The spatially resolved host of GRB 060505 and implications for the nature of the progenitor
aa r X i v : . [ a s t r o - ph ] J un The spatially resolved host of GRB 060505and implications for the nature of theprogenitor
Christina C. Th¨one a , Johan P. U. Fynbo a a Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, JulianeMaries Vej 30, 2100 Copenhagen, Denmark
Abstract
We present a study of the host galaxy of the Gamma-Ray Burst (GRB) of May 52006 (GRB 060505). The host is spatially resolved in both imaging data and in along slit spectrum including the GRB site. We find the galaxy to be a Sbc spiral,which is unusual for a long GRB host galaxy. The site of the GRB is considerablydifferent from the rest of the galaxy with intense star formation, low metallicityand a young age. This suggest a massive stellar progenitor rather than a merger ofcompact objects which has been suggested based on the the relatively short durationof T =4s for the prompt emission. Key words:
GRBs: GRB 060505, GRB host galaxies, emission lines
Until May 2006, the division of GRBs in short (T < s ) and long durationbursts (T > of only 4s could also suggest acompact merger origin. Preprint submitted to Elsevier 31 December 2018 ne way to reveal the nature of the GRB progenitor is to investigate theproperties of the burst site. Due to their distance, we are usually restrictedto analyze the global properties of GRB host galaxies, but as most long GRBhosts are relatively small, the properties of the host and the environment arelikely to be comparable. This is however different for large hosts such as thespiral host of GRB 060505, whose properties can vary significantly over thegalaxy. This we investigate in this paper by analyzing a spatially resolvedspectrum of the host.
We used FORS2 at the VLT and grism 300V with a 1.0” slit (resolution: 11˚A) on May 23 to obtain a series of longslit spectra of the host galaxy whichwere stacked into a single spectrum. From the spectrum covering the bulge ofthe galaxy, the GRB site in the northern arm and a spiral arm south of thebulge, five traces from the different regions were extracted and flux calibrated.Furthermore, we obtained FORS1+ISAAC/VLT images in UBVRIzK takenon Sep. 14, 24 and Oct. 1. Further details on the reduction and calibration ofthe spectra and the images can be found in (7).
The host galaxy of GRB 060505 is classified as an Sbc spiral from the mor-phology, the strength of the emission lines and the colors. Spiral galaxies arethe exceptions among long GRB host galaxies, there are however some earlierexamples, namely GRB 980425 (8), GRB 990705 (9), GRB 020819 (10) andpossibly GRB 051109B (11), of which one, GRB 980425 was clearly connectedto a SN (12). Remarkably, the GRB has been found in the outer parts of theirhosts in all these cases.For GRB 060505 it was possible for the first time to derive a rotation curvefor a GRB host galaxy by tracing the Doppler shift of four strong emissionlines along the spatial profile. The rotation curve flattens at a velocity of212 km/s, taking into account the inclination of the galaxy of 49 deg. Fromthat we derive a mass within the half-light diameter (24 kpc) of 1.3 × M ⊙ .2 The GRB site
GRB 060505 was situated in the northern spiral arm of its host galaxy withina bright star-forming region (4) as confirmed also by HST imaging (13). Thisspeaks in favor of the collapsar origin as short GRBs are expected to befound far away from the birth sites of their compact object progenitors. (13)calculated a minimum merger age of 10 Myr from the size of the star-formingregion assuming a very low kick velocity. This is on the lower limit of the delaytime for a compact object merger (14).The extinction measured from both the afterglow spectral energy distribution(Xu et al., in prep.) and the spectrum of the burst site show a very lowextinction of A V < An important ingredient in the modelling of collapsars that can produce aGRB is the metallicity of the progenitor star. Only metal poor stars are be-lieved to preserve enough angular momentum to be able to lauch a jet (15,e.g.). Long GRBs have genereally been found in sub-solar metallicity environ-ments (16, e.g.) which supports this theory.We determined the metallicity in the five different spectra from the Oxygenemission lines using the so-called R parameter (17), which has been widelyused for other GRB hosts. We further apply the latest recalibration from (18).This tentativlely gives a very low Oxygen abundance of log(12+O/H)=7.8 forthe GRB site whereas it is around or even above solar in the rest of the galaxy(see Fig. 1).From the H α flux which is proportional to the amount of young stars, wederive the star-formation rate (SFR) in the different parts of the galaxy andscale it with the V band luminosity in the same parts. We then find a mod-erate specific SFR of 3 to 8 M ⊙ /yr/(L/L*) in the bulge and a high SF of 18M ⊙ /yr/(L/L*) at the GRB region. Both the low metallicity and the high SFRat the GRB site give an indication for a collapsar origin of the GRB ratherthan a merger event. 3 GRBsiteupperbulgemiddlebulgelowerbulgespiralarm C oun t s i n H α SS F R [ M O • / y r / ( L / L * ) ] + l og ( O / H ) A ge [ M y r ] Fig. 1. Properties of the host galaxy in 5 different parts of the host galaxy, indicatedin the 2D spectrum. For the full 2D spectrum see also (7; 21)
We used several different approaches for determining the age of the stellarpopulation in the different parts of the galaxy. The GRB site is a star-formingHII region which speaks for a low age of the stellar population. For veryyoung populations, the EW of H α gives an upper limit on the age of thepopulation (19). Our H α -EW of −
181 ˚A leads to an age of < yr(see also Fig. 1). The comparison of the GRB site with the rest of the host galaxy reveals asubstantial difference in its properties. Whereas the host galaxy itself turnsout to be a regular late-type Sbc spiral, the GRB site has properties closeto those of the more common type of long GRB host galaxy, low-mass, lowmetallicity, highly starforming dwarf galaxies. The position of the GRB rightwithin a star forming region and the very young age of the stellar populationspeaks much more in favour of a collapsar progenitor than a merger of twocompact objects. The low metallicity is a further hint for the massive starprogenitor while a merger could occure in any environment. (2) had proposedthat if long GRBs occure in spirals, they should be found in the outer spiral4rms in younger populations and low metallicity environments as we see here.We conclude that this GRB was very likely coming from the death of a massivestar rather than a merger and the prognitor might just not have produced a(bright) supernova.
Acknowledgements
The “Dark Cosmology Centre” is funded by the DNRF.