247 research outputs found
A New Galaxy in the Local Group: the Antlia Dwarf Galaxy
We report the discovery of new member of the Local Group in the constellation
of Antlia. Optically the system appears to be a typical dwarf spheroidal galaxy
of type dE3.5 with no apparent young blue stars or unusual features. A
color-magnitude diagram in I, V-I shows the tip of the red giant branch, giving
a distance modulus of 25.3 +/- 0.2 (1.15 Mpc +/- 0.1) and a metallicity of -1.6
+/- 0.3. Although Antlia is in a relatively isolated part of the Local Group it
is only 1.2 degrees away on the sky from the Local Group dwarf NGC3109, and may
be an associated system.Comment: AJ in press, 15 pages, 7 figures, figure 2 in b/w for space saving,
full postscript version available at
http://www.ast.cam.ac.uk/~gkth/antlia-pp.htm
Ca II triplet spectroscopy of RGB stars in NGC 6822: kinematics and metallicities
We present a detailed analysis of the chemistry and kinematics of red giants
in the dwarf irregular galaxy NGC 6822. Spectroscopy at 8500 Angstroms was
acquired for 72 red giant stars across two fields using FORS2 at the VLT. Line
of sight extinction was individually estimated for each target star to
accommodate the variable reddening across NGC 6822. The mean radial velocity
was found to be v_helio = (52.8 +/- 2.2) km/s with dispersion rms = 24.1 km/s,
in agreement with other studies. Ca II triplet equivalent widths were converted
into [Fe/H] metallicities using a V magnitude proxy for surface gravity. The
average metallicity was [Fe/H] = (-0.84 +/- 0.04) with dispersion rms = 0.31
dex and interquartile range 0.48. Our assignment of individual reddening values
makes our analysis more sensitive to spatial variations in metallicity than
previous studies. We divide our sample into metal-rich and metal-poor stars;
the former are found to cluster towards small radii with the metal-poor stars
more evenly distributed across the galaxy. The velocity dispersion of the
metal-poor stars is higher than that of the metal-rich stars; combined with the
age-metallicity relation this indicates that older populations have either been
dynamically heated or were born in a less disclike distribution. The low ratio
(v_rot/v_rms) suggests that within the inner 10', NGC 6822's stars are
dynamically decoupled from the HI gas, possibly in a thick disc or spheroid.Comment: 15 pages, 12 figures, includes tabular dat
Cosmological Evolution of the Universe Neutral Gas Mass Measured by Quasar Absorption Systems
The cosmological evolution of neutral hydrogen is an efficient way of tracing
structure formation with redshift. It indicates the rate of evolution of gas
into stars and hence the gas consumption and rate star formation history of the
Universe. In measuring HI, quasar absorbers have proven to be an ideal tool and
we use observations from a recent survey for high-redshift quasar absorption
systems together with data gathered from the literature to measure the
cosmological comoving mass density of neutral gas. This paper assumes
Omega_M=0.3, Omega_lambda=0.7 and h=0.65.Comment: 3 pages, 2 figures. To appear in the proceedings of the "Cosmic
Evolution" conference, held at l'Institut d'Astrophysique de Paris, November
13-17, 200
Statistical Properties of DLAs and sub-DLAs
Quasar absorbers provide a powerful observational tool with which to probe
both galaxies and the intergalactic medium up to high redshift. We present a
study of the evolution of the column density distribution, f(N,z), and total
neutral hydrogen mass in high-column density quasar absorbers using data from a
recent high-redshift survey for damped Lyman-alpha (DLA) and Lyman limit system
(LLS) absorbers. Whilst in the redshift range 2 to 3.5, ~90% of the neutral HI
mass is in DLAs, we find that at z>3.5 this fraction drops to only 55% and that
the remaining 'missing' mass fraction of the neutral gas lies in sub-DLAs with
N(HI) 10^{19} - 2 * 10^{20} cm^{-2}.Comment: 6 pages, 4 figures, in "Chemical Enrichment of Intracluster and
Intergalactic medium", Proceedings of the Vulcano Workshop, May 14-18, 200
The effect of diffuse background on the spatially-resolved Schmidt relation in nearby spiral galaxies
The global Schmidt law of star formation provides a power-law relation
between the surface densities of star-formation rate (SFR) and gas, and
successfully explains plausible scenarios of galaxy formation and evolution.
However, star formation being a multi-scale process, requires
spatially-resolved analysis for a better understanding of the physics of star
formation. It has been shown that the removal of a diffuse background from SFR
tracers, such as H, far-ultraviolet (FUV), infrared, leads to an
increase in the slope of the sub-galactic Schmidt relation. We reinvestigate
the local Schmidt relations in nine nearby spiral galaxies taking into account
the effect of inclusion and removal of diffuse background in SFR tracers as
well as in the atomic gas.We used multiwavelength data obtained as part of the
surveys such as SINGS, KINGFISH, THINGS, and HERACLES. Making use of a novel
split of the overall light distribution as a function of spatial scale, we
subtracted the diffuse background in the SFR tracers as well as the atomic gas.
Using aperture photometry, we study the Schmidt relations on background
subtracted and unsubtracted data at physical scales varying between 0.5--2 kpc.
The fraction of diffuse background varies from galaxy to galaxy and accounts to
34 % in H, 43 % in FUV, 37 % in 24 m, and
75\% in H I on average. We find that the inclusion of diffuse background
in SFR tracers leads to a linear molecular gas Schmidt relation and a bimodal
total gas Schmidt relation. However, the removal of diffuse background in SFR
tracers leads to a super-linear molecular gas Schmidt relation. A further
removal of the diffuse background from atomic gas results in a slope 1.4
0.1, which agrees with dynamical models of star formation accounting for
flaring effects in the outer regions of galaxies.Comment: 33 pages, 25 figures, 6 tables, accepted for publication in A&
A panoramic VISTA of the stellar halo of NGC 253
Outskirts of large galaxies contain important information about the galaxy
formation and assembly process, and resolved star count studies can probe the
extremely low surface brightness of the outer halos. We use images obtained
with the VISTA telescope to construct spatially resolved J vs Z-J
colour-magnitude diagrams (CMDs) of NGC 253, a nearly edge-on disk galaxy in
the Sculptor group. The very deep photometry, down to J ~ 23.5, and the wide
area covered allows us to trace the red giant branch (RGB) and asymptotic giant
branch (AGB) stars that belong to the outer disk and the halo of NGC 253, out
to 50 kpc along the galaxy minor axis. We confirm the existence of an extra
planar stellar component of the disk, with a very prominent southern shelf and
a symmetrical feature on the north side. The only additional visible
sub-structure is an overdensity in the north-west part of the halo at about 28
kpc from the plane and extending over ~ 20 kpc parallel with the disk of the
galaxy. From the stellar count profile along the major axis we measure the
transition from the disk to the halo at a radial distance of about 25 kpc,
where a clear break appears in the number density profile. The isodensity
contours show that the inner halo is a flattened structure that blends with a
more extended, diffuse, rounder outer halo. Such external structure can be
traced to the very edge of our image out to 50 kpc from the disk plane. The
number density profile of the stars in the stellar halo follows a power law
with index -1.6, as function of radius. The CMD shows a very homogeneous
stellar population across the whole field; by comparison with theoretical
isochrones we conclude that the RGB stars are ~ 8 Gyr old or more, while the
AGB stars trace a population of about 2 x 10^8 Mo, formed from ~ 0.5 to a few
Gyr ago. Surprisingly, part of this latter population appears scattered over a
wide area.Comment: To appear on Astronomy and Astrophysic
- …