HI properties of massive galaxies from stacking

Galaxies have been found to divide into two families: one dominated
by late-type, star forming, blue objects, which are rich in cold
gas and have a low stellar mass surface density (mu*); the other is
made of early-type, red and passive galaxies with higher mu* and on
average low gas content. The physical mechanisms responsible for
the galaxy transition between the active and passive regime are
still debated. In the high mass range, mechanisms proposed to
quench the star formation (SF) through cold gas heating or
depletion are not efficient enough to reproduce the correct red
sequence of passive systems, when implemented in models of galaxy
evolution. Input for a better understanding of the physics of
quenching mechanisms, and of their relative importance and
efficiency, can come from a comparison of the cold atomic neutral
hydrogen (HI) content and SF for a statistically significant sample
of massive systems where quenching is at work. However, existing
surveys do not sample this high mass, gas poor regime well enough.
In this work, we study the HI properties of a volume-limited sample
of ~5000 nearby galaxies with stellar mass M*>10^10 Msun,
selected from the state-of-the-art blind HI survey ALFALFA to have
optical and ultraviolet data so that star formation and galaxy
properties can be derived. As ALFALFA does not sample with
sufficient sensitivity the high mass, gas poorest range, we
developed a software tool to co-add its data, in order to obtain
average gas properties of galaxy classes which individually may be
largely undetected. Using this technique, we study three types of
quenching processes, namely the presence of a bulge component,
feedback from an active galactic nucleus (AGN), and environmental
mechanisms acting on the interstellar medium. Simulations of
early-type galaxies with non star-forming HI disks have suggested
that the presence of a bulge can stabilize the gas, thus preventing
star formation, but on average we do not observe this. We find
that, once mu* and NUV-r colours are fixed, the HI gas fraction in
massive bulge- and disk-dominated galaxies is the same. A similar
negative result is obtained if we compare M_HI/M* of AGN hosts and
control galaxies, despite simulations that invoke feedback from AGN
to heat or deplete cold gas in massive systems. The relation we
observe between the cold gas content and the accretion rate in the
red population actually points towards a co-evolution of SF and AGN
activity, both driven by the amount of gas available. The last
class of quenching mechanisms studied in this work includes
environmental processes, which are known to affect the SF
properties of galaxies and, at least in rich clusters, their cold
gas content. For the first time, though, we study the effect of the
environment on the HI content as a continuous function of local
density, comparing it with global and inner specific star formation
rate. The gradual increase in the suppression of SF from the inner
to the outer regions that we observe, and the even stronger HI
deficiency as a function of increasing local density, can be
explained by a mechanism acting on the disk from the outside-in,
like ram-pressure stripping of the HI. A comparison with mock
catalogs from models, which include only removal of the hot gas,
shows how models underestimate environmental effects, especially on
the cold gas component of galaxies. We therefore suggest that, in
order to improve our understanding of the galaxy bimodality in the
local Universe, observations and models should particularly focus
on environmental mechanisms acting on the cold interstellar medium.
These processes are efficient over a broader range of local
densities than previously thought, and could solve parts of the
puzzle in the formation of massive and passive systems.