A connection between the 2175 Å dust feature and CI? Daniele Malesani Árdís Elíasdóttir Darach J. Watson Johan P.U. Fynbo Jens Hjorth et al. Dark Cosmology Centre Copenhagen, 2009 Jun 29 – Jul 3 – Current Problems in Extragalactic Dust

Using GRB afterglows to probe dust Gamma-ray burst afterglows are useful probes of dust They are bright They span a broad redshift range Sightlines representative of star-forming regions Dense medium (NHI up to 1022 cm-2) Simple spectral shape We also know HI, metals, molecules, … Influence of GRB radiation on the dust? Are GRB host galaxies any special?

Using GRB afterglows to probe dust Jensen et al. 2001 Bump-less extinction curve

Using GRB afterglows to probe dust Kann et al. 2006; Schady et al. 2007

The striking case of GRB 070802 z = 2.45 AV = 0.8-1.5 LMC-like curve Elíasdóttir et al. 2009 See also Krühler et al. 2008

Environmental properties Bright host MB = -20.5 R-K = 3.6

CI in the spectrum 2175 Å rare + CI rare A connection? CI very rare in GRB afterglow spectra (only known case at that time)

Other GRB sightlines Our afterglow spectra archive (Fynbo et al. in preparation) Five afterglows with 2175 Å detected (1 intervening) 3 have CI – 1 does not – 1 don’t know Seven afterglows with CI detected 3 have 2175 Å – 2 do not – 2 don’t know Prochaska et al. 2009 Xu et al. in preparation

Non-GRB sightlines 2175 Å features have been rarely detected in individual systems One lensing galaxy at z=0.83 (Motta et al. 2000) An intervening system to a GRB at z=1 (Ellison et al. 2006) DLA AO 0235+164, @ z=0.524 (Junkkarinen et al. 2004) A few MgII absorbers at z≈1.3-1.4 (Wang et al. 2004; Srianand et al. 2008) MW star HD 185418 (Sonnentrucker et al. 2003) ✔ ? ✔ Submitted program to look for CI in these systems where possible (NOT @ Canaries, VLT + X-shooter)

SDSS J0850+5159 EW = 1 Å

Metallicity? It has been suggested that the presence of the dust bump is a metallicity effect (MW  LMC  SMC) This cannot be the only parameter: the high-metallicity GRB 000926 does not show the 2175 Å feature CI may be the key

Radiation field? CI ionization potential: 11.26 eV: CI is not shielded by HI The radiation field must be weak for CI to survive Alternatively, H2 must be present (e.g. molecular clouds) Suggested correlation between UV flux and presence of the 2175 Å bump (Gordon et al. 2003) This scenario seems consistent with the GRB 070802 properties: high-ionization lines weaker than, e.g., GRB 000926 lack of Lyα emission from the host Generically, high radiation field measured in GRB hosts (which mostly do not have the bump)

Future prospects Extend the set of observations (new GRBs, QSOs, …) Make this quantitative (measure CI and dust column densities Quantify the radiation field (possible only for GRB sightlines)