TY - JOUR
T1 - Interstellar silicate dust in the Z = 0.89 absorber toward PKS1830-211
T2 - Crystalline silicates at high redshift?
AU - Aller, Monique C.
AU - Kulkarni, Varsha P.
AU - York, Donald G.
AU - Vladilo, Giovanni
AU - Welty, Daniel E.
AU - Som, Debopam
PY - 2012/3/20
Y1 - 2012/3/20
N2 - We present evidence of a >10σ detection of the 10 μm silicate dust absorption feature in the spectrum of the gravitationally lensed quasar PKS1830-211, produced by a foreground absorption system at redshift 0.886. We have examined more than 100 optical depth templates, derived from both observations of Galactic and extragalactic sources and laboratory measurements, in order to constrain the chemical structure of the silicate dust. We find that the best fit to the observed absorption profile is produced by laboratory crystalline olivine, with a corresponding peak optical depth of τ10 = 0.27 ± 0.05. The fit is slightly improved upon by including small contributions from additional materials, such as silica, enstatite, or serpentine, which suggests that the dust composition may consist of a blend of crystalline silicates. Combining templates for amorphous and crystalline silicates, we find that the fraction of crystalline silicates needs to be at least 95%. Given the rarity of extragalactic sources with such a high degree of silicate crystallinity, we also explore the possibility that the observed spectral features are produced by amorphous silicates in combination with other molecular or atomic transitions, or by foreground source contamination. While we cannot rule out these latter possibilities, they lead to much poorer profile fits than for the crystalline olivine templates. If the presence of crystalline interstellar silicates in this distant galaxy is real, it would be highly unusual, given that the Milky Way interstellar matter contains essentially only amorphous silicates. It is possible that the z = 0.886 absorber toward PKS1830-211, well known for its high molecular content, has a unique star-forming environment that enables crystalline silicates to form and prevail.
AB - We present evidence of a >10σ detection of the 10 μm silicate dust absorption feature in the spectrum of the gravitationally lensed quasar PKS1830-211, produced by a foreground absorption system at redshift 0.886. We have examined more than 100 optical depth templates, derived from both observations of Galactic and extragalactic sources and laboratory measurements, in order to constrain the chemical structure of the silicate dust. We find that the best fit to the observed absorption profile is produced by laboratory crystalline olivine, with a corresponding peak optical depth of τ10 = 0.27 ± 0.05. The fit is slightly improved upon by including small contributions from additional materials, such as silica, enstatite, or serpentine, which suggests that the dust composition may consist of a blend of crystalline silicates. Combining templates for amorphous and crystalline silicates, we find that the fraction of crystalline silicates needs to be at least 95%. Given the rarity of extragalactic sources with such a high degree of silicate crystallinity, we also explore the possibility that the observed spectral features are produced by amorphous silicates in combination with other molecular or atomic transitions, or by foreground source contamination. While we cannot rule out these latter possibilities, they lead to much poorer profile fits than for the crystalline olivine templates. If the presence of crystalline interstellar silicates in this distant galaxy is real, it would be highly unusual, given that the Milky Way interstellar matter contains essentially only amorphous silicates. It is possible that the z = 0.886 absorber toward PKS1830-211, well known for its high molecular content, has a unique star-forming environment that enables crystalline silicates to form and prevail.
KW - dust, extinction
KW - galaxies: ISM
KW - quasars: absorption lines
UR - http://www.scopus.com/inward/record.url?scp=84858020946&partnerID=8YFLogxK
U2 - 10.1088/0004-637X/748/1/19
DO - 10.1088/0004-637X/748/1/19
M3 - Article
AN - SCOPUS:84858020946
SN - 0004-637X
VL - 748
JO - Astrophysical Journal
JF - Astrophysical Journal
IS - 1
M1 - 19
ER -