A&A 406, L63-L66 (2003)
DOI: 10.1051/0004-6361:20030931
J. P. U. Fynbo 1,2 - P. Jakobsson 2 - P. Møller 3 - J. Hjorth 2 - B. Thomsen 1 - M. I. Andersen 4 - A. S. Fruchter 5 - J. Gorosabel 6,5 - S. T. Holland 7 - C. Ledoux 8 - H. Pedersen 2 - J. Rhoads 5 - M. Weidinger 1 - R. A. M. J. Wijers 9
1 - Department of Physics and Astronomy, University of Aarhus, Ny
Munkegade, 8000 Århus C, Denmark
2 -
Astronomical Observatory, University of
Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
3 -
European Southern Observatory, Karl Schwarzschild-Strasse 2,
85748 Garching, Germany
4 -
Astrophysikalisches Institut, 14482 Potsdam,
Germany
5 -
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD
21218, USA
6 -
Instituto de Astrofísica de Andalucía (IAA-CSIC),
PO Box 03004, 18080 Granada, Spain
7 -
Department of Physics, University of
Notre Dame, Notre Dame, IN 46556-5670, USA
8 -
European Southern Observatory, Casilla 19001, Santiago 19,
Chile
9 -
University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The
Netherlands
Received 30 April 2003 / Accepted 19 June 2003
Abstract
We report on the results of a search for Ly emission from the host
galaxy of the z=2.140 GRB 011211 and other galaxies in its surrounding field.
We detect Ly
emission from the host as well as from six other
galaxies in the field. The restframe equivalent width of the Ly
line
from the GRB 011211 host is about 21 Å. This is the fifth detection of
Ly
emission out of five possible detections from GRB host galaxies,
strongly indicating that GRB hosts, at least at high redshifts,
are Ly
emitters. This is intriguing as only
25% of the
Lyman-Break selected galaxies at similar redshifts have Ly
emission
lines with restframe equivalent width larger than 20 Å. Possible
explanations are i) a preference for GRB progenitors to be metal-poor as
expected in the collapsar model, ii) an optical afterglow selection bias
against dusty hosts, and iii) a higher fraction of Ly
emitters at
the faint end of the luminosity function for high-z galaxies. Of these,
the current evidence seems to favour i).
Key words: gamma rays: bursts - galaxies: high redshift - techniques: photometric
An important aspect of GRB selection compared to other selection mechanisms is that it is not flux
limited. This is obviously the case for most other selection methods;
Lyman-Break selection (Shapley et al. 2003 and references therein) is
continuum flux limited and Ly selection (e.g. Møller & Warren 1993;
Cowie & Hu 1998; Rhoads et al. 2000;
Fynbo et al. 2001;
Ouchi et al. 2003; Fynbo et al. 2003) is
line flux limited. Therefore, GRB selection allows us to probe the faint end
of the luminosity function currently inaccessible to other techniques.
GRB selection is subject to other selection mechanisms, but these are not yet
known in detail (e.g. a relation to the occurrence of star formation).
The precise nature of the GRB selection mechanism provides hints about
the nature of the GRB progenitors (e.g. Woosley 1993; Paczynski 1998;
Hogg & Fruchter 1999).
Ly imaging of GRB hosts is interesting as Ly
emitting galaxies
(in the following we will use the acronym LEGOs - Ly
Emitting
Galaxy-building Objects, Møller & Fynbo 2001) are starburst galaxies with
little or no dust. Ly
imaging is therefore a probe of the star
formation rate and of the dust content of GRB host galaxies. Both of these
parameters are important for our
understanding of GRB progenitors and of how the environment affects the
propagation of afterglow emission out of host galaxies. Furthermore,
Ly
narrow band imaging is an efficient way to probe if the host galaxy
resides in an overdense environment such as a group or a proto-cluster. The
first Ly
narrow band imaging of GRB host galaxies was
presented in Fynbo et al. (2002) where we studied the fields of GRB 000301C and GRB 000926, both at redshift z=2.04. That study resulted in the detection of
Ly
emission from the host of GRB 000926 and 18 additional emitters in
the two fields. The host galaxy of GRB 000301C was too faint,
(Bloom et al. 2002), to allow a detection even if
it has a large Ly
equivalent width (EW). In this Letter we
report on the results of a
search for Ly
emission from the host galaxy of the z=2.140 GRB 011211
and other galaxies in its surrounding field. The properties of the X-ray rich
GRB 011211 and its afterglow are discussed in Holland et al. (2002) and
Jakobsson et al. (2003a). The redshift was measured via absorption lines in
the spectrum of the optical afterglow to be z=2.140 (Fruchter et al. 2001;
Holland et al. 2002). The host galaxy was detected with deep late time imaging
to be a faint
galaxy (Burud et al. 2001; Fox et al.
2002; Jakobsson et al. 2003b).
The observations were carried out during three nights in February
2003 at the 3.5-m New Technology Telescope on La Silla using the
Superb Seeing Imager - 2 (SUSI2). The SUSI2 detector consists of two
thinned, anti-reflection coated EEV CCDs with
a pixel scale of 0
085. The field of GRB 011211 was imaged
in three filters: the standard B and R filters and a special
narrow-band filter manufactured by Omega Optical. The narrow-band
filter (OO3823/59) is tuned to Ly
at z=2.140and has a width of 59 Å (corresponding to a redshift width of
for Ly
or a Hubble flow depth of 4700 km s-1). The total integration
times were 15 hours (OO3823/59), 3.1 hours (B-band), and 1.9 hours
(R-band). The individual exposures were bias-subtracted,
flat-field corrected and combined using standard techniques.
The full-width-at-half-maximum (FWHM) of point sources in
the combined images are 1
10 (R-band), 1
11 (B-band) and 1
22 (OO3823/59).
The narrow-band observations were calibrated using observations of the spectrophotometric standard stars LTT3218, LTT7379, and GD108 (Stone 1996). The broad-band images were calibrated using the secondary standards from Jakobsson et al. (2003b) and brought onto the AB-system using the transformations given in Fukugita et al. (1995).
We used the same methods for photometry and selection of LEGO candidates as those described in Fynbo et al. (2002).
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Figure 1:
A ![]() ![]() ![]() |
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The host galaxy of GRB 011211 is detected in all bands and is a Ly emitter with a restframe EW of
21-8+11 Å. Deep HST/STIS images of
the host have been reported by Fox et al. (2002) and Jakobsson et al.
(2003b). These images show that the host is a multi-component object extending
over almost 1 arcsec or roughly 9 kpc at z=2.14 (assuming H0=65 km s-1 Mpc-1,
and
). In
Fig. 1 we plot the contours of the Ly
emission on top of the
HST/STIS image taken 59 days after the burst. The Ly
emission
weighted centroid of the galaxy is close to the northern knot of the host,
whereas the GRB occurred in the southern end of the host (Fox et al. 2002;
Jakobsson et al. 2003b).
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Figure 2:
Left panel:
calculated colour-colour diagram based on Bruzual &
Charlot (1993) model galaxy spectra.
The filled squares are 0<z<0.6 galaxies with ages from a
few to 15 Gyr and the open triangles are
0.3<z< 3.0 galaxies
with ages from a few Myr to 1 Gyr. The dotted box contains all these
calculated galaxy colours. The full-drawn line corresponds to
objects having the same broad-band colours, but various amounts of
absorption (upper part) or emission (lower part) in the narrow-band filter.
Middle panel:
colour-colour diagram for all objects in the GRB field. The
squares with error-bars indicate objects detected at S/N>5 in the
narrow-band image. As expected, most objects have colours consistent
with being in the dotted box. However, a number of objects, including
the GRB 011211 host, are seen in the lower left part of the diagram.
Right panel: the colours of the seven LEGO candidates which are
selected to have
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Figure 3:
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Table 1:
Photometric properties of the seven LEGO candidates, including the host
galaxy, in the field of GRB 011211. Upper limits are 2.
S1211_6 is
the host galaxy of GRB 011211.
The host galaxy of GRB 011211 has been found to be a Ly emitter
with a restframe EW of
21-8+11 Å. This is somewhat smaller than for GRB 000926 (
71-15+20 Å) and suggests the presence of more dust.
Although
uncertain, the UV continuum of GRB 011211 host is also redder than that
of the GRB 000926 host. The observed
colour corresponds to
,
whereas Fynbo et al. (2002) found
and
for the two main components
of the GRB 000926 host galaxy. Ly
emission has also been
detected from the host galaxies of GRB 971214 at z=3.42 (Kulkarni et al.
1998; Ahn 2000), GRB 021004 at z=2.33 (e.g. Møller et al. 2002 and
references therein), and GRB 030323 at z=3.37 (Vreeswijk et al., in
preparation). For GRB 021004 and GRB 030323 the Ly
emission line is
detected
superimposed on the afterglow spectrum. All current evidence is consistent
with the conjecture that the host galaxies of GRBs, at least at high
redshifts, are Ly
emitters. In contrast, only
25% and
33% of the Lyman-Break selected galaxies at similar redshifts are
Ly
emitters with a restframe EW larger than 20 Å and 10 Å
respectively (Shapley et al. 2003). The median restframe EW of the Ly
line
for Lyman-Break galaxies (LBGs) is
0 Å (about half of the LBGs
have Ly
in absorption).
The restframe EW of the Ly
emission line from the GRB 021004 host is
constrained to be higher than 50 Å (Møller et al. 2002). The restframe EW
for the host galaxy of GRB 971214 is measured spectroscopically to
be 14 Å (Ahn, private communication), whereas it is
unknown for GRB 030323 as its host is still undetected. If we
conservatively assume that the restframe EW is above 10 Å for three hosts
(971214, 011211, 030323) and above 20 Å for two hosts (000926, 021004) then
GRB host galaxies are inconsistent with being drawn randomly from the same
Ly
EW distribution as the LBGs at the
% level.
This remarkable fact can be explained by a preference for GRB progenitors to
be metal-poor. Ly emission with EW larger than 20 Å is
locally only found in starforming galaxies with
(Charlot & Fall 1993, their Fig. 8; Kunth et al. 1998; Kudritzki et al. 2000).
Furthermore, Shapley et al. (2003) find that the collisionally
excited UV nebular emission lines of C III] and [O III] are
stronger than average for the quartile of the their sample with Ly
Å. By analogy with local starbursts this also implies low
metallicity (Heckman et al. 1998). In the collapsar model (Woosley 1993) a
strong stellar wind, which
is the consequence of a high metallicity, makes it difficult to produce a GRB due to mass loss and loss of angular momentum (MacFadyen & Woosley 1999).
Therefore, a preference for GRB hosts to be metal poor is a clear prediction of
the collapsar model. Alternatively, the explanation could be an optical
afterglow selection bias against dusty hosts. For 60-70% of the searches for
optical afterglows since 1997 no detection was made - the dark burst problem
(Fynbo et al. 2001; Berger et al. 2002). Thus, the bursts for which a bright
optical afterglow is detected, including all the bursts with detected
Ly
emission from their hosts, are biased against very
dusty host galaxies (Fynbo et al. 2001; Lazzati et al. 2002; Ramirez-Ruiz
et al. 2002). This is important as even small amounts of dust will
preferentially destroy Ly
photons due to resonant scattering (e.g.
Ferland & Netzer 1979). However, it remains to be shown that the majority of
dark bursts indeed are dust obscured. In fact, several bursts have been found
to be optically dim without significant extinction (Hjorth et al. 2002;
Berger et al. 2002; Fox et al. 2003; Hjorth et al. 2003). The dark bursts are
also generally fainter in X-rays (De Pasquale et al. 2003) again implying that
they are intrinsically dim or very distant. Finally, the fraction of
Ly
emitters could be larger at the faint end of the high-z luminosity
function, where most GRB hosts are found, than the fraction found for the
bright LBGs. Shapley et al. (2003) find that among the LBGs with
Ly
Å the EWs are largest for the faintest
galaxies, but argue that a constant fraction of Ly
emitters down to R=25.5 is consistent with the data when selection effects are taken
into account. Furthermore, a higher
fraction of Ly
emitters at the faint
end of the luminosity function would
also imply a lower metallicity and this
is therefore not in conflict with a low metallicity preference for GRB hosts.
In conclusion, a lower metallicity of GRB hosts compared to LBGs in general
seems to be well established.
Acknowledgements
We thank our anonymous referee for a very constructive report that helped us improve the paper on several important points. We also thank Stan Woosley for helpful comments and the La Silla staff for excellent support during our run. JPUF acknowledges support from the Carlsberg Foundation. PJ acknowledges support from The Icelandic Research Fund for Graduate Students, and from a special grant from the Icelandic Research Council. STH acknowledges support from the NASA LTSA grant NAG5-9364. We acknowledge benefits from collaboration within the EU FP5 Research Training Network "Gamma-Ray Bursts: An Enigma and a Tool''. This work is supported by the Danish Natural Science Research Council (SNF).