8,939 research outputs found
2D Bayesian automated tilted-ring fitting of disk galaxies in large HI galaxy surveys: 2DBAT
We present a novel algorithm based on a Bayesian method for 2D tilted-ring
analysis of disk galaxy velocity fields. Compared to the conventional
algorithms based on a chi-squared minimisation procedure, this new
Bayesian-based algorithm suffers less from local minima of the model parameters
even with highly multi-modal posterior distributions. Moreover, the Bayesian
analysis, implemented via Markov Chain Monte Carlo (MCMC) sampling, only
requires broad ranges of posterior distributions of the parameters, which makes
the fitting procedure fully automated. This feature will be essential when
performing kinematic analysis on the large number of resolved galaxies expected
to be detected in neutral hydrogen (HI) surveys with the Square Kilometre Array
(SKA) and its pathfinders. The so-called '2D Bayesian Automated Tilted-ring
fitter' (2DBAT) implements Bayesian fits of 2D tilted-ring models in order to
derive rotation curves of galaxies. We explore 2DBAT performance on (a)
artificial HI data cubes built based on representative rotation curves of
intermediate-mass and massive spiral galaxies, and (b) Australia Telescope
Compact Array (ATCA) HI data from the Local Volume HI Survey (LVHIS). We find
that 2DBAT works best for well-resolved galaxies with intermediate inclinations
(20 deg < i < 70 deg), complementing three-dimensional techniques better suited
to modelling inclined galaxies.Comment: Accepted for publication in MNRAS; 46 pages, 33 figure
The Panchromatic High-Resolution Spectroscopic Survey of Local Group Star Clusters - I. General Data Reduction Procedures for the VLT/X-shooter UVB and VIS arm
Our dataset contains spectroscopic observations of 29 globular clusters in
the Magellanic Clouds and the Milky Way performed with VLT/X-shooter. Here we
present detailed data reduction procedures for the VLT/X-shooter UVB and VIS
arm. These are not restricted to our particular dataset, but are generally
applicable to different kinds of X-shooter data without major limitation on the
astronomical object of interest. ESO's X-shooter pipeline (v1.5.0) performs
well and reliably for the wavelength calibration and the associated
rectification procedure, yet we find several weaknesses in the reduction
cascade that are addressed with additional calibration steps, such as bad pixel
interpolation, flat fielding, and slit illumination corrections. Furthermore,
the instrumental PSF is analytically modeled and used to reconstruct flux
losses at slit transit and for optimally extracting point sources. Regular
observations of spectrophotometric standard stars allow us to detect
instrumental variability, which needs to be understood if a reliable absolute
flux calibration is desired. A cascade of additional custom calibration steps
is presented that allows for an absolute flux calibration uncertainty of less
than ten percent under virtually every observational setup provided that the
signal-to-noise ratio is sufficiently high. The optimal extraction increases
the signal-to-noise ratio typically by a factor of 1.5, while simultaneously
correcting for resulting flux losses. The wavelength calibration is found to be
accurate to an uncertainty level of approximately 0.02 Angstrom. We find that
most of the X-shooter systematics can be reliably modeled and corrected for.
This offers the possibility of comparing observations on different nights and
with different telescope pointings and instrumental setups, thereby
facilitating a robust statistical analysis of large datasets.Comment: 22 pages, 18 figures, Accepted for publication in Astronomy &
Astrophysics; V2 contains a minor change in the abstract. We note that we did
not test X-shooter pipeline versions 2.0 or later. V3 contains an updated
referenc
HST-COS Observations of AGN. I. Ultraviolet Composite Spectra of the Ionizing Continuum and Emission Lines
The ionizing fluxes from quasars and other active galactic nuclei (AGN) are
critical for interpreting the emission-line spectra of AGN and for
photoionization and heating of the intergalactic medium. Using ultraviolet
spectra from the Cosmic Origins Spectrograph (COS) on the Hubble Space
Telescope (HST), we have directly measured the rest-frame ionizing continua and
emission lines for 22 AGN. Over the redshift range 0.026 < z < 1.44, COS
samples the Lyman continuum and many far-UV emission lines (Lya 1216, C IV
1549, Si IV/OIV] 1400, N V 1240, O VI 1035). Strong EUV emission lines with
14-22 eV excitation energies (Ne VIII 770,780, Ne V 569, O II 834, O III 833,
702, O IV 788,608,554, O V 630, N III 685) suggest the presence of hot gas in
the broad emission-line region. The rest-frame continuum, F_nu ~ nu^{alpha_nu},
shows a break at wavelengths below 1000 A, with spectral index alpha_nu = -0.68
+/- 0.14 in the FUV (1200-2000 A) steepening to alpha_nu = -1.41 +/- 0.21 in
the EUV (500-1000 A). The COS EUV index is similar to that of radio-quiet AGN
in the 2002 HST/FOS survey (alpha_nu = -1.57 +/- 0.17). We see no Lyman edge
(tau_HI < 0.03) or He I 584 emission in the AGN composite. Our 22 AGN exhibit a
substantial range of FUV/EUV spectral indices and a correlation with AGN
luminosity and redshift, likely due to observing below the 1000 A break.Comment: 17 pages, 10 figs, accepted to Astrophysical Journal (revised AGN
luminosities and fluxes, updated figures
Bringing "The Moth" to Light: A Planet-Sculpting Scenario for the HD 61005 Debris Disk
The HD 61005 debris disk ("The Moth") stands out from the growing collection
of spatially resolved circumstellar disks by virtue of its unusual swept-back
morphology, brightness asymmetries, and dust ring offset. Despite several
suggestions for the physical mechanisms creating these features, no definitive
answer has been found. In this work, we demonstrate the plausibility of a
scenario in which the disk material is shaped dynamically by an eccentric,
inclined planet. We present new Keck NIRC2 scattered-light angular differential
imaging of the disk at 1.2-2.3 microns that further constrains its outer
morphology (projected separations of 27-135 AU). We also present complementary
Gemini Planet Imager 1.6 micron total intensity and polarized light detections
that probe down to projected separations less than 10 AU. To test our
planet-sculpting hypothesis, we employed secular perturbation theory to
construct parent body and dust distributions that informed scattered-light
models. We found that this method produced models with morphological and
photometric features similar to those seen in the data, supporting the premise
of a planet-perturbed disk. Briefly, our results indicate a disk parent body
population with a semimajor axis of 40-52 AU and an interior planet with an
eccentricity of at least 0.2. Many permutations of planet mass and semimajor
axis are allowed, ranging from an Earth mass at 35 AU to a Jupiter mass at 5
AU.Comment: Accepted to AJ; added Figure 5 and minor text edit
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