26 research outputs found
Constraints on Radial Migration in Spiral Galaxies - II. Angular momentum distribution and preferential migration
The orbital angular momentum of individual stars in galactic discs can be
permanently changed through torques from transient spiral patterns.
Interactions at the corotation resonance dominate these changes and have the
further property of conserving orbital circularity. We derived in an earlier
paper an analytic criterion that an unperturbed stellar orbit must satisfy in
order for such an interaction to occur i.e. for it to be in a trapped orbit
around corotation. We here use this criterion in an investigation of how the
efficiency of induced radial migration for a population of disc stars varies
with the angular momentum distribution of that population. We frame our results
in terms of the velocity dispersion of the population, this being an easier
observable than is the angular momentum distribution. Specifically, we
investigate how the fraction of stars in trapped orbits at corotation varies
with the velocity dispersion of the population, for a system with an assumed
flat rotation curve. Our analytic results agree with the finding from
simulations that radial migration is less effective in populations with
'hotter' kinematics. We further quantify the dependence of this trapped
fraction on the strength of the spiral pattern, finding a higher trapped
fraction for higher amplitude perturbations.Comment: 28 pages, 15 figure, accepted for publication in MNRA
Constraints on radial migration in spiral galaxies â II. Angular momentum distribution and preferential migration
The orbital angular momentum of individual stars in galactic discs can be permanently changed through torques from transient spiral patterns. Interactions at the corotation resonance dominate these changes and have the further property of conserving orbital circularity. We derived in an earlier paper an analytic criterion that an unperturbed stellar orbit must satisfy in order for such an interaction to occur, i.e. for it to be in a trapped orbit around corotation. We here use this criterion in an investigation of how the efficiency of induced radial migration for a population of disc stars varies with the angular momentum distribution of that population. We frame our results in terms of the velocity dispersion of the population, this being an easier observable than is the angular momentum distribution. Specifically, we investigate how the fraction of stars in trapped orbits at corotation varies with the velocity dispersion of the population, for a system with an assumed flat rotation curve. Our analytic results agree with the finding from simulations that radial migration is less effective in populations with âhotterâ kinematics. We further quantify the dependence of this trapped fraction on the strength of the spiral pattern, finding a higher trapped fraction for higher amplitude perturbations
Constraints on Radial Migration in Spiral Galaxies - II. Angular momentum distribution and preferential migration
The orbital angular momentum of individual stars in galactic discs can be permanently changed through torques from transient spiral patterns. Interactions at the corotation resonance dominate these changes and have the further property of conserving orbital circularity. We derived in an earlier paper an analytic criterion that an unperturbed stellar orbit must satisfy in order for such an interaction to occur i.e. for it to be in a trapped orbit around corotation. We here use this criterion in an investigation of how the efficiency of induced radial migration for a population of disc stars varies with the angular momentum distribution of that population. We frame our results in terms of the velocity dispersion of the population, this being an easier observable than is the angular momentum distribution. Specifically, we investigate how the fraction of stars in trapped orbits at corotation varies with the velocity dispersion of the population, for a system with an assumed flat rotation curve. Our analytic results agree with the finding from simulations that radial migration is less effective in populations with âhotterâ kinematics. We further quantify the dependence of this trapped fraction on the strength of the spiral pattern, finding a higher trapped fraction for higher amplitude perturbations
Constraints on radial migration in spiral galaxies â II. Angular momentum distribution and preferential migration
The orbital angular momentum of individual stars in galactic discs can be permanently changed through torques from transient spiral patterns. Interactions at the corotation resonance dominate these changes and have the further property of conserving orbital circularity. We derived in an earlier paper an analytic criterion that an unperturbed stellar orbit must satisfy in order for such an interaction to occur, i.e. for it to be in a trapped orbit around corotation. We here use this criterion in an investigation of how the efficiency of induced radial migration for a population of disc stars varies with the angular momentum distribution of that population. We frame our results in terms of the velocity dispersion of the population, this being an easier observable than is the angular momentum distribution. Specifically, we investigate how the fraction of stars in trapped orbits at corotation varies with the velocity dispersion of the population, for a system with an assumed flat rotation curve. Our analytic results agree with the finding from simulations that radial migration is less effective in populations with âhotterâ kinematics. We further quantify the dependence of this trapped fraction on the strength of the spiral pattern, finding a higher trapped fraction for higher amplitude perturbations
Bending waves excited by irregular gas inflow along warps
Gaia has revealed clear evidence of bending waves in the vertical kinematics of stars in the solar neighbourhood. We study bending waves in two simulations, one warped, with the warp due to misaligned gas inflow, and the other unwarped. We find slow, retrograde bending waves in both models, with the ones in the warped model having larger amplitudes. We also find fast, prograde bending waves. Prograde bending waves in the unwarped model are very weak, in agreement with the expectation that these waves should decay on short, approximately crossing, time-scales, due to strong winding. However, prograde bending waves are much stronger for the duration of the warped model, pointing to irregular gas inflow along the warp as a continuous source of excitation. We demonstrate that large-amplitude bending waves that propagate through the solar neighbourhood give rise to a correlation between the mean vertical velocity and the angular momentum, with a slope consistent with that found by Gaia. The bending waves affect populations of all ages, but the sharpest features are found in the young populations, hinting that short-wavelength waves are not supported by the older, kinematically hotter, populations. Our results demonstrate the importance of misaligned gas accretion as a recurrent source of vertical perturbations of disc galaxies, including in the Milky Way
An approximate analytic model of a star cluster with potential escapers
In the context of a star cluster moving on a circular galactic orbit, a âpotential escaperâ is a cluster star that has orbital energy greater than the escape energy, and yet is confined within the Jacobi radius of the stellar system. On the other hand analytic models of stellar clusters typically have a truncation energy equal to the cluster escape energy, and therefore explicitly exclude these energetically unbound stars. Starting from the landmark analysis performed by HĂ©non of periodic orbits of the circular Hill equations, we present a numerical exploration of the population of ânon-escapersâ, defined here as those stars which remain within two Jacobi radii for several galactic periods, with energy above the escape energy. We show that they can be characterised by the Jacobi integral and two further approximate integrals, which are based on perturbation theory and ideas drawn from Lidov-Kozai theory. Finally we use these results to construct an approximate analytic model that includes a phase space description of a population resembling that of potential escapers, in addition to the usual bound population
Gemini Deep Deep Survey VI: Massive Hdelta-strong galaxies at z=1
We show that there has been a dramatic decline in the abundance of massive
galaxies with strong Hdelta stellar absorption lines from z=1.2 to the present.
These ``Hdelta-strong'', or HDS, galaxies have undergone a recent and rapid
break in their star-formation activity. Combining data from the Gemini Deep
Deep and the Sloan Digital Sky Surveys to make mass-matched samples
(M*>=10^10.2 Msun), with 25 and 50,255 galaxies, respectively), we find that
the fraction of galaxies in an HDS phase has decreased from about 50% at z=1.2
to a few percent today. This decrease in fraction is due to an actual decrease
in the number density of massive HDS systems by a factor of 2-4, coupled with
an increase in the number density of massive galaxies by about 30 percent. We
show that this result depends only weakly on the threshold chosen for the
Hdelta equivalent width to define HDS systems (if greater than 4 A) and
corresponds to a (1+z)^{2.5\pm 0.7} evolution. Spectral synthesis studies of
the high-redshift population using the PEGASE code, treating Hdelta_A, EW[OII],
Dn4000, and rest-frame colors, favor models in which the Balmer absorption
features in massive Hdelta-strong systems are the echoes of intense episodes of
star-formation that faded about 1 Gyr prior to the epoch of observation. The
z=1.4-2 epoch appears to correspond to a time at which massive galaxies are in
transition from a mode of sustained star formation to a relatively quiescent
mode with weak and rare star-formation episodes. We argue that the most likely
local descendants of the distant massive HDS galaxies are passively evolving
massive galaxies in the field and small groups.Comment: 16 pages, 12 figures, 3 tables, uses emulateapj.sty; updated to match
the version accepted by ApJ. One figure added, conclusions unchange
The pattern speeds of vertical breathing waves
We measure and compare the pattern speeds of vertical breathing, vertical bending, and spiral density waves in two isolated N-body+SPH simulations, using windowed Fourier transforms over 1Gyr time intervals. We show that the pattern speeds of the breathing waves match those of the spirals but are different from those of the bending waves. We also observe matching pattern speeds between the bar and breathing waves. Our results not only strengthen the case that, throughout the disc, breathing motions are driven by spirals but indeed that the breathing motions are part and parcel of the spirals
Chandra Observations of the Pleiades Open Cluster: X-ray Emission from Late-B to Early-F Type Binaries
We present the analysis of a 38.4 ks and a 23.6 ks observation of the core of
the Pleiades open cluster. The Advanced CCD Imaging Spectrometer on board the
Chandra X-ray Observatory detected 99 X-ray sources in a 17'X17' region,
including 18 of 23 Pleiades members. Five candidate Pleiades members have also
been detected, confirming their cluster membership. Fifty-seven sources have no
optical or near-infrared counterparts to limiting magnitudes V=22.5 and J=14.5.
The unidentified X-ray sources are probably background AGN and not stars. The
Chandra field of view contains seven intermediate mass cluster members. Five of
these, HII 980 (B6 + G), HII 956 (A7 + F6), HII 1284 (A9 + K), HII 1338 (F3 +
F6), and HII 1122 (F4 + K), are detected in this study. All but HII 1284 have
high X-ray luminosity and soft X-ray spectra. HII 1284 has X-ray properties
comparable to non-flaring K-type stars. Since all five stars are visual or
spectroscopic binaries with X-ray properties similar to F-G stars, the
late-type binary companions are probably producing the observed coronal X-ray
emission. Strengthening this conclusion is the nondetection by Chandra of two A
stars, HII 1362 (A7, no known companion) and HII 1375 (A0 + A SB) with X-ray
luminosity upper limits 27-54 times smaller than HII 980 and HII 956, the B6-A7
stars with cooler companions. Despite the low number statistics, the Chandra
data appear to confirm the expectation that late-B and A stars are not strong
intrinsic X-ray sources. The ACIS spectra and hardness ratios suggest a gradual
increase in coronal temperature with decreasing mass from F4 to K. M stars
appear to have somewhat cooler coronae than active K stars.Comment: 22 pages, 4 figures, 7 tables, to appear in Ap