A multi-colour study of the dark GRB 000210 host galaxy and its environment

Abstract

We present UBVRIZJsHKs broad band photometry of the host galaxy of the dark gamma-ray burst (GRB) of February 10, 2000. These observations represent the most exhaustive photometry given to date of any GRB host galaxy. A grid of spectral templates have been fitted to the Spectral Energy Distribution (SED) of the host. The derived photometric redshift is z=0.842^+0.054_-0.042, which is in excellent agreement with the spectroscopic redshift (z=0.8463+/-0.0002) proposed by Piro et al. (2002) based on a single emission line. Furthermore, we have determined the photometric redshift of all the galaxies in an area of 6'x6' around the host galaxy, in order to check for their overdensity in the environment of the host. We find that the GRB 000210 host galaxy is a subluminous galaxy (L ~ 0.5+/-0.2 L*), with no companions above our detection threshold of 0.18+/-0.06 L*. Based on the restframe ultraviolet flux a star formation rate of 2.1+/-0.2 Solar Masses per year is estimated. The best fit to the SED is obtained for a starburst template with an age of 0.181^+0.037_-0.026 Gyr and a very low extinction (Av~0). We discuss the implications of the inferred low value of Av and the age of the dominant stellar population for the non detection of the GRB 000210 optical afterglow. Comment: 10 pages with 4 encapsulated PostScript figures included. Accepted for publication in Astronomy & Astrophysics

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arXiv:astro-ph/0212334v1 13 Dec 2002
1
Astronomy & Astrophysics manuscript no.
(will be inserted by hand later)
A multi-colour study of the dark GRB 000210 host galaxy and
its environment⋆
J. Gorosabel1,2,3, L. Christensen4,5, J. Hjorth4, J.U. Fynbo6,4, H. Pedersen4, B.L. Jensen4, M.I. Andersen5, N.
Lund1, A.O. Jaunsen7, J.M. Castro Cer´ on8, A.J. Castro-Tirado2, A. Fruchter9, J. Greiner5,10, E. Pian11, P. M.
Vreeswijk7, I. Burud9, F. Frontera12,13, L. Kaper14, S. Klose15, C. Kouveliotou16, N. Masetti13, E. Palazzi13, J.
Rhoads9, E. Rol14, I. Salamanca14, N. Tanvir17, R.A.M.J. Wijers14, and E. van den Heuvel14
1Danish Space Research Institute, Juliane Maries Vej 30, DK–2100 Copenhagen Ø, Denmark; jgu@dsri.dk,
nl@dsri.dk
2Instituto de Astrof´ ısica de Andaluc´ ıa (IAA-CSIC), P.O. Box 03004, E-18080 Granada, Spain; jgu@iaa.es,
ajct@iaa.es
3Laboratorio de Astrof´ ısica Espacial y F´ ısica Fundamental (LAEFF-INTA), P.O. Box 50727, E-28080, Madrid, Spain;
jgu@laeff.esa.es
4Astronomical Observatory, University of Copenhagen, Juliane Maries Vej 30, DK–2100 Copenhagen Ø, Denmark;
lise@astro.ku.dk, jens@astro.ku.dk, holger@astro.ku.dk, brian j@astro.ku.dk
5Astrophysikalisches Institut,14482 Potsdam,Germany;
jgreiner@aip.de
6Department of Physics and Astronomy, University of
jfynbo@phys.au.dk
7European Southern Observatory, Casilla 19001, Santiago 19, Chile; ajaunsen@eso.org, pvreeswi@eso.org
8Real Instituto y Observatorio de la Armada, Secci´ on de Astronom´ ıa, 11.110 San Fernando-Naval (C´ adiz), Spain;
josemari@alumni.nd.edu
9SpaceTelescopeScience Institute,3700San
fruchter@stsci.edu, burud@stsci.edu, rhoads@stsci.edu
10Max-Planck-Institut f¨ ur extraterrestrische Physik, 85741 Garching, Germany; jcg@mpe.mpg.de
11Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, 34131, Trieste, Italy; pian@tesre.bo.cnr.it
12Dipartimento di Fisica, Universit` a di Ferrara, Via Paradiso 12, I-44100 Ferrara, Italy; frontera@fe.infn.it
13Istituto Tecnologie e Studio Radiazioni Extraterrestri, CNR, Via Gobetti 101, 40129 Bologna, Italy;
filippo@bo.iasf.cnr.it, masetti@bo.iasf.cnr.it, eliana@bo.iasf.cnr.it
14Universityof Amsterdam,Kruislaan403,
lexk@science.uva.nl,evert@science.uva.nl,isabel@science.uva.nl,
rwijers@science.uva.nl, edvdh@science.uva.nl
15Th¨ uringer Landessternwarte Tautenburg, D-07778 Tautenburg, Germany; klose@tls-tautenburg.de
16NASA MSFC, SD-50, Huntsville, AL 35812, USA; kouveliotou@eagles.msfc.nasa.gov
17Department of Physical Sciences, University of Hertfordshire, College Lane, Hatfield, Herts AL10 9AB, UK;
nrt@star.herts.ac.uk
lchristensen@aip.de, mandersen@aip.de,
˚ Arhus, Ny Munkegade, 8000˚ Arhus C, Denmark;
MartinDrive, Baltimore,MD 21218,USA;
1098 SJ Amsterdam,The Netherlands;
Received / Accepted
Abstract. We present UBVRIZJsHKs broad band photometry of the host galaxy of the dark gamma-ray burst (GRB) of
February 10, 2000. These observations represent the most exhaustive photometry given to date of any GRB host galaxy. A grid
of spectral templates have been fitted to the Spectral Energy Distribution (SED) of the host. The derived photometric redshift is
z = 0.842+0.054
al. (2002) based on a single emission line. Furthermore, we have determined the photometric redshift of all the galaxies in an
area of 6′× 6′around the host galaxy, in order to check for their overdensity in the environment of the host. We find that the
GRB 000210 host galaxy is a subluminous galaxy (L ∼ 0.5 ± 0.2L⋆), with no companions above our detection threshold of
0.18 ± 0.06L⋆. Based on the restframe ultraviolet flux a star formation rate of 2.1 ± 0.2M⊙ yr−1is estimated. The best fit to
the SED is obtained for a starburst template with an age of 0.181+0.037
the implications of the inferred low value of AV and the age of the dominant stellar population for the non detection of the
GRB 000210 optical afterglow.
−0.042, which is in excellent agreement with the spectroscopic redshift (z = 0.8463 ± 0.0002) proposed by Piro et
−0.026Gyr and a very low extinction (AV ∼ 0). We discuss
Key words. gamma rays: bursts – galaxies: fundamental parameters – techniques: photometric

Page 2

1. Introduction
The origin of cosmological Gamma-Ray Bursts (GRBs) re-
mains one of the great mysteries of modern astronomy (van
Paradijs et al. 2000). Over the past half decade many advances
have been made in understanding the nature of the bursts
and their afterglows throughout the electromagnetic spectrum.
There are at present mainly two sets of models for GRBs. One
set ofmodels predictsthat GRBs occurwhen two collapsedob-
jects (such as black holes or neutronstars) merge(Eichler et al.
1989;Mochkovitchetal.1993).Thetime-scaleforbinarycom-
pact objects to merge is large (? 1 Gyr), so GRBs can occur
after massive star formation has ended in a galaxy. The other
major set of models predicts that GRBs are associated with the
death of massive stars (supernovae or hypernovae) (Woosley
1993; Paczy´ nski 1998; MacFadyen & Woosley 1999). In this
case GRBs will coincidewith the epochof star formationin the
host. By determining the Spectral Energy Distribution (SED)
andstar formationrate (SFR) of a sample of GRB host galaxies
we can distinguish between these two families of GRB progen-
itor models (see also Belczynski et al. 2002). Substantial in-
sight has already been gained about the galaxies that the bursts
occur in. Radio, optical and/or infrared afterglows have been
observed for ∼40 GRBs, and the majority of these coincide
with starforming galaxies.
As GRB host galaxies tend to be faint (R> 23) spectro-
scopic studies of the SED are only reachablewith 8–10m class
telescopes. A cheaper and elegant alternative to spectroscopy
is to extract information on the properties of the host galaxies
based on multicolour broad band imaging. By determining the
colours of GRB host galaxies we can derive or constrain the
age of the predominant stellar population as well as the extinc-
tion. As part of the global fit, the photometric redshift of the
host galaxies can be derived if the redshift is not known in ad-
vance from spectroscopic observations of the afterglow and/or
the host galaxy. Additional advantages of the multicolour pho-
tometricstudiescomparedtospectroscopictechniquesaretheir
simplicity and their multi-object feasibility. The photometric
technique allows the determinationof the colours of all objects
in the field down to the imaging flux limit, thereby in principle
permitting the study of the host galaxy environment. The pre-
cision of the photometric redshift estimate (which depends on
the photometric accuracy, the spectral coverage and the num-
ber of bands) is evidently not as accurate as the spectroscopic
one, but it is sufficient for a first order study of host galaxy
environments.
So far it has been possible to detect optical afterglows
for only about 30% of localised GRBs (Fynbo et al. 2001;
Lazzati et al. 2002). It is important to understand the nature
of the remaining (rather ill-termed) so-called dark GRBs if
we wish to get a complete understanding on GRB selected
galaxies and thereby constrain the GRB progenitors as well as
Send offprint requests to: J. Gorosabel, e-mail:jgu@dsri.dk
⋆Based on observations collected at the European Southern
Observatory, in La Silla and Paranal (Chile), ESO Large Programmes
165.H-0464(C), 165.H-0464(E), 165.H-0464(I) and 265.D-5742(C),
granted to the GRACE Team. Based on observations made with the
Danish 1.54 m telescope, in La Silla (Chile).
the distribution of cosmic star formation over different modes
(e.g. Ramirez-Ruiz et al. 2002; Venemans & Blain 2001).
GRB 000210 is currently one of only few systems that allow
a detailed study of a galaxy hosting a dark GRB. The burst ex-
hibited the highest γ-ray peak flux among the 54 GRBs local-
ized duringthe entire BeppoSAXoperation,from1996to 2002
(Piro et al. 2002). However, no optical afterglow (OA) was de-
tected in spite of a deep search (R> 23.5) carried out ∼ 16
hrs after the gamma-ray event (Gorosabel et al. 2000a). X-ray
observations performed with the Chandra X-ray telescope 21
hrs after the GRB localised the X-ray afterglow of the burst
to an accuracy of 2′′, later improved by Piro et al. (2002) to a
0.′′6 radius error circle. The optical search revealed an extended
constant source coincident with the X-ray afterglow which was
proposed as the GRB host galaxy (Gorosabel et al. 2000b). In
addition, Piro et al. (2002) have reported the detection of a ra-
dio transient at 8.5 GHz spatially coincident with the X-ray af-
terglow. Based on the detection of a single host galaxy spectral
line,interpretedtobedueto[OII],Piroetal.(2002)proposeda
redshift of z = 0.8463±0.0002. Recently Berger et al. (2002)
and Barnard et al. (2002) have reported ∼2.5 σ detections of
sub-mm emission towards the position of GRB 000210 inter-
preted as emission from the host galaxy and hence suggesting
a SFR of several hundred M⊙yr−1.
In this paper we present the most intensive multi-colour
host galaxy imaging performed to date. The host galaxies SED
studies to date had a limited number of bands (Sokolov et al.
2001; Chary et al. 2002) and no photometric redshift determi-
nations. Throughout, the assumed cosmology will be ΩΛ =
0.7, ΩM = 0.3 and H0= 65 km s−1Mpc−1(except in Sect.
5.3 where the host galaxy luminosity is rescaled to the cosmol-
ogy used by Lilly et al. 1995). At the proposed spectroscopic
redshift (z = 0.8463), the look back time is 7.59 Gyr (52.4%
of the present age) and the luminosity distance is 1.79 × 1028
cm. The physical transverse size of one arcsec at z = 0.8463
corresponds to 8.24 kpc.
2. Observations and photometry
We have used a number of optical/near-IR (NIR) resources in
order to compose a well sampled SED (see Table 1). UBVI
observations were carried out with the 3.6-m ESO telescope
(3.6ESO) equipped with EFOSC2, covering a field of view
(FOV) of 5.′5 × 5.′5. These observations were carried out in
2×2binningmode,providinga pixel scale of 0.′′32/pix.R-band
measurements were obtained with the UT1 of the 8.2-m Very
Large Telescope (8.2VLT) equipped with FORS1 and are pub-
lished in Piro et al. (2002). The Z-band observations were car-
ried out during two consecutive nights with the 1.54-m Danish
Telescope (1.54D) equipped with DFOSC, which provides a
FOV of 13.′7 × 13.′7 and a pixel scale of 0.′′39/pix.
The H-band observations were acquired with the 3.58-m
New TechnologyTelescope (3.58NTT)using SOFI in the large
FOV mode, which provides a FOV of 4.′9 × 4.′9 and a pixel
scale of 0.′′292/pix. The Js and Ks-band observations are based
on the UT1 of the 8.2VLT equipped with ISAAC, allowing us
to cover a FOV of 2.′5 × 2.′5 with a pixel scale of 0.′′148/pix.

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J. Gorosabel et al.: The host galaxy of GRB 0002103
Table 1. Chronologically ordered optical and NIR observations carried out for the GRB 000210 host galaxy.
Telescope
(+Instrument)
8.2VLT (+FORS1)
3.58NTT (+SOFI)
3.6ESO (+EFOSC2)
3.6ESO (+EFOSC2)
3.6ESO (+EFOSC2)
3.6ESO (+EFOSC2)
8.2VLT (+ISAAC)
8.2VLT (+ISAAC)
8.2VLT (+ISAAC)
1.54D (+DFOSC)
1.54D (+DFOSC)
⋆⋆ Published in Piro et al. (2002).
† The images were coadded resulting in just a single Js-band magnitude.
⋆ The images were coadded resulting in just a single Z-band magnitude.
FilterDate UTTexp
(s)
300
182×60
4×600
3×600
3×600
6×600
30×60
15×120
15×120
14×600
21×600
Seeing Limiting magnitude
(4σ)
25.4⋆⋆
22.8
25.4
23.1
25.6
24.7
22.2
24.1†
24.1†
22.9⋆
22.9⋆
R
H
V
I
B
U
Ks
Js
Js
Z
Z
25.237–25.240/10/00
02.251–02.410/09/01
13.219–13.253/09/01
13.256–13.278/09/01
13.280–13.302/09/01
13.304–13.348/09/01
21.159–21.193/09/01
21.194–21.218/09/01
23.193–23.218/09/01
19.090–19.254/12/01
20.042–20.394/12/01
0.70
0.90
1.75
1.45
1.70
1.55
0.45
0.60
0.75
1.10
1.15
Table 2. Magnitudes of the host in the UBVRIZJsHKs bands. Several characteristics of the filters are displayed: filter name (1),
effective wavelength (2) and bandpass width (3). The fourth column shows the measured magnitudes (in the Vega system and
not corrected from Galactic reddening). To facilitate the calculation of the AB magnitudes, and consequently the flux densities
for each band, the AB offsets are provided in the fifth column.
Filter
name
U (ESO#640)
B (ESO#639)
V (ESO#641)
R (ESO R BESSEL+36)
I (ESO#705)
Z (ESO#462)
Js (ISAAC)
H (SOFI)
Ks (ISAAC)
† Published in Piro et al. (2002).
Effective
wavelength (˚ A)
3718.8
4372.6
5563.9
6608.5
7950.2
9477.4
12498.9
16519.6
21638.0
Bandpass
width (˚ A)
172.9
701.4
856.4
1300.3
844.0
985.1
957.8
1732.3
1637.9
MagnitudeABoff
23.54±0.13
24.40±0.13
24.22±0.08
23.46±0.10†
22.49±0.12
22.83±0.28
21.98±0.10
21.51±0.23
20.94±0.14
0.73
−0.07
0.04
0.23
0.45
0.56
0.94
1.41
1.87
In Table 1 we provide the observing log of our optical and NIR
observations.
Given that every extended source shows a different pho-
todensity profile (or FWHM), an unique fixed Aperture
Photometry(orstaticaperturephotometry,APhereafter)would
yield unsatisfactory results. On the other hand, Isophotal
Photometry (IP) would also not provide optimum photometry,
since performing IP we would not consider the same fraction
of each galaxy in the different bands due to colour-dependent
morphologies and seeing. To solve this problem the total in-
tegrated photometry given by SExtractor was used (Bertin
& Arnouts 1996). For each object SExtractor performs two
types of total integrated photometry: the Adaptative Aperture
Photometry (AAP) and the Corrected Isophotal Photometry
(CIP). The AAP and CIP supersede the values given by the
AP and IP, respectively, applying to them an aperture correc-
tion. For each object SExtractor considers the photometry out-
putgivenbytheAAP,exceptif aneighbouris foundbiasingthe
flux by more than 10%. If this is the case, SExtractor chooses
the value given by the CIP (see Bertin & Arnouts 1996 for
details). The host galaxy of GRB 000210 is well isolated and
hence its the photometry is not affected by any neighbours.
The UBVRIZJsHKs-band magnitudes of the host can be
seen in Table 2. The UBVRI-band calibration is based on the
secondary standards given in Table 2 of Piro et al. (2002).
The JHKs-band calibration was performed observing the stan-
dard fields sj9105 and sj9172 (Persson et al. 1998) at several
airmasses. The derived NIR secondary standards are given in
Table 3 and displayed in Fig. 1. The Z-band calibration was
carried out observing the spectro-photometric standard stars
LTT2415 and LTT1788 (Hamuy et al. 1994) with the 1.54D
at an airmass similar to that of the GRB field. The host galaxy
BVRI-band magnitudes reported by Piro et al. (2002) are con-
sistent with our magnitudes displayed in Table 2.
In order to derive the corresponding effective wavelengths
and AB offsets we convolved each filter transmission curve
with the corresponding CCD efficiency curve (see Table 2).
The AB offset is defined as ABoff= mAB− m, where m is

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4J. Gorosabel et al.: The host galaxy of GRB 000210
Fig.1. The image shows the co-added V-band image taken at the 3.6ESO telescope at 13.219–13.253/09/01 UT. The objects
contained in the circles are the ones with redshifts consistent with 0.746 < z < 0.946. As it can be seen there is no obvious
concentration of these galaxies around the host. The circle radius is proportional to 1/|MB|, so the fainter the galaxy the larger
the circle. The host galaxy is indicated by the tick marks. The numbers label the secondary NIR standards shown in Table 3. The
FOV covered by the image corresponds to 5′× 5′.
Table 3. NIR secondary standards in the GRB 000210 field.
Name
1
2
3
α2000
1:59:21.51
1:59:16.72
1:59:16.27
δ2000
JsH Ks
-40:39:33.4
-40:40:20.3
-40:40:27.2
17.94±0.03
16.77±0.07
18.35±0.08
17.19±0.03
16.51±0.03
17.69±0.04
17.00±0.03
16.59±0.04
17.46±0.05
the magnitudein the Vega system and mABis the magnitudein
the AB system (given by mAB= −2.5×logfν−48.60, being
fνthe flux density in erg s−1cm−2Hz−1).
The AB offsets of the nine bands have been derived con-
volving the Vega spectrum taken from the GISSEL98 (Bruzual
& Charlot 1993) library (α Lyrae m = 0 in all bands by defini-
tion) with our UBVRIZJsHKs-band filters and the correspond-
ing CCD efficiency curves. The derived AB offsets (displayed
in the last column of Table 2) are similar to the ones reported
by Fukugita et al. (1995).

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J. Gorosabel et al.: The host galaxy of GRB 0002105
Table 4. The table displays the parameters of the best host galaxy SED fit when several IMFs, indicated in the first column,
are adopted. The rest of the columns display the inferred parameters under the assumed IMF. The second column provides
the confidence of the best fit (given by χ2/dof). The derived photometric redshift is displayed in the third column (and the
corresponding68% and 99% percentile errors). In the fourth and fifth columns the template family of the best fitted SED and the
age of the stellar populationare given.The sixth column displays the derivedvalue of the host galaxy extinctionAV. The seventh
column displays the derived rest frame absolute B-band magnitude, MB. The last two columns give the Luminosity of the host
in units of L⋆, when the luminosity functions of Schechter (1976) and Lilly et al. (1995) are used (see Sect. 5.3 for a detailed
discussion).The extinctionlaw has beenfixed to follow Calzetti et al. (2000) (the effectof the adoptedextinctionlaw is discussed
in Sect. 5.2). As shown in the first two rows of the table, the resolution of our template grid is not able to make a distinction
between most of the properties (Age, AV, MB, L/L⋆) derivedfor the Sa55 and MiSc79 IMFs. The photometricredshifts derived
for the three IMFs are consistent, within the 99% percentile error range, with the spectroscopic redshift. However, within the
68% precentile (∼ 1σ) error range, only the Sa55 and MiSc79 IMFs are consistent, Sc86 is not.
IMF
χ2/dofPhotometric redshift
z+p68%,p99%
−p68%,p99%
Template Age
(Gyr)
AV
MB
L/L⋆
L/L⋆
Salpeter (1955)
1.0960.842+0.054,0.158
−0.042,0.279
Stb0.181 0.00-20.160.67 0.35
Miller & Scalo (1979)
1.0460.836+0.087,0.140
−0.053,0.244
Stb 0.181 0.00-20.16 0.67 0.35
Scalo (1986)
0.9030.757+0.067,0.219
−0.044,0.132
S0 1.0150.00 -19.90 0.52 0.27
3. Method: reproducing the host galaxy
photometry by means of synthetic and
observed SED templates
The fit of the SEDs have been carried out using Hyperz1
(Bolzonella et al. 2000). Eight synthetic spectral types were
used representing Starburst galaxies (Stb), Ellipticals (E),
Lenticulars (S0), Spirals (Sa, Sb, Sc and Sd) and Irregular
galaxies (Im). The time evolution of the SFR for all galaxy
types is represented by an exponential model, i.e. SFR ∝
exp(−t/τ), where τ is the SFR time scale. Each galaxy type
has a value of τ assigned. The SFR of Stb is simulated by an
exponential decay in the limit when τ → 0, in other words an
instantaneous SFR given by a delta function. The early type
galaxy spectra (E, S0) are represented by values of τ between
1 and 2 Gyr. The Spiral galaxies (Sa, Sb, Sc and Sd) have τ
values ranging from 3 to 30 Gyr. The SFR of Im galaxies are
represented by a constant SFR (τ → ∞).
Once thepopulationofstars is generatedfollowingthetime
evolution given by the assigned SFR, the mass of the newly
formed population is distributed in stars following an assumed
Initial Mass Function (IMF). Three IMFs have been consid-
ered:Miller &Scalo (1979), Salpeter(1955),andScalo (1986).
These IMFs will be abbreviated hereafter as MiSc79, Sa55 and
Sc86, respectively.In Sect. 5.1 we discuss the impact of the as-
sumed IMFs in the determination of the photometric redshift.
The newly formed stars evolve depending on their mass
and metallicity following stellar tracks. In each evolutionary
stagethecontributionofalltheindividualstarspectraareadded
yielding an integrated galaxy SED which evolves with time.
For each galaxy type the evolving SEDs can be tabulated and
1
http://webast.ast.obs-mip.fr/hyperz/
stored creating the so called SED libraries. Bruzual & Charlot
(1993) have derived a SED library called GISSEL98 (Galaxy
Isochrone Synthesis Spectral Evolution Library), which is the
base of our SED fits.
In addition to the above mentioned evolutionary templates,
four averaged templates (constructed grouping the SEDs of
the observed local galaxies) from Coleman, Wu & Weedman
(1980) were considered (hereafter named as CWW). These ex-
tra spectral templates work as a backup of the evolutionary fit-
ting SEDs, and give an approximate hint of the galaxy type
when synthetic SED fits fail. The observed CWW templates
can be grouped in four sets: early galaxy types (E/S0), Sbc,
Scd and Im.
Furthermore, the impact of considering different extinc-
tion laws has been studied. Four extinction laws have been
taken into account for the determination of the photometric
redshift and the physical conditions of the GRB 000210 host
galaxy,namelyCalzetti etal.(2000),Seaton(1979),Fitzpatrick
(1986), and Pr´ evot et al. (1984). Each of these laws deter-
mine the dependenceof the extinction on the photon frequency
and are the result of different physical conditions in the in-
terstellar space in the hosts. Thus, Seaton (1979), Fitzpatrick
(1986), and Pr´ evot et al. (1984), are appropriate to describe
the Milky Way (MW), Large Magellanic Cloud (LMC) and the
Small Magellanic Cloud (SMC) extinction laws, respectively.
The Calzetti et al. (2000) extinctionlaw is suitable for starburst
regions. In Sect. 5.2 the effect of the adopted extinction law
on the inferred host galaxy photometric redshift is discussed.
In the SED fits a solar metallicity (Z = Z⊙≃ 0.02, being Z
the mass fractionof heavyelements in the interstellar gas) have
been assumed.
We varied the GRB 000210 host galaxy redshift between
z = 0 and z = 5 with a redshift step of ∆z = 0.01. The host

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6 J. Gorosabel et al.: The host galaxy of GRB 000210
0.0 1.02.03.0 4.0 5.0
Photometric redshift
0.0
10.0
20.0
30.0
40.0
χ
2/dof
Spectroscopic redshift (z=0.846)
Fig.2. The evolution of the fitted SED χ2/dof as a function of
the photometric redshift. The dotted vertical line indicates the
spectroscopicredshift proposedby Piro et al. (2002). As shown
the minimum of χ2/dof (at z = 0.842) is consistent with the
spectroscopic redshift.
galaxy extinction was ranged in a AV = 0 − 5 interval with
a step of ∆AV = 0.005 mag. Table 4 shows several inferred
fit parameters for the assumed extinction law and IMFs: the fit
confidence level (χ2/dof), the photometric redshift z (and the
associated asymmetric uncertainties), the best fitted template,
the dominant stellar age, the extinction AV, the absolute B-
bandmagnitude(MB), and the host galaxyluminosity (in units
of L⋆). As it is shown in Table 4 the resolution of our template
grid is not able to make a distinction between most of the prop-
erties (Age, AV, MB, L/L⋆) derived for the Sa55 and MiSc79
IMFs. Fig. 2 shows the evolution of χ2/dof as a function of
the best fitted SED redshift, when a Sa55 IMF is assumed. The
fit to the UBVRIZJsHK-band photometric points shows a clear
minimumaroundz ∼ 0.85andhas nootheracceptableredshift
solutions.
4. Study of the host galaxy environment
At presentit is unknownifGRB hostgalaxiespreferentiallyare
located in dense environments,or if there is any correlation be-
tween the local density of galaxies and the presence of a GRB.
So far the two z=2.04 bursts, GRB 000301C and GRB 000926,
are the only ones for which the environment of the host galaxy
has been studied (Fynbo et al. 2002). In both of these fields a
number of galaxies at the same redshift were detected, but the
lack of blankfields studied to similar depth preventedthose au-
thors to conclude if the GRB host fields were overdense. The
photometric redshifts of the galaxies in the GRB 000210 field
provide the opportunity to look for other galaxies in its envi-
ronment. The same calibration and photometry software used
to obtain the host galaxy magnitude was applied to the rest of
the objects in the field.
0.01.0 2.03.04.05.0
Photometric redshift
0
5
10
15
20
Galaxies per redshift bin
Spectroscopic redshift (z=0.846)
Fig.3. The plot shows the redshift distribution of galaxies in
a area of 6′× 6′around the GRB 000210 host. The sample
is made up of the 169 galaxies whose SEDs were fitted with
χ2/dof < 2. The vertical dashed line shows the spectroscopic
redshift. As shown there is no special concentrationof galaxies
with redshifts similar to the host galaxy. For the construction
of the histogram we assumed a MiSc79 IMF and the extinction
law given by Calzetti et al. (2000). Other IMFs and extinction
laws yield similar results.
We consider that an object is suitable for redshift determi-
nation when it is detected at least in four bands. Objects de-
tected in less than four filters were rejected due to the large
uncertainty in the determination of their redshifts. The consid-
ered region covers a 6′× 6′area around the host galaxy. At
the redshift of the host galaxy (z = 0.8463)this correspondsto
∼ 3 Mpc ×3 Mpc. The SEDs used to determine the photomet-
ric redshifts of the field objects consist of 8 synthetic templates
(Stb, E, S0, Sa, Sb, Sc, Sd, Im) based on a MiSc79 IMF and
the extinctionlaw given by Calzetti et al. (2000). As in the case
of the host SED, four additional observed spectra from CWW
were considered.
Among the 169 galaxies of the field with acceptable fits
(χ2/dof < 2) we considered the ones with photometric red-
shifts compatible (within 1σ) with a ∆z = ±0.1 redshift
range around the host galaxy spectroscopic redshift. In Fig. 1
a deep V-band image around the host galaxy is displayed.
The FOV covered by the image is 5′× 5′. The circles repre-
sent the galaxies having photometric redshifts consistent with
0.7463 < z < 0.9463.The radius of each circle is proportional
to the inverse of the absolute B-band magnitude (1/|MB|) of
the galaxycontainedinside. From the distributionof the circles
on the image it is clear that there is no obvious concentration
of galaxies around the host (indicated by the tick marks). The
lack of clustering around the host galaxy can also be visualised
in Fig. 3, where the redshift distribution of the galaxies in the
field are plotted. Fig. 3 shows that there is no spike of galaxies
at the redshift of the host. The same study was performed con-

Page 7

J. Gorosabel et al.: The host galaxy of GRB 0002107
sidering the 200 galaxies with SEDs fitted having χ2/dof < 3,
again yielding no apparent concentration of objects around the
host. The exercise was repeated using several extinction laws
and IMFs, giving similar results.
We have calculated the neighbour detectability threshold
of our images; in other words, the minimum luminosity of a
neighbour galaxy for which our data allows a photometric red-
shift determination. With this purpose the GRB 000210 host
galaxy SED has been dimmed until detecting it only in seven
bands (UBVRJsHKs) above the limiting magnitudes given in
Table 1. We consider that the minimum number of bands to
havean acceptableredshiftdeterminationis four.So, if thehost
galaxy SED was dimmed in all filters by 1.16 magnitudes (see
the limiting magnitudes of Table 1 and the host galaxy mag-
nitudes of Table 2), it would have been still detected in seven
bands and a secure photometric redshift determination would
have been still possible. If the SED is dimmed by more than
∼1.29 mag then the host would have been detected only in RJ
(and may be marginally in HK), so no redshift determination
would have been possible.
The absolute B-band magnitude of a host galaxy-like SED
at z = 0.8463,1.16magshallower,is MB= −19.0.So, apho-
tometric redshift of a neighbour galaxy (with a SED similar to
the host) with MB> −19.0 would have been indeterminable.
This magnitude corresponds to a Luminosity of L = 0.23L⋆
(considering M⋆
value deducedfor L based on Lilly et al. (1995) (M⋆
discussed in Sect. 5.3) corresponds to L = 0.12L⋆. Thus
MB = −19.0 implies a luminosity ranging from 0.12 to
0.23L⋆depending on the adopted M⋆
consider L = 0.18 ± 0.06L⋆as an indication of the limit-
ing luminosity of our host environment study. This procedure
assumes that the galaxies in the host environment have simi-
lar SEDs, thus their SEDs can be reproduced by dimming the
GRB 000210 host galaxy SED by the same factor in all bands.
B= −20.6, following Schechter 1976). The
B= 21.33,
Bvalue. Therefore we
5. Discussion
5.1. The impact of the assumed IMF on the
photometric redshift
As shown in Table 4 the spectroscopic redshift is consis-
tent (within the 99% percentile error range) with the inferred
three photometric redshifts. Thus the effect of the assumed
IMF is not crucial to confirm the spectroscopic redshift of the
GRB 000210 host galaxy. However, among the assumed three
IMFs the MiSc79 and Sa55 IMFs are the only ones providing
a photometric redshift consistent within 1σ with the spectro-
scopic redshift. Thus, we consider the Sc86 IMF as the less
appropriate one to describe the predominant stellar population
of the GRB 000210 host galaxy.
According to Bolzonella et al. (2000) the Sa55 IMF pro-
duces an excess of brightblue stars yieldingan UV flux excess.
Onthe otherhandtheSc86IMF generatesan excessivenumber
of solar mass stars, making the spectrum too red to reproduce
the observedSEDs. Intensivephotometricredshift studies have
shown that the MiSc79 IMF is a good compromise between
both tendencies (Bolzonella et al. 2002).
In the particular case of the blue SED of the GRB 000210
host, the potential excess of massive stars given by the Sa55
IMF is not an inconvenient at all. The prominent UV flux pre-
dicted by Sa55 can easily reproduce the blue part of the ob-
served SED. On the contrary, the excess of solar mass stars
given by the Sc86 IMF is not able to reproduce the blue SED
part unless the host galaxy redshift is slightly accommodated.
Thus, the expected impact of the three IMFs (Bolzonella et al.
2002) is in agreement with the photometric redshifts displayed
in Table 4.
5.2. The effect of the adopted extinction law on the
photometric redshift
The host galaxy restframe SED flux density (in a Fλ repre-
sentation as the one of Fig. 4) increases from 3000 to 2000
˚ A (corresponding to the observed SED between the U and the
V band). The detection of this ionising UV continuum implies
a very low extinction in the host. Given the low extinction de-
rived for the host (see the values of AVin Table 4) the inferred
photometricredshiftis basicallyindependentoftheadoptedex-
tinctionlaw forthethree IMFs.The results displayedin Table4
for Sa55 and MiSc79 IMFs remain unchanged if the Calzetti
et al. (2000) extinction law is replaced by another reddening
law, as the ones given by Seaton (1979), Fitzpatrick (1986), or
Pr´ evot et al. (1984). The values of the photometric redshift de-
rived assuming a Sc86 IMF changes slightly from z = 0.757
to z = 0.783, depending on the extinction law.
Therefore, in the particular case of the GRB 000210 host
galaxy,the impact of the adoptedextinction law on the inferred
redshiftis negligibleandhas to be consideredas a secondorder
parameter in comparison to the assumed IMF.
5.3. Is the GRB 000210 host a subluminous
galaxy?
A subluminous galaxy is determined for having a luminos-
ity below the knee of the luminosity function given by L⋆
(Schechter 1976). The characteristic luminosity L⋆can be as-
sociated to a characteristic AB-system B-band absolute mag-
nitude, M⋆
ing on the rest-frame colour of the galaxy (Lilly et al. 1995).
In a more simplified approximation to the luminosity function,
Schechter (1976) reports an unique value of M⋆
the Vega system) for all galaxy types.
The restframe (U−V) colour of the host galaxy is (U−V)=
−0.54, which in the AB system corresponds to (U−V)AB =
0.15 (see the AB offsets given in Table 2). According to Table
1 of Lilly et al. (1995) this (U−V)ABcolour implies a value of
M⋆
0, ΩM = 1 and H0= 50 km s−1Mpc−1) for the redshift bin
corresponding to the host. This B-band AB-system magnitude
correspondsto a B-band absolute magnitude of M⋆
in the Vega system (see Table 2).
The absolute B-band magnitude of the host galaxy for
MiSc79 and Sa55 IMFs (M⋆= −20.16 see Table 4), when
rescaled to the cosmology used by Lilly et al (1995), corre-
B(AB), which ranges from −20.8 to −23.0 depend-
B= −20.6 (in
B(AB) = −21.40 (given for a cosmology defined by ΩΛ=
B= −21.33

Page 8

8 J. Gorosabel et al.: The host galaxy of GRB 000210
20007000 12000 1700022000
Observed wavelength (Å)
0.00
0.05
0.10
0.15
0.20
10
−17erg cm
−2s
−1Å
−1
Starburst at z=0.842, Age=0.181 Gyr, Av=0.0, χ
2/dof=1.096
Fig.4. The points show the measured flux in the UBVRIZJsHKs bands for the GRB 000210host, once the Galactic dereddening
is introduced (Schlegel et al. 1998). The solid curve represent the best SED fitted to the photometric points (χ2/dof = 1.096),
corresponding to a starburst synthetic template at a redshift of z = 0.842 generated with a Sa55 IMF. The derived value of the
starburst age corresponds to 0.181 Gyr. The fit is consistent with a very low extinction, (AV∼ 0). The extinction law used to
construct the plot is given by Calzetti et al. (2000). The SED shows a prominent ∼ 4000 × (1 + z)˚ A break, bracketed between
the R and I-band photometric points, typical of early galaxy types (Stb, E, S0) with stellar population ages > 108yr.
sponds to M⋆= −20.18. Given that M⋆
L = 0.35L⋆. The corresponding value of L derived for a Sc86
IMF is 0.27 L⋆(see last column in Table 4).
The valuesof L,obtainedusing Schechter(1976), basically
double (see the eighth column in Table 4) those obtained when
Lilly et al (1995) is considered. Therefore, considering an av-
eraged value of L = 0.5±0.2L⋆for the host, we concludethat
the host is very likely a subluminous galaxy. This luminosity
value is consistent with the one (L ≈ 0.5L⋆) derived by Piro et
al. (2002).
B= −21.33, then
5.4. Estimation of the star formation rate
The redshiftedspectraof the GRB 000210host galaxyhavethe
restframeUVcontinuumintheobservedopticalrange.TheUV
continuum emission with ongoing star formation is dominated
by bright,short-lived,main-sequenceO and B stars. According
to Kennicutt (1998), for a Sa55 IMF (consistent with our host
galaxy SED, see Table 3), the SFR in a galaxy is directly pro-
portional to the rest frame UV luminosity; SFRUV = 1.4 ×
10−28Lν, where Lνindicates the emitted energy per unit fre-
quency around 2800 ˚ A, measured in ergs s−1Hz−1. SFRUV
gives the amount of stellar mass (measured in solar masses)
created in the host galaxy in a restframe year. The method of
deriving the SFR from the UV continuum flux (named SFRUV
in the present paper) is one of several diagnostic methods used
in the literature to measure SFRs in galaxies (see Kennicutt
(1998) fora comprehensivereview).Obviously,if thereis dust-
enshroudedstar formationthenthisUV-basedmethodwillonly
provide a lower limit to the actual SFR.
At z = 0.8463the 2800˚ A region is redshifted to 5169.6˚ A,
so it is bracketed between the B and V bands. Assuming a
power law SED stretch between both bands, a flux density
of 0.70 ± 0.07 µJy is estimated at 5169.6 ˚ A. This flux den-

Page 9

J. Gorosabel et al.: The host galaxy of GRB 0002109
sity corresponds to a restframe 2800 ˚ A luminosity of Lν =
1.53 ± 0.15 × 1028ergs s−1Hz−1, and therefore to a SFRUV
of2.1±0.2M⊙yr−1. TheSFRUVderivedto dateforGRB host
galaxies range from 1 to 55 M⊙yr−1(see Berger et al. 2002,
Table 3). Thus the SFRUVof the GRB 000210 host galaxy is
in the low end of the distribution for the studied hosts. The
SFRUVper unit luminosity (considering L ∼ 0.35L⋆based on
the Sa55 IMF results of Table 4) is similar to that of other host
galaxies.
As detailed in Kennicutt (1998) the above given SFRUV
estimate is more adequate for galaxies with continuous star
formation (over time scales of 108years or longer), and pro-
vides an upper limit for younger populations such as young
starburst galaxies with ages below 108years. For the esti-
mated stellar population age of the host galaxy (0.181 Gyr,
see Table 4), we consider that the SFRUVexpression gives still
an acceptable approximation to the actual SFRUV. Kennicutt
(1998) estimates that the internal uncertainty of this method is
∼30%. This value is far from the SFR derived by Berger et al.
(2002) based on the tentative sub-millimeter detection of the
host galaxy (SFRsmm≈ 500M⊙yr−1). The apparent discrep-
ancy between SFRsmmand SFRUVcan not be explainedby the
internal uncertainties inherent to the SFRUVor SFRsmmmeth-
ods.
If the tentative detection of sub-mm emission from the host
galaxy of GRB 000210 is real, as opposed to noise or emis-
sion from another source along the line of sight, we need
to conclude that the host of GRB 000210 has two separate
populations of massive stars. One is traced by the rest frame
UV/optical light and shows no sign of extinction and the other
is completely obscured by dust and is only detectable at sub-
mm wavelengths. A possible way to explain this apparently
odd configuration is if the host has a clumpy and opaque ISM
with no thin absorbers, which is able to completely hide part
of the massive stellar population, but does not significantly af-
fect the UV flux of the not hidden massive stars. This sce-
nario would be consistent with the significant line of sight
column density inferred from the afterglow X-ray spectrum
(NH= (5 ± 1) × 1021cm−2, Piro et al. 2002). It would also
naturally explain the lack of optical afterglow emission if the
progenitor was a member of the enshrouded population.
Based on the flux of the [OII] line and assuming several
reasonable hypotheses Piro et al. (2002) deduced a SFR[O II]
of ∼ 3M⊙yr−1. Given the impact of their assumptions (they
calibrated the GRB 000210 [OII] flux relative to the one
of the GRB 970828 host galaxy) and the intrinsic scatter of
the SFRUVmethod (∼30% according to Kennicutt 1998), we
consider that our SFRUV estimate is in agreement with the
SFR[O II]determined by Piro et al. (2002). Thus, the [OII] line
and the UV continuum originate from the same unextincted
blue stellar component.
5.5. Implications of the fitted SED on the GRB
progenitor
The fitted SED assuminga MiSc79 or a Sa55 IMF is consistent
with a Stb, independently of the extinction law used. The Stb
template is characterized by a value of τ → 0, so the SFR can
be expressed by a delta function. In this scenario, the star for-
mation is instantaneous, and occurs at the same time for all the
stars, independently of their masses. Thus all the stars should
have the same age. The local birth places in a host galaxy(even
in the same star forming region) show different physical con-
ditions and, besides, they would be causally separated from
each other, so an instantaneous star formation is physically in-
viable. Therefore, this description should be considered only
as an idealisation of a quasi-simultaneous starburst episode oc-
curredaround0.181Gyr ago (measuredin the restframe) in the
host galaxy.
Several alternatives are possible to explain a GRB progeni-
tor with an age of ∼0.181 Gyr. The first alternative would be a
progenitor made up of a binary merging system. The life time
of such systems is ∼ 0.1 – 1 Gyr, i.e. compatible with the host
galaxy dominant stellar age (Eichler et al. 1989). Thus, the γ-
bright (but optically dark) GRB 000210 would come from the
collapse of a compact binary system. This interpretationwould
support a connection between dark GRBs and binary merg-
ing systems, which would not necessarily invoke a circumburst
dense region and an extinction mechanism to explain the lack
of optical emission (Castro-Tirado et al. 2002). However, a bi-
nary merging origin shows several problems. Piro et al. (2002)
derived a column density of NH = (5 ± 1) × 1021cm−2
along the line of sight to the burst. It is not obvious to conceive
such binary systems located within a high density (NH> 1021
cm−2) region. Each of the components of such systems is the
result of an asymmetric collapse of stellar cores, providing in
the instant of theexplosionkick offvelocitiesup to 900kms−1
to the newly formedcompact object (Frail et al. 1994; Nazin &
Postnov 1997). Thus the binary systems tend to be located far
from their birth places, as they have 0.1 − 1 Gyr to travel be-
fore the binary collapse episode occurs. However, Belczynski
et al. (2002) have recently shown that, although far from the
star forming regions, the binary systems should occur inside
the host galaxies. Besides they find that such systems are more
numerous than previously thought.
In principle a collapsar with an age of ∼0.181 Gyr is not
easy to accommodate.The age of a 8M⊙star when it explodes
as a type II SN is ∼0.05 Gyr (see for instance Portinari et al.
1998). More massive stars, as the progenitors suggested in the
collapsar scenario (Paczy´ nski 1998; MacFadyen & Woosley
1999), have even shorter lifetimes.
The clumpy ISM scenario would be able to reconcile the
difference between the age derived from the SED (∼ 0.181
Gyr) and the age expected for a collapsar (the lifetime of a
∼ 100 M⊙progenitor is ∼ 0.003 Gyr, according to Portinari
et al. 1998). In such scenario the hidden population of young
stars would be able to generate a collapsar but not contribute to
the host galaxy SED.
6. Conclusions
We have presented an intensive UBVRIZJsHKs broad band
photometryof the GRB 000210host galaxy which has allowed
us to determineits photometricredshift.The derivedphotomet-
ric redshift is z = 0.842+0.054
−0.042, in excellent agreement with the

Page 10

10 J. Gorosabel et al.: The host galaxy of GRB 000210
spectroscopicredshift (z = 0.8463±0.0002)proposedby Piro
et al. (2002). The inferred redshift is basically independent of
the extinction law and IMF assumed, although (at least in the
particularcase of GRB 000210)the Scalo (1986) IMF provides
slightly worse results than Miller & Scalo (1979) and Salpeter
(1955) IMFs. The SED of the host galaxy is consistent with a
starburst template with an age of ∼0.181 Gyr and a very low
extinction (AV ∼ 0). Based on the restframe UV flux a star
formation rate of 2.1 ± 0.2M⊙yr−1is estimated.
The absolute restframe B-band magnitude of the host
(MB = −20.16) is consistent with the distribution of the
MBhost galaxy values measured to date (see Djorgovski et al.
2001, Fig. 2). We determinea value of L = 0.5±0.2L⋆for the
luminosity of the host, in agreement with the value estimated
by Piro et al. (2002).
We have tried to the explore the role played by galactic in-
teractions triggering the GRB phenomena. Many host galaxies
observed to date appear as part of complex and interacting sys-
tems (GRB 980613, Hjorth et al. 2002; GRB 001007, Castro
Cer´ on et al. 2002). According to our study the GRB 000210
host galaxy is a subluminous galaxy with no interacting com-
panions above 0.18 ± 0.06L⋆.
The low value of the extinction obtained in the SED fit
(AV ∼ 0) makes difficult to explain the optical darkness
of GRB 000210 in terms of the global host galaxy dust ex-
tinction. If dust extinction is the reason of the lack of opti-
cal afterglow emission, then the circumburst region has to be
very compact and localised around the progenitor. This hy-
pothesis would agree with observations carried out for the
optically-faint GRB 990705. Andersen et al. (2002) have lo-
calised the optically dim GRB 990705 (but NIR bright, see
Masetti et al. 2000) in a face-on spiral galaxy. Thus given the
thin disk of a spiral galaxy (∼ 0.3 kpc), the optical extinction
of GRB 990705 can not be attributed to the global ISM in its
host. This clumpy and fragmented ISM would also explain the
apparent discrepancy between our SFR estimate (derived from
the galaxy UV flux) and the one recently reported based on the
sub-millimeter range (Berger et al. 2002; Barnard et al. 2002).
Several progenitor models have been discussed in order to
explain the inferred stellar population age and the low host
galaxy extinction. Both the collapsar and the binary merging
modelsshowseverelimitationsto explainthevisible stellarage
andthe line ofsight HI columndensity (derivedfromthe after-
glow X-ray spectrum) respectively. A solution to this problem
would be the existence of a younger population of stars (sev-
eral Myr of age) hidden by the clumpy ISM. Such population
(which would include the progenitor massive star) would not
have any impact in the host galaxy SED. Morphological infor-
mation derived by HST could verify the proposed ISM clumpy
scenario present in the host galaxy of the dark GRB 000210.
Acknowledgments
J. Gorosabel acknowledges the receipt of a Marie Curie
Research Grant from the European Commission. This work
wassupportedbytheDanishNaturalScienceResearchCouncil
(SNF). J.M. Castro Cer´ on acknowledges the receipt of a
FPI doctoral fellowship from Spain’s Ministerio de Ciencia
y Tecnolog´ ıa. J.U. Fynbo acknowledges support from the
Carlsberg Foundation. We acknowledge our referee L. Piro
for fruitful comments. The observations presented in this pa-
per were obtained under the ESO Large Programmes 165.H-
0464(E), 165.H-0464(I)and 265.D-5742(C).
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