1,255 research outputs found

### Dark baryons and rotation curves

The best measured rotation curve for any galaxy is that of the dwarf
spiralXXXX DDO 154, which extends out to about 20 disk scale lengths. It
provides an ideal laboratory for testing the universal density profile
prediction from high resolution numerical simulations of hierarchical
clustering in cold dark matter dominated cosmological models. We find that the
observed rotation curve cannot be fit either at small radii, as previously
noted, or at large radii. We advocate a resolution of this dilemma by
postulating the existence of a dark spheroid of baryons amounting to several
times the mass of the observed disk component and comparable to that of the
cold dark matter halo component. Such an additional mass component provides an
excellent fit to the rotation curve provided that the outer halo is still cold
dark matter-dominated with a density profile and mass-radius scaling relation
as predicted by standard CDM-dominated models. The universal existence of such
dark baryonic spheroidal components provides a natural explanation of the
universal rotation curves observed in spiral galaxies, may have a similar
origin and composition to the local counterpart that has been detected as
MACHOs in our own galactic halo via gravitational microlensing, and is
consistent with, and even motivated by, primordial nucleosynthesis estimates of
the baryon fraction.Comment: 16 pages LaTeX, 2 postscript figures. To be published in The
Astrophysical Journal, Letter

### Comment on "Scalar-tensor gravity coupled to a global monopole and flat rotation curves" by Lee and Lee

The recent paper by Lee and Lee (2004) may strongly leave the impression that
astronomers have established that the rotation curves of spiral galaxies are
flat. We show that the old paradigm of Flat Rotation Curves lacks, today, any
observational support and following it at face value leads to intrinsically
flawed alternatives to the Standard Dark Matter Scenario. On the other side, we
claim that the rich systematics of spiral galaxy rotation curves, that reveals,
in the standard Newtonian Gravity framework, the phenomenon of dark matter, in
alternative scenarios, works as a unique benchmark.Comment: 3 pages, 2 figures, accepted in Phys. Rev.

### Dark Matter Scaling Relations

We establish the presence of a dark matter core radius, for the first time in
a very large number of spiral galaxies of all luminosities. Contrary to common
opinion we find that the sizes of these cores and the " DM core problem" are
bigger for more massive spirals. As a result the Burkert profile provides an
excellent mass model for dark halos around disk galaxies. Moreover, we find
that the spiral dark matter core densities $\rho_{0}$ and core radii $r_{0}$
lie in the same scaling relation $\rho_{0}=4.5\times 10^-2 (r_{0}/kpc)^{-2/3}
M_{\odot}pc^{-3}$ of dwarf galaxies with core radii upto ten times more
smaller.Comment: 4 pages, 4 figures, Accepted for Publication in Apj Let

### The Baryonic Mass Function of Spiral Galaxies: Clues to Galaxy Formation

We compute the baryonic mass function (BMF) of disc galaxies using the best
LFs and baryonic M/L ratios reliable for this goal. For baryonic masses (M_b)
ranging between 10^8 and 10^{11} solar masses, the BMF is featureless, i.e. it
scales as M_b^{-1/2}. Outside this mass range, the BMF is a strong inverse
function of M_b. The contributions to the baryon density Omega_b from objects
of different mass highlight a characteristic mass scale of spirals at about
2x10^{11} solar masses, around which >50% of the total baryonic mass is
concentrated. The integral value, Omega_b= 1.4x10^{-3}, confirms, to a higher
accuracy, previous evidence (Persic & Salucci 1992) that the fraction of BBN
baryons locked in disc galaxies is negligible and matches that of high-z Damped
Lyman Alpha systems (DLAs). We investigate the scenario where DLAs are the
progenitors of present-day spirals, and find a simple relationship between
their masses and HI column densities by which the DLA mass function closely
matches the spiral BMF.Comment: MNRAS, in press. Replaces previous, unrefereed version. 10 pages
MNRAS style LaTeX, 7 figure

### Cosmic-Ray Proton to Electron Ratios

A basic quantity in the characterization of relativistic particles is the
proton-to-electron (p/e) energy density ratio. We derive a simple approximate
expression suitable to estimate this quantity, U_p/U_e = (m_p/m_e)^(3-q)/2,
valid when a nonthermal `gas' of these particles is electrically neutral and
the particles' power-law spectral indices are equal -- e.g., at injection. This
relation partners the well-known p/e number density ratio at 1 GeV, i.e.
N_p/N_e = (m_p/m_e)^{(q-1)/2}.Comment: 4 pages; to be published in Proc. of MGM13 (13th Marcel Grossmann
Meeting -- Stockholm July 1-7, 2012

### A STIS Survey for OVI Absorption Systems at 0.12 < z < 0.5 I.: The Statistical Properties of Ionized Gas

We have conducted a systematic survey for intervening OVI absorbers in
available echelle spectra of 16 QSOs at z_QSO = 0.17-0.57. These spectra were
obtained using HST/STIS with the E140M grating. Our search uncovered a total of
27 foreground OVI absorbers with rest-frame absorption equivalent width
W_r(1031) > 25mA. Ten of these QSOs exhibit strong OVI absorbers in their
vicinity. Our OVI survey does not require the known presence of Lya, and the
echelle resolution allows us to identify the OVI absorption doublet based on
their common line centroid and known flux ratio. We estimate the total redshift
survey path, \Delta z, using a series of Monte-Carlo simulations, and find that
\Delta z=1.66, 2.18, and 2.42 for absorbers of strength W_r = 30, 50 and 80mA,
respectively, leading to a number density of dN(W > 50mA)/dz = 6.7 +/- 1.7 and
dN(W > 30mA)/dz = 10.4 +/- 2.2. In contrast, we also measure dN/dz = 27 +/- 9
for OVI absorbers of W_r > 50mA at |\Delta v|< 5000 kms from the background
QSOs. Using the random sample of OVI absorbers with well characterized survey
completeness, we estimate a mean cosmological mass density of the OVI gas
\Omega(OVI)h = 1.7 +/- 0.3 x 10^-7. In addition, we show that <5% of OVI
absorbers originate in underdense regions that do not show a significant trace
of HI. Furthermore, we show that the neutral gas column N(HI) associated with
these OVI absorbers spans nearly five orders of magnitude, and show moderate
correlation with N(OVI). Finally, while the number density of OVI absorbers
varies substantially from one sightline to another, it also appears to be
inversely correlated with the number density of HI absorbers along individual
lines of sight.Comment: 12 pages. ApJ accepte

### The density profile of equilibrium and non-equilibrium dark matter halos

We study the diversity of the density profiles of dark matter halos based on
a large set of high-resolution cosmological simulations of 256^3 particles. The
cosmological models include four scale-free models and three representative
cold dark matter models. The simulations have good force resolution, and there
are about 400 massive halos with more than 10^4 particles within the virial
radius in each cosmological model. Our unbiased selection of all massive halos
enables to quantify how well the bulk of dark matter halos can be described by
the Navarro, Frenk & White (NFW) profile which was established for equilibrium
halos. We find that about seventy percent of the halos can be fitted by the NFW
profile with a fitting residual dvi_{max} less than 30% in Omega_0=1 universes.
This percentage is higher in lower density cosmological models. The rest of the
halos exhibits larger deviations from the NFW profile for more significant
internal substructures. There is a considerable amount of variation in the
density profile even for the halos which can be fitted by the NFW profile (i.e.
dvi_{max}<0.30). The distribution of the profile parameter, the concentration
$c$, can be well described by a lognormal function with the mean value \bar c
slightly smaller (15%) than the NFW result and the dispersion \sigma_c in \ln c
about 0.25. The more virialized halos with dvi_{max}<0.15 have the mean value
\bar c in good agreement with the NFW result and a slightly smaller dispersion
\sigma_c (about 0.2). Our results can alleviate some of the conflicts found
recently between the theoretical NFW profile and observational results.
Implications for theoretical and observational studies of galaxy formation are
discussed.Comment: The final version accepted for publication in ApJ; one figure and one
paragraph added to demonstrate that all the conclusions of the first version
are solid to the resoltuion effects; 19 pages with 6 figure

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