The triply-ionized carbon forest from eBOSS: cosmological correlations with quasars in SDSS-IV DR14

Abstract

We present measurements of the cross-correlation of the triply-ionized carbon (CIV) forest with quasars using Sloan Digital Sky Survey Data Release 14. The study exploits a large sample of new quasars from the first two years of observations by the Extended Baryon Oscillation Spectroscopic Survey (eBOSS). The CIV forest is a weaker tracer of large-scale structure than the Ly$\hat{I}\pm$ forest, but benefits from being accessible at redshifts $z<2$ where the quasar number density from eBOSS is high. Our data sample consists of 287,651 CIV forest quasars in the redshift range $1.4<z<3.5$ and 387,315 tracer quasars with $1.2<z<3.5$. We measure large-scale correlations from CIV absorption occuring in three distinct quasar rest-frame wavelength bands of the spectra referred to as the CIV forest, the SiIV forest and the Ly$\hat{I}\pm$ forest. From the combined fit to the quasar-CIV cross-correlations for the CIV forest and the SiIV forest, the CIV redshift-space distortion parameter is ${\hat{I}}_{\mathrm{CIV}}^2=0.27_{-0.14}^{+0.16}$ and its combination with the CIV linear transmission bias parameter is $b_{\mathrm{CIV}}(1+{\hat{I}}_{\mathrm{CIV}}^2)=-0.0183_{-0.0014}^{+0.0013}$ ($1\ddot{I}ƒ$ statistical error) at the mean redshift $z=2.00$. Splitting the sample at $z=2.2$ to constrain the bias evolution with redshift yields the power-law exponent ${\hat{I}}^3=0.60\pm 0.63$, indicating a significantly weaker redshift-evolution than for the Ly$\hat{I}\pm$ forest linear transmission bias. We demonstrate that CIV absorption has the potential to be used as a probe of baryon acoustic oscillations (BAO). While the current data set is insufficient for a detection of the BAO peak feature, the final quasar samples for redshifts $1.4<z<2.2$ from eBOSS and the Dark Energy Spectroscopic Instrument (DESI) are expected to provide measurements of the isotropic BAO scale to $\sim 7\%$ and $\sim 3\%$ precision, respectively, at $z\simeq 1.6$.

34 pages, 12 figures, matches the published version