GRB Host Galaxies and the Uses of GRBs in Cosmology

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1 GRB Host Galaxies and the Uses of GRBs in Cosmology S. G. Djorgovski for the Caltech-NRAO-CARA GRB Collaboration: S.R. Kulkarni, D.A. Frail, F.A. Harrison, R. Sari, J.S. Bloom, E. Berger, P. Price, D. Reichart, D. Fox, T. Galama, S. Yost, G. Taylor, A. Mahabal, S. Castro, F. Chaffee, R. Goodrich, and many others

2 Issues to be Addressed: What can we learn about GRBs from their host galaxies? Redshifts Physical parameters Location SF, SN connections, progenitors What can GRBs do for cosmology? several things Future prospects

3 What Can GRBs do for Cosmology? A new way to select star-forming galaxies at intermediate/high redshifts A new probe of the faint end of the high-z galaxy luminosity function Absorption probes of the inner portions of high-z galaxies, complementary to QSO abs. Probes of the star formation history and the obscured SF fraction Future: Probes of the primordial star formation and reionization

4 The Observed Magnitude Distribution for the GRB Host Galaxies Why the drop? Selection effects? Median R = 25 mag (33 hosts, as of May 2002)

5 Deep Field Galaxy Counts (Brunner et al.) Red: GRB Hosts (arb. scaling)

6 Examples of GRB Host and Afterglow Spectra GRB z = 1.600

7 GRB Redshift Distribution, May emission 13 absorption 3 x-ray 27 total = { } 5 both } 1 both

8 Morphologies (and most or all other properties) of GRB hosts are normal for their redshifts Holland et al. Bloom et al.

9 Location of GRBs Within Their Host Galaxies

10 Location, location, location Bloom et al. 2001

11 Cumulative Offset Distribution: GRBs Follow the (Young) Starlight Bloom et al. 2001

12 Galaxy Evolution Preliminaries: Selection Modes and Selection Effects Detection in direct emission Optical/NIR (restframe UV): unobscured SF only Sub-mm (restframe FIR): so far ULIRGs only NB: redshift-dependent, sliding selection effects: Low z: luminous mass High z: instanteneous or recent SFR Detection in line absorption Independent of emission properties; counterparts a mixed bag Large galactic radii pref. (larger targets, low extinction) Integrated emission: diffuse backgrounds No redshift information, but bypasses many selection effects

13 GRB Host Spectra are Consistent with Actively Star Forming Galaxies Histograms: GRB host galaxy spectra. Lines: Bruzual-Charlot models GRB GRB

14 [O II] 3727 Doublet Equivalent Widths: A Rough Measure of the SFR per Unit Mass Black points: Field galaxies (Hogg et al.) Red circles : GRB Hosts

15 Neon Lines: A Direct Evidence for Massive Star Formation? [Ne III] / [O II] line ratios: GRB host galaxies: mean = 0.24 median = 0.18 LMC H II regions: mean = 0.06 median = 0.04 Consistent with models with T e > K, low metallicities

16 Star Formation Rates in GRB Hosts From optical spectroscopy: Emission lines, mostly [O II] 3727, some Hα, Hβ UV continuum at λ rest = 2800 A (or 1500 A) Typical measured SFR ~ a few M sun /yr Typical internal extinction (reddening) corrections: a factor of a few (optically thin dust) Blind to a fully obscured component From radio/sub-mm fluxes: Fully obscured component only (opt. thick dust) Typical SFR ~ a few 100 M sun /yr but it is an obvious selection effect

17 High-z Galaxies Detected in Absorption QSO absorbers: Galaxies along random lines of sight Selected by the gas cross section, column density Mainly the outer regions of galaxies/halos GRB hosts: In situ absorption, inner (SF) regions of galaxies: complementary to the QSO absorbers Probing the GRB environments and their possible modification by the bursts Much higher column densities than in QSO abs. Some evidence for the dust depletion (Savaglio et al.)

18 QSO Mg II (metallic line) Absorbers GRB hosts: a few kpc? (Steidel)

19 Distribution of Mg II 2796 Equivalent Widths QSO Absorbers (Steidel & Sargent) GRBs

20 GRB Hosts vs. QSO Absorbers Salamanca et al. 2002

21 GRB Hosts vs. QSO Absorbers Fiore et al. 2001

22 GRBs vs. QSO Absorbers: A Complementary Picture? GRBs show higher gas densities and metallicities, and have significantly lower [(Si,Fe,Cr)/Zn] ratios, implying a higher dust content SF regions? GRBs? disk stars GRBs? DLAs Pettini et al. Lu et al.

23 Evolution of the Field Galaxy Luminosity Function at Moderate Redshifts Deep field redshift surveys show a brightening of L and a steepening of the faint end slope of the GLF out to z ~ 1. GLF is steeper for the late-type galaxies at every redshift. (CFRS, Lilly et al.; CNOC, Lin et al.; LDRS, Ellis et al.; CADIS, Fried et al.; HDF, Poli et al.; etc.) CNOC: Lin et al.

24 GRB Host Galaxy Luminosity Function z ~ 0 GLF GRB hosts: M B computed assuming H 0 = 65, Ω 0 = 0.2, f ν ~ ν -0.5 Models: Luminosity-weighted Schechter functions

25 GRB Host Galaxy Luminosity Function A new probe of the GLF at moderate/high redshifts, especially the faint end (not easily accessible otherwise) A modest fading of L, contrary to the deep field surveys A modest steepening of the faint end slope α, similar to the deep field surveys Consistent with the z ~ 0 LF for the late-type, SF galaxies However, there may be some important biases: GRB hosts: Higher SFR higher L more detectable Field surveys: Flux limited higher L Are these really low-luminosity galaxies, or small, unobscured pieces of big, luminous galaxies? Candidates for hidden giants : , , , , , C,

26 GRBs from Obscured, Star-Forming (Ultraluminous) Galaxies which may have faint optical counterparts GRB : Frail et al. 2001

27 A key issue: Geometry of Dust Obscuration GRB LOS extinction global host galaxy extinction

28 A Prototype Dark Burst: GRB RT We know that some GRBs originate in dusty starburst galaxies, and can thus be used as probes of obscured star formation in the universe Sub-mm/cm detections: , , (Berger, Frail, et al.)

29 A Census of Optical Transients: Constraining the Obscured Fraction of the Total Star Formation in the Universe The number of well-localized GRBs, with adequate searches for optical transients (OTs), as of May 2002: 64 ± 6 The number of OTs found: 35 ± 3 Thus, if GRBs trace (massive) star formation, the OT discovery rate implies that the total obscured SF fraction is at most ~ 45 ± 8 %

30 Diffuse Optical and Sub-mm Backgrounds: Integrated Star Formation History EBL IRAS COBE Gal. cts. Madau Approximately equal amounts of energy in the unobscured and obscured star formation (AGN contribution ~ 10-20%)

31 OT Census and the Obscured SF Fraction A new, independent constraint on the net total obscured SF fraction This is an upper limit; other possible causes for the missing OTs: Fast decline / slow follow-up (cf. GRB ) Intrinsically too faint High redshift High foreground obscuration Note also: only ~ 20% of detected RTs lack OTs Possible loophole: Tunnels through the dust?

32 The Future: High redshift (z " 6) Bursts Substantial numbers of high-z bursts are expected to exist QSOs, 2002 Probes of the primordial star formation, enrichment, and reionization Inadequate OT searches so far Lloyd-Ronning et al. 2002

33 Star Formation History at High Redshifts Madau et al. GRBs may provide a (nearly?) unique probe out to z ~ Bromm & Loeb

34 Primordial Star Formation: a Top-Heavy IMF? A generic expectation in all modern models of Pop III star formation (but will they produce GRBs?) Abel et al. Bromm et al.

35 High-z Bursts as Probes of the Reionization GRBs vs. Quasars: May exist at high redshifts when there are no bright AGN No Lyα line - easier interpretation of the Gunn-Peterson damping wing Different Strömgren spheres (no proximity effect): more representative of the primordial IGM? Propagating reionization front Madau, Norman, et al.

36 What Next for the GRB Cosmology? GRB host galaxies provide new insights into galaxy evolution, complementary to the traditionally selected samples (both in emission and absorption) However, we need a better statistics and understanding of the selection effects GRB afterglows provide a new probe of the star formation history and its obscured fraction However, we must understand better the dust destruction GRBs at z > 6 can provide unique new information about the primordial star formation and reionization However, we must have better follow-up mechanisms for a rapid discovery of high-z OTs