This paper presents an absorption-line study of the multiphase circumgalactic medium ( CGM ) based on observations of { Ly } \alpha , 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 QSO survey data and is characterized by a median redshift of \langle z \rangle = 0.041 , a median projected distance of \langle d \rangle = 362 kpc to the sightline of the background QSO , and a median stellar mass of \log ( M _ { star } / M _ { \odot } ) = 9.7 \pm 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\ > ^ { < } _ { \sim } \ > 0.176 .
First , abundant hydrogen gas is observed out to d \sim 500 kpc , well beyond the dark matter halo radius R _ { h } of individual galaxies , with a mean covering fraction of \approx 60 % .
In contrast , no heavy elements are detected at d\ > ^ { > } _ { \sim } \ > 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 \approx 0.7 R _ { h } or gaseous clumps giving rise to the observed absorption lines can not 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 \approx 3 % ( at a 95 % 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 \sim 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 .