163 research outputs found
Halo assembly bias and its effects on galaxy clustering
The clustering of dark halos depends not only on their mass but also on their
assembly history, a dependence we term `assembly bias'. Using a galaxy
formation model grafted onto the Millennium Simulation of the LCDM cosmogony,
we study how assembly bias affects galaxy clustering. We compare the original
simulation to `shuffled' versions where the galaxy populations are randomly
swapped among halos of similar mass, thus isolating the effects of correlations
between assembly history and environment at fixed mass. Such correlations are
ignored in the halo occupation distribution models often used populate dark
matter simulations with galaxies, but they are significant in our more
realistic simulation. Assembly bias enhances 2-point correlations by 10% for
galaxies with M_bJ-5logh brighter than -17, but suppresses them by a similar
amount for galaxies brighter than -20. When such samples are split by colour,
assembly bias is 5% stronger for red galaxies and 5% weaker for blue ones. Halo
central galaxies are differently affected by assembly bias than are galaxies of
all types. It almost doubles the correlation amplitude for faint red central
galaxies. Shuffling galaxies among halos of fixed formation redshift or
concentration in addition to fixed mass produces biases which are not much
smaller than when mass alone is fixed. Assembly bias must reflect a correlation
of environment with aspects of halo assembly which are not encoded in either of
these parameters. It induces effects which could compromise precision
measurements of cosmological parameters from large galaxy surveys.Comment: 8 pages, 4 figures, accepted for publication in MNRA
A diversity of progenitors and histories for isolated spiral galaxies
We analyze a suite of 33 cosmological simulations of the evolution of Milky
Way-mass galaxies in low-density environments. Our sample spans a broad range
of Hubble types at z=0, from nearly bulgeless disks to bulge-dominated
galaxies. Despite the fact that a large fraction of the bulge is typically in
place by z=1, we find no significant correlation between the morphology at z=1
and at z=0. The z=1 progenitors of disk galaxies span a range of morphologies,
including smooth disks, unstable disks, interacting galaxies and
bulge-dominated systems. By z=0.5, spiral arms and bars are largely in place
and the progenitor morphology is correlated with the final morphology. We next
focus on late-type galaxies with a bulge-to-total ratio B/T<0.3 at z=0. These
show a correlation between B/T at z=0 and the mass ratio of the largest merger
at z1. We find that the
galaxies with the lowest B/T tend to have a quiet baryon input history, with no
major mergers at z<2, and with a low and constant gas accretion rate that keeps
a stable angular-momentum direction. More violent merger or gas accretion
histories lead to galaxies with more prominent bulges. Most disk galaxies have
a bulge Sersic index n<2. The galaxies with the highest bulge Sersic index tend
to have histories of intense gas accretion and disk instability rather than
active mergers.Comment: Accepted for publication in ApJ. 29 pages, 32 figure
Where do "red and dead" early-type void galaxies come from?
Void regions of the Universe offer a special environment for studying
cosmology and galaxy formation, which may expose weaknesses in our
understanding of these phenomena. Although galaxies in voids are observed to be
predominately gas rich, star forming and blue, a sub-population of bright red
void galaxies can also be found, whose star formation was shut down long ago.
Are the same processes that quench star formation in denser regions of the
Universe also at work in voids?
We compare the luminosity function of void galaxies in the 2dF Galaxy
Redshift Survey, to those from a galaxy formation model built on the Millennium
Simulation. We show that a global star formation suppression mechanism in the
form of low luminosity "radio mode" AGN heating is sufficient to reproduce the
observed population of void early-types. Radio mode heating is environment
independent other than its dependence on dark matter halo mass, where, above a
critical mass threshold of approximately M_vir~10^12.5 M_sun, gas cooling onto
the galaxy is suppressed and star formation subsequently fades. In the
Millennium Simulation, the void halo mass function is shifted with respect to
denser environments, but still maintains a high mass tail above this critical
threshold. In such void halos, radio mode heating remains efficient and red
galaxies are found; collectively these galaxies match the observed space
density without any modification to the model. Consequently, galaxies living in
vastly different large-scale environments but hosted by halos of similar mass
are predicted to have similar properties, consistent with observations.Comment: 6 pages, 3 figures, accepted MNRA
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