1,721,804 research outputs found
Dark matter halos around galaxies
No full textWe present evidence that all galaxies, of any Hubble type and luminosity, bear the kinematical signature of a mass component distributed differently from the luminous matter. We review and/or derive the DM halo properties of galaxies of different morphologies: spirals, LSBs, ellipticals, dwarf irregulars and dwarf spheroidals.
We show that the halo density profile M-h(x) = M-h(1)(1+a(2))x(3)/x(2)+a(2) (with x = R/R-opt), across both the Hubble and luminosity sequences: matches all the available data that include, for ellipticals: properties of the X-ray emitting gas and the kinematics of planetary nebulae, stars: and HI disks: for spirals, LSBs and dIrr's: stellar and HI rotation curves: and, finally. for dSph's the motions of individual stars.
The dark + luminous mass structure is obtained: (a) in spirals, LSBs. and dLrr's by modelling the extraordinary properties of the Universal Rotation Curve (URC), to which all these types conform (i.e. the URC luminosity dependence and the smallness of its rms scatter and cosmic variance); (b) in ellipticals and dSph's, by modelling the coadded mass profiles (or the M/L ratios) in terms of a luminous spheroid and the above-specified dark halo.
A main feature of galactic structure is that the dark and visible matter are well mixed already in the luminous region. The transition between the inner, star-dominated regions and the outer, halo-dominated region, moves progressively inwards with decreasing luminosity, to the extent that very-low-L stellar systems (disks or spheroids) are not self-gravitating, while in high-L systems the dark matter becomes a main mass component only beyond the optical edge. A halo core radius, comparable to the optical radius, is detected at all luminosities and for all morphologies. The luminous mass fraction varies with luminosity in a fashion common to all galaxy types: it is comparable with the cosmological baryon fraction at L > L, but it decreases by more than a factor 10(2) at L << L.
For each Hubble type, the central halo density increases with decreasing luminosity: sequences of denser stellar systems (dwarfs, ellipticals, HSBs. LSBs in decreasing order) correspond in turn to sequences of denser halos.
Then, the dark halo structure of galaxies fits into a well ordered pattern underlying a unified picture for the mass distribution of galaxies across the Hubble sequence
THE DISK CONTRIBUTION TO ROTATION CURVES OF SPIRAL GALAXIES
No full textWe formulate analytically the maximum disc hypothesis (MDH) in the framework of a
disc/halo mass decomposition, and apply it to a sample of suitably selected optical
rotation curves. We find that the resulting disc-to-total mass ratios show a definite
trend of increasing dark-to-luminous mass ratio with decreasing luminosity, in very
good agreement with our previous results obtained by means of different decomposition
techniques (Persic & Salucci). The same trend is also clearly discernible when the
mass ratios (at the same radius in disc length-scale units) obtained from published
MDH models are correlated with luminosity. We discuss possible reasons why previous
studies which have assumed a similar framework have missed this fundamental
systematics of dark matter
DARK MATTER IN SPIRAL GALAXIES
No full textThe Tully-Fisher relation is used to probe dark matter (DM) in the optical regions of spiral galaxies. By establishing it at several different isophotal radii in an appropriate sample of 58 galaxies with good B-band photometry and rotation curves, we find that some of its attributes such as scatter, residuals, nonlinearity, and bias dramatically decrease moving from the disk edge inward. We show how this behavior challenges any mass model which assumes no DM or a luminosity-independent DM mass fraction interior to the optical radius of spiral galaxies
ROTATING-DISK GALAXIES - YET ANOTHER CASE FOR DARK MATTER
No full textWe devise a method to probe the systematic variations of the dark matter mass fraction (DM) in the optical region of disk galaxies, based on studying the correlation between galaxy luminosity and the normalized radius where the rotation frequency exceeds a given multiple of the rotation frequency as measured at the optical disk radius. The results we find strongly suggest the ubiquitous existence of DM throughout the luminosity sequence of galaxies, thereby challenging the possibility that luminous matter alone can be responsible for the observed disk dynamics. By modeling the distribution of dark matter by means of a pseudoisothermal halo, we show that the disk-to-halo mass ratio at the optical edge, Adisk/Mhalo, has not the same value in all galaxies, but actually varies with luminosity. Remarkably, galaxies at the knee of the luminosity function have Mdisk/Mhalo ∼ 1
THE UNIVERSAL GALAXY ROTATION CURVE
No full textAnalyzing an extended set of selected galaxy rotation curves (with - 17.5 greater-than-or-equal-to M(B) greater-than-or-equal-to - 23.2), we find that for a given luminosity the rotation curves of spiral galaxies within the optical radius are a universal function, V(R) congruent-to 200(L(B)/L(B*))1/4 {1 + [0.12 - 0.24 log (L(B)/L(B*))](R/R(max) - 1)} km s-1 (where L(B*) = 6 x 10(10)h50(-2)L(B). and R(max) = 2.2 disk length scales are two suitable parameterization constants). This result implies strong systematic variations of both the amplitude and the profile of the circular velocity with luminosity, faint (bright) galaxies having low (high) velocities and steep (shallow) velocity gradients. Because luminous disks are self-similar, the observed progression of the shape of rotation curves with luminosity suggests that the dark-to-visible interplay varies with luminosity
NONLINEARITY OF THE TULLY-FISHER RELATION
No full textA systematic variation of the dark matter abundance in spiral galaxies, previously reported (Persic & Salucci), accounts for the observed non-linearity of the Tully-Fisher relation. Increasing proportions of dark mass at low luminosities, as revealed by optical rotation curves, make faint galaxies shift to higher rotation velocities for a given luminosity, thus inducing a curvature in the velocity-luminosity correlation
Proceedings of the Meeting on Cosmology held in Grado, Italy, 24-27 October 1994 - Guest editors' preface
No full textDark matter, not magnetism
No full textBattaner et al. have recently argued that interstellar magnetic fields can significantly affect the circular motion of gas in spiral galaxies and explain their observed flat rotation curves. This interpretation of galaxy kinematics challenges the conventional dark matter (DM) picture. However, the evidence for DM in galaxies is not restricted to gas motions. Measurements of velocity differences within galaxy pairs (not based on gas rotation and hence not affected by magnetic fields) imply the existence of massive haloes, whose properties are in total agreement with those derived from the DM interpretation of the observed rotation curves. On the other hand, the predictions of the magnetic-field, no-DM scenario cannot be reconciled with observations of pairwise galaxy dynamics. In this paper we discuss the above opposing views of galaxy kinematics. Finally, after giving substantial support to the conventional view that DM plays the key dynamical role on galactic scales, we turn the argument devised to test the Battaner et al. theory into a powerful tool to investigate the properties of DM haloes on scales of several hundred kpc
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