120 research outputs found
Orbital Decay of Supermassive Black Hole Binaries in Clumpy Multiphase Merger Remnants
We simulate an equal-mass merger of two Milky Way-size galaxy discs with
moderate gas fractions at parsec-scale resolution including a new model for
radiative cooling and heating in a multi-phase medium, as well as star
formation and feedback from supernovae. The two discs initially have a
supermassive black hole (SMBH) embedded in
their centers. As the merger completes and the two galactic cores merge, the
SMBHs form a a pair with a separation of a few hundred pc that gradually
decays. Due to the stochastic nature of the system immediately following the
merger, the orbital plane of the binary is significantly perturbed.
Furthermore, owing to the strong starburst the gas from the central region is
completely evacuated, requiring ~Myr for a nuclear disc to rebuild.
Most importantly, the clumpy nature of the interstellar medium has a major
impact on the the dynamical evolution of the SMBH pair, which undergo
gravitational encounters with massive gas clouds and stochastic torquing by
both clouds and spiral modes in the disk. These effects combine to greatly
delay the decay of the two SMBHs to separations of a few parsecs by nearly two
orders of magnitude, yr, compared to previous work. In mergers of
more gas-rich, clumpier galaxies at high redshift stochastic torques will be
even more pronounced and potentially lead to stronger modulation of the orbital
decay. This suggests that SMBH pairs at separations of several tens of parsecs
should be relatively common at any redshift.Comment: submitted to MNRAS; Comments very welcom
YOUNG STARS IN AN OLD BULGE: A NATURAL OUTCOME OF INTERNAL EVOLUTION IN THE MILKY WAY
The center of our disk galaxy, the Milky Way, is dominated by a boxy/peanut-shaped bulge. Numerous studies of the bulge based on stellar photometry have concluded that the bulge stars are exclusively old. The perceived lack of young stars in the bulge strongly constrains its likely formation scenarios, providing evidence that the bulge is a unique population that formed early and separately from the disk. However, recent studies of individual bulge stars using the microlensing technique have reported that they span a range of ages, emphasizing that the bulge may not be a monolithic structure. In this Letter we demonstrate that the presence of young stars that are located predominantly nearer to the plane is expected for a bulge that has formed from the disk via dynamical instabilities. Using an N-body+ smoothed particle hydrodynamics simulation of a disk galaxy forming out of gas cooling inside a dark matter halo and forming stars, we find a qualitative agreement between our model and the observations of younger metal-rich stars in the bulge. We are also able to partially resolve the apparent contradiction in the literature between results that argue for a purely old bulge population and those that show a population comprised of a range in ages; the key is where to look
The impact of COVID-19 on the cancer care of adolescents and young adults and their well-being:Results from an online survey conducted in the early stages of the pandemic
Background: Because of the global spread of coronavirus disease 2019 (COVID-19), oncology departments across the world have rapidly adapted their cancer care protocols to balance the risk of delaying cancer treatments and the risk of COVID-19 exposure. COVID-19 and associated changes may have an impact on the psychosocial functioning of patients with cancer and survivors. This study was designed to determine the impact of the COVID-19 pandemic on young people living with and beyond cancer. Methods: In this cross-sectional study, 177 individuals, aged 18 to 39 years, were surveyed about the impact of COVID-19 on their cancer care and psychological well-being. Participants also reported their information needs with respect to COVID-19. Responses were summarized with a content analysis approach. Results: This was the first study to examine the psychological functioning of young patients and survivors during the first weeks of the COVID-19 pandemic. A third of the respondents reported increased levels of psychological distress, and as many as 60% reported feeling more anxious than they did before COVID-19. More than half also wanted more information tailored to them as young patients with cancer. Conclusions: The COVID-19 pandemic is rapidly evolving and changing the landscape of cancer care. Young people living with cancer are a unique population and might be more vulnerable during this time in comparison with their healthy peers. There is a need to screen for psychological distress and attend to young people whose cancer care has been delayed. As the lockdown begins to ease, the guidelines about cancer care should be updated according to this population's needs.</p
Outer-Disk Populations in NGC 7793: Evidence for Stellar Radial Migration
We analyzed the radial surface brightness profile of the spiral galaxy NGC
7793 using HST/ACS images from the GHOSTS survey and a new HST/WFC3 image
across the disk break. We used the photometry of resolved stars to select
distinct populations covering a wide range of stellar ages. We found breaks in
the radial profiles of all stellar populations at 280" (~5.1 kpc). Beyond this
disk break, the profiles become steeper for younger populations. This same
trend is seen in numerical simulations where the outer disk is formed almost
entirely by radial migration. We also found that the older stars of NGC 7793
extend significantly farther than the underlying HI disk. They are thus
unlikely to have formed entirely at their current radii, unless the gas disk
was substantially larger in the past. These observations thus provide evidence
for substantial stellar radial migration in late-type disks.Comment: 8 pages, 6 figure. Accepted for publication in Ap
The impact of COVID-19 on the cancer care of adolescents and young adults and their well-being:Results from an online survey conducted in the early stages of the pandemic
Background
Because of the global spread of coronavirus disease 2019 (COVID‐19), oncology departments across the world have rapidly adapted their cancer care protocols to balance the risk of delaying cancer treatments and the risk of COVID‐19 exposure. COVID‐19 and associated changes may have an impact on the psychosocial functioning of patients with cancer and survivors. This study was designed to determine the impact of the COVID‐19 pandemic on young people living with and beyond cancer.
Methods
In this cross‐sectional study, 177 individuals, aged 18 to 39 years, were surveyed about the impact of COVID‐19 on their cancer care and psychological well‐being. Participants also reported their information needs with respect to COVID‐19. Responses were summarized with a content analysis approach.
Results
This was the first study to examine the psychological functioning of young patients and survivors during the first weeks of the COVID‐19 pandemic. A third of the respondents reported increased levels of psychological distress, and as many as 60% reported feeling more anxious than they did before COVID‐19. More than half also wanted more information tailored to them as young patients with cancer.
Conclusions
The COVID‐19 pandemic is rapidly evolving and changing the landscape of cancer care. Young people living with cancer are a unique population and might be more vulnerable during this time in comparison with their healthy peers. There is a need to screen for psychological distress and attend to young people whose cancer care has been delayed. As the lockdown begins to ease, the guidelines about cancer care should be updated according to this population's needs.</p
THE IMPRINT of RADIAL MIGRATION on the VERTICAL STRUCTURE of GALAXY DISKS
We use numerical simulations to examine the effects of radial migration on the vertical structure of galaxy disks. The simulations follow three exponential disks of different mass but similar circular velocity, radial scalelength, and (constant) scale height. The disks develop different non-axisymmetric patterns, ranging from feeble, long-lived multiple arms to strong, rapidly evolving few-armed spirals. These fluctuations induce radial migration through secular changes in the angular momentum of disk particles, mixing the disk radially and blurring pre-existing gradients. Migration primarily affects stars with small vertical excursions, regardless of spiral pattern. This "provenance bias" largely determines the vertical structure of migrating stars: inward migrators thin down as they move in, whereas outward migrators do not thicken up but rather preserve the disk scale height at their destination. Migrators of equal birth radius thus develop a strong scale-height gradient, not by flaring out as commonly assumed, but by thinning down as they spread inward. Similar gradients have been observed for low-[α/Fe] mono-abundance populations (MAPs) in the Galaxy, but our results argue against interpreting them as a consequence of radial migration. This is because outward migration does not lead to thickening, implying that the maximum scale height of any population should reflect its value at birth. In contrast, Galactic MAPs have scale heights that increase monotonically outward, reaching values that greatly exceed those at their presumed birth radii. Given the strong vertical bias affecting migration, a proper assessment of the importance of radial migration in the Galaxy should take carefully into account the strong radial dependence of the scale heights of the various stellar populations. © 2016. The American Astronomical Society. All rights reserved
The Evolution of Central Group Galaxies in Hydrodynamical Simulations
We trace the evolution of central galaxies in three ~10^13 M_sun galaxy
groups simulated at high resolution in cosmological hydrodynamical simulations.
The evolution in the group potential leads, at z=0, to central galaxies that
are massive, gas-poor early-type systems supported by stellar velocity
dispersion resembling either elliptical or S0 galaxies. Their z~2-2.5 main
progenitors are massive M* ~ 3-10 x 10^10 M_sun, star forming (20-60 M_sun/yr)
galaxies which host substantial reservoirs of cold gas (~5 x 10^9 M_sun) in
extended gas disks. Our simulations thus show that star forming galaxies
observed at z~2 are likely the main progenitors of central galaxies in galaxy
groups at z=0. Their central stellar densities stay approximately constant from
z~1.5 down to z=0. Instead, the galaxies grow inside-out, by acquiring a
stellar envelope outside the innermost ~2 kpc. Consequently the density within
the effective radius decreases by up to two orders of magnitude. Both major and
minor mergers contribute to most of the mass accreted outside the effective
radius and thus drive the evolution of the half-mass radii. In one of the three
simulated groups the short central cooling time leads to a dramatic
rejuvenation of the central group galaxy at z<1, affecting its morphology,
kinematics and colors. This episode is eventually terminated by a group-group
merger. Our simulations demonstrate that, in galaxy groups, the interplay
between halo mass assembly, galaxy merging and gas accretion has a substantial
influence on the star formation histories and z=0 morphologies of central
galaxies.[Abridged]Comment: 28 pages, 23 figures, 9 tables, accepted to APJ (revised to match
accepted version
ON the CONSERVATION of the VERTICAL ACTION in GALACTIC DISKS
We employ high-resolution N-body simulations of isolated spiral galaxy models, from low-amplitude, multi-armed galaxies to Milky Way-like disks, to estimate the vertical action of ensembles of stars in an axisymmetrical potential. In the multi-armed galaxy the low-amplitude arms represent tiny perturbations of the potential, hence the vertical action for a set of stars is conserved, although after several orbital periods of revolution the conservation degrades significantly. For a Milky Way-like galaxy with vigorous spiral activity and the formation of a bar, our results show that the potential is far from steady, implying that the action is not a constant of motion. Furthermore, because of the presence of high-amplitude arms and the bar, considerable in-plane and vertical heating occurs that forces stars to deviate from near-circular orbits, reducing the degree at which the actions are conserved for individual stars, in agreement with previous results, but also for ensembles of stars. If confirmed, this result has several implications, including the assertion that the thick disk of our Galaxy forms by radial migration of stars, under the assumption of the conservation of the action describing the vertical motion of stars. © 2016. The American Astronomical Society. All rights reserved
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