24 research outputs found
Anomalous Evolution of the Dwarf Galaxy HIPASS J1321-31
We present HST/WFPC2 observations of the dwarf galaxy HIPASS J1321-31. This
unusual galaxy lies in the direction of the Centaurus A group of galaxies, and
has a color-magnitude diagram with a distinctive red plume of luminous stars.
This feature could arise from (a) a red giant branch if the galaxy were much
nearer than previously recognized, (b) a peculiar asymptotic giant branch, or,
(c) an ~1 Gigayear old population of intermediate mass red supergiants, which
we find to be the most likely explanation. However, the lack of equally
luminous blue stars requires that the star formation has dropped substantially
since these stars were formed. Evidently HIPASS J1321-31 experienced an episode
of enhanced star formation rather recently in its star formation history
followed by a period of relative quiescence which has led to the evolution of
the main sequence stars into the red supergiant branch. The stellar populations
in HIPASS J1321-31 reflect a star formation history that is uncommon in star
forming dwarf galaxies. This is the first time such a star formation history
has been noted, although the literature contains a small number of other dwarf
galaxies with similar color-magnitude diagrams. Therefore, HIPASS J1321-31 and
these other galaxies represent a different path of dwarf galaxy evolution that
has not been well-explored and an important probe into how dwarf galaxies
evolve.Comment: 5 pages, including 3 figures and 1 table, emulateapj5/apjfonts style.
Accepted by the Astrophysical Journal Letter
HI Selected Galaxies in the Sloan Digital Sky Survey I: Optical Data
We present the optical data for 195 HI-selected galaxies that fall within
both the Sloan Digital Sky Survey (SDSS) and the Parkes Equatorial Survey (ES).
The photometric quantities have been independently recomputed for our sample
using a new photometric pipeline optimized for large galaxies, thus correcting
for SDSS's limited reliability for automatic photometry of angularly large or
low surface brightness (LSB) galaxies. We outline the magnitude of the
uncertainty in the SDSS catalog-level photometry and derive a quantitative
method for correcting the over-sky subtraction in the SDSS photometric
pipeline. The main thrust of this paper is to present the ES/SDSS sample and
discuss the methods behind the improved photometry, which will be used in
future scientific analysis. We present the overall optical properties of the
sample and briefly compare to a volume-limited, optically-selected sample.
Compared to the optically-selected SDSS sample (in the similar volume),
HI-selected galaxies are bluer and more luminous (fewer dwarf ellipticals and
more star formation). However, compared to typical SDSS galaxy studies, which
have their own selection effects, our sample is bluer, fainter and less
massive.Comment: 14 pages, 8 Figures, accepted for publication in AJ. Complete tables
will be available in the AJ electronic version and on the Vizier sit
HI Selected Galaxies in the Sloan Digital Sky Survey II: The Colors of Gas-Rich Galaxies
We utilize color information for an HI-selected sample of 195 galaxies to
explore the star formation histories and physical conditions that produce the
observed colors. We show that the HI selection creates a significant offset
towards bluer colors that can be explained by enhanced recent bursts of star
formation. There is also no obvious color bimodality, because the HI selection
restricts the sample to bluer, actively star forming systems, diminishing the
importance of the red sequence. Rising star formation rates are still required
to explain the colors of galaxies bluer than g-r < 0.3. We also demonstrate
that the colors of the bluest galaxies in our sample are dominated by emission
lines and that stellar population synthesis models alone (without emission
lines) are not adequate for reproducing many of the galaxy colors. These
emission lines produce large changes in the r-i colors but leave the g-r color
largely unchanged. In addition, we find an increase in the dispersion of galaxy
colors at low masses that may be the result of a change in the star formation
process in low-mass galaxies.Comment: 12 pages, 16 figures, published in AJ (138, 796); replaced Figure 16
with higher resolution versio
Star formation history and evolution of gas-rich dwarf galaxies in the Centaurus A group
We analyse the properties of three unusual dwarf galaxies in the Centaurus A group discovered with the HIPASS survey. From their optical morphology, they appear to be low surface brightness dwarf spheroidals, yet they are gas rich (MH i/LB > 1) with gas-mass-to-stellar light ratios larger than typical dwarf irregular galaxies. Therefore these systems appear different from any dwarfs of the Local Group. They should be favoured hosts for starburst, whereas we find a faint star formation region in only one object. We have obtained 21-cm data and Hubble Space Telescope photometry in V and I bands, and have constructed colour magnitude diagrams (CMDs) to investigate their stellar populations and to set a constraint on their age. From the comparison of the observed and model CMDs, we infer that all three galaxies are at least older than 2 Gyr (possibly even as old as 10 Gyr) and remain gas rich because their star formation rates have been very low (≲10-3 M⊙ yr-1) throughout. In such systems, star formation appears to have been sporadic and local, though one object (HIPASS J1321-31) has a peculiar red plume in its CMD suggesting that many of its stars were formed in a 'miniburst' 300-500 Myr ago. The question of why there are no similar dwarf galaxies in the Local Group remains open
Supernova remnants, planetary nebulae and the distance to NGC 4214
We present narrow band, continuum subtracted Hα, [S II], Hβ, [O III] and [O II] data taken with the Wide Field Camera 3 on the Hubble Space Telescope in the nearby dwarf starburst galaxy NGC 4214. From these images, we identify seventeen new planetary
First Results from the HI Jodrell All Sky Survey: inclination-dependent selection effects in a 21-cm blind survey
Details are presented of the H I Jodrell All Sky Survey (HIJASS). HIJASS is a blind neutral hydrogen (H I) survey of the northern sky (δ > 22°), being conducted using the multibeam receiver on the Lovell Telescope (full width at half-maximum beamwidth
Large-scale shock-ionized and photoionized gas in M83: The impact of star formation
We investigate the ionization structure of the nebular gas in M83 using the line diagnostic diagram, [O III](5007)/Hβ versus [S II](6716+6731)/ Hα, with the newly available narrowband images from the Wide Field Camera 3 (WFC3) of the Hubble Space Teles