arXiv subjects

Felix J. Lockman

Publications and source records attributed to Felix J. Lockman.

At least 37 records · Page 2Linked to original sources

arXiv2019

Observation of Acceleration of HI Clouds Within the Fermi Bubbles

The ~200 HI clouds observed to be entrained in the Fermi Bubble wind show a trend of increasing maximum |VLSR| with Galactic latitude. We analyze previous observations and present new data from the Green Bank Telescope that rule out systematic effects as the source of this phenomenon. Instead, it is likely evidence for acceleration of the clouds. The data suggest that clouds in the lower 2 kpc of the Fermi Bubbles, within the Bubble boundaries established from X-ray studies, have an outflow velocity that rises from ~150 - 200 km/s close to the Galactic Center and reaches ~330 km/s at a distance of 2.5 - 3.5 kpc. These parameters are also consistent with the kinematics of UV absorption lines from highly ionized species observed against two targets behind the Fermi Bubbles at $b = -6.6^{\circ}$, and $b = +11.2^{\circ}$. The implied neutral cloud lifetime is 4 - 10 Myr.

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arXiv2019

The Magellanic Stream as a Probe of Astrophysics

Extending for over 200 degrees across the sky, the Magellanic Stream together with its Leading Arm is the most spectacular example of a gaseous stream in the local Universe. The Stream is an interwoven tail of filaments trailing the Magellanic Clouds as they orbit the Milky Way. Thought to be created by tidal forces, ram pressure, and halo interactions, the Stream is a benchmark for dynamical models of the Magellanic System, a case study for gas accretion and dwarf-galaxy accretion onto galaxies, a probe of the outer halo, and the bearer of more gas mass than all other Galactic high velocity clouds combined. If it survives to reach the Galactic disk, it may maintain or even elevate the Galactic star-formation rate. In this white paper, we emphasize the Stream's importance for many areas of Galactic astronomy, summarize key unanswered questions, and identify future observations and simulations needed to resolve them. We stress the importance of ultraviolet, optical, and radio spectroscopy, and the need for computational models that capture full particle and radiation treatments within an MHD environment.

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arXiv2019

Spectroscopic Observations of the Fermi Bubbles

Two giant plasma lobes, known as the Fermi Bubbles, extend 10 kpc above and below the Galactic Center. Since their discovery in X-rays in 2003 (and in gamma-rays in 2010), the Bubbles have been recognized as a new morphological feature of our Galaxy and a striking example of energetic feedback from the nuclear region. They remain the subject of intense research and their origin via AGN activity or nuclear star formation is still debated. While imaging at gamma-ray, X-ray, microwave, and radio wavelengths has revealed their morphology and energetics, spectroscopy at radio and UV wavelengths has recently been used to study the kinematics and chemical abundances of outflowing gas clouds embedded in the Bubbles (the nuclear wind). Here we identify the scientific themes that have emerged from the spectroscopic studies, determine key open questions, and describe further observations needed in the next ten years to characterize the basic physical conditions in the nuclear wind and its impact on the rest of the Galaxy. Nuclear winds are ubiquitous in galaxies, and the Galactic Center represents the best opportunity to study the constitution and structure of a nuclear wind in close detail.

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arXiv2018

Constraining the Magnetic Field of the Smith High Velocity Cloud using Faraday rotation

The Smith Cloud is a high velocity cloud (HVC) with an orbit suggesting it has made at least one passage through the Milky Way disk. A magnetic field found around this cloud has been thought to provide extra stability as it passes through the Galactic halo. We use the Karl G. Jansky Very Large Array to measure Faraday rotation measures (RMs) towards 1105 extragalactic background point sources behind and next to the Smith Cloud to constrain the detailed geometry and strength of its magnetic field. The RM pattern across the cloud gives a detailed morphology of the magnetic field structure which indicates a field draped over the ionized gas and compressed at the head of the cloud. We constrain the peak line-of-sight magnetic field strength to > +5 uG and create a model of the magnetic field to demonstrate that a draped configuration can qualitatively explain the morphology of the observed RMs.

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arXiv2018

Kinematics of the Atomic ISM in M33 on 80 pc scales

We present new L-band ($1\mbox{--}2$ GHz) observations of the nearby spiral galaxy M33 with 80 pc resolution obtained with the Karl G. Jansky Very Large Array. The HI observations, combined with HI measurements from the Green Bank Telescope, improve the spectral resolution and sensitivity ($2.8$ K rms noise in a $0.2$ km s$^{-1}$ channel) compared to previous observations. We find individual profiles are usually non-Gaussian, harbouring line wings, multiple components, and asymmetries. Given this spectral complexity, we quantify the motions in the atomic ISM through moment analysis of the spectra and fits to aligned, stacked profiles. The measured value of the HI line width depends strongly on the method used, with the velocity stacked profiles aligned to the peak velocity giving the minimum value of $σ= 7$ km s$^{-1}$ and all other methods giving higher values ($σ\sim10$ km s$^{-1}$). All measurements of the line width show a shallow radial trend, with $σ$ decreasing by $\sim2$ km s$^{-1}$ from $R_{\rm gal}=0$ to $R_{\rm gal}=8$ kpc. We consider a number of energy sources that might maintain the line width against turbulent dissipation, but no single source is adequate. We find excess emission relative to a Gaussian in the stacked profile line wings, ranging from 9% to 26% depending on how the spectra are aligned. By splitting the line wings into symmetric and asymmetric components, we find that the lagging rotational disk accounts for one-third of the line wing flux. We also find emission far from the rotation axis of the galaxy in multiple discrete HI clouds, including a filament with a projected length of $\sim8$ kpc.

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arXiv2018

Probing the Southern Fermi Bubble in Ultraviolet Absorption using Distant AGNs

The Fermi Bubbles are two giant gamma-ray emitting lobes extending 55$^{\circ}$ above and below the Galactic Center. While the Northern Bubble has been extensively studied in ultraviolet (UV) absorption, little is known about the gas kinematics of the southern Bubble. We use UV absorption-line spectra from the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope to probe the southern Fermi Bubble using a sample of 17 background AGN projected behind or near the Bubble. We measure the incidence of high-velocity clouds (HVC), finding that four out of six sightlines passing through the Bubble show HVC absorption, versus six out of eleven passing outside. We find strong evidence that the maximum absolute LSR velocity of the HVC components decreases as a function of galactic latitude within the Bubble, for both blueshifted and redshifted components, as expected for a decelerating outflow. We explore whether the column-density ratios SiIV/SiIII, SiIV/SiII and SiIII/SiII correlate with the absolute galactic latitude within the Bubble. These results demonstrate the use of UV absorption-line spectroscopy to characterize the kinematics and ionization conditions of embedded clouds in the Galactic Center outflow.

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arXiv2018

Blowing in the Milky Way wind: neutral hydrogen clouds tracing the Galactic nuclear outflow

We present the results of a new sensitive survey of neutral hydrogen above and below the Galactic Center with the Green Bank Telescope. The observations extend up to Galactic latitude | b | < 10 deg with an effective angular resolution of 9.5' and an average rms brightness temperature noise of 40 mK in a 1 km/s channel. The survey reveals the existence of a population of anomalous high-velocity clouds extending up to heights of about 1.5 kpc from the Galactic Plane and showing no signature of Galactic rotation. These clouds have local standard of rest velocities | Vlsr | < 360 km/s and, assuming a Galactic Center origin, they have sizes of a few tens of parsecs and neutral hydrogen masses spanning $10-10^5 \, M_\odot$. Accounting for selection effects, the cloud population is symmetric in longitude, latitude, and Vlsr. We model the cloud kinematics in terms of an outflow expanding from the Galactic Center and find the population consistent with being material moving with radial velocity Vw ~ 330 km/s distributed throughout a bi-cone with opening angle $α>140$ deg. This simple model implies an outflow luminosity $Lw > 3 \times 10^{40}$ erg/s over the past 10 Myr, consistent with star formation feedback in the inner region of the Milky Way, with a cold gas mass-loss rate $\lesssim 0.1 \, M_\odot$/yr. These clouds may represent the cold gas component accelerated in the nuclear wind driven by our Galaxy, although some of the derived properties challenge current theoretical models of the entrainment process.

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arXiv2018

Chemical Abundances in the Leading Arm of the Magellanic Stream

The Leading Arm (LA) of the Magellanic Stream is a vast debris field of H I clouds connecting the Milky Way and the Magellanic Clouds. It represents an example of active gas accretion onto the Galaxy. Previously only one chemical abundance measurement had been made in the LA. Here we present chemical abundance measurements using Hubble Space Telescope/Cosmic Origins Spectrograph Green Bank Telescope spectra of four sightlines passing through the LA, and three nearby sightlines that may trace outer fragments of the LA. We find low oxygen abundances, ranging from 4.0(+4.0,-2.0) percent solar to 12.6(+6.2,-4.1) percent solar, in the confirmed LA directions, with the lowest values found in the region known as LA III, farthest from the LMC. These abundances are substantially lower than the single previous measurement, S/H=35+/-7 percent solar (Lu et al. 1998), but are in agreement with those reported in the SMC filament of the trailing Stream, supporting a common origin in the SMC (not the LMC) for the majority of the LA and the trailing Stream. This provides important constraints for models of the formation of the Magellanic System. Finally, the HVCs in two of the three nearby sightlines show H I columns, kinematics, and oxygen abundances consistent with LA membership. This suggests that the LA is larger than traditionally thought, extending at least 20 degrees further to the Galactic northwest.

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arXiv2017

Probing the Outflowing Multiphase Gas ~1 kpc Below the Galactic Center

Comparison of ISM absorption in the UV spectrum of LS 4825, a B1 Ib-II star d=21+/-5 kpc from the Sun toward l = 1.67 deg and b = -6.63 deg, with ISM absorption toward an aligned foreground star at d < 7.0+/-1.7 kpc, allows us to isolate and study gas associated with the Milky Way nuclear wind. Spectra from the Space Telescope Imaging Spectrograph (STIS) show low ion absorption out to d < 7 kpc ( e.g., O I, C II, Mg II, Si II, Fe II, S II) only between 0 and 40 km/s, while absorption at d > 7 kpc, ~1 kpc below the galactic plane, is complex and spans -290 to + 94 km/ s. The intermediate and high ions Si III, C IV, Si IV and N V show extremely strong absorption with multiple components from -283 to 107 km/ s implying that the ISM ~1 kpc below the galactic center has a substantial reservoir of plasma, and more gas containing C IV and N V than in the Carina OB1 association at z = 0 kpc. Abundances and physical conditions are presented for many absorption components. The high ion absorption traces cooling transition temperature plasma probably driven by the outflowing hot gas, while the extraordinary large thermal pressure, p/k ~ 10^5 cm^-3 K^-1 in an absorption component at -114 km/s probably arises from the ram pressure of the outflowing hot gas. The observations are consistent with a flow whose ionization structure in the high ions can be understood through a combination of non-equilibrium radiative cooling and turbulent mixing.

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arXiv2017

Project AMIGA: A Minimal Covering Factor for Optically Thick Circumgalactic Gas Around the Andromeda Galaxy

We present a deep search for HI 21-cm emission from the gaseous halo of Messier 31 as part of Project AMIGA, a large program Hubble Space Telescope program to study the circumgalactic medium of the Andromeda galaxy. Our observations with the Robert C. Byrd Green Bank Telesope target sight lines to 48 background AGNs, more than half of which have been observed in the ultraviolet with the Cosmic Origins Spectrograph, with impact parameters $25 \lesssim ρ\lesssim 330$ kpc ($0.1 \lesssim ρ/ R_{\rm vir} \lesssim 1.1$). We do not detect any 21-cm emission toward these AGNs to limits of $N({\rm HI}) \approx 4 \times10^{17}$ cm$^{-2}$ ($5σ$, per 2 kpc diameter beam). This column density corresponds to an optical depth of $\sim2.5$ at the Lyman limit, thus our observations overlap with absorption line studies of Lyman limit systems at higher redshift. Our non-detections place a limit on the covering factor of such optically-thick gas around M31 to $f_c < 0.051$ (at 90\% confidence) for $ρ\leq R_{\rm vir}$. While individual clouds have previously been found in the region between M31 and M33, the covering factor of strongly optically-thick gas is quite small. Our upper limits on the covering factor are consistent with expectations from recent cosmological "zoom" simulations. Recent COS-Halos ultraviolet measurements of \HI\ absorption about an ensemble of galaxies at $z \approx 0.2$ show significantly higher covering factors within $ρ\lesssim 0.5 R_{\rm vir}$ at the same $N({\rm H I})$, although the metal ion-to-H I ratios appear to be consistent with those seen in M31.

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arXiv2016

Nuclear Outflow of the Milky Way: Studying the Kinematics and Spatial Extent of the Northern Fermi Bubble

We report new observations from a systematic, spectroscopic, ultraviolet absorption-line survey that maps the spatial and kinematic properties of the high-velocity gas in the Galactic Center region. We examine the hypothesis that this gas traces the biconical nuclear outflow. We use ultraviolet spectra of 47 background QSOs and halo stars projected inside and outside the northern Fermi Bubble from the Hubble Space Telescope to study the incidence of high velocity absorption around it. We use five lines of sight inside the northern Fermi Bubble to constrain the velocity and column densities of outflowing gas traced by O I, Al II, C II, C IV, Si II, Si III, Si IV and other species. All five lines of sight inside the northern Fermi Bubble exhibit blueshifted high velocity absorption components, whereas only 9 out of the 42 lines of sight outside the northern Fermi Bubble exhibit blueshifted high velocity absorption components. The observed outflow velocity profile decreases with Galactic latitude and radial distance (R) from the Galactic Center. The observed blueshifted velocities change from $v_{GSR}$=-265 km/s at R~2.3 kpc to $v_{GSR}$=-91 km/s at R~6.5 kpc. We derive the metallicity of the entrained gas along the 1H1613-097 sightline, which passes through the center of the northern Fermi Bubble, finding [O/H] $\gtrsim -0.54 \pm 0.15$. A simple kinematic model tuned to match the observed absorption component velocities along the five lines of sight inside the Bubble, constrains the outflow velocities to ~1000$-$1300 km/s, and the age of the outflow to be ~ 6$-$9 Myr. We estimate a minimum mass outflow rate for the nuclear outflow to be $\gtrsim$ 0.2 $\rm{ M_{\odot}\; yr^{-1}}$. Combining the age and mass outflow rates, we determine a minimum mass of total UV absorbing cool gas entrained in the Fermi Bubbles to be $\gtrsim \rm{ 2 \times 10^{6} M_{\odot}}$.

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arXiv2016

Neutral Gas Accretion onto Nearby Galaxies

While there is no lack of evidence for the accretion of stellar systems onto nearby galaxies, direct evidence for the accretion of gas without stars is scarce. Here we consider an inventory of starless gas "clouds" in and around galaxies of the Local Group to discern their general properties and see how they might appear in distant systems. The conclusion is that accreting gas without stars is detected almost entirely within the circumgalactic medium of large galaxies and is rare otherwise. If our Local Group is any example, the best place to detect starless gas clouds is relatively close to galaxies.

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arXiv2016

DHIGLS: DRAO H I Intermediate Galactic Latitude Survey

Observations of Galactic H I gas for seven targeted regions at intermediate Galactic latitude are presented at 1' angular resolution using data from the DRAO Synthesis Telescope (ST) and the Green Bank Telescope (GBT). The DHIGLS data are the most extensive arcminute resolution measurements of the diffuse atomic interstellar medium beyond those in the Galactic plane. The acquisition, reduction, calibration, and mosaicking of the DRAO ST data and the cross calibration and incorporation of the short-spacing information from the GBT are described. The high quality of the resulting DHIGLS products enables a variety of new studies in directions of low Galactic column density. We analyze the angular power spectra of maps of the integrated H I emission (column density) from the data cubes for several distinct velocity ranges. Fitting power spectrum models based on a power law, but including the effects of the synthesized beam and noise at high spatial frequencies, we find exponents ranging from -2.5 to -3.0.. Power spectra of maps of the centroid velocity for these components give similar results. These exponents are interpreted as being representative of the 3D density and 3D velocity fields of the atomic gas, respectively. We find evidence for dramatic changes in the H I structures in channel maps over even small changes in velocity. This narrow line emission has counterparts in absorption spectra against bright background radio sources, quantifying that the gas is cold and dense and can be identified as the cold neutral medium phase. Fully reduced DHIGLS H I data cubes and other data products are available at www.cita.utoronto.ca/DHIGLS.

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arXiv2016

Tracing dense and diffuse neutral hydrogen in the halo of the Milky Way

We have combined observations of Galactic high-velocity HI from two surveys: a very sensitive survey from the Green Bank 140ft Telescope with limited sky coverage, and the less sensitive but complete Galactic All Sky Survey from the 64m Parkes Radio Telescope. The two surveys preferentially detect different forms of neutral gas due to their sensitivity. We adopt a machine learning approach to divide our data into two populations that separate across a range in column density: 1) a narrow line-width population typical of the majority of bright high velocity cloud components, and 2) a fainter, broad line-width population that aligns well with that of the population found in the Green Bank survey. We refer to these populations as dense and diffuse gas respectively, and find that diffuse gas is typically located at the edges and in the tails of high velocity clouds, surrounding dense components in the core. A fit to the average spectrum of each type of gas in the Galactic All Sky Survey data reveals the dense population to have a typical line width of ~20 km/s and brightness temperature of ~0.3 K, while the diffuse population has a typical line width of ~30 km/s and a brightness temperature of ~0.2 K. Our results confirm that most surveys of high velocity gas in the Milky Way halo are missing the majority of the ubiquitous diffuse gas, and that this gas is likely to contribute at least as much mass as the dense gas.

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arXiv2016

The National Science Foundation's AST Portfolio Review of 2012 is Not Relevant to the Green Bank Telescope of 2017: A White Paper

The National Science Foundation (NSF) Astronomy Division's Portfolio Review of 2012 is no longer relevant to the Green Bank Telescope (GBT) of 2017 for two principal reasons, one instrumental and the other astrophysical: 1) The GBT has begun significant operations in the 3mm band, giving it unrivaled capabilities for spectroscopy and continuum studies over 67-116 GHz. It is now an instrument that is unique worldwide and is a critical complement to ALMA for the U.S. scientific community. These capabilities had not been implemented at the time of the review. 2) The detection of gravitational radiation by LIGO in 2015 places the GBT's work on pulsar observations of nano-Hz gravitational radiation at the forefront of modern astrophysics. The Green Bank Telescope of 2017 is not the GBT that was reviewed by the Eisenstein-Miller committee in 2012, a review that was specific to the NSF Astronomy portfolio. The GBT serves a wide spectrum of science areas including physics, chemistry, and planetary studies as well as astronomy. Besides its well-documented intellectual merit, it thus has a significant broader impact. The GBT is making significance advances in our understanding of gravitational waves, the equation-of-state of nuclear matter, the mass of supermassive black holes, the value of H0, and the physics of star-formation, all key science goals for astronomy identified in a recent National Academy study "New Worlds, New Horizons: A Midterm Assessment". In the era of ALMA and LIGO, other countries have bolstered their mm-wave and cm-wave facilities; it is critical that U.S. scientists have ready access to a large filled aperture to remain at the forefront of research.

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arXiv2016

Tracing the Milky Way Nuclear Wind with 21cm Atomic Hydrogen Emission

There is evidence in 21cm HI emission for voids several kpc in size centered approximately on the Galactic centre, both above and below the Galactic plane. These appear to map the boundaries of the Galactic nuclear wind. An analysis of HI at the tangent points, where the distance to the gas can be estimated with reasonable accuracy, shows a sharp transition at Galactic radii $R\lesssim 2.4$ kpc from the extended neutral gas layer characteristic of much of the Galactic disk, to a thin Gaussian layer with FWHM $\sim 125$ pc. An anti-correlation between HI and $γ$-ray emission at latitudes $10^{\circ} \leq |b| \leq 20^{\circ}$ suggests that the boundary of the extended HI layer marks the walls of the Fermi Bubbles. With HI we are able to trace the edges of the voids from $|z| > 2$ kpc down to $z\approx0$, where they have a radius $\sim 2$ kpc. The extended HI layer likely results from star formation in the disk, which is limited largely to $R \gtrsim 3$ kpc, so the wind may be expanding into an area of relatively little HI. Because the HI kinematics can discriminate between gas in the Galactic center and foreground material, 21cm HI emission may be the best probe of the extent of the nuclear wind near the Galactic plane.

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arXiv2015

Sensitive 21cm Observations of Neutral Hydrogen in the Local Group near M31

Very sensitive 21cm HI measurements have been made at several locations around the Local Group galaxy M31 using the Green Bank Telescope (GBT) at an angular resolution of 9.1', with a 5$σ$ detection level of $\rm{N_{HI} = 3.9 \times 10^{17}~cm^{-2}}$ for a 30 $\rm{km~s^{-1}}$ line. Most of the HI in a 12 square degree area almost equidistant between M31 and M33 is contained in nine discrete clouds that have a typical size of a few kpc and HI mass of $10^5$ M$_{\odot}$. Their velocities in the Local Group Standard of Rest lie between -100 and +40 $\rm{km~s^{-1}}$, comparable to the systemic velocities of M31 and M33. The clouds appear to be isolated kinematically and spatially from each other. The total HI mass of all nine clouds is $1.4 \times 10^6$ M$_{\odot}$ for an adopted distance of 800 kpc with perhaps another $0.2 \times 10^6$ M$_{\odot}$ in smaller clouds or more diffuse emission. The HI mass of each cloud is typically three orders of magnitude less than the dynamical (virial) mass needed to bind the cloud gravitationally. Although they have the size and HI mass of dwarf galaxies, the clouds are unlikely to be part of the satellite system of the Local Group as they lack stars. To the north of M31, sensitive HI measurements on a coarse grid find emission that may be associated with an extension of the M31 high-velocity cloud population to projected distances of $\sim 100$ kpc. An extension of the M31 high-velocity cloud population at a similar distance to the south, toward M33, is not observed.

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arXiv2015

On the Metallicity and Origin of the Smith High-Velocity Cloud

The Smith Cloud is a gaseous high-velocity cloud (HVC) in an advanced state of accretion, only 2.9 kpc below the Galactic plane and due to impact the disk in 27 Myr. It is unique among HVCs in having a known distance (12.4+/-1.3 kpc) and a well-constrained 3D velocity (296 km/s), but its origin has long remained a mystery. Here we present the first absorption-line measurements of its metallicity, using HST/COS UV spectra of three AGN lying behind the Cloud together with Green Bank Telescope 21 cm spectra of the same directions. Using Voigt-profile fitting of the S II 1250, 1253, 1259 triplet together with ionization corrections derived from photoionization modeling, we derive the sulfur abundance in each direction; a weighted average of the three measurements gives [S/H]=-0.28+/-0.14, or 0.53+0.21-0.15 solar metallicity. The finding that the Smith Cloud is metal-enriched lends support to scenarios where it represents recycled Galactic material rather than the remnant of a dwarf galaxy or accreting intergalactic gas. The metallicity and trajectory of the Cloud are both indicative of an origin in the outer disk. However, its large mass and prograde kinematics remain to be fully explained. If the cloud has accreted cooling gas from the corona during its fountain trajectory, as predicted in recent theoretical work, its current mass would be higher than its launch mass, alleviating the mass concern.

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