2,194 research outputs found
Large-Scale Asymmetry of Rotation Curves in Lopsided Spiral Galaxies
Many spiral galaxies show a large-scale asymmetry with a cos\phi dependence
in their rotation curves as well as in their morphology, such as M101 and NGC
628. We show that both these features can be explained by the response of a
galactic disk to an imposed lopsided halo potential. A perturbation potential
of 5 % is deduced for the morphologically lopsided galaxies in the Rix &
Zaritsky (1995) sample. This is shown to result in a difference of 10 % or
about 20-30 kms^{-1} in the rotation velocity on the two sides of the major
axis. Interestingly, the observed isophotal asymmetry in a typical spiral
galaxy is not much smaller and it results in a velocity asymmetry of 7 % or
about 14-21 kms^{-1} . Hence, we predict that most galaxies show a fairly
significant rotational asymmetry. The rotation velocity is shown to be maximum
along the elongated isophote - in agreement with the observations along the SW
in M101, while it is minimum along the opposite direction. This result leads to
the distinctive asymmetric shape of the rotation curve which rises more steeply
in one half of the galaxy than the other, as observed by Swaters et al. (1999).
This shape is shown to be a robust feature and would result for any centrally
concentrated disk. The net disk lopsidedness and hence the asymmetry in the
rotation curve is predicted to increase with radius and hence can be best
studied using HI gas as the tracer.Comment: 30 pages, accepted for publication in A &
Q criterion for disc stability modified by external tidal field
The standard Q criterion (with Q > 1) describes the local stability of a disc
supported by rotation and random motion. Most astrophysical discs, however, are
under the influence of an external gravitational field which can affect their
stability. A typical example is a galactic disc embedded in a dark matter halo.
Here we do a linear perturbation analysis for a disc in an external field, and
obtain a generalized dispersion relation and a modified stability criterion. An
external field has two effects on the disc dynamics: first, it contributes to
the unperturbed rotational field, and second, it adds a tidal field term in the
stability parameter. A typical disruptive tidal field results in a higher
modified Q value and hence leads to a more stable disc. We apply these results
to the Milky Way, and to a low surface brightness galaxy UGC 7321. We find that
in each case the stellar disc by itself is barely stable and it is the dark
matter halo that stabilizes the disc against local, axisymmetric gravitational
instabilities. This result has been largely missed so far because in practice
the value for Q for a galactic disc is obtained in a hybrid fashion using the
observed rotational field that is set by both the disc and the halo, and hence
is higher than for a pure disc.Comment: 7 pages, 3 figures, submitted to MNRA
Angular momentum transport and evolution of lopsided galaxies
The surface brightness distribution in the majority of stellar galactic discs
falls off exponentially. Often what lies beyond such a stellar disc is the
neutral hydrogen gas whose distribution also follows a nearly exponential
profile at least for a number of nearby disc galaxies. Both the stars and gas
are commonly known to host lopsided asymmetry especially in the outer parts of
a galaxy. The role of such asymmetry in the dynamical evolution of a galaxy has
not been explored so far.
Following Lindblad's original idea of kinematic density waves, we show that
the outer part of an exponential disc is ideally suitable for hosting lopsided
asymmetry. Further, we compute the transport of angular momentum in the
combined stars and gas disc embedded in a dark matter halo. We show that in a
pure star and gas disc, there is a transition point where the free precession
frequency of a lopsided mode, , changes from retrograde to
prograde and this in turn reverses the direction of angular momentum flow in
the disc leading to an unphysical behaviour. We show that this problem is
overcome in the presence of a dark matter halo, which sets the angular momentum
flow outwards as required for disc evolution, provided the lopsidedness is
leading in nature. This, plus the well-known angular momentum transport in the
inner parts due to spiral arms, can facilitate an inflow of gas from outside
perhaps through the cosmic filaments.Comment: 13 pages, 11 figures, accepted for publication in MNRA
Suppression of gravitational instabilities by dominant dark matter halo in low surface brightness galaxies
The low surface brightness galaxies are gas-rich and yet have a low star
formation rate, this is a well-known puzzle. The spiral features in these
galaxies are weak and difficult to trace, although this aspect has not been
studied much. These galaxies are known to be dominated by the dark matter halo
from the innermost regions. Here we do a stability analysis for the galactic
disc of UGC 7321, a low surface brightness, superthin galaxy, for which the
various observational input parameters are available. We show that the disc is
stable against local, linear axisymmetric and non-axisymmetric perturbations.
The Toomre Q parameter values are found to be large (>> 1) mainly due to the
low disc surface density and the high rotation velocity resulting due to the
dominant dark matter halo, which could explain the observed low star formation
rate. For the stars-alone case, the disc shows finite swing amplification but
the addition of dark matter halo suppresses that amplification almost
completely. Even the inclusion of the low-dispersion gas which constitutes a
high disc mass fraction does not help in causing swing amplification. This can
explain why these galaxies do not show strong spiral features. Thus the
dynamical effect of a halo that is dominant from inner regions can naturally
explain why star formation and spiral features are largely suppressed in low
surface brightness galaxies, making these different from the high surface
brightness galaxies.Comment: 7 pages, 2 figures, 1 table, accepted for publication in MNRA
Role of gas in supporting grand spiral structure
The density wave theory for the grand-design two-armed spiral pattern in
galaxies is successful in explaining several observed features. However, the
long-term persistence of this spiral structure is a serious problem since the
group transport would destroy it within about a billion years as shown in a
classic paper by Toomre. In this paper we include the low velocity dispersion
component, namely gas, on an equal footing with stars in the formulation of the
density wave theory, and obtain the dispersion relation for this coupled
system. We show that the inclusion of gas makes the group transport slower by a
factor of few, thus allowing the pattern to persist longer - for several
billion years. Though still less than the Hubble time, this helps in making the
spiral structure more long-lived. Further we show that addition of gas is
essential to get a stable wave for the observed pattern speed for the Galaxy,
which otherwise is not possible for a one-component stellar disc.Comment: 6 pages, 3 figures, 1 table, accepted for publication in MNRA
Measurement of non-axisymmetry in centres of advanced mergers of galaxies
We measure the non-axisymmetry in the luminosity distribution in the inner
few kpc of the remnants of advanced mergers of galaxies with a view to
understand the relaxation in the central regions. For this, we analyze the
images from the 2MASS archival data for a selected sample of 12 merging
galaxies, which show signs of interaction but have a single nucleus. The
central regions are fitted by elliptical isophotes whose centres are allowed to
vary to get the best fit. The centres of isophotes show a striking sloshing
pattern with a spatial variation of up to 20-30 % within the central 1 kpc.
This indicates mass asymmetry and a dynamically unrelaxed behaviour. Next, we
Fourier-analyze the galaxy images while keeping the centre constant and measure
the deviation from axisymmetry in terms of the fractional Fourier amplitudes
(A_1, A_2 etc) as a function of radius. All mergers show a high value of
lopsidedness (upto A_1 ~ 0.2) in the central 5 kpc. The m=2 asymmetry is even
stronger, with values of A_2 upto ~ 0.3, and in three cases these are shown to
represent bars. The corresponding values denoting non-axisymmetry in inner
regions of a control sample of eight non-merger galaxies are found to be
several times smaller. Surprisingly, this central asymmetry is seen even in
mergers where the outer regions have relaxed into a smooth elliptical-like
r^{1/4} profile or a spiral-like exponential profile. Thus the central
asymmetry is long-lived, estimated to be ~ 1 Gyr, and hence lasts for over 100
local dynamical timescales. These central asymmetries are expected to play a
key role in the future dynamical evolution of the central region of a merger,
and can help in feeding a central AGN.Comment: 14 pages, 3 figures, accepted for publication in MNRA
Collisional removal of HI from the inner disks of Virgo cluster galaxies
There is sufficient observational evidence to show that many Virgo Cluster spirals are HI deficient in their inner disks (in addition to being HI deficient globally, as previously established). It is shown here that collisions between galaxies in a cluster can lead to the removal of HI gas from these galaxies while leaving the H2 gas, undisturbed. This follows directly from the application of the Spitzer-Baade collisional gas removal mechanism to galaxies consisting of stars and a two-component interstellar medium (ISM) consisting of HI and H2, with HI having the largest filling factor. This can account for both the observed HI deficiency in the inner regions and the normal H2 content of these galaxies. The frequency of galaxy collisions in the Virgo Cluster is shown to be large enough to make collisional gas removal a viable mechanism
Why are some galaxy disks extremely thin?
Some low surface brightness galaxies are known to have extremely thin stellar
disks with the vertical to planar axes ratio 0.1 or less, often referred to as
superthin disks. Although their existence is known for over three decades, the
physical origin for the thin distribution is not understood. We model the
stellar thickness for a two-component (gravitationally coupled stars and gas)
disk embedded in a dark matter halo, for a superthin galaxy UGC 7321 which has
a dense, compact halo, and compare with a typical dwarf galaxy HoII which has a
non-compact halo. We show that while the presence of gas does constrain the
disk thickness, it is the compact dark matter halo which plays the decisive
role in determining the superthin disk distribution in low-mass disks. Thus the
compact dark matter halo significantly affects the disk structure and this
could be important for the early evolution of galaxies.Comment: Submitted to MNRA
Off-centred dark matter halo leading to strong central disc lopsidedness
There is increasing evidence now from simulations and observations that the
centre of dark matter halo in a Milky Way type galaxy could be off-centred by a
few 100 pc w.r.t. the galactic disc. We study the effect of such an offset halo
on the orbits and kinematics in the central few kpc of the disc. The equations
of motion in the disc plane can be written in terms of the disc and halo
potentials when these two are concentric and a perturbation term due to the
offset halo. This perturbation potential shows an m=1 azimuthal variation, or
is lopsided, and its magnitude increases at small radii. On solving these
equations, we find that the perturbed orbit shows a large deviation of ~ 40 %
in radius at R = 1.5 kpc, and also strong kinematical lopsidedness. Thus even a
small halo offset of 350 pc can induce surprisingly strong spatial and
kinematical lopsidedness in the central region within ~ 3 kpc radius. Further,
the disc would remain lopsided for several Gyr, as long as the halo offset
lasts. This would have important implications for the dynamical evolution of
this region.Comment: 6 pages, 6 figures, Submitted to MNRA
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