143 research outputs found
The Extinction Distribution in the Galaxy UGC 5041
We probe the dust extinction through the foreground disk of the overlapping
galaxy pair UGC 5041 by analyzing B,I, and H band images. The inclined
foreground disk of this infrared-selected pair is almost opaque in B at a
projected distance of ~8kpc. From the images, we estimate directly the
area-weighted distribution of differential near-IR extinction: it is nearly
Gaussian with =0.6 and sigma=0.27. For a homogenous dust
distribution and a Milky Way extinction curve, this corresponds to a face-on
distribution p(tau) with a mean of =0.34 and sigma_V=0.15. For a clumpy
dust model the optical depth estimate increases to =0.41 and
sigma_V=0.19. Even though the galaxy pair is subject to different selection
biases and our analysis is subject to different systematics, the result is
consistent with existing case studies, indicating that ~0.3 is generic
for late-type spirals near their half-light radii.
We outline how to estimate from p(tau) by how much background quasars are
underreresented, where projected within ~10kpc of nearby spirals, such as
damped Ly-alpha absorbers or gravitational lenses; from our data we derive a
factor of two deficit for flux-limited, optical surveys.Comment: 18 pages, 3 figures; To appear in the Astronomical Journa
Pitch Angle Restrictions in Late Type Spiral Galaxies Based on Chaotic and Ordered Orbital Behavior
We built models for low bulge mass spiral galaxies (late type as defined by
the Hubble classification) using a 3-D self-gravitating model for spiral arms,
and analyzed the orbital dynamics as a function of pitch angle, going from
10 to 60. Testing undirectly orbital self-consistency, we search
for the main periodic orbits and studied the density response. For pitch angles
up to approximately , the response supports closely the potential
permitting readily the presence of long lasting spiral structures. The density
response tends to "avoid" larger pitch angles in the potential, by keeping
smaller pitch angles in the corresponding response. Spiral arms with pitch
angles larger than , would not be long-lasting structures but
rather transient. On the other hand, from an extensive orbital study in phase
space, we also find that for late type galaxies with pitch angles larger than
, chaos becomes pervasive destroying the ordered phase space
surrounding the main stable periodic and quasi-periodic orbits and even
destroying them. This result is in good agreement with observations of late
type galaxies, where the maximum observed pitch angle is .Comment: ApJL accepted (12 pages, 3 figures
The origin of galaxy scaling laws in LCDM
It has long been recognized that tight relations link the mass, size, and
characteristic velocity of galaxies. These scaling laws reflect the way in
which baryons populate, cool, and settle at the center of their host dark
matter halos; the angular momentum they retain in the assembly process; as well
as the radial distribution and mass scalings of the dark matter halos. There
has been steady progress in our understanding of these processes in recent
years, mainly as sophisticated N-body and hydrodynamical simulation techniques
have enabled the numerical realization of galaxy models of ever increasing
complexity, realism, and appeal. These simulations have now clarified the
origin of these galaxy scaling laws in a universe dominated by cold dark
matter: these relations arise from the tight (but highly non-linear) relations
between (i) galaxy mass and halo mass, (ii) galaxy size and halo characteristic
radius; and (iii) from the self-similar mass nature of cold dark matter halo
mass profiles. The excellent agreement between simulated and observed galaxy
scaling laws is a resounding success for the LCDM cosmogony on the highly
non-linear scales of individual galaxies.Comment: Contribution to the Proceedings of the Simons Conference
"Illuminating Dark Matter", held in Kruen, Germany, in May 2018, eds. R.
Essig, K. Zurek, J. Fen
Dark Matter Halos of Disk Galaxies: Constraints from the Tully-Fisher Relation
We investigate structural properties of dark matter halos of disk galaxies in
LCDM cosmology, using a well-defined sample of 81 disk-dominated galaxies from
the SDSS redshift survey. We model the mass-velocity and fundamental plane
relations of these galaxies, which are constructed from the galaxy stellar
mass, disk scale length, and optical Halpha rotation velocity at 2.2 scale
lengths. We calculate a sequence of model galaxy populations, defined by the
distribution of the stellar disk-to-total mass fraction, m_d. We include the
effect of adiabatic contraction of dark matter halos in response to
condensation of baryons. We find that models with constant m_d underpredict the
intrinsic scatter of the TF and FP relations and predict an (unobserved) strong
correlation between TF residuals. Introducing a scatter of disk mass fractions
and allowing the mean value m_d to scale with the stellar surface density
significantly improves observational match of both the slope and intercept of
the TF relation and reduces the predicted residual correlation enough to be
consistent with the data. Our best-fit models with a Kroupa stellar IMF
over-produce the galaxy stellar mass function and predict the virial r-band
mass-to-light ratios, M_vir/L_r, systematically lower than those inferred from
galaxy-galaxy weak lensing and satellite dynamics. We investigate three
possible solutions to these problems: (1) ignoring the effects of adiabatic
contraction, (2) adopting a ``light'' stellar IMF, or (3) considering the lower
halo concentrations predicted for a low cosmological power spectrum
normalization. Any of these solutions yields acceptable residual correlations
and relieves most of the observational tension between the TF relation and the
galaxy stellar mass function (abridged).Comment: 20 pages, submitted to Ap
Fitting functions for a disk-galaxy model with different LCDM-halo profiles
We present an adaptation of the standard scenario of disk-galaxy formation to
the concordant LCDM cosmology aimed to derive analytical expressions for the
scale length and rotation speed of present-day disks that form within four
different, cosmologically motivated protogalactic dark matter halo-density
profiles. We invoke a standard galaxy-formation model that includes virial
equilibrium of spherical dark halos, specific angular momentum conservation
during gas cooling, and adiabatic halo response to the gas inflow. The mean
mass-fraction and mass-to-light ratio of the central stellar disk are treated
as free parameters whose values are tuned to match the zero points of the
observed size-luminosity and circular speed-luminosity relations of galaxies.
We supply analytical formulas for the characteristic size and rotation speed of
disks built inside Einasto r^{1/6}, Hernquist, Burkert, and Navarro-Frenk-White
dark matter halos. These expressions match simultaneously the observed zero
points and slopes of the different correlations that can be built in the RVL
space of disk galaxies from plausible values of the galaxy- and star-formation
efficiencies
The morphological dependent Tully-Fisher relation of spiral galaxies
The Tully-Fisher relation of spiral galaxies shows notable dependence on
morphological types, with earlier type spirals having systematically lower
luminosity at fixed maximum rotation velocity . This decrement of
luminosity is more significant in shorter wavelengths. By modeling the rotation
curve and stellar population of different morphological type spiral galaxies in
combination, we find the of spiral galaxies is weakly dependent on
the morphological type, whereas the difference of the stellar population
originating from the bulge disk composition effect mainly account for the
morphological type dependence of the Tully-Fisher relation.Comment: 8 pages, 3 figures, ApJ accepte
Evidence for the disky origin of luminous Virgo dwarf ellipticals from the kinematics of their globular cluster systems
We report evidence for dynamically significant rotation in the globular
cluster systems of two luminous Virgo dwarf ellipticals, VCC1261 and VCC1528.
Including previous results for VCC1087, the globular cluster systems of all
three Virgo dwarf ellipticals studied in detail to date exhibit v_rot/sigma >
1. Taking the rotation seen in the globular clusters as maximal disk rotation,
we find all three dEs lie on the r-band Tully-Fisher relation. We argue that
these data support the hypothesis that luminous dEs are the remnants of
transformed disk galaxies. We also obtained deep, longslit data for the stars
in VCC1261 and VCC1528. Both these galaxies show rapid rotation in their inner
regions, with spatial scales of ~0.5 kpc. These rotation velocities are similar
to those seen in the GC systems. Since our longslit data for Virgo dEs extend
out to 1-2 effective radii (typical of deep observations), whereas the globular
clusters extend out to 4--7 effective radii, we conclude that non-detections of
rotation in many luminous dEs may simply be due to a lack of radial coverage in
the stellar data, and that globular clusters represent singularly sensitive
probes of the dynamics of dEs. Based on these data, we suggest that gas disks
are significant sites of globular cluster formation in the early universe.Comment: To appear in the AJ, corrected typographical errors in Table 1, added
a referenc
Dark Matter and Stellar Mass in the Luminous Regions of Disk Galaxies
We investigate the correlations among stellar mass (M_*), disk scale length
(R_d), and rotation velocity at 2.2 disk scale lengths (V_2.2) for a sample of
81 disk-dominated galaxies (disk/total >= 0.9) selected from the SDSS. We
measure V_2.2 from long-slit H-alpha rotation curves and infer M_* from galaxy
i-band luminosities (L_i) and g-r colors. We find logarithmic slopes of
2.60+/-0.13 and 3.05+/-0.12 for the L_i-V_2.2 and M_*-V_2.2 relations, somewhat
shallower than most previous studies, with intrinsic scatter of 0.13 dex and
0.16 dex. Our direct estimates of the total-to-stellar mass ratio within
2.2R_d, assuming a Kroupa IMF, yield a median ratio of 2.4 for M_*>10^10 Msun
and 4.4 for M_*=10^9-10^10 Msun, with large scatter at a given M_* and R_d. The
typical ratio of the rotation speed predicted for the stellar disk alone to the
observed rotation speed at 2.2R_d is ~0.65. The distribution of R_d at fixed
M_* is broad, but we find no correlation between disk size and the residual
from the M_*-V_2.2 relation, implying that this relation is an approximately
edge-on view of the disk galaxy fundamental plane. Independent of the assumed
IMF, this result implies that stellar disks do not, on average, dominate the
mass within 2.2R_d. We discuss our results in the context of infall models of
disk formation in cold dark matter halos. A model with a disk-to-halo mass
ratio m_d=0.05 provides a reasonable match to the R_d-M_* distribution for spin
parameters \lambda ranging from ~0.04-0.08, and it yields a reasonable match to
the mean M_*-V_2.2 relation. A model with m_d=0.1 predicts overly strong
correlations between disk size and M_*-V_2.2 residual. Explaining the wide
range of halo-to-disk mass ratios within 2.2R_d requires significant scatter in
m_d values, with systematically lower m_d for galaxies with lower .Comment: 18 pages, 2 tables, 7 figures, Accepted to ApJ, Table 1 updated,
otherwise minor change
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