The clustering of massive galaxies at z~0.5 from the first semester of BOSS data

TL;DR

This paper measures the clustering of 44,000 massive galaxies at $z\sim0.5$ from the BOSS CMASS sample using two-point statistics and interprets the results with a halo-occupation distribution model. It finds that most galaxies are central occupants in halos of roughly $M\sim(2-3)\times10^{13}\,h^{-1}\,M_\odot$, with about 10% as satellites in halos an order of magnitude more massive, yielding a large-scale bias of $b\approx2$ and a number density $\bar n\approx3\times10^{-4}\,h^3\mathrm{Mpc}^{-3}$. The authors demonstrate that CMASS galaxies are excellent tracers of large-scale structure and that their clustering is well described by an HOD framework, as validated by extensive mock catalogs and redshift-space analyses. While the current data do not yet decisively detect the acoustic peak, the planned expansion of the survey will enable precise measurements of the distance scale and growth of structure at redshift $z\sim0.5$. Overall, the work establishes a robust baseline for using BOSS to study large-scale structure and massive galaxy evolution at intermediate redshifts.

Abstract

We calculate the real- and redshift-space clustering of massive galaxies at z~0.5 using the first semester of data by the Baryon Oscillation Spectroscopic Survey (BOSS). We study the correlation functions of a sample of 44,000 massive galaxies in the redshift range 0.4<z<0.7. We present a halo-occupation distribution modeling of the clustering results and discuss the implications for the manner in which massive galaxies at z~0.5 occupy dark matter halos. The majority of our galaxies are central galaxies living in halos of mass 10^{13}Msun/h, but 10% are satellites living in halos 10 times more massive. These results are broadly in agreement with earlier investigations of massive galaxies at z~0.5. The inferred large-scale bias (b~2) and relatively high number density (nbar=3e-4 h^3 Mpc^{-3}) imply that BOSS galaxies are excellent tracers of large-scale structure, suggesting BOSS will enable a wide range of investigations on the distance scale, the growth of large-scale structure, massive galaxy evolution and other topics.

Figures (13)

• Figure 1: The (comoving) number density of galaxies, $\bar{n}(z)$ for the sample described in the text (§\ref{['sec:obs']}). The vertical dashed lines indicate the redshift limits we use in our analysis: $0.4<z<0.7$.
• Figure 2: The distribution of absolute magnitudes for the sample analyzed in this paper. We have $k+e$ corrected the $r$-band magnitudes to $z\simeq 0.55$ using the $g-i$ color assuming a passively evolving galaxy -- since the redshift range is small this amounts to a small correction. This sample consists of intrinsically very bright, and massive, galaxies with stellar masses several times the characteristic mass in a Schechter fit. The luminosity function of Fab07 at $z=0.5$, converted from $B$ to $r$ band assuming a redshifted $z=0$ elliptical galaxy template, has a characteristic luminosity of $-19.8$. Converting the Bel04 luminosity function using a high-$z$ single burst model gives $-20$. So all of the CMASS galaxies are brighter than this characteristic luminosity.
• Figure 3: (Top) The sky coverage of the galaxies used in this analysis, in orthographic projection centered on $\alpha_{J2000}=180^\circ$ and $\delta_{J2000}=0^\circ$. The regions A, B and C described in the text are marked. (Bottom) A zoom in of region A with the greyscale showing completeness. This region is the most contiguous of the three, and region B is the least contiguous owing to hardware problems in the early part of the year.
• Figure 4: Contours of the redshift-space correlation function, $\xi(R,Z)$, for our $0.4<z<0.7$ galaxy sample (see text). Note the characteristic elongation in the $Z$ direction at small $R$ (fingers-of-god) and squashing at large $R$ (super-cluster infall). The upper panel shows the results from the BOSS data, while the lower panel is from our mock catalogs. The level of agreement is quite good, as can be seen more quantitatively in later figures.
• Figure 5: The projected correlation function for the $0.4<z<0.7$ sample in regions A, B and C (lines) and for the combined sample (points with errors). The errors on the individual samples have been suppressed for clarity. The data are combined using the full covariance matrix, but only the diagonal elements are plotted. The $w_p$ implied by a power-law correlation function of slope $-1.8$ and correlation length of $7.5\,h^{-1}$Mpc forms a reasonable fit to the data with $1<R_p<10\,h^{-1}$Mpc but we do not plot it here for clarity. The (thick) long-dashed-dotted line shows the prediction of the best-fitting HOD model (§\ref{['sec:hod']}), which provides a reasonable fit on all scales plotted (recall the errors are correlated).