837 research outputs found
Low Surface Brightness Galaxies around the HDF-S: II. Distances and volume densities
With this study we aim at the spectroscopic verification of a photometrically
selected sample of Low Surface Brightness (LSB) galaxy candidates in a field
around the Hubble Deep Field-South (HDF-S). The sample helps to extend the
parameter space for LSB galaxies to lower central surface brightnesses and to
provide better estimates on the volume densities of these objects. To derive
redshifts for the LSB candidates, long-slit spectra were obtained covering a
spectral range from 3400{\AA} to 7500{\AA}. The observations have been obtained
using the ESO 3.6m telescope, equipped with the EFOSC2 spectrograph. From the
measured radial velocities, distances could be estimated. With this distance
information, it is possible to differentiate between true LSB galaxies and
higher redshift High Surface Brightness (HSB) galaxies which may contaminate
the sample. A correction for the surface brightnesses can then be applied,
accounting for the cosmological dimming effect (``Tolman Dimming''). We show
that ~70% of the LSB candidates, selected based on their location in the
color-color space, are real LSB galaxies. Their position in the color-color
diagrams, therefore, indicate that the LSB galaxies have a different stellar
population mix resulting from a different star formation history compared to
HSBs. Our LSB galaxy sample consists only of large disk galaxies with
scale-length between 2.5kpc and 7.3kpc. We confirm the flat central surface
brightness distribution of previous surveys and extend this distribution down
to central surface brightnesses of 27 B mag arcsec^-2.Comment: 12 pages, 20 figures, accepted by A&
The Baryon Content of Extremely Low Mass Dwarf Galaxies
We investigate the gas content and baryonic Tully-Fisher relationship for
extremely low luminosity dwarf galaxies in the absolute magnitude range -13.5 >
Mr > -16. The sample is selected from the Sloan Digital Sky Survey and consists
of 101 galaxies for which we have obtained follow-up HI observations using the
Arecibo Observatory and Green Bank Telescope. This represents the largest
homogeneous sample of dwarfs at low luminosities with well-measured HI and
optical properties. The sample spans a range of environments, from dense groups
to truly isolated galaxies. The average neutral gas fraction is f_gas=0.6,
significantly exceeding that of typical gas-rich galaxies at higher
luminosities. Dwarf galaxies are therefore less efficient at turning gas into
stars over their lifetimes. The strong environmental dependence of the gas
fraction distribution demonstrates that while internal processes can reduce the
gas fractions to roughly f_gas=0.4, external processes are required to fully
remove gas from a dwarf galaxy. The average rotational velocity of our sample
is vrot=50 km/s. Including more massive galaxies from the literature, we fit a
baryonic Tully-Fisher slope of M_baryon \propto vrot^(3.70+/- 0.15). This slope
compares well with CDM models that assume an equal baryon to dark matter ratio
at all masses. While gas stripping or other processes may modify the baryon to
dark matter ratio for dwarfs in the densest environments, the majority of dwarf
galaxies in our sample have not preferentially lost significant baryonic mass
relative to more massive galaxies.Comment: 33 pages, 8 figures. Accepted to ApJ. Data available at
http://www.ociw.edu/~mgeha/researc
Milky Way Mass Models and MOND
Using the Tuorla-Heidelberg model for the mass distribution of the Milky Way,
I determine the rotation curve predicted by MOND. The result is in good
agreement with the observed terminal velocities interior to the solar radius
and with estimates of the Galaxy's rotation curve exterior thereto. There are
no fit parameters: given the mass distribution, MOND provides a good match to
the rotation curve. The Tuorla-Heidelberg model does allow for a variety of
exponential scale lengths; MOND prefers short scale lengths in the range 2.0 to
2.5 kpc. The favored value of scale length depends somewhat on the choice of
interpolation function. There is some preference for the `simple' interpolation
function as found by Famaey & Binney. I introduce an interpolation function
that shares the advantages of the simple function on galaxy scales while having
a much smaller impact in the solar system. I also solve the inverse problem,
inferring the surface mass density distribution of the Milky Way from the
terminal velocities. The result is a Galaxy with `bumps and wiggles' in both
its luminosity profile and rotation curve that are reminiscent of those
frequently observed in external galaxies.Comment: Accepted for publication in the Astrophysical Journal. 31 pages
including 8 figures and 3 table
A lower limit on the dark particle mass from dSphs
We use dwarf spheroidal galaxies as a tool to attempt to put precise lower
limits on the mass of the dark matter particle, assuming it is a sterile
neutrino. We begin by making cored dark halo fits to the line of sight velocity
dispersions as a function of projected radius (taken from Walker et al. 2007)
for six of the Milky Way's dwarf spheroidal galaxies. We test Osipkov-Merritt
velocity anisotropy profiles, but find that no benefit is gained over constant
velocity anisotropy. In contrast to previous attempts, we do not assume any
relation between the stellar velocity dispersions and the dark matter ones, but
instead we solve directly for the sterile neutrino velocity dispersion at all
radii by using the equation of state for a partially degenerate neutrino gas
(which ensures hydrostatic equilibrium of the sterile neutrino halo). This
yields a 1:1 relation between the sterile neutrino density and velocity
dispersion, and therefore gives us an accurate estimate of the Tremaine-Gunn
limit at all radii. By varying the sterile neutrino particle mass, we locate
the minimum mass for all six dwarf spheroidals such that the Tremaine-Gunn
limit is not exceeded at any radius (in particular at the centre). We find
sizeable differences between the ranges of feasible sterile neutrino particle
mass for each dwarf, but interestingly there exists a small range 270-280eV
which is consistent with all dSphs at the 1- level.Comment: 13 pages, 2 figures, 1 tabl
Chemical abundances in spiral and irregular galaxies. O and N abundances derived from global emission--line spectra
The validity of oxygen and nitrogen abundances derived from the global
emission-line spectra of galaxies via the P-method has been investigated using
a collection of published spectra of individual HII regions in irregular and
spiral galaxies. The conclusions of Kobulnicky, Kennicutt & Pizagno (1999) that
global emission-line spectra can reliably indicate the chemical properties of
galaxies has been confirmed. It has been shown that the comparison of the
global spectrum of a galaxy with a collection of spectra of individual HII
regions can be used to distinguish high and low metallicity objects and to
estimate accurate chemical abundances in a galaxy. The oxygen and nitrogen
abundances in samples of UV-selected and normal nearby galaxies have been
determined. It has been found that the UV-selected galaxies occupy the same
area in the N/O -- O/H diagram as individual HII regions in nearby galaxies.
Finally, we show that intermediate-redshift galaxies systematically deviate
from the metallicity -- luminosity trend of local galaxies.Comment: 15 pages, 17 figures, accepted for publication in Astronomy and
Astrophysic
New HI-detected Galaxies in the Zone of Avoidance
We present the first results of a blind HI survey for galaxies in the
southern Zone of Avoidance with a multibeam receiver on the Parkes telescope.
This survey is eventually expected to catalog several thousand galaxies within
Galactic latitude |b|<5 degrees, mostly unrecognised before due to Galactic
extinction and confusion. We present here results of the first three detections
to have been imaged with the Australia Telescope Compact Array (ATCA). The
galaxies all lie near Galactic longitude 325 degrees and were selected because
of their large angular sizes, up to 1.3 degrees. Linear sizes range from 53 to
108 kpc. The first galaxy is a massive 5.7x10^11 solar mass disk galaxy with a
faint optical counterpart, SGC 1511.1--5249. The second is probably an
interacting group of galaxies straddling the Galactic equator. No optical
identification is possible. The third object appears to be an interacting pair
of low column density galaxies, possibly belonging to an extended Circinus or
Centaurus A galaxy group. No optical counterpart has been seen despite the
predicted extinction (A(B) = 2.7 - 4.4 mag) not being excessive. We discuss the
implications of the results, in particular the low HI column densities (~10^19
atoms/sq.cm) found for two of the three galaxies.Comment: 17 pages, 8 figures (Fig.1 in three parts, Fig.5 in two parts). To
appear in Astronomical Journal (Dec 1998). See
http://www.atnf.csiro.au/research/multibea
Reconciling MOND and dark matter?
Observations of galaxies suggest a one-to-one analytic relation between the
inferred gravity of dark matter at any radius and the enclosed baryonic mass, a
relation summarized by Milgrom's law of modified Newtonian dynamics (MOND).
However, present-day covariant versions of MOND usually require some additional
fields contributing to the geometry, as well as an additional hot dark matter
component to explain cluster dynamics and cosmology. Here, we envisage a
slightly more mundane explanation, suggesting that dark matter does exist but
is the source of MOND-like phenomenology in galaxies. We assume a canonical
action for dark matter, but also add an interaction term between baryonic
matter, gravity, and dark matter, such that standard matter effectively obeys
the MOND field equation in galaxies. We show that even the simplest realization
of the framework leads to a model which reproduces some phenomenological
predictions of cold dark matter (CDM) and MOND at those scales where these are
most successful. We also devise a more general form of the interaction term,
introducing the medium density as a new order parameter. This allows for new
physical effects which should be amenable to observational tests in the near
future. Hence, this very general framework, which can be furthermore related to
a generalized scalar-tensor theory, opens the way to a possible unification of
the successes of CDM and MOND at different scales.Comment: 9 page
A large sample of low surface brightness disc galaxies from the SDSS- II. Metallicities in surface brightness bins
We study the spectroscopic properties of a large sample of Low Surface
Brightness galaxies (LSBGs) (with B-band central surface brightness mu0(B)>22
mag arcsec^(-2)) selected from the Sloan Digital Sky Survey Data Release 4
(SDSS-DR4) main galaxy sample. A large sample of disk-dominated High Surface
Brightness galaxies (HSBGs, with mu0(B)<22 mag arcsec^(-2)) are also selected
for comparison simultaneously. To study them in more details, these sample
galaxies are further divided into four subgroups according to mu0(B) (in units
of mag arcsec^(-2)): vLSBGs (24.5-22.75),iLSBGs (22.75-22.0), iHSBGs
(22.0-21.25), and vHSBGs (<21.25). The diagnostic diagram from spectral
emission-line ratios shows that the AGN fractions of all the four subgroups are
small (<9%). The 21,032 star-forming galaxies with good quality spectroscopic
observations are further selected for studying their dust extinction,
strong-line ratios, metallicities and stellar mass-metallicities relations. The
vLSBGs have lower extinction values and have less metal-rich and massive
galaxies than the other subgroups. The oxygen abundances of our LSBGs are not
as low as those of the HII regions in LSBGs studied in literature, which could
be because our samples are more luminous, and because of the different
metallicity calibrations used. We find a correlation between 12+log(O/H) and
mu0(B) for vLSBGs, iLSBGs and iHSBGs but show that this could be a result of
correlation between mu0(B) and stellar mass and the well-known mass-metallicity
relation. This large sample shows that LSBGs span a wide range in metallicity
and stellar mass, and they lie nearly on the stellar mass vs. metallicity and
N/O vs. O/H relations of normal galaxies. This suggests that LSBGs and HSBGs
have not had dramatically different star formation and chemical enrichment
histories.Comment: 14 pages, 11 figures, accepted for publication in MNRA
The spacetime structure of MOND with Tully-Fisher relation and Lorentz invariance violation
It is believed that the modification of Newtonian dynamics (MOND) is possible
alternate for dark matter hypothesis. Although Bekenstein's TeVeS supplies a
relativistic version of MOND, one may still wish a more concise covariant
formulism of MOND. In this paper, within covariant geometrical framwork, we
present another version of MOND. We show the spacetime structure of MOND with
properties of Tully-Fisher relation and Lorentz invariance violation.Comment: 6 pages. arXiv admin note: substantial text overlap with
arXiv:1111.1383 and arXiv:1108.344
The dark matter halo of NGC 1399 - CDM or MOND?
Central galaxies in galaxy clusters may be key discriminants in the
competition between the cold dark matter (CDM) paradigm and modified Newtonian
dynamics (MOND). We investigate the dark halo of NGC 1399, the central galaxy
of the Fornax cluster, out to a galactocentric distance of 80 kpc. The data
base consists of 656 radial velocities of globular clusters obtained with
MXU/VLT and GMOS/Gemini, which is the largest sample so far for any galaxy. We
performed a Jeans analysis for a non-rotating isotropic model. An NFW halo with
the parameters r_s = 50 kpc and rho_s = 0.0065 M_sun/pc^3 provides a good
description of our data, fitting well to the X-ray mass. More massive halos are
also permitted that agree with the mass of the Fornax cluster as derived from
galaxy velocities. We compare this halo with the expected MOND models under
isotropy and find that additional dark matter on the order of the stellar mass
is needed to get agreement. A fully radial infinite globular cluster system
would be needed to change this conclusion. Regarding CDM, we cannot draw firm
conclusions. To really constrain a cluster wide halo, more data covering a
larger radius are necessary. The MOND result appears as a small-scale variant
of the finding that MOND in galaxy clusters still needs dark matter.Comment: 4 pages, 2 figures, accepted for publication as a Letter in A&
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