1,231 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 and core radii
lie in the same scaling relation 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 LCO/Palomar 10,000 km/sec Cluster Survey. I. Properties of the Tully-Fisher Relation
The first results from a Tully-Fisher (TF) survey of cluster galaxies are
presented. The galaxies are drawn from fifteen Abell clusters that lie in the
redshift range 9000-12,000 km/sec and are distributed uniformly around the
celestial sky. The data set consists of R-band CCD photometry and long- slit
H-alpha spectroscopy. The rotation curves (RCs) are characterized by a turnover
radius (r_t) and an asymptotic velocity v_a, while the surface brightness
profiles are characterized in terms of an effective exponential surface
brightness I_e and a scale length r_e. The TF scatter is minimized when the
rotation velocity is measured at 2.0 +/- 0.2 r_e; a significantly larger
scatter results when the rotation velocity is measured at > 3 or < 1.5 scale
lengths. This effect demonstrates that RCs do not have a universal form, as has
been suggested by Persic, Salucci, and Stel. In contrast to previous studies, a
modest but statistically significant surface-brightness dependence of the TF
relation is found, log v = const + 0.28*log L + 0.14*log I_e. This indicates a
stronger parallel between the TF relation and the FP relations of elliptical
galaxies than has previously been recognized. Future papers in this series will
consider the implications of this cluster sample for deviations from Hubble
flow on 100-200 Mpc scales.Comment: 35 pages, 8 figures, uses aaspp4.sty. Submitted to ApJ. Also
available at http://astro.stanford.edu/jeff
- âŠ