Lyman Alpha Signatures from Direct Collapse Black Holes
  `Direct collapse black holes' (DCBHs) provide possible seeds for supermassive
black holes that exist at redshifts as high as z~7. We study Lyman Alpha (Lya)
radiative transfer through simplified representations of the DCBH-scenario. We
find that gravitational heating of the collapsing cloud gives rise to a Lya
cooling luminosity of up to ~ 1e38(M_gas/1e6 Msun)^2 erg/s. The Lya production
rate can be significantly larger during the final stages of collapse, but
collisional deexcitation efficiently suppresses the emerging Lya flux.
Photoionization by a central source boosts the Lya luminosity to
L~1e43(M_BH/1e6 M_sun) erg/s during specific evolutionary stages of the cloud,
where M_BH denotes the mass of the black hole powering this source. We predict
that the width and velocity off-set of the Lya spectral line range from a few
tens to few thousands km/s, depending sensitively on the evolutionary state of
the cloud. We also compare our predictions to observations of CR7 (Sobral et
al. 2015), a luminous Lya emitter at z~7, which is potentially associated with
a DCBH. If CR7 is powered by a black hole, then its Lya flux alone requires
that M_BH> 1e7 M_sun, which exceeds the mass of DCBHs when they first form. The
observed width of the Lya spectrum favors the presence of only a low column
density of hydrogen, log [N_HI/cm^-2]~19-20. The shape of the Lya spectrum
indicates that this gas is outflowing. These requirements imply that if CR7
harbors a DCBH, then the physical conditions that enabled its formation have
been mostly erased, which is in agreement with theoretical expectations.
