arXiv subjects

Sangmo Tony Sohn

Publications and source records attributed to Sangmo Tony Sohn.

At least 73 records · Page 4Linked to original sources

arXiv2017

The Proper Motion of Pyxis: the first use of Adaptive Optics in tandem with HST on a faint halo object

We present a proper motion measurement for the halo globular cluster Pyxis, using HST/ACS data as the first epoch, and GeMS/GSAOI Adaptive Optics data as the second, separated by a baseline of about 5 years. This is both the first measurement of the proper motion of Pyxis and the first calibration and use of Multi-Conjugate Adaptive Optics data to measure an absolute proper motion for a faint, distant halo object. Consequently, we present our analysis of the Adaptive Optics data in detail. We obtain a proper motion of mu_alpha cos(delta)=1.09+/-0.31 mas/yr and mu_delta=0.68+/-0.29 mas/yr. From the proper motion and the line-of-sight velocity we find the orbit of Pyxis is rather eccentric with its apocenter at more than 100 kpc and its pericenter at about 30 kpc. We also investigate two literature-proposed associations for Pyxis with the recently discovered ATLAS stream and the Magellanic system. Combining our measurements with dynamical modeling and cosmological numerical simulations we find it unlikely Pyxis is associated with either system. We examine other Milky Way satellites for possible association using the orbit, eccentricity, metallicity, and age as constraints and find no likely matches in satellites down to the mass of Leo II. We propose that Pyxis probably originated in an unknown galaxy, which today is fully disrupted. Assuming that Pyxis is bound and not on a first approach, we derive a 68% lower limit on the mass of the Milky Way of 0.95*10^12 M_sun.

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arXiv2016

Hubble Space Telescope Proper Motions of Individual Stars in Stellar Streams: Orphan, Sagittarius, Lethe, and the New "Parallel" Stream

We present a multi-epoch Hubble Space Telescope (HST) study of stellar proper motions (PMs) for four fields along the Orphan Stream. We determine absolute PMs of several individual stars per target field using established techniques that utilize distant background galaxies to define a stationary reference frame. Five Orphan Stream stars are identified in one of the four fields based on combined color-magnitude and PM information. The average PM is consistent with the existing model of the Orphan stream by Newberg et al. In addition to the Orphan stream stars, we detect stars that likely belong to other stellar streams. To identify which stellar streams these stars belong to, we examine the 2-d bulk motion of each group of stars on the sky by subtracting the PM contribution of the solar motion (which is a function of position on the sky and distance) from the observed PMs, and comparing the vector of net motion with the spatial extent of known stellar streams. By doing this, we identify candidate stars in the Sagittarius and Lethe streams, and a newly-found stellar stream at a distance of ~17 kpc, which we tentatively name the "Parallel stream". Together with our Sagittarius stream study (Sohn et al., 2015, ApJ, 803, 56), this work demonstrates that even in the Gaia era, HST will continue to be advantageous in measuring PMs of old stellar populations on a star-by-star basis, especially for distances beyond ~10 kpc.

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arXiv2016

Isotropic at the Break? 3D Kinematics of Milky Way Halo Stars in the Foreground of M31

We present the line-of-sight (LOS) velocities for 13 distant main sequence Milky Way halo stars with published proper motions. The proper motions were measured using long baseline (5-7 years) multi-epoch HST/ACS photometry, and the LOS velocities were extracted from deep (5-6 hour integrations) Keck II/DEIMOS spectra. We estimate the parameters of the velocity ellipsoid of the stellar halo using a Markov chain Monte Carlo ensembler sampler method. The velocity second moments in the directions of the Galactic $(l,b,$ LOS) coordinate system are $\langle v^2_l \rangle^{1/2} = 138^{+43}_{-26}$ km/s, $\langle v^2_b \rangle^{1/2} = 88^{+28}_{-17}$ km/s, and $\langle v^2_{\rm{LOS}} \rangle^{1/2} = 91^{+27}_{-14}$ km/s. We use these ellipsoid parameters to constrain the velocity anisotropy of the stellar halo. Ours is the first measurement of the anisotropy parameter $β$ using 3D kinematics outside of the solar neighborhood. We find $β=-0.3^{+0.4}_{-0.9}$, consistent with isotropy and lower than solar neighborhood $β$ measurements by 2$σ$ ($β_{SN} \sim 0.5-0.7$). We identify two stars in our sample that are likely members of the known TriAnd substructure, and excluding these objects from our sample increases our estimate of the anisotropy to $β=0.1^{+0.4}_{-1.0}$, which is still lower than solar neighborhood measurements by $1σ$. The potential decrease in $β$ with Galactocentric radius is inconsistent with theoretical predictions, though consistent with recent observational studies, and may indicate the presence of large, shell-type structure (or structures) at $r \sim 25$ kpc. The methods described in this paper will be applied to a much larger sample of stars with 3D kinematics observed through the ongoing HALO7D program.

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arXiv2016

An HST Proper-Motion Study of the Large-scale Jet of 3C273

The radio galaxy 3C 273 hosts one of the nearest and best-studied powerful quasar jets. Having been imaged repeatedly by the Hubble Space Telescope (HST) over the past twenty years, it was chosen for an HST program to measure proper motions in the kiloparsec-scale resolved jets of nearby radio-loud active galaxies. The jet in 3C 273 is highly relativistic on sub-parsec scales, with apparent proper motions up to 15$c$ observed by VLBI (Lister et al., 2013). In contrast, we find that the kpc-scale knots are compatible with being stationary, with a mean speed of $-$0.2$\pm$0.5$c$ over the whole jet. Assuming the knots are packets of moving plasma, an upper limit of 1c implies a bulk Lorentz factor $Γ<$2.9. This suggests that the jet has either decelerated significantly by the time it reaches the kpc scale, or that the knots in the jet are standing shock features. The second scenario is incompatible with the inverse Compton off the Cosmic Microwave Background (IC/CMB) model for the X-ray emission of these knots, which requires the knots to be in motion, but IC/CMB is also disfavored in the first scenario due to energetic considerations, in agreement with the recent finding of Meyer & Georganopoulos (2014) which ruled out the IC/CMB model for the X-ray emission of 3C 273 via gamma-ray upper limits.

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arXiv2015

Hubble Space Telescope Proper Motions along the Sagittarius Stream: I. Observations and Results for Stars in Four Fields

We present a multi-epoch Hubble Space Telescope (HST) study of stellar proper motions (PMs) for four fields spanning 200 degrees along the Sagittarius (Sgr) stream: one trailing arm field, one field near the Sgr dwarf spheroidal tidal radius, and two leading arm fields. We determine absolute PMs of dozens of individual stars per field, using established techniques that use distant background galaxies as stationary reference frame. Stream stars are identified based on combined color-magnitude diagram and PM information. The results are broadly consistent with the few existing PM measurements for the Sgr galaxy and the trailing arm. However, our new results provide the highest PM accuracy for the stream to date, the first PM measurements for the leading arm, and the first PM measurements for individual stream stars; we also serendipitously determine the PM of the globular cluster NGC~6652. In the trailing-arm field, the individual PMs allow us to kinematically separate trailing-arm stars from leading-arm stars that are 360 degrees further ahead in their orbit. Also, in three of our fields we find indications that two distinct kinematical components may exist within the same arm and wrap of the stream. Qualitative comparison of the HST data to the predictions of the Law & Majewski and Penarrubia et al. N-body models show that the PM measurements closely follow the predicted trend with Sgr longitude. This provides a successful consistency check on the PM measurements, as well as on these N-body approaches (which were not tailored to fit any PM data).

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arXiv2015

A Hubble Astrometry Initiative: Laying the Foundation for the Next-Generation Proper-Motion Survey of the Local Group

High-precision astrometry throughout the Local Group is a unique capability of the Hubble Space Telescope (HST), with potential for transformative science, including constraining the nature of dark matter, probing the epoch of reionization, and understanding key physics of galaxy evolution. While Gaia will provide unparalleled astrometric precision for bright stars in the inner halo of the Milky Way, HST is the only current mission capable of measuring accurate proper motions for systems at greater distances (> 80 kpc), which represents the vast majority of galaxies in the Local Group. The next generation of proper-motion measurements will require long time baselines, spanning many years to decades and possibly multiple telescopes, combining HST with the James Webb Space Telescope (JWST) or the Wide-Field Infrared Survey Telescope (WFIRST). However, the current HST allocation process is not conducive to such multi-cycle/multi-mission science, which will bear fruit primarily over many years. We propose an HST astrometry initiative to enable long-time-baseline, multi-mission science, which we suggest could be used to provide comprehensive kinematic measurements of all dwarf galaxies and high surface-density stellar streams in the Local Group with HST's Advanced Camera for Surveys (ACS) or Wide Field Camera 3 (WFC3). Such an initiative not only would produce forefront scientific results within the next 5 years of HST's life, but also would serve as a critical anchor point for future missions to obtain unprecedented astrometric accuracy, ensuring that HST leaves a unique and lasting legacy for decades to come.

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arXiv2014

Reddening, Distance, and Stellar Content of the Young Open Cluster Westerlund 2

We present deep $UBVI_C$ photometric data of the young open cluster Westerlund 2. An abnormal reddening law of $R_{V,cl}=4.14\pm0.08$ was found for the highly reddened early-type members ($E(B-V)\geq 1.45$), whereas a fairly normal reddening law of $R_{V,fg}=3.33\pm0.03$ was confirmed for the foreground early-type stars ($E(B-V)_{fg}<1.05$). The distance modulus was determined from zero-age main-sequence (ZAMS) fitting to the reddening-corrected colour-magnitude diagram of the early-type members to be $V_0-M_V=13.9\pm0.14$ (random error) $_{-0.1}^{+0.4}$ (the upper limit of systematic error) mag ($d = 6.0 \pm 0.4 _{-0.3}^{+1.2}$ kpc). To obtain the initial mass function, pre-main-sequence (PMS) stars were selected by identifying the optical counterparts of Chandra X-ray sources and mid-infrared emission stars from the Spitzer GLIMPSE source catalog. The initial mass function shows a shallow slope of $Γ=-1.1 \pm 0.1$ down to $\log m = 0.7$. The total mass of Westerlund 2 is estimated to be at least 7,400 $M_{\odot}$. The age of Westerlund 2 from the main-sequence turn-on and PMS stars is estimated to be $\lesssim$ 1.5 Myr. We confirmed the existence of a clump of PMS stars located $\sim1$ arcmin north of the core of Westerlund 2, but we could not find any clear evidence for an age difference between the core and the northern clump.

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arXiv2014

Investigating Nearby Star-Forming Galaxies in the Ultraviolet with HST/COS Spectroscopy. I: Spectral Analysis and Interstellar Abundance Determinations

This is the first in a series of three papers describing a project with the Cosmic Origins Spectrograph on the Hubble Space Telescope to measure abundances of the neutral interstellar medium (ISM) in a sample of 9 nearby star-forming galaxies. The goal is to assess the (in)homogeneities of the multiphase ISM in galaxies where the bulk of metals can be hidden in the neutral phase, yet the metallicity is inferred from the ionized gas in the HII regions. The sample, spanning a wide range in physical properties, is to date the best suited to investigate the metallicity behavior of the neutral gas at redshift z=0. ISM absorption lines were detected against the far-ultraviolet spectra of the brightest star-forming region(s) within each galaxy. Here we report on the observations, data reduction, and analysis of these spectra. Column densities were measured by a multi-component line-profile fitting technique, and neutral-gas abundances were obtained for a wide range of elements. Several caveats were considered including line saturation, ionization corrections, and dust depletion. Ionization effects were quantified with `ad-hoc' CLOUDY models reproducing the complex photoionization structure of the ionized and neutral gas surrounding the UV-bright sources. An `average spectrum of a redshift z=0 star-forming galaxy' was obtained from the average column densities of unsaturated profiles of neutral-gas species. This template can be used as a powerful tool for studies of the neutral ISM at both low and high redshift.

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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.

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arXiv2013

Optical Proper Motion Measurements of the M87 Jet: New Results from the Hubble Space Telescope

We report new results from an HST archival program to study proper motions in the optical jet of the nearby radio galaxy M87. Using over 13 years of archival imaging, we reach accuracies below 0.1c in measuring the apparent velocities of individual knots in the jet. We confirm previous findings of speeds up to 4.5c in the inner 6" of the jet, and report new speeds for optical components in the outer part of the jet. We find evidence of significant motion transverse to the jet axis on the order of 0.6c in the inner jet features, and superluminal velocities parallel and transverse to the jet in the outer knot components, with an apparent ordering of velocity vectors possibly consistent with a helical jet pattern. Previous results suggested a global deceleration over the length of the jet in the form of decreasing maximum speeds of knot components from HST-1 outward, but our results suggest that superluminal speeds persist out to knot C, with large differentials in very nearby features all along the jet. We find significant apparent accelerations in directions parallel and transverse to the jet axis, along with evidence for stationary features in knots D, E, and I. These results are expected to place important constraints on detailed models of kpc-scale relativistic jets.

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arXiv2013

The Space Motion of Leo I: Hubble Space Telescope Proper Motion and Implied Orbit

We present the first absolute proper motion measurement of Leo I, based on two epochs of HST ACS/WFC images separated by ~5 years. The average shift of Leo I stars with respect to ~100 background galaxies implies a proper motion of (mu_W, mu_N) = (0.1140 +/- 0.0295, -0.1256 +/- 0.0293) mas/yr. The implied Galactocentric velocity vector, corrected for the reflex motion of the Sun, has radial and tangential components V_rad = 167.9 +/- 2.8 km/s and V_tan = 101.0 +/- 34.4 km/s, respectively. We study the detailed orbital history of Leo I by solving its equations of motion backward in time for a range of plausible mass models for the Milky Way and its surrounding galaxies. Leo I entered the Milky Way virial radius 2.33 +/- 0.21 Gyr ago, most likely on its first infall. It had a pericentric approach 1.05 +/- 0.09 Gyr ago at a Galactocentric distance of 91 +/- 36 kpc. We associate these time scales with characteristic time scales in Leo I's star formation history, which shows an enhanced star formation activity ~2 Gyr ago and quenching ~1 Gyr ago. There is no indication from our calculations that other galaxies have significantly influenced Leo I's orbit, although there is a small probability that it may have interacted with either Ursa Minor or Leo II within the last ~1 Gyr. For most plausible Milky Way masses, the observed velocity implies that Leo I is bound to the Milky Way. However, it may not be appropriate to include it in models of the Milky Way satellite population that assume dynamical equilibrium, given its recent infall. Solution of the complete (non-radial) timing equations for the Leo I orbit implies a Milky Way mass M_MW,vir = 3.15 (-1.36, +1.58) x 10^12 Msun, with the large uncertainty dominated by cosmic scatter. In a companion paper, we compare the new observations to the properties of Leo I subhalo analogs extracted from cosmological simulations.

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arXiv2013

The Space Motion of Leo I: The Mass of the Milky Way's Dark Matter Halo

We combine our Hubble Space Telescope measurement of the proper motion of the Leo I dwarf spheroidal galaxy (presented in a companion paper) with the highest resolution numerical simulations of Galaxy-size dark matter halos in existence to constrain the mass of the Milky Way's dark matter halo (M_MW). Despite Leo I's large Galacto-centric space velocity (200 km/s) and distance (261 kpc), we show that it is extremely unlikely to be unbound if Galactic satellites are associated with dark matter substructure, as 99.9% of subhalos in the simulations are bound to their host. The observed position and velocity of Leo I strongly disfavor a low mass Milky Way: if we assume that Leo I is the least bound of the Milky Way's classical satellites, then we find that M_MW > 10^{12} M_sun at 95% confidence for a variety of Bayesian priors on M_MW. In lower mass halos, it is vanishingly rare to find subhalos at 261 kpc moving as fast as Leo I. Should an additional classical satellite be found to be less bound than Leo I, this lower limit on M_MW would increase by 30%. Imposing a mass weighted LCDM prior, we find a median Milky Way virial mass of M_MW=1.6 x 10^{12} M_sun, with a 90% confidence interval of [1.0-2.4] x 10^{12} M_sun. We also confirm a strong correlation between subhalo infall time and orbital energy in the simulations and show that proper motions can aid significantly in interpreting the infall times and orbital histories of satellites.

astro-ph.CO↗

arXiv2013

Wide-Field Multiband Photometry of Globular Cluster Systems in the Fornax Galaxy Cluster

We present wide-field multiband photometry of globular cluster (GC) systems in NGC 1399, NGC 1404, and NGC 1387 located at the central region of the Fornax galaxy cluster. Observation was carried out through U, B, V, and I bands, which marks one of the widest and deepest U-band studies on extragalactic GC systems. The present U-band photometry enables us to significantly reduce the contamination by a factor of two for faint sources (Vo~23.5). The main results based on some 2000 GC candidates around NGC 1399, NGC 1404, and NGC 1387 are as follows: (1) the GC system in each galaxy exhibits bimodal color distributions in all colors examined, but the shape of color histograms varies systematically depending on colors; (2) NGC 1399 shows that the mean colors of both blue and red GCs become bluer with increasing galactocentric radius; (3) NGC 1399 shows overabundance of GCs in the directions of NGC 1404 and NGC 1387, indicating their ongoing interactions; and (4) NGC 1399 also exhibits a ~0.5' offset between the center of the inner GC distribution and the galaxy's optical center, suggesting that NGC 1399 is not yet dynamically relaxed and may be undergoing merger events.

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arXiv2012

The M31 Velocity Vector. I. Hubble Space Telescope Proper Motion Measurements

We present the first proper motion measurements for the galaxy M31. We obtained new V-band imaging data with the HST ACS/WFC and WFC3/UVIS of a spheroid field near the minor axis, an outer disk field along the major axis, and a field on the Giant Southern Stream. The data provide 5-7 year time baselines with respect to pre-existing deep first-epoch observations. We measure the positions of thousands of M31 stars and hundreds of compact background galaxies in each field. High accuracy and robustness is achieved by building and fitting a unique template for each individual object. The average proper motion for each field is obtained from the average motion of the M31 stars between the epochs with respect to the background galaxies. For the three fields, the observed proper motions (mu_W,mu_N) are (-0.0458, -0.0376), (-0.0533, -0.0104), and (-0.0179,-0.0357) mas/yr, respectively. The ability to average over large numbers of objects and over the three fields yields a final accuracy of 0.012 mas/yr. The robustness of the proper-motion measurements and uncertainties are supported by the fact that data from different instruments, taken at different times and with different telescope orientations, as well as measurements of different fields, all yield statistically consistent results. Papers II and III explore the implications for our understanding of the history, future, and mass of the Local Group. (Abridged)

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arXiv2012

The M31 Velocity Vector. II. Radial Orbit Towards the Milky Way and Implied Local Group Mass

We determine the velocity vector of M31 with respect to the Milky Way and use this to constrain the mass of the Local Group, based on HST proper-motion measurements presented in Paper I. We construct N-body models for M31 to correct the measurements for the contributions from stellar motions internal to M31. We also estimate the center-of-mass motion independently, using the kinematics of satellite galaxies of M31 and the Local Group. All estimates are mutually consistent, and imply a weighted average M31 heliocentric transverse velocity of (v_W,v_N) = (-125.2+/-30.8, -73.8+/-28.4) km/s. We correct for the reflex motion of the Sun using the most recent insights into the solar motion within the Milky Way. This implies a radial velocity of M31 with respect to the Milky Way of V_rad = -109.3+/-4.4 km/s, and a tangential velocity V_tan = 17.0 km/s (<34.3 km/s at 1-sigma confidence). Hence, the velocity vector of M31 is statistically consistent with a radial (head-on collision) orbit towards the Milky Way. We revise prior estimates for the Local Group timing mass, including corrections for cosmic bias and scatter. Bayesian combination with other mass estimates yields M_LG = M_MW(vir) + M_M31(vir) = (3.17 +/- 0.57) x 10^12 solar masses. The velocity and mass results imply at 95% confidence that M33 is bound to M31, consistent with expectation from observed tidal deformations. (Abridged)

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arXiv2012

The M31 Velocity Vector. III. Future Milky Way-M31-M33 Orbital Evolution, Merging, and Fate of the Sun

We study the future orbital evolution and merging of the MW-M31-M33 system, using a combination of collisionless N-body simulations and semi-analytic orbit integrations. Monte-Carlo simulations are used to explore the consequences of varying the initial phase-space and mass parameters within their observational uncertainties. The observed M31 transverse velocity implies that the MW and M31 will merge t = 5.86 (+1.61-0.72) Gyr from now, after a first pericenter at t = 3.87 (+0.42-0.32) Gyr. M31 may (probability p=41%) make a direct hit with the MW (defined here as a first-pericenter distance less than 25 kpc). Most likely, the MW and M31 will merge first, with M33 settling onto an orbit around them. Alternatively, M33 may make a direct hit with the MW first (p=9%), or M33 may get ejected from the Local Group (p=7%). The MW-M31 merger remnant will resemble an elliptical galaxy. The Sun will most likely (p=85%) end up at larger radius from the center of the MW-M31 merger remnant than its current distance from the MW center, possibly further than 50 kpc (p=10%). The Sun may (p=20%) at some time in the next 10 Gyr find itself moving through M33 (within 10 kpc), but while dynamically still bound to the MW-M31 merger remnant. The arrival and possible collision of M31 (and possibly M33) with the MW is the next major cosmic event affecting the environment of our Sun and solar system that can be predicted with some certainty. (Abridged)

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arXiv2011

A Comprehensive GALEX Ultraviolet Catalog of Star Clusters in M31 and a Study of the Young Clusters

We present a comprehensive catalog of 700 confirmed star clusters in the field of M31 compiled from three major existing catalogs. We detect 418 and 257 star clusters in Galaxy Evolution Explorer (GALEX) near-ultraviolet (NUV) and far-ultraviolet (FUV) imaging, respectively. Our final catalog includes photometry of star clusters in up to 16 passbands ranging from FUV to NIR as well as ancillary information such as reddening, metallicity, and radial velocities. In particular, this is the most extensive and updated catalog of UV integrated photometry for M31 star clusters. Ages and masses of star clusters are derived by fitting the multi-band photometry with model spectral energy distribution (SED); UV photometry enables more accurate age estimation of young clusters. Our catalog includes 182 young clusters with ages less than 1 Gyr. Our estimated ages and masses of young clusters are in good agreement with previously determined values in the literature. The mean age and mass of young clusters are about 300 Myr and 10^4 M_sun, respectively. We found that the compiled [Fe/H] values of young clusters included in our catalog are systematically lower (by more than 1 dex) than those from recent high-quality spectroscopic data and our SED fitting result. We confirm that most of the young clusters kinematics show systematic rotation around the minor axis and association with the thin disk of M31. The young clusters distribution exhibits a distinct peak in the M31 disk around 10 - 12 kpc from the center and follow a spatial distributions similar to other tracers of disk structure such as OB stars, UV star-forming regions, and dust. Some young clusters also show concentration around the ring splitting regions found in the southern part of the M31 disk and most of them have systematically younger (< 100 Myr) ages.

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arXiv2010

Multi-Element Abundance Measurements from Medium-Resolution Spectra. IV. Alpha Element Distributions in Milky Way Dwarf Satellite Galaxies

We derive the star formation histories of eight dwarf spheroidal (dSph) Milky Way satellite galaxies from their alpha element abundance patterns. Nearly 3000 stars from our previously published catalog (Paper II) comprise our data set. The average [alpha/Fe] ratios for all dSphs follow roughly the same path with increasing [Fe/H]. We do not observe the predicted knees in the [alpha/Fe] vs. [Fe/H] diagram, corresponding to the metallicity at which Type Ia supernovae begin to explode. Instead, we find that Type Ia supernova ejecta contribute to the abundances of all but the most metal-poor ([Fe/H] < -2.5) stars. We have also developed a chemical evolution model that tracks the star formation rate, Types II and Ia supernova explosions, and supernova feedback. Without metal enhancement in the supernova blowout, massive amounts of gas loss define the history of all dSphs except Fornax, the most luminous in our sample. All six of the best-fit model parameters correlate with dSph luminosity but not with velocity dispersion, half-light radius, or Galactocentric distance.

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