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Neutral Hydrogen Gas in Star Forming Galaxies at z=0.24
Philip Lah1, Jayaram Chengalur2, Frank Briggs1, Matthew Colless3, Roberto De Propris4, Michael Pracy1 & Erwin de Blok1
1Australian
National University, 2National Centre for Radio Astrophysics, 3Anglo-Australian Observatory, 4Cerro Tololo Inter-American Observatory
Introduction
Cosmic Neutral Gas Density
We are conducting a study of the relationship between neutral gas content
and star formation in galaxies as a function of redshift and environment.
Our study combines deep radio observations of HI 21cm emission using
the Giant Metrewave Radio Telescope (GMRT) in India with optical
imaging and spectroscopy. The main results presented here are from a
sample of 348 Hα-emitting, field galaxies at z = 0.24 selected from a
narrow-band imaging survey on the Subaru Telescope (Fujita et al. 2003,
ApJ, 586, L115). The figure to the left shows 10'' by 10'' thumbnails of
the normalised B band images of these 'Fujita galaxies' ordered by
decreasing Hα luminosity (the brightest at the top left, the faintest at the
bottom right). From these galaxies the star formation rate density at
z = 0.24 was measured to be ~3 times that at z  0. Spectroscopic followup of these galaxies was done using the Anglo-Australian Telescope.
Radio observations of the galaxies totaling ~44 hours were obtained using
the GMRT at 1150 MHz (HI 21cm at z = 0.24).
Radio Continuum at z=0.24
The figure to right shows the Hα emission line luminosity
plotted against the restframe 1.4 GHz radio continuum
luminosity for all 348 'Fujita galaxies' at z = 0.24. The
plotted line is the observed relationship found at z  0
(Sullivan et al. 2001, ApJ, 558, 72). The black triangle is a
interacting system of two galaxies. The blue square in the
'bright L(Hα)' region is the combined signal from 45
galaxies. The medium and faint subsamples of galaxies (55
and 236 galaxies respectively) provide only 2σ radio
continuum upper limits. These measurements at z = 0.24 are
consistent with those at z  0 suggesting that there has been
no significant change in the star formation-radio continuum
correlation in the past ~3 Gyrs.
The neutral gas density of the universe as a function of redshift (on the above left) and look-back time (on the above right).
The small, black triangle at z = 0 is the HIPASS 21cm emission measurement from Zwaan et al. 2005, MNRAS, 359, L30.
The blue, open circles are damped Lyα measurements made using HST by Rao et al. 2006, ApJ, 636, 610. The red, filled
circles are damped Lyα measurements from Prochaska et al. 2005, ApJ, 635, 123. The large, black triangle at z = 0.24 is
our HI 21cm measurement, Ωgas = (0.91  0.42)  10-3. This is made using our coadded HI signal and the known density of
Hα emission galaxies at z = 0.24. Our value is consistent with that from damped Lyα at similar redshifts. However our
measurement has the advantage of being over a much narrower redshift range.
line from
Sullivan et al. 2001
For further information see:
Lah et al. 2007, MNRAS, 376, 1357
Coadded HI Signal at z=0.24
The individual HI emission for any one galaxyy at z = 0.24
is below the detection limits of our radio observations.
However, we can measure the average HI signal of the
galaxies by coadding the signal from multiple galaxies using
their known optical positions and redshifts,. The figure to
the left shows the average HI galaxy spectrum created from
coadding the 121 galaxies with useful optical redshifts. The
measured average HI mass is (2.26  0.90)109 M (M* at
z  0 is 6.3  109 M ). The top spectrum has no smoothing
or binning. The bottom spectrum has been binned to
~500 kms-1. For both spectra the 1σ error is shown as
dashed lines.
BONUS SECTION
HI in Abell 370, a galaxy cluster at z=0.37
With increasing redshift, galaxy clusters and their surroundings show a higher fraction
of optically blue galaxies (the Butcher-Oemler effect). This effect is noticeable from
z  0.1 and is a strong effect by z = 0.4. We are using the coadding HI technique to see
whether there is a similar increase in the amount of HI gas in cluster galaxies. Here I
present preliminary results for HI in Abell 370, a galaxy cluster at z = 0.37. We have
obtained ~34 hours of GMRT radio observations of the galaxy cluster at 1040 MHz.
For optical imaging we used SSO 40 inch telescope and for spectroscopic follow-up,
4 nights with AAOmega on the Anglo-Australian Telescope. The image to the right is
from the VLT UT1+FORS1 and shows the cluster centre of Abell 370.
Coadded HI Signal at z=0.37
HI Mass vs.
Star Formation Rate
The figure to the right shows the average galaxy HI mass
plotted against the average galaxy star formation rate for the
'Fujita galaxies'. The blue triangle is the average of all 121
galaxies with redshifts. The black circular points are the
values for bright, medium and faint L(Hα) subsamples. The
line is the correlation found at z  0 by Doyle &
Drinkwater 2006, MNRAS, 372, 977. Our measurements
at z = 0.24 are consistent with those at z  0. This means
that the increase in the star formation rates in field galaxies
~3 Gyr ago is matched with a corresponding increase in the
amount of neutral gas in the galaxies.
line from Doyle
& Drinkwater
2006
The figure to the left shows the average HI galaxy spectrum
created from coadding the signal of 278 redshifts. The top
spectrum has no smoothing or binning. The bottom
spectrum has been binned to ~500 kms-1. For both spectra
the 1σ error is shown as dashed lines. The measured
average HI mass is (7.4  2.6)  109 M  (M* at z  0 is
6.3  109 M ). The galaxies are all within 10 Mpc
projected distance of the cluster centre and have
0.366 < z < 0.387. The galaxies are very bright in the
optical; their luminosities are L* or brighter. The average
galaxy HI mass for blue galaxies with V-I colour  0.9 is
(10.1  4.1)  109 M  from 128 redshifts. The average
galaxy HI mass for red galaxies with V-I colour > 0.9 is
(5.5  3.4)  109 M  from 150 redshifts. From these results
it is clear that there is a large amount of HI gas in galaxies in
the cluster and its surroundings and that the gas is
concentrated in the blue galaxies.