arXiv subjects

R. A. Benjamin

Publications and source records attributed to R. A. Benjamin.

At least 19 recordsLinked to original sources

arXiv2024

Kinetic tomography of the Galactic plane within 1.25 kiloparsecs from the Sun. The interstellar flows revealed by HI and CO line emission and 3D dust

We present a reconstruction of the line-of-sight motions of the local interstellar medium (ISM) based on the combination of a model of the three-dimensional dust density distribution within 1.25 kpc from the Sun and the HI and CO line emission within Galactic latitudes $|b| < 5^{\circ}$. We used the histogram of oriented gradient (HOG) method, a computer vision technique for evaluating the morphological correlation between images, to match the plane-of-the-sky dust distribution across distances with the atomic and molecular line emission. We identified a significant correlation between the 3D dust model and the line emission. We employed this correlation to assign line-of-sight velocities to the dust across density channels and produce a face-on map of the local ISM radial motions with respect to the local standard of rest (LSR). We find that most of the material in the 3D dust model follows the large-scale pattern of Galactic rotation; however, we also report local departures from the rotation pattern with standard deviations of 10.8 and 6.6 km/s for the HI and CO line emission, respectively. The mean kinetic energy densities corresponding to these streaming motions are around 0.11 and 0.04 eV/cm$^{3}$ from either gas tracer. Assuming homogeneity and isotropy in the velocity field, these values are within a factor of a few of the total kinetic energy density. These kinetic energy values are roughly comparable to other energy densities, thus confirming the near-equipartition in the local ISM. Yet, we identify energy and momentum overdensities of around a factor of ten concentrated in local density structures. Although we do not find evidence of the local spiral arm's impact on these energy overdensities, their distribution suggests the influence of large-scale effects that, in addition to supernova feedback, shape the energy distribution and dynamics in the solar neighborhood.

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arXiv2024

The VISTA Variables in the Vía Láctea eXtended (VVVX) ESO public survey: Completion of the observations and legacy

The ESO public survey VISTA Variables in the Vía Láctea (VVV) surveyed the inner Galactic bulge and the adjacent southern Galactic disk from $2009-2015$. Upon its conclusion, the complementary VVV eXtended (VVVX) survey has expanded both the temporal as well as spatial coverage of the original VVV area, widening it from $562$ to $1700$ sq. deg., as well as providing additional epochs in $JHK_{\rm s}$ filters from $2016-2023$. With the completion of VVVX observations during the first semester of 2023, we present here the observing strategy, a description of data quality and access, and the legacy of VVVX. VVVX took $\sim 2000$ hours, covering about 4% of the sky in the bulge and southern disk. VVVX covered most of the gaps left between the VVV and the VISTA Hemisphere Survey (VHS) areas and extended the VVV time baseline in the obscured regions affected by high extinction and hence hidden from optical observations. VVVX provides a deep $JHK_{\rm s}$ catalogue of $\gtrsim 1.5\times10^9$ point sources, as well as a $K_{\rm s}$ band catalogue of $\sim 10^7$ variable sources. Within the existing VVV area, we produced a $5D$ map of the surveyed region by combining positions, distances, and proper motions of well-understood distance indicators such as red clump stars, RR Lyrae, and Cepheid variables. In March 2023 we successfully finished the VVVX survey observations that started in 2016, an accomplishment for ESO Paranal Observatory upon 4200 hours of observations for VVV+VVVX. The VVV+VVVX catalogues complement those from the Gaia mission at low Galactic latitudes and provide spectroscopic targets for the forthcoming ESO high-multiplex spectrographs MOONS and 4MOST.

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arXiv2021

A high pitch angle structure in the Sagittarius Arm

Context: In spiral galaxies, star formation tends to trace features of the spiral pattern, including arms, spurs, feathers, and branches. However, in our own Milky Way, it has been challenging to connect individual star-forming regions to their larger Galactic environment owing to our perspective from within the disk. One feature in nearly all modern models of the Milky Way is the Sagittarius Arm, located inward of the Sun with a pitch angle of ~12 deg. Aims: We map the 3D locations and velocities of star-forming regions in a segment of the Sagittarius Arm using young stellar objects (YSOs) from the Spitzer/IRAC Candidate YSO (SPICY) catalog to compare their distribution to models of the arm. Methods: Distances and velocities for these objects are derived from Gaia EDR3 astrometry and molecular line surveys. We infer parallaxes and proper motions for spatially clustered groups of YSOs and estimate their radial velocities from the velocities of spatially associated molecular clouds. Results: We identify 25 star-forming regions in the Galactic longitude range l~4.0-18.5 deg arranged in a narrow, ~1 kpc long linear structure with a high pitch angle of $ψ= 56$ deg and a high aspect ratio of ~7:1. This structure includes massive star-forming regions such as M8, M16, M17, and M20. The motions in the structure are remarkably coherent, with velocities in the direction of Galactic rotation of $240\pm3$ km/s (slightly higher than average) and slight drifts toward the Galactic center (-4.3 km/s) and in the negative Z direction (-2.9 km/s). The rotational shear experienced by the structure is 4.6 km/s/kpc. Conclusions: The observed 56 deg pitch angle is remarkably high for a segment of the Sagittarius Arm. We discuss possible interpretations of this feature as a substructure within the lower pitch angle Sagittarius Arm, as a spur, or as an isolated structure.

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arXiv2021

Assessing the stellar population and the environment of an HII region on the far side of the Galaxy

We have investigated the stellar and interstellar content of the distant star formation region IRAS 17591-2228 (WISE HII region GAL 007.47+0.06). It is associated to a water maser, whose parallax distance is d=20.4^{+2.8} {-2.2} kpc, supported by independent measurements of proper motion and radial velocity. It is projected in the same direction as an extremely red (J-Ks ~ 6 mag) group of stars, and a shell of mid-infrared emission. We qualify the group of stars as a cluster candidate, VVV CL177. Its radius spans between 0.45' and 1' and contains at least two young stellar objects with an extreme extinction near Av ~ 40 mag. Yet more analysis will be required to determine is it is a real single cluster associated with the water maser. The 13CO emissions at the radial velocity of the maser corresponds to the mid-infrared emission.

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arXiv2020

Discovery of a Thirty-Degree Long Ultraviolet Arc in Ursa Major

Our view of the interstellar medium of the Milky Way and the universe beyond is affected by the structure of the local environment in the Solar neighborhood. Here, we present the discovery of a thirty-degree long arc of ultraviolet emission with a thickness of only a few arcminutes: the Ursa Major Arc. It consists of several arclets seen in the near- and far-ultraviolet bands of the GALEX satellite. A two-degree section of the arc was first detected in the Hα optical spectral line in 1997; additional sections were seen in the optical by the team of amateur astronomers included in this work. This direction of the sky is known for very low hydrogen column density and dust extinction; many deep fields for extra-galactic and cosmological investigations lie in this direction. Diffuse ultraviolet and optical interstellar emission are often attributed to scattering of light by interstellar dust. The lack of correlation between the Ursa Major Arc and thermal dust emission observed with the Planck satellite, however, suggests that other emission mechanisms must be at play. We discuss the origin of the Ursa Major Arc as the result of an interstellar shock in the Solar neighborhood.

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arXiv2014

Chasing the Identification of ASCA Galactic Objects (ChIcAGO) - An X-ray Survey of Unidentified Sources in the Galactic Plane I: Source Sample and Initial Results

We present the Chasing the Identification of ASCA Galactic Objects (ChIcAGO) survey, which is designed to identify the unknown X-ray sources discovered during the ASCA Galactic Plane Survey (AGPS). Little is known about most of the AGPS sources, especially those that emit primarily in hard X-rays (2-10 keV) within the F_x ~ 10^-13 to 10^-11 erg cm^-2 s^-1 X-ray flux range. In ChIcAGO, the subarcsecond localization capabilities of Chandra have been combined with a detailed multi-wavelength follow-up program, with the ultimate goal of classifying the >100 unidentified sources in the AGPS. Overall to date, 93 unidentified AGPS sources have been observed with Chandra as part of the ChIcAGO survey. A total of 253 X-ray point sources have been detected in these Chandra observations within 3' of the original ASCA positions. We have identified infrared and optical counterparts to the majority of these sources, using both new observations and catalogs from existing Galactic plane surveys. X-ray and infrared population statistics for the X-ray point sources detected in the Chandra observations reveal that the primary populations of Galactic plane X-ray sources that emit in the F_x ~ 10^-13 to 10^-11 erg cm^-2 s^-1 flux range are active stellar coronae, massive stars with strong stellar winds that are possibly in colliding-wind binaries, X-ray binaries, and magnetars. There is also a fifth population that is still unidentified but, based on its X-ray and infrared properties, likely comprise partly of Galactic sources and partly of active galactic nuclei.

astro-ph.HE↗

arXiv2012

Continuum Halos in Nearby Galaxies -- an EVLA Survey (CHANG-ES) -- I: Introduction to the Survey

We introduce a new survey to map the radio continuum halos of a sample of 35 edge-on spiral galaxies at 1.5 GHz and 6 GHz in all polarization products. The survey is exploiting the new wide bandwidth capabilities of the Karl G. Jansky Very Large Array (i.e. the Expanded Very Large Array, or EVLA) in a variety of array configurations (B, C, and D) in order to compile the most comprehensive data set yet obtained for the study of radio halo properties. This is the first survey of radio halos to include all polarization products. In this first paper, we outline the scientific motivation of the survey, the specific science goals, and the expected improvements in noise levels and spatial coverage from the survey. Our goals include investigating the physical conditions and origin of halos, characterizing cosmic ray transport and wind speed, measuring Faraday rotation and mapping the magnetic field, probing the in-disk and extraplanar far-infrared - radio continuum relation, and reconciling non-thermal radio emission with high-energy gamma-ray models. The sample size allows us to search for correlations between radio halos and other properties, including environment, star formation rate, and the presence of AGNs. In a companion paper (Paper II) we outline the data reduction steps and present the first results of the survey for the galaxy, NGC 4631.

astro-ph.CO↗

arXiv2012

Continuum Halos in Nearby Galaxies -- an EVLA Survey (CHANG-ES) -- II: First Results on NGC 4631

We present the first results from the CHANG-ES survey, a new survey of 35 edge-on galaxies to search for both in-disk as well as extra-planar radio continuum emission. The motivation and science case for the survey are presented in a companion paper (Paper I). In this paper (Paper II), we outline the observations and data reduction steps required for wide-band calibration and mapping of EVLA data, including polarization, based on C-array test observations of NGC 4631. With modest on-source observing times (30 minutes at 1.5 GHz and 75 minutes at 6 GHz for the test data) we have achieved best rms noise levels of 22 and 3.5 $μ$Jy beam$^{-1}$ at 1.5 GHz and 6 GHz, respectively. New disk-halo features have been detected, among them two at 1.5 GHz that appear as loops in projection. We present the first 1.5 GHz spectral index map of NGC 4631 to be formed from a single wide-band observation in a single array configuration. This map represents tangent slopes to the intensities within the band centered at 1.5 GHz, rather than fits across widely separated frequencies as has been done in the past and is also the highest spatial resolution spectral index map yet presented for this galaxy. The average spectral index in the disk is $\barα_{1.5 GHz}\,=\,-0.84\,\pm\,0.05$ indicating that the emission is largely non-thermal, but a small global thermal contribution is sufficient to explain a positive curvature term in the spectral index over the band. Two specific star forming regions have spectral indices that are consistent with thermal emission. Polarization results (uncorrected for internal Faraday rotation) are consistent with previous observations and also reveal some new features. On broad scales, we find strong support for the notion that magnetic fields constrain the X-ray emitting hot gas.

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arXiv2010

Identification of a Population of X-ray Emitting Massive Stars in the Galactic Plane

We present X-ray, infrared, optical and radio observations of four previously unidentified Galactic plane X-ray sources, AX J163252-4746, AX J184738-0156, AX J144701-5919 and AX J144547-5931. Detection of each source with the Chandra X-ray Observatory has provided sub-arcsecond localizations, which we use to identify bright infrared counterparts to all four objects. Infrared and optical spectroscopy of these counterparts demonstrate that all four X-ray sources are extremely massive stars, with spectral classifications Ofpe/WN9 (AX J163252-4746), WN7 (AX J184738-0156 = WR121a), WN7-8h (AX J144701-5919) and OIf+ (AX J144547-5931). AX J163252-4746 and AX J184738-0156 are both luminous, hard, X-ray emitters with strong Fe XXV emission lines in their X-ray spectra at ~6.7 keV. The multi-wavelength properties of AX J163252-4746 and AX J184738-0156 are not consistent with isolated massive stars or accretion onto a compact companion; we conclude that their X-ray emission is most likely generated in a colliding-wind binary system. For both AX J144701-5919 and AX J144547-5931, the X-ray emission is an order of magnitude less luminous and with a softer spectrum. These properties are consistent with a colliding-wind binary interpretation for these two sources also, but other mechanisms for the generation of X-rays cannot be excluded. There are many other as yet unidentified X-ray sources in the Galactic plane, with X-ray properties similar to those seen for AX J163252-4746, AX J184738-0156, AX J144701-5919 and AX J144547-5931. This may indicate a substantial population of X-ray-emitting massive stars and colliding-wind binaries in the Milky Way.

astro-ph.HE↗

arXiv2006

The Frequency of Mid-Infrared Excess Sources in Galactic Surveys

We have identified 230 Tycho-2 Spectral Catalog stars that exhibit 8 micron mid-infrared extraphotospheric excesses in the MidCourse Space Experiment (MSX) and Spitzer Space Telescope Galactic Legacy MidPlane Survey Extraordinaire (GLIMPSE) surveys. Of these, 183 are either OB stars earlier than B8 in which the excess plausibly arises from a thermal bremsstrahlung component or evolved stars in which the excess may be explained by an atmospheric dust component. The remaining 47 stars have spectral classifications B8 or later and appear to be main sequence or late pre-main-sequence objects harboring circumstellar disks. Six of the 47 stars exhibit multiple signatures characteristic of pre-main-sequence circumstellar disks, including emission lines, near-infrared K-band excesses, and X-ray emission. Approximately one-third of the remaining 41 sources have emission lines suggesting relative youth. Of the 25 GLIMPSE stars with SST data at >24 microns, 20 also show an excess at 24 microns. Three additional objects have 24 micron upper limits consistent with possible excesses, and two objects have photospheric measurements at 24 microns. Six MSX sources had a measurement at wavelengths >8 microns. We modeled the excesses in 26 stars having two or more measurements in excess of the expected photospheres as single-component blackbodies. We determine probable disk temperatures and fractional infrared luminosities in the range 191 < T < 787 and 3.9x10^-4 < L_IR/L_* < 2.7x10^-1. We estimate a lower limit on the fraction of Tycho-2 Spectral Catalog main-sequence stars having mid-IR, but not near-IR, excesses to be 1.0+-0.3%.

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arXiv2006

Discovery of Two Galaxies Deeply Embedded in the Great Attractor Wall

We report on the discovery of two spiral galaxies located behind the southern Milky Way, within the least explored region of the Great Attractor. They lie at 317, -0.5 deg galactic, where obscuration from Milky Way stars and dust exceeds 13 to 15 mag of visual extinction. The galaxies were the most prominent of a set identified using mid-infrared images of the low-latitude (|b| < 1 deg) Spitzer Legacy program GLIMPSE. Follow-up HI radio observations reveal that both galaxies have redshifts that place them squarely in the Norma Wall of galaxies, which appears to extend diagonally across the Galactic Plane from Norma in the south to Centaurus & Vela in the north. We report on the near-infrared, mid-infrared and radio properties of these newly discovered galaxies, and discuss their context in the larger view of the Great Attractor. The work presented here demonstrates that mid-infrared surveys open up a new window to study galaxies in the Zone of Avoidance.

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arXiv2006

MHD Turbulent Mixing Layers: Equilibrium Cooling Models

We present models of turbulent mixing at the boundaries between hot (T~10^{6-7} K) and warm material (T~10^4 K) in the interstellar medium, using a three-dimensional magnetohydrodynamical code, with radiative cooling. The source of turbulence in our simulations is a Kelvin-Helmholtz instability, produced by shear between the two media. We found, that because the growth rate of the large scale modes in the instability is rather slow, it takes a significant amount of time (~1 Myr) for turbulence to produce effective mixing. We find that the total column densities of the highly ionized species (C IV, N V, and O VI) per interface (assuming ionization equilibrium) are similar to previous steady-state non-equilibrium ionization models, but grow slowly from log N ~10^{11} to a few 10^{12} cm^{-2} as the interface evolves. However, the column density ratios can differ significantly from previous estimates, with an order of magnitude variation in N(C IV)/N(O VI) as the mixing develops.

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arXiv2005

Imaging Fabry-Perot Spectroscopy of NGC 5775: Kinematics of the Diffuse Ionized Gas Halo

We present imaging Fabry-Perot observations of Halpha emission in the nearly edge-on spiral galaxy NGC 5775. We have derived a rotation curve and a radial density profile along the major axis by examining position-velocity (PV) diagrams from the Fabry-Perot data cube as well as a CO 2-1 data cube from the literature. PV diagrams constructed parallel to the major axis are used to examine changes in azimuthal velocity as a function of height above the midplane. The results of this analysis reveal the presence of a vertical gradient in azimuthal velocity. The magnitude of this gradient is approximately 1 km/s/arcsec, or about 8 km/s/kpc, though a higher value of the gradient may be appropriate in localized regions of the halo. The evidence for an azimuthal velocity gradient is much stronger for the approaching half of the galaxy, although earlier slit spectra are consistent with a gradient on both sides. There is evidence for an outward radial redistribution of gas in the halo. The form of the rotation curve may also change with height, but this is not certain. We compare these results with those of an entirely ballistic model of a disk-halo flow. The model predicts a vertical gradient in azimuthal velocity which is shallower than the observed gradient, indicating that an additional mechanism is required to further slow the rotation speeds in the halo.

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arXiv2005

SparsePak Observations of Diffuse Ionized Gas Halo Kinematics in NGC 891

We present WIYN SparsePak observations of the diffuse ionized gas (DIG) halo of NGC 891. Preliminary results of an analysis of the halo velocity field reveal a clear gradient of the azimuthal velocity with z which agrees with results for the neutral gas. The magnitude of the gradient has been determined, using two independent methods, to be approximately 15 km/s/kpc.

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arXiv2005

Identification of Main Sequence Stars with Mid-Infrared Excesses Using GLIMPSE: Beta-Pictoris Analogs?

Spitzer IRAC 3.6-8 micron photometry obtained as part of the GLIMPSE survey has revealed mid-infrared excesses for 33 field stars with known spectral types in a 1.2 sq. degree field centered on the southern Galactic HII region RCW49. These stars comprise a subset of 184 stars with known spectral classification, most of which were pre-selected to have unusually red IR colors. We propose that the mid-IR excesses are caused by circumstellar dust disks that are either very late remnants of stellar formation or debris disks generated by planet formation. Of these 33 stars, 29 appear to be main-sequence stars based on optical spectral classifications. Five of the 29 main-sequence stars are O or B stars with excesses that can be plausibly explained by thermal bremsstrahlung emission, and four are post main-sequence stars. The lone O star is an O4V((f)) at a spectrophotometric distance of 3233+ 540- 535 pc and may be the earliest member of the Westerlund 2 cluster. Of the remaining 24 main-sequence stars, 18 have SEDs that are consistent with hot dusty debris disks, a possible signature of planet formation. Modeling the excesses as blackbodies demonstrates that the blackbody components have fractional bolometric disk-to-star luminosity ratios, LIR/L*, ranging from 10^-3 to 10^-2 with temperatures ranging from 220 to 820 K. The inferred temperatures are more consistent with asteroid belts rather than the cooler temperatures expected for Kuiper belts. Mid-IR excesses are found in all spectral types from late B to early K.

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arXiv2004

The Wavelength Dependence of Interstellar Extinction from 1.25 to 8.0 microns Using GLIMPSE Data

We determine and tabulate A(lambda)/A(K), the wavelength dependence of interstellar extinction, in the Galactic plane for 1.25um<lambda<8.0um along two lines of sight: l~42 and l~284. The first is a relatively quiescent and unremarkable region; the second contains the giant HII region RCW 49 as well as a ``field'' region unrelated to the cluster and nebulosity. Areas near these Galactic longitudes were imaged at J, H, and K bands by 2MASS and at 3--8um by Spitzer for the GLIMPSE Legacy program. We measure the mean values of the color excess ratios (A(lambda)-A(K))/(A(J)-A(K)) directly from the color distributions of observed stars. The extinction ratio between two of the filters, e.g. A(J)/A(K), is required to calculate A(lambda)/A(K) from those measured ratios. We use the apparent JHK magnitudes of giant stars along our two sightlines, and fit the reddening as a function of magnitude (distance) to determine A(J)/kpc, A(K)/kpc, and A(J)/A(K). Our values of A(lambda)/A(K) show a flattening across the 3--8um wavelength range, roughly consistent with the Lutz 1996 extinction measurements derived for the sightline toward the Galactic center.

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arXiv2004

Discovery of a New Low-Latitude Milky Way Globular Cluster using GLIMPSE

Spitzer Space Telescope imaging as part of the Galactic Legacy Mid-Plane Survey Extraordinaire (GLIMPSE) reveals a previously unidentified low-latitude rich star cluster near l=31.3 degrees, b=-0.1 degrees. Near-infrared JHK' photometry from the Wyoming Infrared Observatory indicates an extinction of A_V ~ 15+/-3 mag for cluster members. Analysis of 13CO features along the same sightline suggests a probable kinematic distance of 3.1 -- 5.2 kpc. The new cluster has an angular diameter of ~1-2 pc, a total magnitude m_{K_0}=2.1 corrected for extinction, and a luminosity of M_K ~ -10.3 at 3.1 kpc. In contrast to young massive Galactic clusters with ages <100 Myr, the new cluster has no significant radio emission. Comparison to theoretical K-band luminosity functions indicates an age of at least several Gyr and a mass of at least 10^5 solar masses. Unlike known old open clusters, this new cluster lies in the inner Galaxy at R_{GC} ~ 6.1 kpc. We designate this object ``GLIMPSE-C01'' and classify it as a Milky Way globular cluster passing through the Galactic disk. We also identify a region of star formation and fan-shaped outflows from young stellar objects in the same field as the cluster. The cluster's passage through the Galactic molecular layer may have triggered this star formation activity.

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arXiv2004

Kinematics of the Ionized Halo of NGC 5775

Studies of the kinematics of gaseous halos in some spiral galaxies have shown that the rotational velocity of the gas decreases with height above the midplane (z). This vertical velocity gradient has been observed in both the neutral and ionized components of halos. We present imaging Fabry-Perot Halpha observations of the edge-on galaxy NGC 5775. These observations allow us to study the variation in the rotation curve (and, to some extent, the radial profile) of the ionized gas component as a function of z. We have used an iterative technique to infer the major axis rotation curve from the ionized gas observations as well as CO data. A preliminary analysis of position-velocity (PV) diagrams parallel to the major axis implies the presence of a vertical velocity gradient. The magnitude of this gradient is tentatively found to be ~2 km/s/arcsec, consistent with the results of Rand (2000) and Tullmann et al. (2000).

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