609 research outputs found
The portrait of Malin 2: a case study of a giant low surface brightness galaxy
The low surface brightness disc galaxy Malin2 challenges the standard theory
of galaxy evolution by its enormous total mass ~2 10^12 Ms which must have been
formed without recent major merger events. The aim of our work is to create a
coherent picture of this exotic object by using the new optical multicolor
photometric and spectroscopic observations at Apache Point Observatory as well
as archival datasets from Gemini and wide-field surveys. We performed the
Malin2 mass modelling, estimated the contribution of the host dark halo and
found that it had acquired its low central density and the huge isothermal
sphere core radius before the disc subsystem was formed. Our spectroscopic data
analysis reveals complex kinematics of stars and gas in the very inner region.
We measured the oxygen abundance in several clumps and concluded that the gas
metallicity decreases from the solar value in the centre to a half of that at
20-30 kpc. We found a small satellite and measured its mass (1/500 of the host
galaxy) and gas metallicity. One of the unique properties of Malin2 turned to
be the apparent imbalance of ISM: the molecular gas is in excess with respect
to the atomic gas for given values of the gas equilibrium turbulent pressure.
We explain this imbalance by the presence of a significant portion of the dark
gas not observable in CO and the Hi 21 cm lines. We also show that the
depletion time of the observed molecular gas traced by CO is nearly the same as
in normal galaxies. Our modelling of the UV-to-optical spectral energy
distribution favours the exponentially declined SFH over a single-burst
scenario. We argue that the massive and rarefied dark halo which had formed
before the disc component well describes all the observed properties of Malin2
and there is no need to assume additional catastrophic scenarios proposed
previously to explain the origin of giant LSB galaxies. [Abbreviated]Comment: 17 pages, 10 figures, accepted for publication in MNRA
The chemical composition of Ultracompact Dwarf Galaxies in the Virgo and Fornax Clusters
We present spectroscopic observations of ultra compact dwarf (UCD) galaxies
in the Fornax and Virgo Clusters made to measure and compare their stellar
populations. The spectra were obtained on the Gemini-North (Virgo) and
Gemini-South (Fornax) Telescopes using the respective Gemini Multi-Object
Spectrographs.
We estimated the ages, metallicities and abundances of the objects from mea-
surements of Lick line-strength indices in the spectra; we also estimated the
ages and metallicities independently using a direct spectral fitting technique.
Both methods re- vealed that the UCDs are old (mean age 10.8 \pm 0.7 Gyr) and
(generally) metal-rich (mean [Fe/H] = -0.8 \pm 0.1). The alpha-element
abundances of the objects measured from the Lick indices are super-Solar.
We used these measurements to test the hypothesis that UCDs are formed by the
tidal disruption of present-day nucleated dwarf elliptical galaxies. The data
are not consistent with this hypothesis because both the ages and abundances
are significantly higher than those of observed dwarf galaxy nuclei (this does
not exclude disruption of an earlier generation of dwarf galaxies). They are
more consistent with the properties of globular star clusters, although at
higher mean metallicity. The UCDs display a very wide range of metallicity
(-1.7 <[Fe/H]< 0.0), spanning the full range of both globular clusters and
dwarf galaxy nuclei.
We confirm previous reports that most UCDs have high metalliticities for
their luminosities, lying significantly above the canonical
metallicitiy-luminosity relation followed by early-type galaxies. In contrast
to previous work we find that there is no significant difference in either the
mean ages or the mean metallicities of the Virgo and Fornax UCD populations.Comment: 15 pages (including references and appendix), 8 figures (including
appendix
Study on Cosmic Ray Background Rejection with a 30 m Stand-Alone IACT using Non-parametric Multivariate Methods in a sub-100 GeV Energy Range
During the last decade ground-based very high-energy gamma-ray astronomy
achieved a remarkable advancement in the development of the observational
technique for the registration and study of gamma-ray emission above 100 GeV.
It is widely believed that the next step in its future development will be the
construction of telescopes of substantially larger size than the currently used
10 m class telescopes. This can drastically improve the sensitivity of the
ground-based detectors for gamma rays of energy from 10 to 100 GeV. Based on
Monte Carlo simulations of the response of a single stand-alone 30 m imaging
atmospheric Cherenkov telescope (IACT) the maximal rejection power against
background cosmic ray showers for low energy gamma-rays was investigated in
great detail. An advanced Bayesian multivariate analysis has been applied to
the simulated Cherenkov light images of the gamma-ray- and proton-induced air
showers. The results obtained here quantitatively testify that the separation
between the signal and background images degrades substantially at low
energies, and consequently the maximum overall quality factor can only be about
3.1 for gamma rays in the 10-30 GeV energy range. Various selection criteria as
well as optimal combinations of the standard image parameters utilized for
effective image separation have been also evaluated.Comment: Accepted for publication in the Journal of Physics
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