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Stellar Disc (stellar + disc)
Selected AbstractsStructure and dynamics of galaxies with a low surface-brightness disc , I. The stellar and ionized-gas kinematicsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2008A. Pizzella ABSTRACT Photometry and long-slit spectroscopy are presented for a sample of six galaxies with a low surface-brightness stellar disc and a bulge. The characterizing parameters of the bulge and disc components were derived by means of a two-dimensional photometric decomposition of the images of the sample galaxies. Their surface-brightness distribution was assumed to be the sum of the contribution of a Sérsic bulge and an exponential disc, with each component being described by elliptical and concentric isophotes of constant ellipticity and position angle. The stellar and ionized-gas kinematics were measured along the major and minor axes in half of the sample galaxies, whereas the other half was observed only along two diagonal axes. Spectra along two diagonal axes were obtained also for one of the objects with major and minor axis spectra. The kinematic measurements extend in the disc region out to a surface-brightness level ,R, 24 mag arcsec,2, reaching in all cases the flat part of the rotation curve. The stellar kinematics turns out to be more regular and symmetric than the ionized-gas kinematics, which often shows the presence of non-circular, off-plane and non-ordered motions. This raises the question about the reliability of the use of the ionized gas as the tracer of the circular velocity in the modelling of the mass distribution, in particular in the central regions of low surface-brightness galaxies. [source] Global lopsided instability in a purely stellar galactic discMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2007Kanak Saha ABSTRACT It is shown that pure exponential discs in spiral galaxies are capable of supporting slowly varying discrete global lopsided modes, which can explain the observed features of lopsidedness in the stellar discs. Using linearized fluid dynamical equations with the softened self-gravity and pressure of the perturbation as the collective effect, we derive self-consistently a quadratic eigenvalue equation for the lopsided perturbation in the galactic disc. On solving this, we find that the ground-state mode shows the observed characteristics of the lopsidedness in a galactic disc, namely the fractional Fourier amplitude A1, increases smoothly with the radius. These lopsided patterns precess in the disc with a very slow pattern speed with no preferred sense of precession. We show that the lopsided modes in the stellar disc are long-lived because of a substantial reduction (approximately a factor of 10 compared to the local free precession rate) in the differential precession. The numerical solution of the equations shows that the ground-state lopsided modes are either very slowly precessing stationary normal mode oscillations of the disc or growing modes with a slow growth rate depending on the relative importance of the collective effect of the self-gravity. N -body simulations are performed to test the spontaneous growth of lopsidedness in a pure stellar disc. Both approaches are then compared and interpreted in terms of long-lived global m= 1 instabilities, with almost zero pattern speed. [source] Cold dark matter microhalo survival in the Milky WayMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007G. W. Angus ABSTRACT A special purpose N -body simulation has been built to understand the tidal heating of the smallest dark matter substructures (10,6 M, and 0.01 pc) from the grainy potential of the Milky Way due to individual stars in the disc and the bulge. To test the method, we first run simulations of single encounters of microhaloes with an isolated star, and compare with analytical predictions of the dark particle bound fraction as a function of impact parameter. We then follow the orbits of a set of microhaloes in a realistic flattened Milky Way potential. We concentrate on (detectable) microhaloes passing near the Sun with a range of pericentre and apocentre. Stellar perturbers near the orbital path of a microhalo would exert stochastic impulses, which we apply in a Monte Carlo fashion according to the Besançon model for the distribution of stars of different masses and ages in our Galaxy. Also incorporated are the usual pericentre tidal heating and disc shocking. We give a detailed diagnosis of typical microhaloes and find microhaloes with internal tangential anisotropy are slightly more robust than the ones with radial anisotropy. In addition, the dark particles generally go through of a random walk in velocity space and diffuse out of the microhaloes. We show that the typical destruction time-scales are strongly correlated with the stellar density averaged along a microhalo's orbit over the age of the stellar disc. We also present the morphology of a microhalo at several epochs which may hold the key to dark matter detections. We checked our results against different choices of microhalo mass, virial radius and anisotropy. [source] Simulations of the heating of the Galactic stellar discMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2002Jyrki Hänninen ABSTRACT The velocity dispersion of nearby stars in the Galactic disc is well known to increase substantially with age; this is the so-called age,velocity relation, and is interpreted as a ,heating' of the disc as a function of time. We have studied the heating of the Galactic stellar disc caused by giant molecular clouds and halo black holes, via simulations of the orbits of tracer stars embedded in a patch of the local Galactic disc. We examine a range of masses and number densities of the giant molecular cloud and halo black hole perturbers. The heating of the stellar disc in the simulations is fitted with a simple power law of the form ,,t,, where , is the velocity dispersion of the tracer stars as a function of time, t. We also fit this form to the best determinations of the increase in the velocity dispersion as a function of time as derived from stars in the solar neighbourhood for which ages can be reliably assigned. Observationally, , is found to lie in the range 0.3,0.6, i.e. it remains poorly constrained and its determination is probably still dominated by systematic errors. Better constrained observationally is the ratio of the velocity dispersions of the stars in the vertical z and horizontal x directions (i.e. towards the Galactic Centre), ,z/,x= 0.5 ± 0.1. For the heating of the stellar disc caused by giant molecular clouds (GMCs) we derive a heating ,,t0.21, which differs somewhat from early (analytic) studies in which ,,t1/4. This confirms the well-known results that there are insufficient GMCs to heat the Galactic disc appropriately. A range of dark halo black hole scenarios are verified to heat the stellar disc as ,,t1/2 (as expected from analytical studies), and give ,z/,x in the range 0.5,0.6, which is consistent with observations. Black holes with a mass of 107 M, are our favoured disc heaters, although they are only marginally consistent with observations. Simulations featuring a combination of giant molecular clouds and halo black holes can explain the observed heating of the stellar disc, but since other perturbing mechanisms, such as spiral arms, are yet to be included, we regard this solution as being ad hoc. [source] The baryonic and dark matter properties of high-redshift gravitationally lensed disc galaxiesMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2007P. Salucci ABSTRACT We present a detailed study of the structural properties of four gravitationally lensed disc galaxies at z= 1. Modelling the rotation curves on sub-kpc scales, we derive the values for the disc mass, the reference dark matter density and core radius, and the angular momentum per unit mass. The derived models suggest that the rotation curve profile and amplitude are best fitted with a dark matter component similar to those of local spiral galaxies. The stellar component also has a similar length-scale, but with substantially smaller masses than similarly luminous disc galaxies in the local Universe. Comparing the average dark matter density inside the optical radius, we find that the disc galaxies at z= 1 have larger densities (by up to a factor of ,7) than similar disc galaxies in the local Universe. Furthermore, the angular momentum per unit mass versus reference velocity is well matched to the local relation, suggesting that the angular momentum of the disc remains constant between high redshifts and the present day. Though statistically limited, these observations point towards a spirals' formation scenario in which stellar discs are slowly grown by the accretion of angular momentum conserving material. [source] Global lopsided instability in a purely stellar galactic discMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2007Kanak Saha ABSTRACT It is shown that pure exponential discs in spiral galaxies are capable of supporting slowly varying discrete global lopsided modes, which can explain the observed features of lopsidedness in the stellar discs. Using linearized fluid dynamical equations with the softened self-gravity and pressure of the perturbation as the collective effect, we derive self-consistently a quadratic eigenvalue equation for the lopsided perturbation in the galactic disc. On solving this, we find that the ground-state mode shows the observed characteristics of the lopsidedness in a galactic disc, namely the fractional Fourier amplitude A1, increases smoothly with the radius. These lopsided patterns precess in the disc with a very slow pattern speed with no preferred sense of precession. We show that the lopsided modes in the stellar disc are long-lived because of a substantial reduction (approximately a factor of 10 compared to the local free precession rate) in the differential precession. The numerical solution of the equations shows that the ground-state lopsided modes are either very slowly precessing stationary normal mode oscillations of the disc or growing modes with a slow growth rate depending on the relative importance of the collective effect of the self-gravity. N -body simulations are performed to test the spontaneous growth of lopsidedness in a pure stellar disc. Both approaches are then compared and interpreted in terms of long-lived global m= 1 instabilities, with almost zero pattern speed. [source] Weighing the young stellar discs around Sgr A*MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006Sergei Nayakshin ABSTRACT It is believed that young massive stars orbiting Sgr A* in two stellar discs on scales of , 0.1,0.2 parsec were formed either farther out in the Galaxy and then quickly migrated inwards or in situ in a massive self-gravitating disc. Comparing N -body evolution of stellar orbits with observational constraints, we set upper limits on the masses of the two stellar systems. These masses turn out to be a few times lower than the expected total stellar mass estimated from the observed young high-mass stellar population and the standard galactic initial mass function (IMF). If these stars were formed in situ, in a massive self-gravitating disc, our results suggest that the formation of low-mass stars was suppressed by a factor of at least a few, requiring a top-heavy IMF for stars formed near Sgr A*. [source] Thick gas discs in faint dwarf galaxiesMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2010Sambit Roychowdhury ABSTRACT We determine the intrinsic axial ratio distribution of the gas discs of extremely faint MB < ,14.5 dwarf irregular galaxies. We start with the measured (beam corrected) distribution of apparent axial ratios in the H i 21-cm images of dwarf irregular galaxies observed as part of the Faint Irregular Galaxy GMRT Survey (FIGGS). Assuming that the discs can be approximated as oblate spheroids, the intrinsic axial ratio distribution can be obtained from the observed apparent axial ratio distribution. We use a variety of methods to do this, and our final results are based on using Lucy's deconvolution algorithm. This method is constrained to produce physically plausible distributions, and also has the added advantage of allowing for observational errors to be accounted for. While one might a priori expect that gas discs would be thin (because collisions between gas clouds would cause them to quickly settle down to a thin disc), we find that the H i discs of faint dwarf irregulars are quite thick, with mean axial ratio ,q,, 0.6. While this is substantially larger than the typical value of ,0.2 for the stellar discs of large spiral galaxies, it is consistent with the much larger ratio of velocity dispersion to rotational velocity (,/vc) in dwarf galaxy H i discs as compared to that in spiral galaxies. Our findings have implications for studies of the mass distribution and the Tully,Fisher relation for faint dwarf irregular galaxies, where it is often assumed that the gas is in a thin disc. [source] Numerical modelling of the vertical structure and dark halo parameters in disc galaxiesASTRONOMISCHE NACHRICHTEN, Issue 7 2010A. Khoperskov Abstract The non-linear dynamics of bending instability and vertical structure of a galactic stellar disc embedded into a spherical halo are studied with N-body numerical modelling. Development of the bending instability in stellar galactic disc is considered as the main factor that increases the disc thickness. Correlation between the disc vertical scale height and the halo-to-disc mass ratio is predicted from the simulations. The method of assessment of the spherical-to-disc mass ratio for edge-on spiral galaxies with a small bulge is considered. Modelling of eight edge-on galaxies: NGC 891, NGC 4738, NGC 5170, UGC 6080, UGC 7321, UGC 8286, UGC 9422 and UGC 9556 is performed. Parameters of stellar discs, dark haloes and bulges are estimated. The lower limit of the dark-to-luminous mass ratio in our galaxies is of the order of one within the limits of their stellar discs. The dark haloes dominate by mass in the galaxies with very thin stellar discs (NGC 5170, UGC 7321 and UGC 8286) (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||