494 research outputs found
Quantified HI Morphology V: HI Disks in the Virgo Cluster
We explore the quantified morphology of atomic hydrogen (HI) disks in the
Virgo cluster. These galaxies display a wealth of phenomena in their Hi
morphology, e.g., tails, truncation and warps. These morphological disturbances
are related to the ram-pressure stripping and tidal interaction that galaxies
undergo in this dense cluster environment. To quantify the morphological
transformation of the HI disks, we compute the morphological parameters of CAS,
Gini, and M20 and our own GM for 51 galaxies in 48 HI column density maps from
the VIVA project. Some morphological phenomena can be identified in this space
of relatively low resolution HI data. Truncation of the HI disk can be cleanly
identified via the Concentration parameter (C<1) and Concentration can also be
used to identify HI deficient disks (1<C<5). Tidal interaction is typically
identified using combinations of these morphological parameters, applied to
(optical) images of galaxies. We find that some selection criteria (Gini-M20,
Asymmetry, and a modified Concentration-M20) are still applicable for the
coarse (~15" FWHM) VIVA HI data. The phenomena of tidal tails can be reasonably
well identified using the Gini-M20 criterion (60% of galaxies with tails
identified but with as many contaminants). Ram-pressure does move HI disks into
and out of most of our interaction criteria: the ram-pressure sequence
identified by Vollmer et al. (2009) tracks into and out of some of these
criteria (Asymmetry based and the Gini-M20 selections, but not the
Concentration-M20 or the GM based ones). Therefore, future searches for
interaction using HI morphologies should take ram-pressure into account as a
mechanism to disturb HI disks enough to make them appear as gravitationally
interacting. One mechanism would be to remove all the HI deficient (C<5) disks
from the sample, as these have undergone more than one HI removal mechanism.Comment: 10 pages, 3 figures, accepted by MNRAS, appendixes not include
Quantified Morphology of HI Disks in the Universe
he upcoming new perspective of the high redshift Universe in the 21 cm line
of atomic hydrogen opens possibilities to explore topics of spiral disk
evolution, hitherto reserved for the optical regime. The growth of spiral gas
disks over Cosmic time can be explored with the new generation of radio
telescopes, notably the SKA, and its precursors, as accurately as with the
Hubble Space Telescope for stellar disks. Since the atomic hydrogen gas is the
building block of these disks, it should trace their formation accurately.
Morphology of HI disks can now equally be quantified over Cosmic time. In
studies of HST deep fields, the optical or UV morphology of high-redshift
galaxy disks have been characterized using a few quantities: concentration (C),
asymmetry (A), smoothness (S), second-order-moment (M20), the GINI coefficient
(G), and Ellipticity (E). We have applied these parameters across wavelengths
and compared them to the HI morphology over the THINGS sample. NGC 3184, an
unperturbed disk, and NGC 5194, the canonical 3:1 interaction, serve as
examples for quantified morphology. We find that morphology parameters
determined in HI are as good or better a tracer of interaction compared to
those in any other wavelength, notably in Asymmetry, Gini and M20. This opens
the possibility of using them in the parameterization pipeline for SKA
precursor catalogues to select interacting or harassed galaxies from their HI
morphology. Asymmetry, Gini and M20 may be redefined for use on data-cubes
rather than HI column density image.Comment: 6 pages, 3 figures, proceeding of the conference "Panoramic Radio
Astronomy: Wide-field 1-2 GHz research on galaxy evolution", June 02 - 05
2009, Groningen, update after small edit
Quantified HI Morphology VII: star-formation and tidal influence on local dwarf HI morphology
Scale-invariant morphology parameters applied to atomic hydrogen maps (HI) of
galaxies can be used to quantify the effects of tidal interaction or
star-formation on the ISM. Here we apply these parameters, Concentration,
Asymmetry, Smoothness, Gini, M20, and the GM parameter, to two public surveys
of nearby dwarf galaxies, the VLA-ANGST and LITTLE-THINGS survey, to explore
whether tidal interaction or the ongoing or past star-formation is a dominant
force shaping the HI disk of these dwarfs.
Previously, HI morphological criteria were identified for ongoing
spiral-spiral interactions. When we apply these to the Irregular dwarf
population, they either select almost all or none of the population. We find
that only the Asymmetry-based criteria can be used to identify very isolated
dwarfs (i.e., these have a low tidal indication). Otherwise, there is little or
no relation between the level of tidal interaction and the HI morphology. We
compare the HI morphology to three star-formation rates based on either Halpha,
FUV or the resolved stellar population, probing different star-formation
time-scales.
The HI morphology parameters that trace the inequality of the distribution,
the Gini, GM, and M20 parameters, correlate weakly with all these
star-formation rates. This is in line with the picture that local physics
dominates the ISM appearance and not tidal effects. Finally, we compare the
SDSS measures of star-formation and stellar mass to the HI morphological
parameters for all four HI surveys. In the two lower-resolution HI surveys
(12"), there is no relation between star-formation measures and HI morphology.
The morphology of the two high-resolution HI surveys (6"), the Asymmetry,
Smoothness, Gini, M20, and GM, do show a link to the total star-formation, but
a weak one.Comment: 26 figures, 4 tables, two appendices. Third appendix (HI maps of all
galaxies) omitted. Accepted by MNRA
Quantified HI Morphology III: Merger Visibility Times from HI in Galaxy Simulations
Major mergers of disk galaxies are thought to be a substantial driver in
galaxy evolution. To trace the fraction and the rate galaxies are in mergers
over cosmic times, several observational techniques, including morphological
selection criteria, have been developed over the last decade. We apply this
morphological selection of mergers to 21 cm radio emission line (HI) column
density images of spiral galaxies in nearby surveys. In this paper, we
investigate how long a 1:1 merger is visible in HI from N-body simulations. We
evaluate the merger visibility times for selection criteria based on four
parameters: Concentration, Asymmetry, M20, and the Gini parameter of second
order moment of the flux distribution (GM). Of three selection criteria used in
the literature, one based on Concentration and M20 works well for the HI
perspective with a merger time scale of 0.4 Gyr. Of the three selection
criteria defined in our previous paper, the GM performs well and cleanly
selects mergers for 0.69 Gyr. The other two criteria (A-M20 and C-M20), select
isolated disks as well, but perform best for face-on, gas-rich disks (T(merger)
~ 1 Gyr). The different visibility scales can be combined with the selected
fractions of galaxies in any large HI survey to obtain merger rates in the
nearby Universe. All-sky surveys such as WALLABY with ASKAP and the Medium Deep
Survey with the APETIF instrument on Westerbork are set to revolutionize our
perspective on neutral hydrogen and will provide an accurate measure of the
merger fraction and rate of the present epoch.Comment: 12 pages, 6 figures, 4 tables, accepted by MNRAS, appendix not
include
Quantified H i Morphology VII: The Morphology of Extended Disks in UV and H i
Extended UltraViolet (xuv) disks have been found in a substantial fraction of
late-type --S0, spiral and irregular-- galaxies. Similarly, most late-type
spirals have an extended gas disk, observable in the 21cm radio line (HI). The
morphology of galaxies can be quantified well using a series of scale-invariant
parameters; Concentration- Asymmetry-Smoothness (CAS), Gini, M20, and GM
parameters. In this paper, we compare the quantified morphology and effective
radius (R50) of the Westerbork observations of neutral Hydrogen in Irregular
and Spiral galaxies Project (WHISP) HI maps to those of far-and
near-ultraviolet images obtained with galex, to explore how close the
morphology and scales of HI and UV in these disks correlate. We find that xuv
disks do not stand out by their effective radii in UV or HI. However, the
concentration index in FUV appears to select some xuv disks. And known xuv
disks can be identified via a criterion using Asymmetry and M20; 80% of xuv
disks are included but with 55% contamination. This translates into 61
candidate xuv disk out of our 266 galaxies, (23%) consistent with previous
findings. We consider three scenarios; tidal features from major mergers, the
typical extended Hi disk is a photo- dissociation product of the xuv regions
and both Hi and UV features originate in cold flows fueling the main galaxy. We
define extended HI and UV disks based on their concentration (CHI > 5 and CFUV
> 4 respectively), but note that these two subsamples never overlap in the
WHISP sample. This appears to discount a simple photo-dissociation origin of
the outer HI disk. Previously, we identified the morphology space occupied by
ongoing major mergers. Known xuv disks rarely reside in the merger dominated
part of HI morphology space but those that do are Type 1. This suggests cold
flows as the origin for the xuv complexes and their surrounding HI structures.Comment: 18 pages, 13 figures, 2 table
Quantified HI Morphology II : Lopsidedness and Interaction in WHISP Column Density Maps
Lopsidedness of the gaseous disk of spiral galaxies is a common phenomenon in
disk morphology, profile and kinematics. Simultaneously, the asymmetry of a
galaxy's stellar disk, in combination with other morphological parameters, has
seen extensive use as an indication of recent merger or interaction in galaxy
samples. Quantified morphology of stellar spiral disks is one avenue to
determine the merger rate over much of the age of the Universe. In this paper,
we measure the quantitative morphology parameters for the HI column density
maps from the Westerbork observations of neutral Hydrogen in Irregular and
SPiral galaxies (WHISP). These are Concentration, Asymmetry, Smoothness, Gini,
M20, and one addition of our own, the Gini parameter of the second order moment
(GM). Our aim is to determine if lopsided or interacting disks can be
identified with these parameters. Our sample of 141 HI maps have all previous
classifications on their lopsidedness and interaction. We find that the
Asymmetry, M20, and our new GM parameter correlate only weakly with the
previous morphological lopsidedness quantification. These three parameters may
be used to compute a probability that an HI disk is morphologically lopsided
but not unequivocally to determine it. However, we do find that that the
question whether or not an HI disk is interacting can be settled well using
morphological parameters. Parameter cuts from the literature do not translate
from ultraviolet to HI directly but new selection criteria using combinations
of Asymmetry and M20 or Concentration and M20, work very well. We suggest that
future all-sky HI surveys may use these parameters of the column density maps
to determine the merger fraction and hence rate in the local Universe with a
high degree of accuracy.Comment: 12 pages, 5 figures, 1 table, accepted by MNRAS, appendix not
include
Quantified HI Morphology I: Multi-Wavelengths Analysis of the THINGS Galaxies
Galaxy evolution is driven to a large extent by interactions and mergers with
other galaxies and the gas in galaxies is extremely sensitive to the
interactions. One method to measure such interactions uses the quantified
morphology of galaxy images. Well-established parameters are Concentration,
Asymmetry, Smoothness, Gini, and M20 of a galaxy image. Thus far, the
application of this technique has mostly been restricted to restframe
ultra-violet and optical images. However, with the new radio observatories
being commissioned (MeerKAT, ASKAP, EVLA, WSRT/APERTIF, and ultimately SKA), a
new window on the neutral atomic hydrogen gas (HI) morphology of a large
numbers of galaxies will open up. The quantified morphology of gas disks of
spirals can be an alternative indicator of the level and frequency of
interaction. The HI in galaxies is typically spatially more extended and more
sensitive to low-mass or weak interactions. In this paper, we explore six
morphological parameters calculated over the extent of the stellar (optical)
disk and the extent of the gas disk for a range of wavelengths spanning UV,
Optical, Near- and Far-Infrared and 21 cm (HI) of 28 galaxies from The HI
Nearby Galaxy Survey (THINGS). Though the THINGS sample is small and contains
only a single ongoing interaction, it spans both non-interacting and
post-interacting galaxies with a wealth of multi-wavelength data. We find that
the choice of area for the computation of the morphological parameters is less
of an issue than the wavelength at which they are measured. The signal of
interaction is as good in the HI as in any of the other wavelengths in which
morphology has been used to trace the interaction rate to date, mostly
star-formation dominated ones (near- and far-ultraviolet). The Asymmetry and
M20 parameters are the ones which show the most promise as tracers of
interaction in 21 cm line observations.Comment: 16 pages, 11 figure, table 1, accepted by MNRAS, appendix not
include
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