Calibration of the line width scaling relations derived from different
techniques with different tracers, as a function of redshift
The determination of the rotational parameters of disk galaxies is of a crucial
importance for several areas of observational cosmology, ranging from galactic structure
and dynamics, to galaxy formation and evolution across cosmic time. In particular, scaling
relations such as the Tully-Fisher relation are used to determine the extragalactic
distance scale, map the large-scale velocity and mass distribution, and constrain N-body
simulations of cosmological scenarios. The latter suggest that disk scaling relations
should change as galaxies evolve. For example, even at modest redshifts (z ~ 0.4),
it is suggested that the comoving star formation rate is higher than today so that, for a
given gravitational potential, the associated luminosity may be higher by a factor of
3. It is thus important to ascertain the scale within which a relation
such as TF yields cosmologically unbiased results and to determine how the mass--to--light
ratio M/L varies with lookback time, a diagnostic of galaxy evolution. Several
recent studies based on optical spectrocopy reveal an intriguing state
of conflict in this field. Results vary between the inference that, even at
modest redshifts, M/L is substantially different (in excess of one magnitude)
from that at z=0, to ones that infer no significant change up to z ~ 1.
At low redshifts, rotational widths are usually derived either from HI line profiles or
from optical \ha rotation curves. At higher redshifts, long-slit/IFU spectroscopy currently
exists for only a few dozen objects. In contrast, large on-going surveys like
Sloan Digital Sky Survey (SDSS) and VIRMOS promise to deliver, over the next
few years, millions of galaxy spectra from which linewidths
can be derived. The main obstacle to applying such linewidths for
comparison with similar scaling relations at low redshift arises from the need
for common calibration of the width measures obtained using fundamentally different
techniques and over the largest possible redshift range. For example, the SDSS fiber diameter
limits the spatial scale over which the linewidth is measured,
so that its measures are most applicable at intermediate to high redshift. Since the most
prominent H-alpha line can be traced only up to z < 0.4,
application of the method to higher redshifts will require use of linewidths from
other species such as [OII] 3727 angstroms and [OIII] 5007 angstroms. It is
therefore important to establish a cross calibration of the Tully-Fisher
relation by comparing linewidths measured using different kinematic tracers
for a single set of galaxies to redshifts as large as possible, typically beyond
the distances included in previous peculiar velocity studies conducted using Arecibo.
We are involved in several projects that will allow us to extend our results
on use of the Tully-Fisher relation at low redshift to higher redshifts:
We are conducting Arecibo Observations of galaxies in the Sloan Survey
to provide a cross-calibration with our low-redshift
HI and H-alpha surveys.