616 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
Radial B-V/V-K color gradients, extinction-free Q_BVK combined color indices, and the history of star formation in the Cartwheel ring galaxy
In this paper we model and analyse the B-V/V-K radial color gradients
observed in the Cartwheel ring galaxy. Along with the color-color diagrams, we
use the Q_BVK combined color indices, which minimise the uncertainties in the
observed B-V and V-K colors introduced by dust extinction. To model the optical
and near-infrared color properties of the Cartwheel galaxy, we assume that an
intruder galaxy generates an expanding ring density wave in the Cartwheel's
disk, which in its turn triggers massive star formation along the wave's
perimeter according to the Schmidt law. We use the population synthesis to
calculate the color properties of stellar populations formed in the expanding
density wave. The results of color modelling suggest that the pre-collision
Cartwheel was a late-type spiral, embedded in an extensive gaseous disk of
sub-critical surface density. The properties of the old stellar disk are
typical for the late-type Freeman disks, with the central surface brightness in
V-band and the scale length being 21.0 mag arcsec^-2 and 2 kpc respectively.
The pre-collision gaseous disk has a metallicity gradient ranging from
z=z_sun/5 at the outer regions to z=z_sun in the central regions. At present,
the wave of star formation has passed the initial extent of the pre-collision,
old stellar disk and is currently moving in the predominantly gaseous,
low-metallicity disk at the radius of 16 kpc. Neither young stellar populations
formed in an expanding density wave, nor their mixture with the old,
pre-collision stellar populations can reproduce the B-V and V-K colors of the
Cartwheel's nucleus+inner ring. We find that an additional 10-Myr-old burst of
star formation in the nuclear regions, along with the visual extinction of 1.3
mag, might be responsible for the peculiar colors of the Cartwheel's nucleus.Comment: 11 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
WSRT observations and surface photometry of two unusual spiral galaxies
We discuss the results of a mass decomposition of two spiral galaxies, NGC
6824 and UGC 11919. In a previous analysis of the Hyperleda catalog, the
galaxies were identified as having a peculiar dynamical . The aim of this
study is to confirm or disprove the preliminary findings, indicating a
non-standard stellar initial mass function (IMF) for the galaxies. The surface
photometry in B, V, and R bands was carried out with the Apache Point 0.5-m
telescope and the \ion{H}{I} data cubes were obtained with the Westerbork
Synthesis Radio Telescope (WSRT). Photometric profiles were decomposed into
bulge and exponential disk components. Using the obtained \ion{H}{I} data
cubes, rotation curves of both galaxies were constructed. Employing the
photometric profiles, the mass distribution of the galaxies was decomposed into
mass components: bulge, stellar disk, gas, and pseudo-isothermal dark halo. We
conclude that NGC 6824 possesses a stellar disk with mass-to-light ratio
, in agreement with its color . On the
contrary, UGC 11919 appears to have a very lightweight disk. Its dynamically
estimated mass corresponds to a low stellar disk mass-to-light ratio
. Under standard assumptions, this ratio does
not agree with the relatively red color of the disk, while a bottom light
stellar initial mass function is needed to explain the observations.Comment: 14 pages, 14 figures, accepted for publication in Astronomy and
Astrophysic
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