Detection of the 2175

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Detection of the 2175Å Dust Extinction Feature at
high z

• Junfeng Wang (Penn State/UFL)
• Collaborators Jian Ge (U. of Florida), Pat Hall
  (Princeton U./York U.), Jason Prochaska
  (UCSC/Lick Observatory),
• Aigen Li (U. of Arizona/U. of Missouri), Don
  Schneider (PSU), Don York (U. of Chicago), Scott
  Anderson (U. of Washington)

IAUC199_at_SHAO, Mar. 17, 2005
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The Broad Absorption Bump
A dramatic piece of spectroscopic evidence which
should have much to tell us about at least a part
of the interstellar grain population.Draine
(1989)

• The 2175Å dust extinction feature was first
  discovered by Aerobee rocket observations
  (Stecher 1965)
• It is seen in extinction curves along lines of
  sight in the Milky Way (MW), the Large Magellanic
  Cloud (LMC), and even some regions of the Small
  Magellanic Cloud (SMC).
• Extinction curves in the SMC bar region lack the
  2175Å feature.

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Empirical Extinction Curves
Pei (1992) and references within
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Analytical Extinction Curves

• CCM Galactic extinction law from Cardelli,
  Clayton, Mathis (1989)

RvA(V)/E(B-V) x?-1
Fitzpatrick (1999)
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The Broad Absorption Bump
A dramatic piece of spectroscopic evidence which
should have much to tell us about at least a part
of the interstellar grain population.Draine
(1989)

• The 2175Å dust extinction feature was first
  discovered by Aerobee rocket observations
  (Stecher 1965)
• It is seen in extinction curves along lines of
  sight in the Milky Way (MW), the Large Magellanic
  Cloud (LMC), and even some regions of the Small
  Magellanic Cloud (SMC).
• Extinction curves in the SMC bar region lack the
  2175Å feature.
• The central wavelength of the feature varies by
  only 0.46(2?) around 2175Å, while its FWHM
  varies by 12(2?) around 469Å. (Fitzpatrick
  Massa 1986)
• Although the exact carrier is unknown, some
  form of graphitic carbon is responsible, most
  likely polycyclic aromatic hydrocarbons (PAHs
  Joblin, Leger, Martin 1992 Duley Seahra
  1998 Weingartner Draine 2001 Li Draine
  2001 Draine 2003).

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Previous Detections

• Malhotra (1997) reports a statistical detection
  in a sample of 92 QSOs at redshift 0.2ltzlt2.2
• Cohen et al. (1999) detected the 2175Å feature in
  a DLA at redshift z0.524 toward the BL Lac
  object AO 0235164 at z0.94
• -- the only previous detection from an
  individual intervening absorption system
• Falco et al. (1999) determined extinction laws in
  galaxies z ? 1.01, with Rv estimates from 1.5 to
  7.2, using gravitational lensing of background
  quasars.

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Previous Detections

• Munoz et al. (2004) reported that the dust in the
  z0.68 lens galaxy of B 0218357 shows very flat
  UV extinction curve (Rv122)
• Motta et al. (2002) detected a strong 2175Å bump
  in a lensing galaxy at z0.83

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Previous Detections
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SDSS DR1 and DR2

• Large and well-selected samples extending to high
  redshifts are needed to characterize the
  diversity and evolution of dust properties in the
  early universe SDSS quasar spectra
• -- See Khare et al. poster
• The DR2 spectra distributed by the SDSS have been
  sky subtracted, corrected for telluric absorption
  but not for galactic extinction.
• Visual inspection of the SDSS spectra of 22000
  DR2 (include DR1) quasars with z 0.9 uncovered
  several quasars with possible 2175Å signature.
• -- 3 candidates from DR1 and 12 candidates
  from DR2

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A Cartoon Illustration
Quasar absorption line technique is also a
powerful tool for studying dust properties at
high z
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Fitting and Results

• Templates
• -Composite spectrum from Vanden Berk et al.
  (2001)

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Fitting and Results

• Templates
• -Composite spectrum from Vanden Berk et al.
  (2001)
• -Reddest-quartile and bluest-quartile composites
  from Richards et al. (2003)

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Comparison of Spectral Indices
f?? ?-(a2)
Wang et al. (2004), ApJ, 609, 589
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DR2 sample showing possible bump features
Mode g-i from Hopkins et al.(2004)
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Fitting and Results

• Templates
• -Composite spectrum from Vanden Berk et al.
  (2001)
• -Reddest-quartile and bluest-quartile composites
  from Richards et al. (2003)
• Extinction curves
• -Empirical extinction curves for the MW, LMC and
  SMC from Pei (1992) free par. E(B-V)
• -CCM Galactic extinction law from Cardelli,
  Clayton, Mathis (1989) free par. E(B-V), Rv

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SDSS J14590024
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• Reddened spectra with LMC or SMC curves can not
  produce sufficiently deep bump to match the
  observed spectrum.
• The best fit is obtained with the reddest
  composite using CCM extinction law.
• Rv1.9(0.3, -0.2)
• E(B-V)0.130.01
• Detected by 2MASS, and its J-K color of 1.22
  places it in the reddest quartile of z3 quasars
  (Barkhouse Hall 2001).

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SDSS J01210027
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Diversity of Grain Sizes?

• Surprisingly small Rv values for SDSS J14590024
  (Rv1.9) and SDSS J14460351 (Rv0.7)
• Most extreme extinction curve with Rv2.1 in MW
  line of sight towards HD 210121 high latitude
  translucent cloud
• Previously reported minimum Rv1.5 is associated
  with a lensing galaxy at z0.96 (Falco et al.
  1999)
• Preferential removal of small dust grains will
  result in a gray extinction law with large Rv,
  while in star forming galaxies finer grains
  dominate the dust size distribution.

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• Fe/Zn is very large for SDSS J01210027,
  similar to heavily depleted diffuse clouds in MW,
  e.g. ? Oph (Savage Sembach 1996)
• Very likely DLAs indicated by the strong MgII and
  FeII lines (Rao Turnshek 2000)
• Among the largest in all the high-redshift DLAs
  searched for dust and molecular hydrogen to date
  (e.g., Ge et al. 2001 Ledoux et al. 2003)

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The 2175 Å selected dusty intervening systems are
all strong MgII absorbers
Wang et al. (2005), submitted to ApJ
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Keck ESI spectra (?v 33km/s)
J01210027
J14590024
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Voigt Profile fitting (MgII ??2796,2803)
Relative Intensity
Wavelength (Å)
VPFIT 6.0, R. Carswell et al.
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Dust Depletion Pattern
Savage and Sembach (1996)
Wang, Prochaska, Ge et al. (2005), in
preparation
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J1459
J0121
Ledoux, Petitjean and Srianand (2003), also see
their poster in the next room
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Summary

• Several spectroscopic detections of the 2175Å
  dust extinction feature in quasar absorption
  systems at redshifts z 1.4
• The first detection of this feature in individual
  MgII absorption system.
• Dust depletion for SDSS J1459, J0121 is very
  large, similar to heavily depleted diffuse clouds
  in MW
• Various Rv values indicates a wide range of
  dominant grain sizes among intervening absorption
  systems.
• The sample showing the 2175Å feature are all
  strong MgII systems with strong FeII lines
  (likely DLAs) and heavily reddened.
• It implies that DLAs with heavy dust content
  should be rare (but, any evolution with z?)
• It may be evidence of complex organic molecules
  in the young universe, if the presently favored
  PAH model is correct.