Diapositive 1

1
Evidence of dust in DLAs from abundance studies
Miroslava Dessauges-Zavadsky Geneva Observatory,
Switzerland
2
General idea

• How can we detect the presence of dust from
  abundance studies ?
• Observations in the local ISM
• elemental abundances measured in the
  interstellar gas of the solar
• vicinity are generally depleted with respect to
  the solar values
• (e.g. Morton 74 Jenkins, Savage Spitzer 86)
• ? Commonly accepted interpretation
• dust depletion a fraction of these elements
  is not detected in the gas,
• because it is locked into dust grains
• ? Different elements have different depletion
  values
• the propensity of a given element to be depleted
  appears to be correlated
• with its condensation temperature
• (e.g. Savage Sembach 96)
• ? Elemental depletions vary among different lines
  of sight
• dust properties depend heavily on the local
  physical conditions
• (e.g. Draine McKee 93)

3
Depletion values from element to element
Observations in the Galactic ISM
Savage Sembach 96 Welty et al. 99
Summary table of the detected elements in DLAs
4
1st evidence of dust in DLAs

• Comparison of gas phase abundances of elements
• which in the Galactic ISM clouds are depleted by
  different amounts
• ! consider elements with the same
  nucleosynthetic type to avoid
• problems of mixing of nucleosynthesis and dust
  depletion effects

DLA at zabs 2.309 toward Q010013
DLA at zabs 1.776 toward Q133117
? In DLAs ?Zn/Fe? 0.45, ?Zn/Cr?
0.30 In Galactic stars Fe, Cr and Ni track
each other and
Zn/Fe 00.2 (e.g. Mishenina et al. 02)
5
2nd evidence of dust in DLAs
Correlations between different abundance
ratios (Prochaska Wolfe 02) ? Correlations
are expected if the observed enhancement in
iron-peak element ratios are really due to
differential dust depletion.

• Anti-correlation Cr/Fe, Cr/Ni vs Zn/Fe
• - observed in the Galactic ISM at much higher
  level
• - Fe more readily depleted into dust grain
  cores,
• while Cr more efficiently adsorbed into grain
  
• mantles which form in denser/more depleted
  regions
• Correlation Si/Ti vs Zn/Fe
• - 2 ?-elements and 2 iron-peak elements
• - due to dust because no star exhibits Si/Ti gt
  0.2

6
3rd evidence of dust in DLAs
Anti-correlation between X/Zn and F(zn)
Zn/Hlog N(HI) (Hou, Boissier Prantzos
01) ? a tight anti-correlation between log
(X/H) vs log N(HI) is observed in the ISM
high column densities correspond to a larger
amount of dust depletion
? this
anti-correlation not observed in DLAs,
but X/Zn is anti-correlated with F(Zn)
log (X/H) ? X/Zn / log N(HI) ? F(Zn)
because DLAs have different metallicities
high amounts of metals ? high amounts of dust
Wakker Mathis 00
7
4th evidence of dust in DLAs

• Detection of the H2 molecules
• H2 molecules efficiently formed on dust grains
• ? the detection of H2 suggests the presence of
  dust
• In our Galaxy the molecular fraction, f (H2),
  correlates with E(B?V)
• (Spitzer Jenkins 75)
• In DLAs f (H2) correlates with dust indicators
  
• H2 mostly detected at the highest
  metallicities and largest depletions
• (Levshakov, Dessauges-Zavadsky et al. 02
  Ledoux, Petitjean Srianand 03)

Dust-to-gas ratio
f (H2) 2 N(H2) / 2 N(H2) N(HI)
8
Dust is indeed present in DLAs

• The presence of dust in DLAs is now firmly
  established
• The dust depletion levels observed in DLAs are
  small, lower than those
• measured in the Galactic ISM clouds with similar
  HI column densities
• They mostly resemble to the dust depletion of
  the Galactic halo warm gas
• (Kulkarni, Fall Truran 97 Savaglio, Panagia
  Stiavelli 00)
• For most DLAs the molecular fraction is very
  small log f(H2) lt 6
• The mean log f (H2), where H2 is detected, is
  0.01, about the same as in the LMC and SMC,
  and a factor of 10 smaller than in the Galactic
  disk (Savage et al. 77 Tumlinson et al. 02)
• The low dust depletion level and molecular
  fraction are likely a consequence of low
  metallicities and higher temperatures in the DLA
  ISM (Petitjean, Srianand Ledoux
  00 Kanekar Chengalur 01)