Misc,
Surface density effects in quenching: cause or effect?
(2016)cite arxiv:1604.06459Comment: 17 pages of text plus 13 figures. Submitted to The Astrophysical Journal on April 18 2016.
Abstract
There are very strong observed correlations between the specific
star-formation rates (sSFR) of galaxies and their mean surface mass densities,
\Sigma, as well as other aspects of their internal structure. These strong
correlations have often been taken to indicate that the internal structure of a
galaxy must play a major physical role, directly or indirectly, in the control
of star-formation. In this paper we show by means of a very simple toy model
that these correlations can arise naturally without any such physical role once
the observed evolution of the size-mass relation for star-forming galaxies is
taken into account. In particular, the model reproduces the sharp threshold in
\Sigma between galaxies that are star-forming and those that are quenched,
and the evolution of this threshold with redshift. Similarly, it produces
iso-quenched-fraction contours in the $f_Q(m,R_e)$ plane that are almost
exactly parallel to lines of constant \Sigma for centrals and shallower for
satellites. It does so without any dependence on quenching on size or \Sigma,
and without invoking any differences between centrals and satellites, beyond
the different mass-dependences of their quenching laws. The toy-model also
reproduces several other observations, including the sSFR gradients within
galaxies and the appearance of inside-out build-up of passive galaxies.
Finally, it is shown that curvature in the Main Sequence sSFR-mass relation can
produce curvature in the apparent B/T ratios with mass. Our analysis therefore
suggests that many of the strong correlations that are observed between galaxy
structure and sSFR may well be a consequence of things unrelated to quenching
and should not be taken as evidence of the physical processes that drive
quenching.
