HOPCAT, a complete Southern Sky Neutral
Hydrogen-Optical Galaxy Catalogue
and the Star Formation Rate-Density Relation.
Marianne T. Doyle
School of Physical Sciences
University of Queensland
November, 2006
Abstract
We investigate the star formation rate (SFR)-density relation using a
neutral hydrogen (HI) galaxy sample. The SFR-density relation is the observation
that the SFR decreases with increasing local galaxy density. We determine
which of the two main alternatives explains what causes the cessation of
star formation in regions with high galaxy density. Is it that fewer star
forming HI galaxies are present in galaxy dense regions or are galaxies capable
of star formation present but some physical process is suppressing their
star formation? We use the HI Parkes All Sky survey catalogue (HICAT) and
find the optical counterparts for the HI detections. We produce the HICAT
Optical Catalogue, HOPCAT which contains 4315 HI detections. Of these we
find optical counterparts for 3618 (84 per cent) galaxies. For 481 (11 per
cent), multiple galaxies are present but no single optical counterpart can
be chosen and 216 (5 per cent) have no obvious optical galaxy present. We
conduct a search for possible isolated dark galaxies in HOPCAT and find no
isolated optically dark galaxies within the limits of the HI Parkes All Sky
survey. A compact disk copy of HOPCAT is provided with this thesis.
The SFR-density relation is investigated using the optically matched
HOPCAT galaxies. We use these galaxies along with two background catalogues,
one optical and one k-band, to calculate the local surface density, Σ10
Sigma10. To calculate the SFRs we use infrared (IR) and radio
flux. We have four data sets: two with densities calculated using the optical
background catalogue each with IR and radio SFRs, and two with densities
calculated using the K-band background catalogue with IR and radio SFRs.
Using the largest HI detected data set of the whole southern sky we investigate
the relationship between HI mass and the SFR and find that, for nearby
(velocity < 10000 km s -1) HI galaxies, we find a strong
correlation. We find a larger population of HI galaxies compared to optical
galaxies for Sigma10 < 2 galaxies Mpc-2. We also
find the number of HI galaxies decreases by 50 per cent over a Sigma10
range between 0 and 11 galaxies Mpc-2. For the same density
range the optical galaxy population only decreases by 2.9 per cent. We do
not find a change in the HI content of our galaxy sample with increasing
local galaxy density.
For HI galaxies (1000 < velocity < 6000 kms-1 there
is no significant change in the star formation rate or the efficiency of
star formation with respect to local surface density. We find that the SFR-density
relation as reported in previous studies can be explained by their optical
data sample. Their samples contain galaxies detected by their stellar population
and not their star formation potential. The SFR is less in regions of high
local galaxy density because the galaxies present in these regions are predominately
elliptical galaxies that display little or no star formation. This density/morphology
mix is confirmed by the morphology-density relation.
For the first time we can explain the reasons behind the SFR-density
relation and we find that the decrease in the SFR with increasing local
galaxy density is explained by a decrease in the population of HI star
forming galaxies and not due to the suppression of star formation in the
HI galaxies.
Thesis
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Created : July 11th 2005 | Last Updated : December
11th 2006 | VERSION : 1.0