1. 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

2. 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?

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

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

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

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

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

8. 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

9. 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 z=0.524 (Junkkarinen et al. 2004)
A few MgII absorbers at z≈ (Wang et al. 2004; Srianand et al. 2008)
MW star HD (Sonnentrucker et al. 2003) ✔ ? ✔
Submitted program to look for CI in these systems where possible Canaries, VLT + X-shooter)

10. SDSS J
EW = 1 Å

11. 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 does not show the 2175 Å feature
CI may be the key

12. Radiation field?
CI ionization potential: 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 properties:
high-ionization lines weaker than, e.g., GRB
lack of Lyα emission from the host
Generically, high radiation field measured in GRB hosts (which mostly do not have the bump)

13. 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)