246 research outputs found
Origin of the high v_los feature in the Galactic bar
We analyse a controlled N-body + smoothed particle hydrodynamics simulation
of a growing disc galaxy within a non-growing, live dark halo. The disc is
continuously fed with gas and star particles on near-circular orbits and
develops a bar comparable in size to the one of the Milky Way (MW). We extract
line of sight velocity v_los distributions from the model and compare it to
data recently obtained from the APOGEE survey which show distinct high velocity
features around v_los ~ 200 km/s. With an APOGEE like selection function, but
without any scaling nor adjustment, we find v_los distributions very similar to
those in APOGEE. The stars that make up the high v_los features at positive
longitudes l are preferentially young bar stars (age <~ 2-3 Gyr) which move
away from us along the rear side of the bar. At negative l, we find the
corresponding low v_los feature from stars moving towards us. At l>10 degrees
the highest v_los stars are a mixture of bar and background disc stars which
complicates the interpretation of observations. The main peak in v_los is
dominated by fore- and background stars. At a given time, ~40-50 per cent of
high v_los stars occupy x_1 like orbits, but a significant fraction are on more
complex orbits. The observed feature is likely due to a population of
dynamically cool, young stars formed from gas just outside the bar and
subsequently captured by the growing bar. The high v_los features disappear at
high latitudes |b|>~2 degrees which explains the non-detection of such features
in other surveys.Comment: 21 pages, 17 figures, accepted for publication in MNRA
Quantifying Resonant Structure in NGC 6946 from Two-dimensional Kinematics
We study the two-dimensional kinematics of the H-alpha-emitting gas in the
nearby barred Scd galaxy, NGC 6946, in order to determine the pattern speed of
the primary m=2 perturbation mode. The pattern speed is a crucial parameter for
constraining the internal dynamics, estimating the impact velocities of the
gravitational perturbation at the resonance radii, and to set up an
evolutionary scenario for NGC 6946. Our data allows us to derive the best
fitting kinematic position angle and the geometry of the underlying gaseous
disk, which we use to derive the pattern speed using the Tremaine-Weinberg
method. We find a main pattern speed Omega_p=22 km/s/kpc, but our data clearly
reveal the presence of an additional pattern speed Omega_p=47 km/s/kpc in a
zone within 1.25 kpc of the nucleus. Using the epicyclic approximation, we
deduce the location of the resonance radii and confirm that inside the outer
Inner Lindblad Resonance radius of the main oval, a primary bar has formed
rotating at more than twice the outer pattern speed. We further confirm that a
nuclear bar has formed inside the Inner Lindblad Resonance radius of the
primary bar, coinciding with the inner Inner Lindblad Resonance radius of the
large-scale m=2 mode oval.Comment: Accepted for publication in ApJ Letter
The Black Hole Mass of NGC 4151. II. Stellar Dynamical Measurement from Near-Infrared Integral Field Spectroscopy
We present a revised measurement of the mass of the central black hole (Mbh)
in the Seyfert 1 galaxy NGC 4151. The new stellar dynamical mass measurement is
derived by applying an axisymmetric orbit-superposition code to near-infrared
integral field data obtained using adaptive optics with the Gemini NIFS
spectrograph. When our models attempt to fit both the NIFS kinematics and
additional low spatial resolution kinematics, our results depend sensitively on
how chi-squared is computed--probably a consequence of complex bar kinematics
that manifest immediately outside the nuclear region. The most robust results
are obtained when only the high spatial resolution kinematic constraints in the
nuclear region are included in the fit. Our best estimates for the BH mass and
H-band mass-to-light ratio are Mbh~(3.76+/-1.15)E7 Msun (1-sigma error) and
M/L(H-band)~0.34+/-0.03 Msun/Lsun (3-sigma error), respectively (the quoted
errors reflect the model uncertainties). Our BH mass measurement is consistent
with estimates from both reverberation mapping (3.57[+0.45/-0.37]E7 Msun) and
gas kinematics (3.0[+0.75/-2.2]E7 Msun; 1-sigma errors), and our best-fit
mass-to-light ratio is consistent with the photometric estimate of
M/L(H-band)=0.4+/-0.2 Msun/Lsun. The NIFS kinematics give a central bulge
velocity dispersion sigma_c=116+/-3 km/s, bringing this object slightly closer
to the M-sigma relation for quiescent galaxies. Although NGC 4151 is one of
only a few Seyfert 1 galaxies in which it is possible to obtain a direct
dynamical BH mass measurement--and thus, an independent calibration of the
reverberation mapping mass scale--the complex bar kinematics makes it less than
ideally suited for this purpose.Comment: 21 pages, 15 figures. Accepted for publication in Ap
The Milky Way's Stellar Disk
A suite of vast stellar surveys mapping the Milky Way, culminating in the
Gaia mission, is revolutionizing the empirical information about the
distribution and properties of stars in the Galactic stellar disk. We review
and lay out what analysis and modeling machinery needs to be in place to test
mechanisms of disk galaxy evolution and to stringently constrain the Galactic
gravitational potential, using such Galactic star-by-star measurements. We
stress the crucial role of stellar survey selection functions in any such
modeling; and we advocate the utility of viewing the Galactic stellar disk as
made up from `mono-abundance populations' (MAPs), both for dynamical modeling
and for constraining the Milky Way's evolutionary processes. We review recent
work on the spatial and kinematical distribution of MAPs, and lay out how
further study of MAPs in the Gaia era should lead to a decisively clearer
picture of the Milky Way's dark matter distribution and formation history.Comment: Astron. Astrophys. Rev., in pres
Structure of sunspot penumbral filaments: a remarkable uniformity of properties
The sunspot penumbra comprises numerous thin, radially elongated filaments
that are central for heat transport within the penumbra, but whose structure is
still not clear. To investigate the fine-scale structure of these filaments, we
perform a depth-dependent inversion of spectropolarimetric data of a sunspot
very close to solar disk center obtained by Hinode (SOT/SP). We have used a
recently developed spatially coupled 2D inversion scheme which allows us to
analyze the fine structure of individual penumbral filaments up to the
diffraction limit of the telescope. Filaments of different sizes in all parts
of penumbra display very similar magnetic field strengths, inclinations and
velocity patterns. The similarities allowed us to average all these filaments
and to extract the physical properties common to all of them. This average
filament shows upflows associated with an upward pointing field at its inner,
umbral end and along its axis, downflows along the lateral edge and strong
downflows in the outer end associated with a nearly vertical, strong and
downward pointing field. The upflowing plasma is significantly hotter than the
downflowing plasma. The hot, tear-shaped head of the averaged filament can be
associated with a penumbral grain. The central part of the filament shows
nearly horizontal fields with strengths of ~1kG. The field above the filament
converges, whereas a diverging trend is seen in the deepest layers near the
head of the filament. We put forward a unified observational picture of a
sunspot penumbral filament. It is consistent with such a filament being a
magneto-convective cell, in line with recent MHD simulations. The uniformity of
its properties over the penumbra sets constraints on penumbral models and
simulations. The complex and inhomogeneous structure of the filament provides a
natural explanation for a number of long-running controversies in the
literature.Comment: 19 pages; 12 figures; accepted for publication in A&
The formation of stellar nuclear discs in bar-induced gas inflows
The role of gas in the mass assembly at the nuclei of galaxies is still subject to some uncertainty. Stellar nuclear discs bridge the gap between the large-scale galaxy and the central massive objects that reside there. Using a high-resolution simulation of a galaxy forming out of gas cooling and settling into a disc, we study the formation and properties of nuclear discs. Gas, driven to the centre by a bar, settles into a rotating star-forming nuclear disc (ND). This ND is thinner, younger, kinematically cooler and more metal rich than the surrounding bar. The ND is elliptical and orthogonal to the bar. The complex kinematics in the region of the ND are a result of the superposition of older stars streaming along the bar and younger stars circulating within the ND. The signature of the ND is therefore subtle in the kinematics. Instead the ND stands out clearly in metallicity and age maps. We compare the model to the density and kinematics of real galaxies with NDs finding qualitative similarities. Our results suggest that gas dissipation is very important for forming nuclear structure
- …