occured in a rather enriched envirorment.In conclusion, GRB 090205 clearly shows that GRBs can beused as signpost of young, starburst galaxies at high- z that arethought to be the dominant galaxy population at those epochs.Thanks to the brightness of their afterglow, metal lines can beeasily identified providing, together with follow-up photomet-ric observation of their host galaxies, a new way to measure themass-metallicity relation and its evolution through cosmic times. Acknowledgements.
We thank the referee for his / her useful comments and sug-gestions. We acknowledge support by ASI grant SWIFT I / / /
0. This re-search has made use of the XRT Data Analysis Software (XRTDAS) developed
7. D’Avanzo et al.: The afterglow and host galaxy of GRB 090205: evidence for a Ly − α emitter at z = . Fig. 7.
Observations of the host galaxy of GRB 090205 shownwith data points and upper limits are compared with syntheticSEDs for a starburst galaxy with di ff erent stellar ages τ . Fromtop to bottom, lines corresponds to τ = M ⊙ , Z = . Z ⊙ and no dust extinction in the host galaxy. The SEDare normalized to reproduce the observation in the I band. under the responsibility of the ASI Science Data Center (ASDC), Italy. We ac-knowledge the invaluable help from the ESO sta ff at Paranal in carrying out ourtarget-of-opportunity observations. References