A connection between the 2175 Å dust feature and CI?

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)

A connection between the 2175 Å dust feature and CI?