Presentazione di PowerPoint

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Metal Abundances in GRB Host Galaxies
Valerio DElia
F. Fiore, S.Piranomonte (INAF OAR) E.J.A.
Meurs, L. Norci, S.D. Vergani, P.Ward
(Dublin Observatory/Universi
ty)
December 20, 2006
Rome
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TALK OUTLINE

• Metal abundances in high redshift galaxies
• High resolution spectroscopy of GRB afterglows
  (GRB050730)
• The circumburst environment
• Host regions far away from the GRB
• Metallicity in GRB hosts
• The dust depletion problem
• Conclusions and perspectives

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METAL ABUNDANCES IN HIGH z GALAXIES

• The study of the metal abundances in high z
  galaxies gives us precious information on the
  metal enrichment history of the galaxies, which
  in turn is linked to the mass function evolution.
• To now, metal enrichment in galaxies at high z
  has been studied using
• Lyman Break Galaxies
• Galaxies along the line of sight of quasars
  (Damped Lyman-? systems)
• The first class can not be studied up to very
  high redshift.The second one is entangled by
  selection effects the radiation from the QSOs
  probes preferentially the halos of the galaxies.
• Moreover, it is difficult to identify the
  redshift of the intervening galaxies, since the
  light from QSOs overwhelms the galaxy emission
  lines.

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METAL ABUNDANCES IN HIGH z GALAXIES
GRBs provide an independent way of studing the
metal enrichment of galaxies at z gt 1.

• Advantages
• Probing central galaxy regions
• Redshift of the host galaxy is always obtained.
• ISM can be studied up to higher redshift than
  DLA systems.
• GRB hosts appear to be more metal rich than DLA
  systems (Savaglio 2005)

(Savaglio 2005)
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METAL ABUNDANCES IN HIGH z GALAXIES
GRB explosion site
Circumburst environment
Host gas far away
To Earth
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HIGH RESOLUTION SPECTROSCOPY OF GRBs

• High resolution spectroscopy can disentangle the
  absorption features in components, allowing a
  more accurate study.

GRB 021004 FORS1 R1000 CIV z 2.296 and z
2.328
GRB 021004 UVES R40000 CIV z2.296 e 2.328
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HIGH RESOLUTION SPECTROSCOPY OF GRBs
2. High resolution spectroscopy is necessary to
disentangle host regions far away from the GRB
and the absorption coming from the GRB
surroundings.
GRB 050922C
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GRB 050730 LIGHT CURVE AND UVES/VLT OBSERVATION
3000 s Dichroic 1 3000 s Dichroic 2 4 hr after
the GRB
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GRB 050730 ABSORBING SYSTEMS AND LINE FITTING
Five intervening absorbers identified
z1 3.967 (GRB host) z2 3.564 z3
2.262 z4 2.253 (d) z5 1.772 (d)
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GRB 050730 THE SYSTEM AT THE HOST REDSHIFT
The main system presents 5 ( 1) components
C IV 5 components 1) 32.6 2) 2.4 3)
-44.0 4) -90.2 5) -154.6
Si IV 4 components 2) 2.4 3a) -44.0
a 3b) -44.0 b 4) -90.2
CIV and Si IV components used as reference to fit
the other ions
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GRB 050730 THE CIRCUMBURST ENVIRONMENT
Fine structure transitions
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C II
Si II
3
2
Fe II
2
2
3
OI
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GRB 050730 CONSTRAINING THE PHYSICAL PARAMETERS

• Detailed balance equation for a two levels
  system
• n density of the states - w radiative terms -
  Q collisional terms
• Fine structure, assuming electron-ion collisions
  is main process
• (For C II) (For Si II)
• ne electron density - T temperature - N
  density of the states
• INFORMATIONS ON T AND ne can be obtained.
• If indirect UV pumping is instead at work,
  we can gather informations on the strength of the
  radiation field G.

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GRB 050730 CONSTRAINING THE PHYSICAL PARAMETERS
Second component, assuming electron-ion
collisions
From C II and Si II fine structure doublets
103ltTlt104K ne gt 300 cm-3
Assuming indirect UV pumping, G/G0 105 ? 106
where G0 1.6 X 10-3 erg cm-2 s-1.
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GRB 050730 CONSTRAINING THE PHYSICAL PARAMETERS
Third component, assuming electron-ion collisions
From C II fine structure doublet 103 lt Tlt
104K 10 lt ne lt 60 cm-3 For Si II third
component is uncertain
T 10 3 K
Assuming indirect UV pumping, G/G0 105 (O I)
and G/G0 106 (Si II) Third component of Si II
is uncertain, but if UV pumping is at work, we
shoud observe similar columns for O I and Fe II
(not observed).
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GAS DISTANCE FROM THE GRB SITE
Disentangling circumburst gas from far away host
regions

• Component 1 is present only with very high
  ionization states (CIV and OVI) it experience
  strong radiation field and it is probably the
  closest to the GRB site.

• Scaling arguments both in case of electron-ion
  collisions (d ? n-1/2) and indirect UV pumping (d
  ? G/G0-1/2) suggest that the second component is
  closer to the GRB site by a factor from a few to
  a few tens with respect to the third.
• A more accurate estimate of the distance of the
  shells from the GRB site needs the comparison of
  the data with a time dependent photoionization
  model.

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METALLICITY IN GRB HOST GALAXIES
Fitting the H absorption features, the
metallicity can be estimated
We used the Ly-? and Ly-? absorption features to
constrain the H column density. We find NH
22.05?0.29. The Hydrogen line profiles are too
broad to disentangle the contributions from the
five components. Metallicity values of Z ? 10-3
? 10-2 with respect to solar are obtained.
GRB 050730

• Z can be underestimated, since
• we are probing Hydrogen from the outer regions
  of the host.
• Heavy elements may form dust.

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THE DUST DEPLETION PROBLEM
Heavy elements form dust grains, invisible to UV
absorption
The ISM of the Milky Way shows that Zn tends to
stay in the gas phase (max 20 in dust). On the
other hand, Fe tends to form dust (up to more
than 99 of total). The ratio Zn/Fe is an
excellent dust depletion indicator because
(Savaglio 2005)

• Fe and Zn are extreme in their refractory
  properties.
• They are easy to detect.
• They have similar formation timescales.

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THE DUST DEPLETION PROBLEM
Using Zn as indicator of metallicity, or the
correlation Zn vs Fe
GRB 060418
Z(Cr) -1.8 ? 0.2 Z(Zn) -1.3 ? 0.2
(See also Vreeswijk et al. 2006)
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THE DUST DEPLETION PROBLEM
The C/Fe ratio
C/Fe ratio is 0.08?0.24, consistent with values
predicted for a galaxy younger than a Gyr
undergoing star formation, but uncertainties due
to dust depletion can still be important. C/Fe
in component 3 is 0.53?0.23, larger than in 2
(-0.15?0.13), with C roughly constant. Since
Fe dust is more efficiently destroyed than C
(Perna, Lazzati Fiore 2003), this confirms that
component 2 is closer to the GRB than component
3.
GRB 050730
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CONCLUSIONS AND PERSPECTIVES

• Metal abundances in high z galaxies play a key
  role in the study of galaxy evolution.
• GRB high resolution spectra are excellent tools
  to explore the metal enrichment history in high z
  galaxies, because they can disentangle the GRB
  environment from gas regions far away from the
  explosion site.
• For what concerns the GRB environment, fine
  structure absorption features (always present in
  GRB spectra), help to put constrains on physical
  parameters (T, ne and G), and to discriminate
  their production mechanism.
• Informations on the relative distance of the
  shells from the GRB can be obtained, but absolute
  values need time dependent photoionization codes.
• Metals in GRB hosts are underabundant with
  respect to the solar abundances, but large
  uncertainties are present (H column and dust
  depletion).
• Dust depletion problem can be reduced using Zn
  as metallicity indicator or the correlation Zn vs
  Fe, when Zn is not detected.
• C/Fe ratio can be a powerful diagnostic to study
  the dust depletion in GRBs.