The Starburst-AGN connection: quenching the fire and feeding the beast
  The merger of two spiral galaxies is believed to be one of the main channels
for the production of elliptical and early-type galaxies. In the process, the
system becomes an (ultra) luminous infrared galaxy, or (U)LIRG, that morphs to
a quasar, to a K+A galaxy, and finally to an early-type galaxy. The time scales
for this metamorphosis are only loosely constrained by observations. In
particular, the K+A phase should follow immediately after the QSO phase during
which the dust and gas remaining from the (U)LIRG phase are expelled by the
AGN. An intermediate class of QSOs with K+A spectral signatures, the
post-starburst QSOs or PSQ, may represent the transitional phase between QSOs
and K+As. We have compiled a sample of 72 {bona fide} $z<0.5$ PSQ from the SDSS
DR7 QSO catalogue. We find the intermediate age populations in this sample to
be on average significantly weaker and metal poorer than their putative
descendants, the K+A galaxies. The typical spectral energy distribution of PSQ
is well fitted by three components: starlight; an obscured power-law; and a hot
dust component required to reproduce the mid-IR fluxes. From the slope and
bolometric luminosity of the power-law component we estimate typical masses and
accretion rates of the AGN, but we find little evidence of powerful radio-loud
or strong X-ray emitters in our sample. This may indicate that the power-law
component originates in a nuclear starburst rather than in an AGN, as expected
if the bulk of their young stars are still being formed, or that the AGN is
still heavily enshrouded in dust and gas. We find that both alternatives are
problematic and that more and better optical, X-ray, and mm-wave observations
are needed to elucidate the evolutionary history of PSQ.
