Astronomical Science

GHostS – Gamma-Ray Burst Host Studies

Sandra Savaglio1 shift was measured in 1997, when GRBs The GRB host population Tamás Budavári 2 were finally confirmed to be cosmologi- Karl Glazebrook 3 cal sources. Today there are 108 GRBs There is no doubt that studies of GRB Damien Le Borgne 4 with measured , the highest being host are entering into a realm of Emeric Le Floc’h 5 GRB 050904, at z = 6.3. The typical galaxy formation that was hardly known Hsiao-Wen Chen 6 energy emitted by a GRB, in a couple of before. The reason is that GRB hosts Jochen Greiner1 minutes, is 1051 ergs, equivalent to the are generally faint and detected at high Aybuk Küpcü Yoldas¸ 1 energy emitted by the sun in 10 billion . Similar galaxies are very hard years. to find using conventional techniques, because they would require very long 1 Max-Planck Institute for Extraterrestrial Today GRBs are primarily discovered by integration times, even for the largest and Physics, Garching, Germany the satellite Swift (http://swift.gsfc.nasa. most efficient telescopes. 2 Johns Hopkins University, Baltimore, gov), the dedicated NASA mission which USA in two years of operation helped to double GRB events offer a shortcut to the quest 3 Swinburne University, Melbourne, the number of measured redshifts. With for faint and distant galaxies. They are Australia the growing data collected from space detected as short and energetic events. 4 CEA-Saclay, Gif-sur-Yvette Cedex, and ground telescopes, and the advent The localisation (and for a fraction of France of Swift, our group decided to create a them the redshift) is measured from the 5 Institute for Astronomy, Honolulu, database, available to scientists, which bright X-ray and optical afterglow. We Hawaii collects observational results on the gal- know that long-duration GRBs occur in 6 University of Chicago, USA axies hosting GRB events. We called it regions of formation, therefore, they GHostS, or GRB Host Studies. Our focus are associated with galaxies. Dedicat- is to explore and unveil the nature of gal- ed programmes can observationally com- GHostS is the largest public data-base axies in which GRBs occur. Our goal is to plete the investigation. on gamma-ray burst (GRB) host galax- answer fundamental questions, such as: ies and is accessible at the URL http:// are these galaxies different from the gen- In our GHostS search, we want to explore www.grbhosts.org. Started in 2005, it eral galaxy population; are they special in many of the most interesting galaxy pa- currently contains photometric and any way; can we use them to understand rameters, such as metallicity, star-forma- spectroscopic information on 39 GRB galaxy formation under extreme condi- tion rate, stellar mass, age of the stellar hosts, almost 2/5 of the total number tions? population and dust extinction. Each of of GRBs with measured redshift. It will these parameters are very difficult to de- continue to grow, together with the un- Today the GHostS database includes rive. One difficulty is the faintness of the stoppable data flow from the obser- results for 39 GRB host galaxies. This is typical GRB host. Moreover, the data ob- vatories all over the world, every time a large number, considering that only tained by the community are often not a new event is discovered. Among 108 GRBs have known redshift (see Fig- homogeneous. To complicate the picture, other features, GHostS uses the Virtual ure 1). the tools often used to derive the physical Observatory resources. quantities are affected by systematic un- certainties which are sometimes greater than the relations being derived. The Gamma-Ray Burst phenomenon

Gamma-ray bursts (GRBs) are the most energetic events in the Universe, and also among the fastest. They are associ- 12 ated with the death of massive (core collapse supernovae) or the merging of 10 compact objects, such as neutron stars and black holes. During the explosive 8 phase, they emit most of their energy as a collimated flux of gamma-ray photons from neutrino-antineutrino annihilations, 6 Number and as gravitational waves. The gamma emission lasts from a few milliseconds to 4 a few minutes. 2 The first GRB was discovered in 1967, by the US military satellite Vela, but it took Figure 1: Redshift distribution of all 0 GRBs (empty histogram) and the six years, until 1973, for the first paper to 0 321 4 5 6 7 appear in a scientific journal. The first red- subsample of GRBs included in the Redshift GHostS database (blue histogram).

The Messenger 128 – June 2007 47 Astronomical Science Savaglio S. et al., GHostS – Gamma-Ray Burst Host Studies

The typical GRB host is a star-forming, 9 Figure 2: Histogram of the stellar mass low-mass and low-metallicity galaxy, de- of GRB host galaxies in the redshift 8 interval 0 < z < 6.3, from the GHostS tected at redshift below z = 2. The mean sample. The mean stellar mass is stellar mass we derived in our sample 7 of the order of the stellar mass of the is similar to the stellar mass of the Large Large Magellanic Cloud. This is ten Magellanic Cloud (Figure 2). Observation- 6 times smaller than the stellar-mass limit reached by the deepest high- al limitations prevent us from fully explor- redshift galaxy surveys, performed by ing the same parameters in GRBs at 5 todays’ largest telescopes. larger redshift, but in exceptional cases. 4 The detection of the cold interstellar me- Number dium in high-redshift hosts indicates that 3 there the population is different, with larger stellar masses, higher star-forma- 2 tion rates and higher metallicities (Berger et al. 2005, Fynbo et al. 2006, Savaglio 1 2006). It is still unclear if this means a 0 different population, or different observa- 7 98 10 11 12 tional biases. Such issues will be faced log M�(M�) and fully exploited by the future genera- tion of telescopes and instruments. Figure 3: Example of the In general, we know that GRB hosts are GHostS database web pages. Each GRB entry very peculiar galaxies. We still do not includes basic informa- know whether this is because our tradi- tion on the event, togeth- tional observational technique cannot er with access to rele- reach the extreme limits attained by GRB vant papers available in the literature. It reports detections, or because GRB hosts are in- multi-band photometry, trinsically different. The main limitation is fluxes of emission lines the small number of galaxies discovered and images of the field. so far. This number is still below 100, while the number of galaxies in today’s sur- veys exceed 105.

The GHostS database

GHostS includes GRB hosts for which photometric and spectroscopic informa- tion is available from the literature (Fig- ure 3). In total 39 galaxies, out of a total of 108 GRBs whose redshift is known, have been gathered from 81 different pa- pers and sources. 7 10–17 Figure 4: Very Large Telescope spectrum of the galaxy hosting –17 One of the spectra in the GHostS data- 6 10 GRB 990712 at redshift base is shown in Figure 4. The galaxy z = 0.433 (Küpcü Yoldas¸ hosting GRB 990712 at redshift z = 0.433 5 10–17 et al. 2006). was observed at the Very Large Telescope /s/Å) (Küpcü Yoldas¸, Greiner, Perna, 2006). It 2 4 10–17

has very strong emission lines, originating ] ] in regions of the galaxy where star forma- III III [O 3 10–17 [O tion is very active. We derived a stellar Flux (erg/cm ]

9 II mass of the galaxy of a few times 10 MA, –17 β

2 10 [O H similar to the stellar mass of the Large ] III

Magellanic Cloud, but it forms stars γ

–17 [Ne at a rate that is ten times higher, about 1 10 H –1 6 MA yr . This gives a specific star for- 0 mation (the SFR per unit stellar mass) of 5000 5500 6000 6500 7000 7500 100 times that in the Milky Way. Similar Wavelength (Å)

48 The Messenger 128 – June 2007 galaxies hosting GRBs are detected up to a distance when the Universe was less 1� N than 1 Gyr old. They are very hard to find using normal tools of investigation. Most surveys, probing the high-redshift Uni- verse today, can reach, in terms of stellar mass, galaxies similar to or larger than the Milky Way. E W It is generally not easy to collect results published by the GRB community, be- cause the number of papers available for each event can be large. One extreme case was GRB 060218, which occurred in February 2006, at redshift z = 0.0335, S associated with a supernova, detected three days after the alert (Masetti et al. 2006). In about a year and a half, more vey (POSS, SERC). Other types of auto- Figure 5: One example of the tools offered than 20 articles dedicated to this event mated searches are also in the works, by GHostS. The images show the field of GRB 050509b, at redshift z = 0.2248 (Gehrels et al. have been published (or are in the proc- e.g., for finding spectra at the given posi- 2005), obtained from the Sloan Digital Sky Sur- ess of being published) in refereed jour- tion. These VO additions, the efficient vey (left) and Digitalized Sky Survey (right) archives, nals, four of which appeared in Nature. database, and the presentation of the through the Virtual Observatory. scientific data, make the website not only This proliferation of material is one of an invaluable research tool for GRB host bers would be larger if we were able to the main motivations for the existence of studies, but also very enjoyable to navi- observe all GRB afterglows early on. In GHostS. Observational results are gate. GHostS will feature in the future ad- fact, 2/3, of the Swift GRBs have no mea- searched, collected, homogenised and vanced search capabilities for the com- sured redshift. The fractions of z > 4 and made easily accessible for the whole munity, fine-tuned for astronomical data z > 5 known QSOs are 0.8 %. and 0.03 % community. The database is constantly and will eventually dynamically search VO of the total, with only 17 QSOs known growing in terms of total number of resources for relevant observations, such with redshift larger than 5. objects, and in terms of tools offered. We as images and spectra. plan to eventually include all host galax- Particularly impressive was GRB 050904, ies discovered in the past and in the fu- the record holder at z = 6.3 (Kawai et al. ture for the years of operation of Swift, The future 2005). This GRB was observed with the and it will likely contain a final sample of a Japanese telescope Subaru more than few hundred galaxies. The future of GRB science fully depends three days after the Swift alert, when it on the life of GRB hunting machines, had already faded 11 magnitudes from its i.e. gamma-ray detectors. Today it almost initial brightness. Fortunately at high red- The Virtual Observatory exclusively relies on the existence of shifts the brightness of a fading object Swift, which has been funded for an ad- is helped by time dilation, the effect de- The Virtual Observatory (VO) is one of the ditional four years. scribed in Einstein’s theory of General features offered in our database. The Relativity. If this GRB had occurred in the VO is one of the newest and most prom- Among the many unexplored territories, local Universe, it would have faded at ising tools introduced in the astronomy we mention the discovery of the very a rate more than seven times faster. The world, allowing scientists to easily access high redshift GRBs, at z > 6. We know spectrum of this spectacular event is data from multiple astronomical obser- that there are collapsed objects at those shown in Figure 6 (from Totani et al. 2006). vatories, both ground- and space-based times. We do not know under which The host galaxy is barely detected by the facilities, through one portal (see for in- physical conditions these objects formed: Hubble Space Telescope and Spitzer stance http://www.us-vo.org/). Its com- are they stars, galaxies or massive black Space Telescope (Berger et al. 2006). We ponents have been added to the GHostS holes? At z > 6, QSO searching has failed estimate a stellar mass which is half web portal (Figure 5). Using information in providing a satisfactory picture, and that of the Large Magellanic Cloud, and a about the GRBs and their hosts stored galaxies are incredibly hard to find. GRBs rate of star formation ten times higher. Al- inside our SQL database, one can auto- on the other hand have shown some though this event happened less than matically enhance the view of the cata- very promising results. In the first two 900 million years after the Big Bang, the logued hosts by using various VO re- years of operations, Swift has discovered absorption lines detected in its spectrum sources. The details pages currently show 8 GRBs at redshift larger than 4, and 4 indicate a surprisingly high pollution of images of the relevant pieces of the sky at redshift larger than 5. This is 7% and heavy elements in the interstellar medium as seen by the Sloan Digital Sky Survey 3.5 % of the total GRB population with in the host galaxy, of the order of 1/10 (SDSS) and the legacy Digitized Sky Sur- measured redshift, and probably the num- that of the solar vicinity (Totani et al. 2006;

The Messenger 128 – June 2007 49 Astronomical Science

Figure 6: Subaru spectrum of the most distant GRB ever discovered, GRB 050904 at z = 6.3 (Totani et al. 2006). It was observed more than three days after the alert given by the gamma-ray burst finder Swift, when it had already faded 11 magnitudes from its initial brightness. Savaglio 2006). This is hardly explained by most theories of chemical enrich- ment in the primordial Universe. Galaxies 1.5 like these are very likely faint and dormant today, because they would have con- ) 1 sumed their gas reservoir for star forma- –1 tion in less than one Gyr, i.e. by redshift Å –1 z = 3. s

–2 0.5 If GRB 050904 were observed two hours GRB 050904 after the Swift alert, its spectrum would 3.4 days

erg cm 0

have been much more spectacular, with –18

a signal-to-noise ratio ten times better (10 λ (C IV) S II O I C II than that of the Subaru spectrum. Noth- F –0.5 ing like this has ever been achieved in the Lyα + Si II 1 errors remote Universe with normal galaxies σ or QSOs. It is only a matter of time that, –1 8600 8 800 9 000 9 200 9 400 9 600 9800 10 000 sooner or later, Swift will trigger similar Observed wavelength (Å) events at higher redshift. Then telescopes from the ground will be ready to catch the fading sources and deliver a unique data set to the scientific community, for References Kawai N. et al. 2005, GRB Coordinates Network new exciting discoveries. We will be there 3937, 1 Berger E. et al. 2005, ApJ 642, 979 Küpcü Yoldas¸ A., Greiner J. and Perna R. 2006, to continue our service to the community Berger E. et al. 2006, ArXiv Astrophysics e-prints, A&A 457, 115 with GHostS, in the attempt to fully char- arXiv: astro-ph/0603689 Masetti N. et al. 2006, GRB Coordinates Network acterise those galaxies parenting one of Fynbo J. P. U. et al. 2006, A&A 451, L47 4803, 1 the most extraordinary objects in the Uni- Gehrels N. et al. 2005, Nature 437, 851 Savaglio S. 2006, New Journal of Physics 8, 195 Totani T. et al. 2006, PASJ 58, 485 verse.

The Rapid-Eye Mount (REM) 0.6-m telescope at the ESO La Silla Ob- servatory, which is dedi- cated to the follow-up of gamma-ray bursts, is shown in operation. ESO PR 26/07 provides details of the telescope and some science re- sults.

50 The Messenger 128 – June 2007