Detection of the 2175Å Dust Extinction Feature at

high zJunfeng 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@SHAO, Mar. 17, 2005IAUC199@SHAO, Mar. 17, 2005

The Broad Absorption “Bump”

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.

”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)

Empirical Extinction Curves

Pei (1992) and references within

Analytical Extinction Curves

VR

xbxa

VA

A )()(

)(

)(

Rv=A(V)/E(B-V); x=λ-1

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

Fitzpatrick (1999)

The Broad Absorption “Bump”

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).

”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)

Malhotra (1997)

Previous Detections…

• Malhotra (1997) reports a statistical detection in a sample of 92 QSOs at redshift 0.2<z<2.2

• Cohen et al. (1999) detected the 2175Å feature in a DLA at redshift z=0.524 toward the BL Lac object AO 0235+164 at z=0.94

-- 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.

• Munoz et al. (2004) reported that the dust in the z=0.68 lens galaxy of B 0218+357 shows very flat UV extinction curve (Rv=12±2)

• Motta et al. (2002) detected a strong 2175Å bump in a lensing galaxy at z=0.83

Previous Detections

Munoz et al. (2004)~

Motta et al. (2002)

Previous Detections

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

Quasar absorption line technique is also a powerful tool for studying dust properties at high z

A Cartoon Illustration

• Templates:-Composite spectrum from Vanden Berk et al.

(2001)

Fitting and ResultsVanden Berk et al. (2001)

• Templates:-Composite spectrum from Vanden Berk et al.

(2001)

-Reddest-quartile and bluest-quartile composites from Richards et al. (2003)

Fitting and ResultsRichards et al. (2003)

Comparison of Spectral Indices

Wang et al. (2004), ApJ, 609, 589

fλ λ-(α+2)

Mode g-i from Hopkins et al.(2004)

DR2 sample showing possible bump features

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

SDSS J1459+0024

•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.

•Rv=1.9(+0.3, -0.2)

•E(B-V)=0.13±0.01

•Detected by 2MASS, and its J-K color of 1.22 places it in the reddest quartile of z=3 quasars (Barkhouse & Hall 2001).

SDSS J0121+0027

Diversity of Grain Sizes?

• Surprisingly small Rv values for SDSS J1459+0024 (Rv=1.9) and SDSS J1446+0351 (Rv=0.7)

• Most extreme extinction curve with Rv=2.1 in MW: line of sight towards HD 210121 high latitude translucent cloud

• Previously reported minimum Rv=1.5 is associated with a lensing galaxy at z=0.96 (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.

•[Fe/Zn] is very large for SDSS J0121+0027, 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)

•The 2175 Å selected dusty intervening systems are all strong MgII absorbers

Wang et al. (2005), submitted to ApJ

DR1 sample

DR2 sample

Random MgII systems

Keck ESI spectra (∆v ~ 33km/s)

J0121+0027 J1459+0024

Wavelength (Å)

Rel

ativ

e In

tens

ityVoigt Profile fitting (MgII λλ2796,2803)

VPFIT 6.0, R. Carswell et al.

Dust Depletion Pattern

Wang, Prochaska, & Ge et al. (2005), in preparation

Savage and Sembach (1996)

J1459J0121

Ledoux, Petitjean and Srianand (2003), also see their poster in the next room

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.