Halo Masses of MgII absorbers at z\sim 0.5 from SDSS DR7
  We present the cross-correlation function of MgII absorbers with respect to a
volume-limited sample of luminous red galaxies (LRGs) at z=0.45-0.60 using the
largest MgII absorber sample and a new LRG sample from SDSS DR7. We present the
clustering signal of absorbers on projected scales r_p = 0.3-35 Mpc/h in four
Wr(2796) bins spanning Wr(2796)=0.4-5.6A. We found that on average MgII
absorbers reside in halos < log M_h > \approx 12.1, similar to the halo mass of
an L_* galaxy. We report that the weakest absorbers in our sample with
W_r(2796)=0.4-1.1A reside in relatively massive halos with < log M_h > \approx
12.5^{+0.6}_{-1.3}, while stronger absorbers reside in halos of similar or
lower masses < log M_h > \approx 11.6^{+0.9}. We compared our bias data points,
b, and the frequency distribution function of absorbers, f_{W_r}, with a simple
model incorporating an isothermal density profile to mimic the distribution of
absorbing gas in halos. We also compared the bias data points with Tinker &
Chen (2008) who developed halo occupation distribution models of MgII absorbers
that are constrained by b and f_{W_r}. The simple isothermal model can be ruled
at a \approx 2.8\sigma level mostly because of its inability to reproduce
f_{W_r}. However, b values are consistent with both models, including TC08. In
addition, we show that the mean b of absorbers does not decrease beyond
W_r(2796) \approx 1.6A. The flat or potential upturn of b for Wr(2796) \gtrsim
1.6A absorbers suggests the presence of additional cool gas in massive halos.
