The blue host galaxy of the red GRB 000418^*

¹ Instituto de Astrofísica de Andalucía (IAA-CSIC), PO Box 03004, 18080 Granada, Spain; e-mail: jgu@iaa.es, ajct@iaa.es
² Danish Space Research Institute, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark e-mail: jgu@dsri.dk, nl@dsri.dk
³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA e-mail: gorosabel@stsci.edu, fruchter@stsci.edu, burud@stsci.edu, rhoads@stsci.edu
⁴ Thüringer Landessternwarte Tautenburg, 07778 Tautenburg, Germany; e-mail: klose@tls-tautenburg.de
⁵ Astrophysikalisches Institut, 14482 Potsdam, Germany; e-mail: lchristensen@aip.de
⁶ Department of Physics and Astronomy, University of Aarhus, Ny Munkegade, 8000 Århus C, Denmark e-mail: jfynbo@phys.au.dk
⁷ Astronomical Observatory, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark e-mail: jens@astro.ku.dk, brian_j@astro.ku.dk, holger@astro.ku.dk
⁸ Max-Planck-Institut für extraterrestrische Physik, 85741 Garching, Germany; e-mail: jcg@mpe.mpg.de
⁹ Department of Physical Sciences, University of Hertfordshire, College Lane, Hatfield, Herts AL10 9AB, UK e-mail: nrt@star.herts.ac.uk
¹⁰ Department of Physics, University of Notre Dame, Notre Dame, IN 46556–5670, USA; e-mail: sholland@nd.edu
¹¹ European Southern Observatory, Casilla 19001, Santiago 19, Chile; e-mail: ajaunsen@eso.org, pvreeswi@eso.org
¹² Real Instituto y Observatorio de la Armada, Sección de Astronomía, 11.110 San Fernando-Naval, Cádiz, Spain e-mail: josemari@alumni.nd.edu
¹³ Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34131, Trieste, Italy; e-mail: pian@tesre.bo.cnr.it
¹⁴ Dipartimento di Fisica, Università di Ferrara, via Paradiso 12, 44100 Ferrara, Italy; e-mail: frontera@fe.infn.it
¹⁵ Istituto Tecnologie e Studio Radiazioni Extraterrestri, CNR, via Gobetti 101, 40129 Bologna, Italy e-mail: filippo@bo.iasf.cnr.it, masetti@bo.iasf.cnr.it, eliana@bo.iasf.cnr.it
¹⁶ University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands e-mail: lexk@science.uva.nl,evert@science.uva.nl,isabel@science.uva.nl, rwijers@science.uva.nl, edvdh@science.uva.nl
¹⁷ NASA MSFC, SD–50, Huntsville, AL 35812, USA; e-mail: kouveliotou@eagles.msfc.nasa.gov

Corresponding author: J. Gorosabel, jgu@iaa.es

Received: 30 April 2003
Accepted: 7 July 2003

Abstract

We report on multi-band () observations of the host galaxy of the April 18, 2000 gamma-ray burst. The Spectral Energy Distribution (SED) is analysed by fitting empirical and synthetic spectral templates. We find that: (i) the best SED fit is obtained with a starburst template, (ii) the photometric redshift is consistent with the spectroscopic redshift, (iii) the colours of the host are inconsistent with an old stellar population, and (iv) the global extinction is constrained to be in the range –0.61 mag. The derived global extinction agrees with the one reported for the afterglow (–0.9 mag), suggesting a homogeneous distribution of the interstellar medium (ISM) in the host galaxy. These findings are supplemented by morphological information from Hubble Space Telescope (HST) imaging: the surface brightness profile is smooth, symmetric and compact with no underlying structures (like dust lanes, spiral arms or disks). A natural scenario which accounts of all the above results is a nuclear starburst that harbours a young population of stars from which the GRB originated.