Publication Date:
2014-10-17
Description:
This paper presents an absorption-line study of the multiphase circumgalactic medium (CGM) based on observations of Lyα, C ii , C iv , Si ii , Si iii , and Si iv absorption transitions in the vicinities of 195 galaxies at redshift z 〈 0.176. The galaxy sample is established based on a cross-comparison between public galaxy and quasi-stellar object (QSO) survey data and is characterized by a median redshift of 〈 z 〉 = 0.041, a median projected distance of 〈 d 〉 = 362 kpc to the sightline of the background QSO, and a median stellar mass of log ( M star /M ) = 9.7 ± 1.1. Comparing the absorber features identified in the QSO spectra with known galaxy properties has led to strong constraints for the CGM absorption properties at z 0.176. First, abundant hydrogen gas is observed out to d ~ 500 kpc, well beyond the dark matter halo radius R h of individual galaxies, with a mean covering fraction of 60 per cent. In contrast, no heavy elements are detected at d 0.7 R h from either low-mass dwarfs or high-mass galaxies. The lack of detected heavy elements in low- and high-ionization states suggests that either there exists a chemical enrichment edge at d 0.7 R h or gaseous clumps giving rise to the observed absorption lines cannot survive at these large distances. Considering all galaxies at d 〉 R h leads to a strict upper limit for the covering fraction of heavy elements of 3 per cent (at a 95 per cent confidence level) over d = (1 – 9) R h . At d 〈 R h , differential covering fraction between low- and high-ionization gas is observed, suggesting that the CGM becomes progressively more ionized from d 〈 0.3 R h to larger distances. Comparing CGM absorption observations at low and high redshifts shows that at a fixed fraction of R h the CGM exhibits stronger mean absorption at z = 2.2 than at z ~ 0, and that the distinction is most pronounced in low-ionization species traced by C ii and Si ii absorption lines. We discuss possible pseudo-evolution of the CGM as a result of misrepresentation of halo radius, and present a brief discussion on the implications of these findings.
Print ISSN:
0035-8711
Electronic ISSN:
1365-2966
Topics:
Physics
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