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It appears that central supermassive black holes are a nearly universal
component of galactic bulges. Do the central black holes form first and
serve as condensates for galaxies? Or do they build up as galaxies grow
and merge? SAFIR will be a powerful tool to answer these questions. The
low lying H2 lines at 17 and 28.2um are one of the few ways to study warm
molecular gas condensations prior to the formation of metals, for example
molecular gas around primordial massive black holes. Line widths and
profiles will indicate whether the central mass is highly compact
(suggesting a black hole), or if the molecular cloud is just in a mild
state of turbulence. At the current epoch, galaxy mergers produce huge
far-infrared fluxes through a combination of violent starbursts and AGNs
associated with their central black holes. The distinction of starbursts
from supermassive black holes as power sources for young and distant
galaxies was identified in the Decade Report (NAS Decade Report 2001) as a
major goal for new far-infrared telescopes.
What happens during the much more common mergers that build galaxies in the
early Universe? COBE showed that the far-infrared-to-submillimeter energy
density in the early Universe is comparable to that in the
visible-to-near-infrared. What are the relative roles of dust-embedded
AGNs and starbursts in producing this luminosity? Do AGNs at high redshift
differ in basic properties from nearby ones? Models of the cosmic X-ray
background indicate that the great majority of AGNs at high redshift are
heavily absorbed (Gilli, Salvati, & Hasinger 2001; Comastri et al. 2001).
Thus, these answers must be sought in the far-infrared where optical depths
are low (interstellar medium, ISM, optical depths are similar at 20m and 6
kev and rapidly decrease at longer infrared wavelengths and higher X-ray
energies). The fine structure lines of NeII (12.8um), NeIII (15.6um) and
NeV (14.3um) are the best tool to distinguish unambiguously whether the ISM
of a dusty galaxy is ionized by a starburst or by an AGN. Not only are the
line ratios very well separated, but their extinction is reduced by more
than a factor of thirty compared with the visible. At the epoch of peak
quasar activity, these lines will be redshifted to the 45 to 55m range.
Figure 1 shows the spectrum of the nearby active galaxy, Centaurus A,
illustrating the wealth of spectroscopic features that will be redshifted
into the 25 - 100um spectral region in early galaxies. Using these fine
structure lines, a 10m far-infrared telescope will have the necessary
resolution and sensitivity to determine the roles of star formation and
nuclear activity in the early Universe. The full suite of infrared fine
structure lines probes a very wide range of excitation energy, allowing
SAFIR to constrain the UV spectra of AGNs and extending work done with ISO
on a few nearby Seyfert galaxies to large lookback times. In addition,
many of these lines have relatively high critical densities (up to ~10^10
cm^-3), so they have a unique ability to probe the density of the gas around
AGNs. The angular resolution of SAFIR is a critical contribution to these
studies, and complements that of forthcoming NASA investments in telescopes
operating at shorter wavelengths. This capability is also illustrated in
Figure 1 with a comparison of the SAFIR and SIRTF beams on a simulated JWST
image.
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