Are the hosts of Gamma-Ray Bursts sub-luminous and blue galaxies?

Astrophysics, abstract
astro-ph/0301149

From: Emeric Le Floc’h <elefloch@discovery.saclay.cea.fr>

Date: Thu, 9 Jan 2003 12:15:08 GMT   (513kb)

Authors:
E. Le Floc’h (CEA-Saclay, France),
P.-A. Duc,
I.F. Mirabel,
D.B. Sanders,
G. Bosch,
R.J. Diaz,
C.J. Donzelli,
I. Rodrigues,
T.J.-L. Courvoisier,
J. Greiner,
S. Mereghetti,
J. Melnick,
J. Maza,
D. Minniti

Comments: 15 pages, 6 figures, accepted for publication in Astronomy &
Astrophysics

We present K-band imaging observations of ten Gamma-Ray Burst (GRB) host
galaxies. We compare their observed and absolute K magnitudes as well as their
R-K colours with those of other distant sources detected in various optical,
near-infrared, mid-infrared and submillimeter deep surveys. We find that the
GRB host galaxies, most of them lying at 0.5<z<1.5, exhibit very blue colours,
comparable to those of the faint blue star-forming sources at high redshift.
They are sub-luminous in the K-band, suggesting a low stellar mass content. We
do not find any GRB hosts harbouring R- and K-band properties similar to those
characterizing the luminous infrared/submillimeter sources and the extremely
red starbursts. Should GRBs be regarded as an unbiased probe of star-forming
activity, this lack of luminous and/or reddened objects among the GRB host
sample might reveal that the detection of GRB optical afterglows is likely
biased toward unobscured galaxies. It would moreover support the idea that a
large fraction of the optically-dark GRBs occur within dust-enshrouded regions
of star formation. On the other hand, our result might also simply reflect
intrinsic properties of GRB host galaxies experiencing a first episode of very
massive star formation and characterized by a rather weak underlying stellar
population. Finally, we compute the absolute B magnitudes for the whole sample
of GRB host galaxies with known redshifts and detected at optical wavelengths.
We find that the latter appear statistically even less luminous than the
sub-luminous blue sources which mostly contributed to the B-band light emitted
at high redshift. This indicates that the formation of GRBs could be favoured
in particular systems with very low luminosities and, therefore, low
metallicities. (Abridged)

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