199 research outputs found
Normalization of Gravitational Acceleration Models
Unlike the uniform density spherical shell approximations of Newton, the con- sequence of spaceflight in the real universe is that gravitational fields are sensitive to the nonsphericity of their generating central bodies. The gravitational potential of a nonspherical central body is typically resolved using spherical harmonic approximations. However, attempting to directly calculate the spherical harmonic approximations results in at least two singularities which must be removed in order to generalize the method and solve for any possible orbit, including polar orbits. Three unique algorithms have been developed to eliminate these singularities by Samuel Pines [1], Bill Lear [2], and Robert Gottlieb [3]. This paper documents the methodical normalization of two1 of the three known formulations for singularity-free gravitational acceleration (namely, the Lear [2] and Gottlieb [3] algorithms) and formulates a general method for defining normalization parameters used to generate normalized Legendre Polynomials and ALFs for any algorithm. A treatment of the conventional formulation of the gravitational potential and acceleration is also provided, in addition to a brief overview of the philosophical differences between the three known singularity-free algorithms
The Lyman Alpha Reference Sample: III. Properties of the Neutral ISM from GBT and VLA Observations
We present new H I imaging and spectroscopy of the 14 UV-selected
star-forming galaxies in the Lyman Alpha Reference Sample (LARS), aimed for a
detailed study of the processes governing the production, propagation, and
escape of Ly photons. New H I spectroscopy, obtained with the 100m
Green Bank Telescope (GBT), robustly detects the H I spectral line in 11 of the
14 observed LARS galaxies (although the profiles of two of the galaxies are
likely confused by other sources within the GBT beam); the three highest
redshift galaxies are not detected at our current sensitivity limits. The GBT
profiles are used to derive fundamental H I line properties of the LARS
galaxies. We also present new pilot H I spectral line imaging of 5 of the LARS
galaxies obtained with the Karl G. Jansky Very Large Array (VLA). This imaging
localizes the H I gas and provides a measurement of the total H I mass in each
galaxy. In one system, LARS 03 (UGC 8335 or Arp 238), VLA observations reveal
an enormous tidal structure that extends over 160 kpc from the main interacting
systems and that contains 10 M of H I. We compare various H I
properties with global Ly quantities derived from HST measurements. The
measurements of the Ly escape fraction are coupled with the new direct
measurements of H I mass and significantly disturbed H I velocities. Our
robustly detected sample reveals that both total H I mass and linewidth are
tentatively correlated with key Ly tracers. Further, on global scales,
these data support a complex coupling between Ly propagation and the H
I properties of the surrounding medium.Comment: Preprint form, 16 figures, accepted in Ap
Star Formation Histories of the LEGUS dwarf galaxies. II. Spatially resolved star formation history of the Magellanic irregular NGC 4449
We present a detailed study of the Magellanic irregular galaxy NGC 4449 based
on both archival and new photometric data from the Legacy Extragalactic UV
Survey, obtained with the Hubble Space Telescope Advanced Camera for Surveys
and Wide Field Camera 3. Thanks to its proximity ( Mpc) we
reach stars 3 magnitudes fainter than the tip of the red giant branch in the
F814W filter. The recovered star formation history spans the whole Hubble time,
but due to the age-metallicity degeneracy of the red giant branch stars, it is
robust only over the lookback time reached by our photometry, i.e.
Gyr. The most recent peak of star formation is around 10 Myr ago. The average
surface density star formation rate over the whole galaxy lifetime is
M yr kpc. From our study it emerges that NGC 4449 has
experienced a fairly continuous star formation regime in the last 1 Gyr with
peaks and dips whose star formation rates differ only by a factor of a few. The
very complex and disturbed morphology of NGC 4449 makes it an interesting
galaxy for studies of the relationship between interactions and starbursts, and
our detailed and spatially resolved analysis of its star formation history does
indeed provide some hints on the connection between these two phenomena in this
peculiar dwarf galaxy.Comment: 16 pages, 25 figures. Accepted for publication in the Astrophysical
Journa
Candidate LBV stars in galaxy NGC 7793 found via HST photometry + MUSE spectroscopy
Only about 19 Galactic and 25 extragalactic bonafide luminous blue variables (LBVs) are known to date. This incomplete census prevents our understanding of this crucial phase of massive star evolution which leads to the formation of heavy binary black holes via the classical channel. With large samples of LBVs one could better determine the duration and maximum stellar luminosity which characterize this phase. We search for candidate LBVs (cLBVs) in a new galaxy, NGC 7793. For this purpose, we combine high spatial resolution images from two Hubble Space Telescope (HST) programs with optical spectroscopy from the Multi Unit Spectroscopic Explorer (MUSE). By combining PSF-fitting photometry measured on F547M, F657N, and F814W images, with restrictions on point-like appearance (at HST resolution) and Hâα luminosity, we find 100 potential cLBVs, 36 of which fall in the MUSE fields. Five of the latter 36 sources are promising cLBVs which have MV †â7 and a combination of: Hâα with a P-Cygni profile; no [OâI]λ6300 emission; weak or no [OâIII]λ5007 emission; large [NâII]/Hâα relative to HâII regions; and [SâII]λ6716/[SâII]λ6731âŒ1â . It is not clear if these five cLBVs are isolated from O-type stars, which would favour the binary formation scenario of LBVs. Our study, which approximately covers one fourth of the optical disc of NGC 7793, demonstrates how by combining the above HST surveys with multi-object spectroscopy from 8-m class telescopes, one can efficiently find large samples of cLBVs in nearby galaxies
An Open System for Social Computation
Part of the power of social computation comes from using the collective intelligence of humans to tame the aggregate uncertainty of (otherwise) low veracity data obtained from human and automated sources. We have witnessed a surge in development of social computing systems but, ironically, there have been few attempts to generalise across this activity so that creation of the underlying mechanisms themselves can be made more social. We describe a method for achieving this by standardising patterns of social computation via lightweight formal specifications (we call these social artifacts) that can be connected to existing internet architectures via a single model of computation. Upon this framework we build a mechanism for extracting provenance meta-data across social computations
The spatially resolved star formation history of the dwarf spiral galaxy NGC 5474
We study the resolved stellar populations and derive the star formation
history of NGC 5474, a peculiar star-forming dwarf galaxy at a distance of
Mpc, using Hubble Space Telescope Advanced Camera for Surveys data
from the Legacy Extragalactic UV Survey (LEGUS) program. We apply an improved
colour-magnitude diagram fitting technique based on the code SFERA and use the
latest PARSEC-COLIBRI stellar models. Our results are the following. The
off-centre bulge-like structure, suggested to constitute the bulge of the
galaxy, is dominated by star formation (SF) activity initiated Gyr ago and
lasted at least up to Gyr ago. Nevertheless, this component shows clear
evidence of prolonged SF activity (lasting until Myr ago). We
estimate the total stellar mass of the bulge-like structure to be \MSUN. Such a mass is consistent with published suggestions
that this structure is in fact an independent system orbiting around and not
within NGC 5474's disc. The stellar over-density located to the South-West of
the bulge-like structure shows a significant SF event older than Gyr, while
it is characterised by two recent peaks of SF, around and
Myr ago. In the last Gyr, the behavior of the stellar disc is consistent with
what is known in the literature as `gasping'. The synchronised burst at
Myr in all components might hint to the recent gravitational interaction
between the stellar bulge-like structure and the disc of NGC 5474.Comment: 18 pages, 12 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Societ
Star Formation Histories of the LEGUS Spiral Galaxies. I. the Flocculent Spiral NGC 7793
We present a detailed study of the flocculent spiral galaxy NGC 7793, part of the Sculptor group. By analyzing the resolved stellar populations of the galaxy, located at a distance of ~3.7 Mpc, we infer for the first time its radial star formation history (SFH) from Hubble Space Telescope photometry, thanks to both archival and new data from the Legacy ExtraGalactic UV Survey. We determine an average star formation rate (SFR) for the galaxy portion covered by our F555W and F814W data of 0.23 ± 0.02 M â yrâ1 over the whole Hubble time, corresponding to a total stellar mass of (3.09 ± 0.33) Ă 109 M â, in agreement with previous determinations. Thanks to the new data extending to the F336W band, we are able to analyze the youngest stellar populations with a higher time resolution. Most importantly, we recover the resolved SFH in different radial regions of the galaxy; this shows an indication of a growing trend of the present-to-past SFR ratio, increasing from internal to more external regions, supporting previous findings of the inside-out growth of the galaxy.Support for this program was provided by
NASA through grants from the Space Telescope Science
Institute. M.C. and M.T. acknowledge funding from the INAF
PRIN-SKA 2017 program 1.05.01.88.04. A.A. acknowledges
the support of the Swedish Research Council, VetenskapsrÄdet
and the Swedish National Space Agency (SNSA)
Revisiting Attenuation Curves: the Case of NGC 3351
Multi-wavelength images from the farUV (~0.15 micron) to the sub-millimeter
of the central region of the galaxy NGC 3351 are analyzed to constrain its
stellar populations and dust attenuation. Despite hosting a ~1 kpc
circumnuclear starburst ring, NGC 3351 deviates from the IRX-beta relation, the
relation between the infrared-to-UV luminosity ratio and the UV continuum slope
(beta) that other starburst galaxies follow. To understand the reason for the
deviation, we leverage the high angular resolution of archival nearUV-to-nearIR
HST images to divide the ring into ~60-180 pc size regions and model each
individually. We find that the UV slope of the combined intrinsic (dust-free)
stellar populations in the central region is redder than what is expected for a
young model population. This is due to the region's complex star formation
history, which boosts the nearUV emission relative to the farUV. The resulting
net attenuation curve has a UV slope that lies between those of the starburst
attenuation curve (Calzetti et al. 2000) and the Small Magellanic Cloud
extinction curve; the total-to-selective attenuation value, R'(V)=4.93, is
larger than both. As found for other star-forming galaxies, the stellar
continuum of NGC 3351 is less attenuated than the ionized gas, with
E(B-V)_{star}=0.40 E(B-V)_{gas}. The combination of the `red' intrinsic stellar
population and the new attenuation curve fully accounts for the location of the
central region of NGC 3351 on the IRX-beta diagram. Thus, the observed
characteristics result from the complex mixture of stellar populations and dust
column densities in the circumnuclear region. Despite being a sample of one,
these findings highlight the difficulty of defining attenuation curves of
general applicability outside the regime of centrally-concentrated starbursts.Comment: 45 page, 17 figures; accepted for publication on the Astrophysical
Journal, March 21st, 202
Exploring the IMF of star clusters: a joint SLUG and LEGUS effort
We present the implementation of a Bayesian formalism within the Stochastically Lighting Up Galaxies (SLUG) stellar population synthesis code, which is designed to investigate variations in the initial mass function (IMF) of star clusters. By comparing observed cluster photometry to large libraries of clusters simulated with a continuously varying IMF, our formalism yields the posterior probability distribution function (PDF) of the cluster mass, age and extinction, jointly with the parameters describing the IMF. We apply this formalism to a sample of star clusters from the nearby galaxy NGC 628, for which broad-band photometry in five filters is available as part of the Legacy ExtraGalactic UV Survey (LEGUS). After allowing the upper-end slope of the IMF (α3) to vary, we recover PDFs for the mass, age and extinction that are broadly consistent with what is found when assuming an invariant Kroupa IMF. However, the posterior PDF for α3 is very broad due to a strong degeneracy with the cluster mass, and it is found to be sensitive to the choice of priors, particularly on the cluster mass. We find only a modest improvement in the constraining power of α3 when adding Hα photometry from the companion Hα-LEGUS survey. Conversely, Hα photometry significantly improves the age determination, reducing the frequency of multi-modal PDFs. With the aid of mock clusters, we quantify the degeneracy between physical parameters, showing how constraints on the cluster mass that are independent of photometry can be used to pin down the IMF properties of star clusters.GA acknowledges support from the Science and Technology Facilities Council (ST/L00075X/1 and ST/M503472/1). MF acknowledges support by the Science and Technology Facilities Council
[grant number ST/L00075X/1]. DAG kindly acknowledges financial support by the German Research Foundation (DFG) through
programme GO 1659/3-2
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