arXiv subjects

David Andersen

Publications and source records attributed to David Andersen.

27 records · Page 2Linked to original sources

arXiv2015

Spatio-angular Minimum-variance Tomographic Controller for Multi-Object Adaptive Optics systems

Multi-object astronomical adaptive-optics (MOAO) is now a mature wide-field observation mode to enlarge the adaptive-optics-corrected field in a few specific locations over tens of arc-minutes. The work-scope provided by open-loop tomography and pupil conjugation is amenable to a spatio-angular Linear-Quadratic Gaussian (SA-LQG) formulation aiming to provide enhanced correction across the field with improved performance over static reconstruction methods and less stringent computational complexity scaling laws. Starting from our previous work [1], we use stochastic time-progression models coupled to approximate sparse measurement operators to outline a suitable SA-LQG formulation capable of delivering near optimal correction. Under the spatio-angular framework the wave-fronts are never explicitly estimated in the volume,providing considerable computational savings on 10m-class telescopes and beyond. We find that for Raven, a 10m-class MOAO system with two science channels, the SA-LQG improves the limiting magnitude by two stellar magnitudes when both Strehl-ratio and Ensquared-energy are used as figures of merit. The sky-coverage is therefore improved by a factor of 5.

astro-ph.IM↗

arXiv2014

NFIRAOS First Facility AO System for the Thirty Meter Telescope

NFIRAOS, the Thirty Meter Telescope's first adaptive optics system is an order 60x60 Multi-Conjugate AO system with two deformable mirrors. Although most observing will use 6 laser guide stars, it also has an NGS-only mode. Uniquely, NFIRAOS is cooled to -30 C to reduce thermal background. NFIRAOS delivers a 2-arcminute beam to three client instruments, and relies on up to three IR WFSs in each instrument. We present recent work including: robust automated acquisition on these IR WFSs; trade-off studies for a common-size of deformable mirror; real-time computing architectures; simplified designs for high-order NGS-mode wavefront sensing; modest upgrade concepts for high-contrast imaging.

astro-ph.IM↗

arXiv2011

Two-dimensional H_alpha kinematics of bulgeless disk galaxies

We present two-dimensional H_alpha velocity fields for 20 late-type, disk-dominated spiral galaxies, the largest sample to date with high-resolution H_alpha velocity fields for bulgeless disks. From these data we derive rotation curves and the location of the kinematic centers. The galaxy sample was selected to contain nucleated and non-nucleated galaxies, which allows us to investigate what impact the gas kinematics in the host disk have on the presence (or absence) of a nuclear star cluster. In general, we find that the velocity fields span a broad range of morphologies. While some galaxies show regular rotation, most have some degree of irregular gas motions. There appears to be no systematic difference in the kinematics of nucleated and non-nucleated disks. Due to the large fields of view of the integral field units we use, we are able to observe the flattening of the rotation curve in almost all of our sample galaxies. This makes modeling of the velocity fields relatively straight-forward. Due to the complexities of the velocity fields, we obtain reliable determinations of the kinematic center for only 6 of our 20 sample galaxies. For all of these the locations of the nuclear star cluster/photometric center and the kinematic center agree within the uncertainties. If we disregard all kinematically irregular galaxies, our study concludes that nuclear star clusters truly occupy the nuclei, or dynamical centers, of their hosts. Our results are thus consistent with in-situ formation of nuclear star clusters. Yet, many well-motivated formation scenarios for nuclear clusters invoke off-center cluster formation and subsequent sinking of clusters due to dynamical friction. In that case, our results imply that dynamical friction in the centers of bulgeless galaxies must be very effective in driving massive clusters to the kinematic center. (abridged)

astro-ph.CO↗

arXiv2011

Using Site Testing Data for Adaptive Optics Simulations

Astronomical Site testing data plays a vital role in the simulation, design, evaluation and operation of adaptive optics systems for large telescope. We present the example of TMT and its first light facilitiy adaptive optics system NFIRAOS, and illustrate the many simulations done based on site testing data.

astro-ph.IM↗

arXiv2004

The Disk Mass project; science case for a new PMAS IFU module

We present our Disk Mass project as the main science case for building a new fiber IFU-module for the PMAS spectrograph, currently mounted at the Cassegrain focus of the 3.5m telescope on Calar Alto. Compared to traditional long-slit observations, the large light collecting power of 2-dimensional Integral Field Units dramatically improves the prospects for performing spectroscopy on extended low surface brightness objects with high spectral resolution. This enables us to measure stellar velocity dispersions in the outer disk of normal spiral galaxies. We describe some results from a PMAS pilot study using the existing lenslet array, and provide a basic description of the new fiber IFU-module for PMAS.

astro-ph↗

arXiv2003

A Face-On Tully-Fisher Relation

We construct the first "face-on" Tully-Fisher (TF) relation for 24 galaxies with inclinations between 16 degrees and 41 degrees. The enabling measurements are integral-field, echelle spectroscopy from the WIYN 3.5m telescope, which yield accurate kinematic estimates of disk inclination to 15 degrees. Kinematic inclinations are of sufficient accuracy that our measured TF scatter of 0.42 mag is comparable to other surveys even without internal-absorption corrections. Three of four galaxies with significant kinematic and photometric asymmetries also have the largest deviations from our TF relation, suggesting that asymmetries make an important contribution to TF scatter. By measuring inclinations below 40 degrees, we establish a direct path to linking this scatter to the unprojected structure of disks and making non-degenerate dynamical mass-decompositions of spiral galaxies.

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arXiv2002

WIYN Integral-Field Kinematics of Disk Galaxies

We have completed a new fiber array, SparsePak, optimized for low-surface-brightness studies of extended sources on the WIYN telescope. We are now using this array as a measuring engine of velocity and velocity-dispersion fields of stars and ionized gas in disk galaxies from high to low surface-brightness. Here we present commissioning data on the velocity ellipsoids, surface densities and mass-to-light ratios in two blue, high surface-brightness, yet small disks. If our preliminary results survive further observation and more sophisticated analysis, then NGC 3949 has sigma_z/sigma_R >> 1, implying strong vertical heating, while NGC 3982's disk is substantially sub-maximal. These galaxies are strikingly unlike the Milky Way, and yet would be seen more easily at high redshift.

astro-ph↗

arXiv2002

Measuring Galaxy Disk Mass with the SparsePak Integral Field Unit on WIYN

We present first results from commissioning data of the SparsePak Integral Field Unit on the WIYN telescope. SparsePak is a bundle of 82 fibers, each 5 arcsec in diameter and arranged in a sparsely filled, 76x77 arcsec hexagonal grid. NGC 3982, a nearly face-on, blue and very high surface brightness galaxy, was mapped in three spectral regions; around 5130A (MgI), 6680A (H-alpha) and 8660A (CaII-triplet) with FWHM spectral resolutions of 24, 16 and 37 km/s respectively. At 3.5 disk scale lengths, the spectra of 18 fibers were averaged after deprojection of the rotation curve. Stellar velocity dispersions were determined in the CaII region by convolving the spectrum of a K0.5-III template star with a Gaussian of varying FWHM and finding the best match to the azimuthally avaraged galaxy spectra. A radial mass surface density profile has been derived for a variety of scale heights assuming an isothermal vertical density profile. At two disk scale lengths, the mass surface density of N3982 is ~3 times higher than that of the Milky Way in the solar neighborhood. Surprisingly, the stellar rotation curve of this compact and high surface brightness galaxy is substantially sub-maximum.

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arXiv2000

The Assembly and Evolution of Spiral Disks

We explore how the growth rate of spiral disks can be measured via analyses of the scatter in the Tully-Fisher (TF) relation of local and intermediate redshift galaxies. As an initial step, we show it is possible to construct a low-dispersion TF relation for nearly face-on, nearby spirals. We find these spiral disks are non-circular, (a mean ellipticity of 6%), which accounts for about 0.1 mag of the intrinsic scatter in the nearby TF relation of even the most ``normal'' looking spirals. If this ellipticity is induced by matter accretion, we expect to find greater disk ellipticity or disturbance in the past. We find evidence that more extreme outliers of the intermediate-redshift TF relation are more morphologically and kinematically disturbed. Whether this effect reflects a redshift trend or selection bias of local samples needs to be addressed.

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