145 research outputs found
Estels, galà xies, cosmos: la dècada passada, la dècada vinent
La nostra visió de l'univers ha canviat molt pel descobriment de nous objectes i nous fenòmen
Stellar Motions in the Polar Ring Galaxy NGC 4650A
We present the first measurement of the stellar kinematics in the polar ring
of NGC 4650A. There is well defined rotation, with the stars and gas rotating
in the same direction, and with similar amplitude. The gaseous and stellar
kinematics suggest an approximately flat rotation curve, providing further
support for the hypothesis that the polar material resides in a disk rather
than in a ring. The kinematics of the emission line gas at and near the center
of the S0 suggests that the polar disk lacks a central hole. We have not
detected evidence for two, equal mass, counterrotating stellar polar streams,
as is predicted in the resonance levitation model proposed by Tremaine & Yu. A
merger seems the most likely explanation for the structure and kinematics of
NGC 4650A.Comment: 4 pages, accepted for publication in ApJ Letter
The Stellar and Gas Kinematics of Several Irregular Galaxies
We present long-slit spectra of three irregular galaxies from which we
determinethe stellar kinematics in two of the galaxies (NGC 1156 and NGC 4449)
and ionized-gas kinematics in all three (including NGC 2366). We compare this
to the optical morphology and to the HI kinematics of the galaxies. In the
ionized gas, we see a linear velocity gradient in all three galaxies. In NGC
1156 we also detect a weak linear velocity gradient in the stars of (5+/-1/sin
i) km/s/kpc to a radius of 1.6 kpc. The stars and gas are rotating about the
same axis, but this is different from the major axis of the stellar bar which
dominates the optical light of the galaxy. In NGC 4449 we do not detect
organized rotation of the stars and place an upper limit of (3/sin i) km/s/kpc
to a radius of 1.2 kpc. For NGC 4449, which has signs of a past interaction
with another galaxy, we develop a model to fit the observed kinematics of the
stars and gas. In this model the stellar component is in a rotating disk seen
nearly face-on while the gas is in a tilted disk with orbits whose planes
precess in the gravitational potential. This model reproduces the apparent
counter-rotation of the inner gas of the galaxy. The peculiar orbits of the gas
are presumed due to acquisition of gas in the past interaction.Comment: To be published in ApJ, November 20, 200
The stellar velocity dispersion in the inner 1.3 disk scale-lengths of the irregular galaxy NGC 4449
We present measurements of the stellar velocity dispersion in the inner 1
arcmin radius (1.3 disk scale-lengths) of the irregular galaxy NGC 4449
determined from long-slit absorption-line spectra. The average observed
dispersion is 29 +/-2 km/s, the same as predicted from NGC 4449's luminosity.
No significant rotation in the stars is detected. If we assume a maximum
rotation speed of the stars from the model determined from the gas kinematics
of Hunter et al. (2002), the ratio V_max/sigma_z measured globally is 3. This
ratio is comparable to values measured in spiral galaxies, and implies that the
stellar disk in NGC 4449 is kinematically relatively cold. The intrinsic
minor-to-major axis ratio (b/a)_0 is predicted to be in the range 0.3-0.6,
similar to values derived from the distribution of observed b/a of Im galaxies.
However, V/sigma_z measured locally is 0.5-1.1, and so the circular velocity of
NGC 4449 is comparable or less than the velocity of the stars within the
central 1.3 disk scale-lengths of the galaxy.Comment: To be published in ApJ, Nov 200
Mass Density Profiles of LSB Galaxies
We derive the mass density profiles of dark matter halos that are implied by
high spatial resolution rotation curves of low surface brightness galaxies. We
find that at small radii, the mass density distribution is dominated by a
nearly constant density core with a core radius of a few kpc. For rho(r) ~ r^a,
the distribution of inner slopes a is strongly peaked around a = -0.2. This is
significantly shallower than the cuspy a < -1 halos found in CDM simulations.
While the observed distribution of alpha does have a tail towards such extreme
values, the derived value of alpha is found to depend on the spatial resolution
of the rotation curves: a ~ -1 is found only for the least well resolved
galaxies. Even for these galaxies, our data are also consistent with constant
density cores (a = 0) of modest (~ 1 kpc) core radius, which can give the
illusion of steep cusps when insufficiently resolved. Consequently, there is no
clear evidence for a cuspy halo in any of the low surface brightness galaxies
observed.Comment: To be published in ApJ Letters. 6 pages. Uses aastex and
emulateapj5.sty Typo in Eq 1 fixe
Breaking new ground: Estimates of crack depth along the axial zone of the East Pacific Rise (9°12′–54′N)
High-resolution rotation curves of low surface brightness galaxies: Data
We present long slit Halpha observations of 50 low surface brightness
galaxies. Of these, 36 are of sufficient quality to form rotation curves. These
data provide a large increase in the number of low surface brightness galaxies
for which accurate rotation curves are available. They also represent an order
of magnitude improvement in spatial resolution over previous 21 cm studies (1"
to 2" instead of 13" to 45"). The improved resolution and accuracy of the data
extend and strengthen the scientific conclusions previously inferred from 21 cm
data.Comment: Accepted for publication in the Astronomical Journal. Electronic
versions of the data are available at http://www.astro.umd.edu/~ssm/data &
http://www.atnf.csiro.au/~edeblok/dat
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