arXiv subjects

Chanda J. Jog

Publications and source records attributed to Chanda J. Jog.

50 records · Page 3Linked to original sources

arXiv2006

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.

astro-ph↗

arXiv2006

Self-consistent response of a galactic disc to vertical perturbations

We study the self-consistent, linear response of a galactic disc to vertical perturbations as induced say by a tidal interaction. We calculate the self-gravitational potential corresponding to a non-axisymmetric, self-consistent density response of the disc using the Green's function method. The response potential is shown to oppose the perturbation potential because the self-gravity of the disc resists the imposed potential, and this resistence is stronger in the inner parts of a galactic disc. For the m=1 azimuthal wavenumber, the disc response opposes the imposed perturbation upto a radius that spans a range of 4-6 disc scale-lengths, so that the disc shows a net warp only beyond this region. This physically explains the well-known but so far unexplained observation (Briggs 1990) that warps typically set in beyond this range of radii. We show that the inclusion of a dark matter halo in the calculation only marginally changes (by ~ 10 %) the radius for the onset of warps. For perturbations with higher azimuthal wavenumbers, the net signature of the vertical perturbations can only be seen at larger radii - for example beyond 7 exponential disc scale-lengths for m=10. Also, for high m cases, the magnitude of the negative disc response due to the disc self-gravity is much smaller. This is shown to result in corrugations of the mid-plane density, which explains the puzzling scalloping with m=10 detected in HI in the outermost regions ~ 30 kpc in the Galaxy by Kulkarni et al. (1982).

astro-ph↗

arXiv2005

On the generation of asymmetric warps in disk galaxies

The warps in many spiral galaxies are now known to asymmetric. Recent sensitive observations have revealed that asymmetry of warps may be the norm rather than exception. However there exists no generic mechanism to generate these asymmetries in warps. We have derived the dispersion relation in a compact form for the S-shaped warps(described by the m=1 mode) and the bowl-shaped distribution(described by the m=0 mode) in galactic disk embedded in a dark matter halo. We then performed the numerical modal analysis and used the linear and time-dependent superposition principle to generate asymmetric warps in the disk. On doing the modal analysis we find the frequency of the $m=0$ mode is much larger than that of the $m=1$ mode. The linear and time-dependent superposition of these modes with their unmodulated amplitudes(that is, the coefficients of superposition being unity) results in an asymmetry in warps of ~ 20 - 40 %, whereas a smaller coefficient for the m=0 mode results in a smaller asymmetry. The resulting values agree well with the recent observations. We study the dependence of the asymmetry index on the dark matter halo parameters. This approach can also naturally produce U-shaped warps and L-shaped warps. We show that a rich variety of possible asymmetries in the z-distribution of the spiral galaxies can naturally arise due to a dynamical wave interference between the first two bending modes(i.e. m=0 and m=1) in the disk. This is a simple but general method for generating asymmetric warps that is independent of how the individual modes arise in the disk.

astro-ph↗

arXiv2005

Vertical distribution of stars and gas in a galactic disk

We study the vertical density distribution of stars and gas (HI and H_2) in a galactic disk which is embedded in a dark matter halo. The new feature of this work is the inclusion of gas, and the gravitational coupling between stars and gas, which has led to a more realistic treatment of a multi-component galactic disk. The gas gravity is shown to be crucially important despite the low gas mass fraction. This approach physically explains the observed scaleheight distribution of all the three disk components, including the long-standing puzzle (Oort 1962) of a constant HI scaleheight observed in the inner Galaxy. The above model is applied to two external galaxies: NGC 891 and NGC 4565, and the stellar disk is shown to be not strictly flat as was long believed but rather it shows a moderate flaring of a factor of about 2 within the optical radius.

astro-ph↗

arXiv2004

Unequal-mass galaxy merger remnants: spiral-like morphology but elliptical-like kinematics

It is generally believed that major galaxy mergers with mass ratios in the range 1:1-3:1 result in remnants that have properties similar to elliptical galaxies, and minor mergers below 10:1 result in disturbed spiral galaxies. The intermediate range of mass ratios 4:1-10:1 has not been studied so far. Using N-body simulations, we show that such mergers can result in very peculiar systems, that have the morphology of a disk galaxy with an exponential profile, but whose kinematics is closer to that of elliptical systems. These objects are similar to those recently observed by Jog & Chitre (2002). We present two cases with mass ratios 4.5:1 and 7:1, and show that the merging causes major heating and results in the appearance of elliptical-type kinematics, while surprisingly the initial spiral-like mass profile is conserved.

astro-ph↗

arXiv2003

The puzzle about the radial cut-off in galactic disks

The stellar disk in a spiral galaxy is believed to be truncated physically because the disk surface brightness is observed to fall faster than that for an exponential in the outer, faint regions. We review the literature associated with this phenomenon and find that a number of recent observations contradict the truncation picture. Hence we question the very existence of a physical outer cut-off in stellar disks. We show, in this paper, that the observed drop in the surface brightness profiles in fact corresponds to a negligible decrease in intensity, and that this minor change at the faint end appears to be exaggerated on a log-normal plot. Since minor deviations from a perfect exponential are common throughout the disk, we suggest that such a deviation at the faint end could easily give rise to the observed sharp drop.

astro-ph↗

arXiv2002

Vertical scaleheights in a gravitationally coupled, three-component Galactic disk

The vertical scaleheight of the atomic hydrogen gas shows a remarkably flat distribution with the galactocentric radius in the inner Galaxy. This has been a long-standing puzzle (Oort 1962) because the gas scaleheight should increase with radius when treated as responding to the gravitational potential of the exponential stellar disk. We argue that the gravitational force of the molecular and atomic hydrogen gas should also be brought into the picture to explain this. We treat the stars, the HI and H_2 gas as three gravitationally coupled components in the Galactic disk, and find the response of each component to the joint potential and thus obtain their vertical distribution in a self-consistent fashion. The effect of the joint potential is different for the three components because of their different velocity dispersions. We show that this approach cohesively and naturally explains the observed scaleheight distribution of all the three components, namely, the HI and H_2 gas and the stars, in the region studied (2-12 kpc). This includes the constant scaleheight for the HI seen in the inner Galaxy. The effect of H_2 dominates in the molecular ring region of 4-8.5 kpc, while that due to HI is dominant in the outer Galaxy.

astro-ph↗

arXiv2002

Spiral-like Light Profiles but Elliptical-like Kinematics in Mergers of Galaxies

It is commonly accepted that a merger of two spiral galaxies results in a remnant with an elliptical-like surface-brightness profile. Surprisingly, our recent study (Chitre & Jog 2002) of the 2MASS data for twenty-seven advanced mergers of galaxies has shown that half of these have a light distribution that decreases exponentially with radius. Such a distribution normally characterizes a rotationally supported disk in a spiral galaxy. Here we show from kinematic data for two of these mergers, Arp 224 and Arp 214, that the main support against gravitational collapse comes from pressure due to random motion of stars as seen in an elliptical galaxy rather than from rotation. The origin of the unusual combination of properties seen here is a puzzle. The standard theoretical N-body models in the literature cannot account for these systems. Further observational and dynamical studies of this new class of merger remnants are needed, and would be important for understanding merger dynamics and galaxy evolution.

astro-ph↗

arXiv2002

Origin of Radially Increasing Stellar Scaleheight in a Galactic Disk

For the past twenty years, it has been accepted that the vertical scaleheight of the stellar disk in spiral galaxies is constant with radius. However, there is no clear physical explanation for this in the literature. Here we calculate the vertical stellar scaleheight for a self-gravitating stellar disk including the additional gravitational force of the HI and H_2 gas and the dark matter halo. We apply our model to two edge-on galaxies, NGC 891 and NGC 4565, and find that the resulting scaleheight shows a linear increase of nearly a factor of two within the optical disk for both these galaxies. Interestingly, we show that the observed data when looked at closely, do not imply a constant scaleheight but actually support this moderate flaring in scaleheight.

astro-ph↗

arXiv2002

Large-Scale Asymmetry of Rotation Curves in Lopsided Spiral Galaxies

Many spiral galaxies show a large-scale asymmetry with a cosϕdependence 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.

astro-ph↗

arXiv2002

Luminosity profiles of advanced mergers of galaxies using 2MASS data

A sample of 27 disturbed galaxies that show signs of interaction but have a single nucleus were selected from the Arp and the Arp-Madore catalogues. For these, the K_s band images from the Two Micron All Sky Survey (2MASS) are analysed to obtain their radial luminosity profiles and other structural parameters. We find that in spite of their similar optical appearance, the sample galaxies vary in their dynamical properties, and fall into two distinct classes. The first class consists of galaxies which can be described by a single r^{1/4} law and the second class consists of galaxies that show an outer exponential disk. A few galaxies that have disturbed profiles cannot be fit into either of the above classes. However, all the galaxies are similar in all other parameters such as the far-infrared colours, the molecular hydrogen content and the central velocity dispersion. Thus, the dynamical parameters of these sets seem to be determined by the ratio of the initial masses of the colliding galaxies. We propose that the galaxies in the first class result from a merger of spiral galaxies of equal masses whereas the second class of galaxies results from a merger of unequal mass galaxies. The few objects that do not fall into either category show a disturbed luminosity profile and a wandering centre, which is indicative of these being unrelaxed mergers. Of the 27 galaxies in our sample, 9 show elliptical-like profiles and 13 show an outer exponential. Interestingly, Arp 224, the second oldest merger remnant of the Toomre sequence shows an exponential disk in the outer parts.

astro-ph↗

arXiv2001

Vertical Distribution of Galactic Disc Stars and Gas Constrained by a Molecular Cloud Complex

We investigate the dynamical effects of a molecular cloud complex with a mass of about 10**7 M_sun and a size of a few 100 pc on the vertical distribution of stars and atomic hydrogen gas in a spiral galactic disc. Such massive complexes have now been observed in a number of spiral galaxies. The extended mass distribution in a complex, with an average mass density 6 times higher than the Oort limit, is shown to dominate the local gravitational field. This results in a significant redistribution of the surrounding disc components towards the mid-plane, with a resulting decrease in their vertical scaleheights. A surprising result is the large radial distance of about 500 pc from the complex centre over which the complex influences the disc. The complex has a comparable effect on the vertical distribution of HI in the galactic disc. This `pinching' or constraining effect should be detectable in the nearby spiral galaxies. Thus the gravitational field of a complex results in local corrugations of the stellar and HI vertical scaleheights, and the galactic disc potential is highly non-uniform on scales of the inter-complex separation of about 1 kpc.

astro-ph↗

arXiv2000

Self-Consistent Response of a Galactic Disk to an Elliptical Perturbation Halo Potential

We calculate the self-consistent response of an axisymmetric galactic disk perturbed by an elliptical halo potential of harmonic number m = 2, and obtain the net disk ellipticity. Such a potential is commonly expected to arise due to a galactic tidal encounter and also during the galaxy formation process. The self-gravitational potential corresponding to the self-consistent, non-axisymmetric density response of the disk is obtained by inversion of Poisson equation for a thin disk. This response potential is shown to oppose the perturbation potential, because physically the disk self-gravity resists the imposed potential. This results in a reduction in the net ellipticity of the perturbation halo potential in the disk plane. The reduction factor denoting this decrease is independent of the strength of the perturbation potential, and has a typical minimum value of 0.75 - 0.9 for a wide range of galaxy parameters. The reduction is negligible at all radii for higher harmonics (m > or = 3) of the halo potential. (abridged).

astro-ph↗

arXiv1999

Tidally Compressed Gas in Centers of Early Type and Ultraluminous Galaxies

In this paper we propose that the compressive tidal field in the centers of flat-core early type galaxies and ultraluminous galaxies compresses molecular clouds producing dense gas obseved in the centers of these galaxies. The effect of galactic tidal fields is usually considered disruptive in the literature. However, for some galaxies, the mass profile flattens towards the center and the resulting galactic tidal field is not disruptive but instead it is compressive within the flat-core region. We have used the virial theorem to determine the minimum density of a molecular cloud to be stable and gravitationally bound within the tidally compressive region of a galaxy. We have applied the mechanism to determine the mean molecular cloud densities in the centers of a sample of flat-core, early-type galaxies and ultraluminous galaxies.

astro-ph↗