arXiv subjects

Tuan Do

Publications and source records attributed to Tuan Do.

At least 109 records · Page 6Linked to original sources

arXiv2015

Mapping the Outer Edge of the Young Stellar Cluster in the Galactic Center

We present new near-infrared spectroscopic observations of the outer edges of the young stellar cluster around the supermassive black hole at the Galactic center. The observations show a break in the surface-density profile of young stars at approximately 13 arcsec (0.52 pc). These observations spectroscopically confirm previous suggestions of a break based on photometry. Using Gemini North's Near-Infrared Integral Field Spectrometer (NIFS) we are able to detect and separate early- and late-type stars with a 75% completeness at Ks = 15.5. We sample a region with radii between 7" to 23" (0.28 pc to 0.92 pc) from Sgr A*, and present new spectral classifications of 144 stars brighter than Ks = 15.5, where 140 stars are late-type (> 1 Gyr) and only four stars are early-type (young, 4-6 Myr). A broken power-law fit of the early-type surface-density matches well with our data and previously published values. The projected surface-density of late-type stars is also measured and found to be consistent with previous results. We find that the observed early-type surface-density profile is inconsistent with the theory of the young stars originating from a tightly bound infalling cluster, as no significant trail of young stars is found at radii above 13". We also note that either a simple disk instability criterion or a cloud-cloud collision could explain the location of the outer edge, though we lack information to make conclusive remarks on either alternative. If this break in surface-density represents an edge to the young stellar cluster it would set an important scale for the most recent episode of star formation at the Galactic center.

astro-ph.GA↗

arXiv2014

Detection of Galactic Center source G2 at 3.8 $μ$m during periapse passage

We report new observations of the Galactic Center source G2 from the W. M. Keck Observatory. G2 is a dusty red object associated with gas that shows tidal interactions as it nears closest approach with the Galaxy's central black hole. Our observations, conducted as G2 passed through periapse, were designed to test the proposal that G2 is a 3 earth mass gas cloud. Such a cloud should be tidally disrupted during periapse passage. The data were obtained using the Keck II laser guide star adaptive optics system (LGSAO) and the facility near-infrared camera (NIRC2) through the K' [2.1 $μ$m] and L' [3.8 $μ$m] broadband filters. Several results emerge from these observations: 1) G2 has survived its closest approach to the black hole as a compact, unresolved source at L'; 2) G2's L' brightness measurements are consistent with those over the last decade; 3) G2's motion continues to be consistent with a Keplerian model. These results rule out G2 as a pure gas cloud and imply that G2 has a central star. This star has a luminosity of $\sim$30 $L_{\odot} $ and is surrounded by a large ($\sim$2.6 AU) optically thick dust shell. The differences between the L' and Br-$γ$ observations can be understood with a model in which L' and Br-$γ$ emission arises primarily from internal and external heating, respectively. We suggest that G2 is a binary star merger product and will ultimately appear similar to the B-stars that are tightly clustered around the black hole (the so-called S-star cluster).

astro-ph.HE↗

arXiv2014

The Infrared Imaging Spectrograph (IRIS) for TMT: Instrument Overview

We present an overview of the design of IRIS, an infrared (0.84 - 2.4 micron) integral field spectrograph and imaging camera for the Thirty Meter Telescope (TMT). With extremely low wavefront error (<30 nm) and on-board wavefront sensors, IRIS will take advantage of the high angular resolution of the narrow field infrared adaptive optics system (NFIRAOS) to dissect the sky at the diffraction limit of the 30-meter aperture. With a primary spectral resolution of 4000 and spatial sampling starting at 4 milliarcseconds, the instrument will create an unparalleled ability to explore high redshift galaxies, the Galactic center, star forming regions and virtually any astrophysical object. This paper summarizes the entire design and basic capabilities. Among the design innovations is the combination of lenslet and slicer integral field units, new 4Kx4k detectors, extremely precise atmospheric dispersion correction, infrared wavefront sensors, and a very large vacuum cryogenic system.

astro-ph.IM↗

arXiv2014

The InfraRed Imaging Spectrograph (IRIS) for TMT: Overview of innovative science programs

IRIS (InfraRed Imaging Spectrograph) is a first light near-infrared diffraction limited imager and integral field spectrograph being designed for the future Thirty Meter Telescope (TMT). IRIS is optimized to perform astronomical studies across a significant fraction of cosmic time, from our Solar System to distant newly formed galaxies (Barton et al. [1]). We present a selection of the innovative science cases that are unique to IRIS in the era of upcoming space and ground-based telescopes. We focus on integral field spectroscopy of directly imaged exoplanet atmospheres, probing fundamental physics in the Galactic Center, measuring 10^4 to 10^10 Msun supermassive black hole masses, resolved spectroscopy of young star-forming galaxies (1 < z < 5) and first light galaxies (6 < z < 12), and resolved spectroscopy of strong gravitational lensed sources to measure dark matter substructure. For each of these science cases we use the IRIS simulator (Wright et al. [2], Do et al. [3]) to explore IRIS capabilities. To highlight the unique IRIS capabilities, we also update the point and resolved source sensitivities for the integral field spectrograph (IFS) in all five broadband filters (Z, Y, J, H, K) for the finest spatial scale of 0.004" per spaxel. We briefly discuss future development plans for the data reduction pipeline and quicklook software for the IRIS instrument suite.

astro-ph.IM↗

arXiv2014

Prospects for measuring supermassive black hole masses with future extremely large telescopes

The next generation of giant-segmented mirror telescopes ($>$ 20 m) will enable us to observe galactic nuclei at much higher angular resolution and sensitivity than ever before. These capabilities will introduce a revolutionary shift in our understanding of the origin and evolution of supermassive black holes by enabling more precise black hole mass measurements in a mass range that is unreachable today. We present simulations and predictions of the observations of nuclei that will be made with the Thirty Meter Telescope (TMT) and the adaptive optics assisted integral-field spectrograph IRIS, which is capable of diffraction-limited spectroscopy from $Z$ band (0.9 $μ$m) to $K$ band (2.2 $μ$m). These simulations, for the first time, use realistic values for the sky, telescope, adaptive optics system, and instrument, to determine the expected signal-to-noise ratio of a range of possible targets spanning intermediate mass black holes of $\sim10^4$ \msun to the most massive black holes known today of $>10^{10}$ $M_\odot$. We find that IRIS will be able to observe Milky Way-mass black holes out the distance of the Virgo cluster, and will allow us to observe many more brightest cluster galaxies where the most massive black holes are thought to reside. We also evaluate how well the kinematic moments of the velocity distributions can be constrained at the different spectral resolutions and plate scales designed for IRIS. We find that a spectral resolution of $\sim8000$ will be necessary to measure the masses of intermediate mass black holes. By simulating the observations of galaxies found in SDSS DR7, we find that over $10^5$ massive black holes will be observable at distances between $0.005 < z < 0.18$ with the estimated sensitivity and angular resolution provided by access to $Z$-band (0.9 $μ$m) spectroscopy from IRIS and the TMT adaptive optics system. (Abridged)

astro-ph.GA↗

arXiv2014

Properties of the Remnant Clockwise Disk of Young Stars in the Galactic Center

We present new kinematic measurements and modeling of a sample of 116 young stars in the central parsec of the Galaxy in order to investigate the properties of the young stellar disk. The measurements were derived from a combination of speckle and laser guide star adaptive optics imaging and integral field spectroscopy from the Keck telescopes. Compared to earlier disk studies, the most important kinematic measurement improvement is in the precision of the accelerations in the plane of the sky, which have a factor of six smaller uncertainties (~10 uas/yr/yr). We have also added the first radial velocity measurements for 8 young stars, increasing the sample at the largest radii (6"-12") by 25%. We derive the ensemble properties of the observed stars using Monte-Carlo simulations of mock data. There is one highly significant kinematic feature (~20 sigma), corresponding to the well-known clockwise disk, and no significant feature is detected at the location of the previously claimed counterclockwise disk. The true disk fraction is estimated to be ~20%, a factor of ~2.5 lower than previous claims, suggesting that we may be observing the remnant of what used to be a more densely populated stellar disk. The similarity in the kinematic properties of the B stars and the O/WR stars suggests a common star formation event. The intrinsic eccentricity distribution of the disk stars is unimodal, with an average value of = 0.27 +/- 0.07, which we show can be achieved through dynamical relaxation in an initially circular disk with a moderately top-heavy mass function.

astro-ph.GA↗

arXiv2014

Efficiency Measurements and Installation of a New Grating for the OSIRIS Spectrograph at Keck Observatory

OSIRIS is a near-infrared integral field spectrograph operating behind the adaptive optics system at W. M. Keck Observatory. While OSIRIS has been a scientifically productive instrument to date, its sensitivity has been limited by a grating efficiency that is less than half of what was expected. The spatially averaged efficiency of the old grating, weighted by error, is measured to be 39.5 +/- 0.8 % at λ = 1.310 μm, with large field dependent variation of 11.7 % due to efficiency variation across the grating surface. Working with a new vendor, we developed a more efficient and uniform grating with a weighted average efficiency at λ = 1.310 μm of 78.0 +/- 1.6 %, with field variation of only 2.2 %. This is close to double the average efficiency and five times less variation across the field. The new grating was installed in December 2012, and on- sky OSIRIS throughput shows an average factor of 1.83 improvement in sensitivity between 1 and 2.4 microns. We present the development history, testing, and implementation of this new near-infrared grating for OSIRIS and report the comparison with the predecessors. The higher sensitivities are already having a large impact on scientific studies with OSIRIS.

astro-ph.IM↗

arXiv2013

Astrometry in the Galactic Center with the Thirty Meter Telescope

We report on the expected astrometric performance of the Thirty Meter Telescope's InfraRed Imaging Spectrometer (IRIS) as determined using simulated images of the Galactic center. This region of the Galaxy harbors a supermassive black hole and a dense nuclear stellar cluster, thus providing an ideal laboratory for testing crowded-field astrometry with the IRIS imager. Understanding the sources of astrometric error is also important for making precision measurements of the short-period stars orbiting the supermassive black hole in order to probe the curvature of space-time as predicted by General Relativity. Various sources of error are investigated, including read-out and photon noise, spatially variable point spread functions, confusion, static distortion for the IRIS imager, and the quadratic probe arm distortion. Optical distortion is the limiting source of error for bright stars (K < 15), while fainter sources will be limited by the effects of source confusion. A detailed astrometric error budget for the Galactic center science case is provided.

astro-ph.IM↗

arXiv2013

A reconnaissance of the possible donor stars to the Kepler supernova

The identity of Type Ia supernova progenitors remains a mystery, with various lines of evidence pointing towards either accretion from a non-degenerate companion, or the rapid merger of two degenerate stars leading to the thermonuclear destruction of a white dwarf. In this paper we spectroscopically scrutinize 24 of the brightest stars residing in the central 38" x 38" of the SN 1604 (Kepler) supernova remnant to search for a possible surviving companion star. We can rule out, with high certainty, a red giant companion star - a progenitor indicated by some models of the supernova remnant. Furthermore, we find no star that exhibits properties uniquely consistent with those expected of a donor star down to L>10Lsun. While the distribution of star properties towards the remnant are consistent with unrelated stars, we identify the most promising candidates for further astrometric and spectroscopic follow-up. Such a program would either discover the donor star, or place strong limits on progenitor systems to luminosities with L<<Lsun.

astro-ph.SR↗

arXiv2013

Local Group and Star Cluster Dynamics from HSTPROMO (The Hubble Space Telescope Proper Motion Collaboration)

The Hubble Space Telescope (HST) has proven to be uniquely suited for the measurement of proper motions (PMs) of stars and galaxies in the nearby Universe. Here we summarize the main results and ongoing studies of the HSTPROMO collaboration, which over the past decade has executed some two dozen observational and theoretical HST projects on this topic. This is continuing to revolutionize our dynamical understanding of many objects, including: globular clusters; young star clusters; stars and stellar streams in the Milky Way halo; Local Group galaxies, including dwarf satellite galaxies, the Magellanic Clouds, and the Andromeda galaxy; and AGN Black Hole Jets.

astro-ph.CO↗

arXiv2013

Stellar Populations in the Central 0.5 pc of the Galaxy I: A New Method for Constructing Luminosity Functions and Surface-Density Profiles

We present new high angular resolution near-infrared spectroscopic observations of the nuclear star cluster surrounding the Milky Way's central supermassive black hole. Using the integral-field spectrograph OSIRIS on Keck II behind the laser-guide-star adaptive optics system, this spectroscopic survey enables us to separate early-type (young, 4-6 Myr) and late-type (old, >1 Gyr) stars with a completeness of 50% down to K' = 15.5 mag, which corresponds to ~10 \msun for the early-type stars. This work increases the radial extent of reported OSIRIS/Keck measurements by more than a factor of 3 from 4" to 14" (0.16 pc to 0.56 pc), along the projected disk of young stars. For our analysis, we implement a new method of completeness correction using a combination of star-planting simulations and Bayesian inference. We assign probabilities for the spectral type of every source detected in deep imaging down to K' = 15.5 mag using information from spectra, simulations, number counts, and the distribution of stars. The inferred radial surface-density profiles, $Σ(R) \propto R^{-Γ}$, for the young stars and late-type giants are consistent with earlier results ($Γ_{early} = 0.93 \pm 0.09$, $Γ_{late} = 0.16 \pm 0.07$). The late-type surface-density profile is approximately flat out to the edge of the survey. While the late-type stellar luminosity function is consistent with the Galactic bulge, the completeness-corrected luminosity function of the early-type stars has significantly more young stars at faint magnitudes compared to previous surveys with similar depth. This luminosity function indicates that the corresponding mass function of the young stars is likely less top-heavy than that inferred from previous surveys.

astro-ph.SR↗

arXiv2013

Stellar Populations in the Central 0.5 pc of the Galaxy II: The Initial Mass Function

The supermassive black hole at the center of the Milky Way plays host to a massive, young cluster that may have formed in one of the most inhospitable environments in the Galaxy. We present new measurements of the global properties of this cluster, including the initial mass function (IMF), age, and cluster mass. These results are based on Keck laser-guide-star adaptive optics observations used to identify the young stars and measure their Kp-band luminosity function as presented in Do et al. 2013. A Bayesian inference methodology is developed to simultaneously fit the global properties of the cluster utilizing the observations and extensive simulations of synthetic star clusters. We find that the slope of the mass function for this cluster is α= 1.7 +/- 0.2, which is steeper than previously reported, but still flatter than the traditional Salpeter slope of 2.35. The age of the cluster is between 2.5-5.8 Myr with 95% confidence, which is a younger age than typically adopted but consistent within the uncertainties of past measurements. The exact age of the cluster is difficult to determine since our results show two distinct age solutions (3.9 Myr and 2.8 Myr) due to model degeneracies in the relative number of Wolf-Rayet and OB stars. The total cluster mass is between 14,000 - 37,000 \msun above 1 \msun and it is necessary to include multiple star systems in order to fit the observed luminosity function and the number of observed Wolf-Rayet stars. The new IMF slope measurement is now consistent with X-ray observations indicating a factor of 10 fewer X-ray emitting pre-main-sequence stars than expected when compared with a Salpeter IMF. The young cluster at the Galactic center is one of the few definitive examples of an IMF that deviates significantly from the near-universal IMFs found in the solar neighborhood.

astro-ph.SR↗

arXiv2011

Proper motions of the Arches cluster with Keck-LGS Adaptive Optics: the first kinematic mass measurement of the Arches

We report the first detection of the intrinsic velocity dispersion of the Arches cluster - a young (~2 Myr), massive (~10,000 Solar Mass) starburst cluster located near the Galactic center. This was accomplished using proper motion measurements within the central region of the cluster, obtained with the laser guide star adaptive optics system at Keck Observatory over a 3 year time baseline (2006-2009). This uniform dataset results in proper motion measurements that are improved by a factor ~5 over previous measurements from heterogeneous instruments, yielding internal velocity dispersion estimates 0.15 +/- 0.01 mas/yr, which corresponds to 5.4 +/- 0.4 km/s at a distance of 8.4 kpc. Projecting a simple model for the cluster onto the sky to compare with our proper motion dataset, in conjunction with surface density data, we estimate the total present-day mass of the cluster to be 15,000 (+7400 -6000) Solar masses. The mass in stars observed within a cylinder of radius R=0.4 pc is found to be 9000 (+4000 -3500) Solar Masses at formal 3-sigma confidence. This mass measurement is free from assumptions about the mass function of the cluster, and thus may be used to check mass estimates from photometry and simulation. When we conduct this check, we find that the present-day mass function of the Arches cluster is likely either top-heavy or truncated at low-mass, or both. Collateral benefits of our data and analysis include: 1. cluster membership probabilities, which may be used to extract a clean cluster sample for future photometric work; 2. a refined estimate of the bulk motion of the Arches cluster with respect to the field, which we find to be 172 +/- 15 km/s, which is slightly slower than suggested by previous VLT-Keck measurements; and 3. a velocity dispersion estimate for the field itself, which is likely dominated by the inner galactic bulge and the nuclear disk.

astro-ph.GA↗

arXiv2011

The Adaptive Optics Summer School Laboratory Activities

Adaptive Optics (AO) is a new and rapidly expanding field of instrumentation, yet astronomers, vision scientists, and general AO practitioners are largely unfamiliar with the root technologies crucial to AO systems. The AO Summer School (AOSS), sponsored by the Center for Adaptive Optics, is a week-long course for training graduate students and postdoctoral researchers in the underlying theory, design, and use of AO systems. AOSS participants include astronomers who expect to utilize AO data, vision scientists who will use AO instruments to conduct research, opticians and engineers who design AO systems, and users of high-bandwidth laser communication systems. In this article we describe new AOSS laboratory sessions implemented in 2006-2009 for nearly 250 students. The activity goals include boosting familiarity with AO technologies, reinforcing knowledge of optical alignment techniques and the design of optical systems, and encouraging inquiry into critical scientific questions in vision science using AO systems as a research tool. The activities are divided into three stations: Vision Science, Fourier Optics, and the AO Demonstrator. We briefly overview these activities, which are described fully in other articles in these conference proceedings (Putnam et al., Do et al., and Harrington et al., respectively). We devote attention to the unique challenges encountered in the design of these activities, including the marriage of inquiry-like investigation techniques with complex content and the need to tune depth to a graduate- and PhD-level audience. According to before-after surveys conducted in 2008, the vast majority of participants found that all activities were valuable to their careers, although direct experience with integrated, functional AO systems was particularly beneficial.

physics.ed-ph↗

arXiv2010

Improving Galactic Center Astrometry by Reducing the Effects of Geometric Distortion

We present significantly improved proper motion measurements of the Milky Way's central stellar cluster. These improvements are made possible by refining our astrometric reference frame with a new geometric optical distortion model for the W. M. Keck II 10 m telescope's Adaptive Optics camera (NIRC2) in its narrow field mode. For the first time, this distortion model is constructed from on-sky measurements, and is made available to the public. When applied to widely dithered images, it produces residuals in the separations of stars that are a factor of ~3 smaller compared to the outcome using previous models. By applying this new model, along with corrections for differential atmospheric refraction, to widely dithered images of SiO masers at the Galactic center, we improve our ability to tie into the precisely measured radio Sgr A*-rest frame. The resulting infrared reference frame is ~2-3 times more accurate and stable than earlier published efforts. In this reference frame, Sgr A* is localized to within a position of 0.6 mas and a velocity of 0.09 mas/yr, or ~3.4 km/s at 8 kpc (1 sigma). While earlier proper motion studies defined a reference frame by assuming no net motion of the stellar cluster, this approach is fundamentally limited by the cluster's intrinsic dispersion and therefore will not improve with time. We define a reference frame with SiO masers and this reference frame's stability should improve steadily with future measurements of the SiO masers in this region. This is essential for achieving the necessary reference frame stability required to detect the effects of general relativity and extended mass on short-period stars at the Galactic center.

astro-ph.GA↗

arXiv2010

Recent Results and Perspectives for Precision Astrometry and Photometry with Adaptive Optics

Large ground-based telescopes equipped with adaptive optics (AO) systems have ushered in a new era of high-resolution infrared photometry and astrometry. Relative astrometric accuracies of <0.2 mas have already been demonstrated from infrared images with spatial resolutions of 55-95 mas resolution over 10-20'' fields of view. Relative photometric accuracies of 3% and absolute photometric accuracies of 5%-20% are also possible. I will review improvements and current limitations in astrometry and photometry with adaptive optics of crowded stellar fields. These capabilities enable experiments such as measuring orbits for brown dwarfs and exoplanets, studying our Galaxy's supermassive black hole and its environment, and identifying individual stars in young star clusters, which can be used test the universality of the initial mass function.

astro-ph.IM↗

arXiv2010

Disks in the Arches cluster -- survival in a starburst environment

Deep Keck/NIRC2 HK'L' observations of the Arches cluster near the Galactic center reveal a significant population of near-infrared excess sources. We combine the L'-band excess observations with K'-band proper motions, to confirm cluster membership of excess sources in a starburst cluster for the first time. The robust removal of field contamination provides a reliable disk fraction down to our completeness limit of H=19 mag, or about 5 Msun at the distance of the Arches. Of the 24 identified sources with K'-L' > 2.0 mag, 21 have reliable proper motion measurements, all of which are proper motion members of the Arches cluster. VLT/SINFONI K'-band spectroscopy of three excess sources reveals strong CO bandhead emission, which we interpret as the signature of dense circumstellar disks. The detection of strong disk emission from the Arches stars is surprising in view of the high mass of the B-type main sequence host stars of the disks and the intense starburst environment. We find a disk fraction of 6 +/- 2% among B-type stars in the Arches cluster. A radial increase in the disk fraction from 3 to 10% suggests rapid disk destruction in the immediate vicinity of numerous O-type stars in the cluster core. A comparison between the Arches and other high- and low-mass star-forming regions provides strong indication that disk depletion is significantly more rapid in compact starburst clusters than in moderate star-forming environments.

astro-ph.SR↗

arXiv2010

Testing stellar cusp formation theories with observations of the Milky Way nuclear star cluster

(Abridged) We report on the structure of the nuclear star cluster in the innermost 0.16 pc of the Galaxy as measured by the number density profile of late-type giants. Using laser guide star adaptive optics in conjunction with the integral field spectrograph, OSIRIS, at the Keck II telescope, we are able to differentiate between the older, late-type (~ 1 Gyr) stars, which are presumed to be dynamically relaxed, and the unrelaxed young (~ 6 Myr) population. This distinction is crucial for testing models of stellar cusp formation in the vicinity of a black hole, as the models assume that the cusp stars are in dynamical equilibrium in the black hole potential. We find that contamination from young stars is significant, with more than twice as many young stars as old stars in our sensitivity range (K < 15.5) within the central arcsecond. Based on the late-type stars alone, the surface stellar number density profile, is flat, with a projected power law slope of -0.26+-0.24. These results are consistent with the nuclear star cluster having no cusp, with a core profile that is significantly flatter than predicted by most cusp formation theories. Here, we also review the methods for further constraining the true three-dimensional radial profile using kinematic measurements. Precise acceleration measurements in the plane of the sky as well as along the line of sight has the potential to directly measure the density profile to establish whether there is a "hole" in the distribution of late-type stars in the inner 0.1 pc.

astro-ph.GA↗