A unified model for age-velocity dispersion relations in Local Group
  galaxies: Disentangling ISM turbulence and latent dynamical heating
  We analyze age-velocity dispersion relations (AVRs) from kinematics of
individual stars in eight Local Group galaxies ranging in mass from Carina
($M_{*} \sim 10^{6}$) to M31 ($M_{*} \sim 10^{11}$). Observationally the
$\sigma$ vs. stellar age trends can be interpreted as dynamical heating of the
stars by GMCs, bars/spiral arms, or merging subhalos; alternatively the stars
could have simply been born out of a more turbulent ISM at high redshift and
retain that larger velocity dispersion till present day - consistent with
recent IFU studies. To ascertain the dominant mechanism and better understand
the impact of instabilities and feedback, we develop models based on observed
SFHs of these Local Group galaxies in order to create an evolutionary formalism
which describes the ISM velocity dispersion due to a galaxy's evolving gas
fraction. These empirical models relax the common assumption that the stars are
born from gas which has constant velocity dispersion at all redshifts. Using
only the observed SFHs as input, the ISM velocity dispersion and a mid-plane
scattering model fits the observed AVRs of low mass galaxies without fine
tuning. Higher mass galaxies above $M_{vir} > 10^{11}$ need a larger
contribution from latent dynamical heating processes (for example minor
mergers), in excess of the ISM model. Using the SFHs we also find that
supernovae feedback does not appear to be a dominant driver of the gas velocity
dispersion compared to gravitational instabilities - at least for dispersions
$\sigma \gtrsim 25$ km/s. Together our results point to stars being born with a
velocity dispersion close to that of the gas at the time of their formation,
with latent dynamical heating operating with a galaxy mass-dependent
efficiency. These semi-empirical relations may help constrain the efficiency of
feedback and its impact on the physics of disk settling in galaxy formation
simulations.
