Bart P. Wakker

University of Wisconsin–Madison

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Blair D. Savage

University of Wisconsin–Madison

Andrew J. Fox

Space Telescope Science Institute

P. Richter

University of Potsdam

Kenneth R. Sembach

Space Telescope Science Institute

Nicolas Lehner

University of Notre Dame

E. B. Jenkins

Princeton University

J. Christopher Howk

University of Notre Dame

Felix J. Lockman

National Radio Astronomy Observatory

Hugo van Woerden

University of the Highlands and Islands

Joss Bland‐Hawthorn

University of Sydney

Cooperative Institutions

Space Telescope Science Institute

Astronomy and Space

Johns Hopkins University

Commonwealth Scientific and Industrial Research Organisation

University of Wisconsin–Madison

Australia Telescope National Facility

ARC Centre of Excellence for All-sky Astrophysics

University of Western Australia

International Centre for Radio Astronomy Research

Australian National University

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The Metallicity of High-Velocity Cloud Complex C

HST Proposal (1996)

Bart P. Wakker

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COSMIC ORIGINS SPECTROGRAPH DETECTION OF Ne VIII TRACING WARM-HOT GAS TOWARD PKS 0405–123

The Astrophysical Journal (2011)

A. Narayanan Blair D. Savage Bart P. Wakker Charles Danforth Yangsen Yao

We report on the detection of Ne viii in the Hubble Space Telescope/Cosmic Origins Spectrograph (COS) spectrum of the intervening absorption system at z = 0.495096 toward PKS 0405−123 (zem = 0.5726). The high signal-to-noise COS spectrum also covers absorption from H i, C iii, O iii, O iv, and O vi associated with this multiphase system. The Ne viii is detected with high significance in both lines of the doublet, with integrated column densities of log Na(Ne viii 770) = 13.96 ± 0.06 and log Na(Ne viii 780) = 14.08 ± 0.07. We find the origin of Ne viii consistent with collisionally ionized gas at T ∼ 5 × 105 K with a large baryonic column density of N(H) ∼ 1019–1020 cm−2. The metallicity in the Ne viii gas phase is estimated to be [Ne/H] ∼−0.6 ± 0.3 dex. The intermediate ions such as C iii, O iii, O iv, and H i are consistent with photoionization in lower ionization gas at T ∼ 104 K. The O v and O vi in this absorber can have contributions from both the photoionized and collisionally ionized gas phases. The absorber is at |Δv| = 180 km s−1 systematic velocity and ρ = 110h−170 kpc projected separation from an MR = −19.6 galaxy of extended morphology. The collisionally ionized gas at T ∼ 5 × 105 K detected in Ne viii and O vi points to an origin in multiphase gas embedded in the hot halo of the galaxy, or in a nearby WHIM structure. The high-sensitivity UV spectroscopy afforded by COS has opened up new opportunities for discovering large reservoirs of missing baryons in the low-z universe through the detection of Ne viii systems.

10.1088/0004-637x/730/1/15

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Spectroscopic Observations of the Fermi Bubbles

arXiv (Cornell University) (2019)

Andrew J. Fox Trisha Ashley Robert A. Benjamin Joss Bland‐Hawthorn Rongmon Bordoloi

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.

10.48550/arxiv.1903.04833

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The Metallicities of Five Small High-Velocity Clouds

arXiv (Cornell University) (2023)

Trisha Ashley Andrew J. Fox Felix J. Lockman Bart P. Wakker P. Richter

High-velocity clouds (HVCs) are multi-phase gas structures whose velocities (|v_LSR|>100 km/s) are too high to be explained by Galactic disk rotation. While large HVCs are well characterized, compact and small HVCs (with HI angular sizes of a few degrees) are poorly understood. Possible origins for such small clouds include Milky Way halo gas or fragments of the Magellanic System, but neither their origin nor their connection to the Milky Way halo has been confirmed. We use new Hubble Space Telescope/Cosmic Origins Spectrograph UV spectra and Green Bank Telescope HI spectra to measure the metallicities of five small HVCs in the southern Galactic sky projected near the Magellanic System. We build a set of distance-dependent Cloudy photoionization models for each cloud and calculate their ionization-corrected metallicities. All five small HVCs have oxygen metallicities <0.17 Z_sun, indicating they do not originate in the disk of the Milky Way. Two of the five have metallicities of 0.16-0.17 Z_sun, similar to the Magellanic Stream, suggesting these clouds are fragments of the Magellanic System. The remaining three clouds have much lower metallicities of 0.02-0.04 Z_sun. While the origin of these low-metallicity clouds is unclear, they could be gaseous mini-halos or gas stripped from dwarf galaxies by ram pressure or tidal interactions. These results suggest that small HVCs do not all reside in the inner Milky Way halo or the Magellanic System, but instead can trace more distant structures.

Small Magellanic Cloud

10.48550/arxiv.2311.09377

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Nearby galaxy filaments and the Lya forest: confronting simulations and the UV background with observations

arXiv (Cornell University) (2015)

Bart P. Wakker Audra K. Hernandez David French Tae‐Sun Kim Benjamin D. Oppenheimer

Simulations of the formation of large-scale structure predict that dark matter, low density highly ionized gas, and galaxies form 10 40 Mpc scale filaments. These structure are easily recognized in the distribution of galaxies, but have not been directly observed in the distribution of the gas. We use Ly-alpha absorption lines in the spectra of 24 AGN to present a new way to probe these filaments. We use a new catalogue of nearby (cz<10,000 km/s) galaxies, complete down to a luminosity of about 0.05 L* for the region of space analyzed here. Using HST spectra of 24 AGN we sample the gas associated with a 30x5 Mpc galaxy filament at cz~3500 km/s. All of our sightlines pass outside the virial radius of any known filament galaxy. Within 500 kpc of the filament axis the detection rate is ~80%, while no detections are seen more than 2.1 Mpc from the filament. The width of the Lya lines correlates with filament impact parameter and the four BLAs in our sample all occur within 400 kpc of the filament axis, indicating increased temperature and/or turbulence. Comparing to simulations, we find that the recent Haardt & Madau (2012) extragalactic ionizing background predicts a factor 3-5 too few ionizing photons. Using a more intense radiation field matches the hydrogen density profile within 2.1 Mpc of the filament axis, but the simulations still overpredict the detection rate between 2.1 and 5 Mpc from the axis.

10.48550/arxiv.1504.02539

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Exploring Hot Gas in the Galactic Halo and High Velocity Clouds

arXiv (Cornell University) (2004)

Kenneth R. Sembach Bart P. Wakker Blair D. Savage P. Richter

In the five years since its launch, the Far Ultraviolet Spectroscopic Explorer has given the astronomical community a marvelous glimpse into the realm of previously unexplored regions of the Milky Way and its surroundings. Absorption lines produced by gas along entire paths through the nearby universe have now been recorded in the spectra of distant QSOs and active galactic nuclei. Of the lines recorded, the O VI 1031.926, 1037.617 Angstrom doublet is the best tracer of highly ionized regions. Exciting discoveries include a definitive confirmation of the hot Galactic halo postulated by Lyman Spitzer nearly 50 years ago, the discovery of a hot, highly extended Galactic corona enveloping the Milky Way out to distances of several tens of kiloparsecs, and the discovery of an extensive network of highly ionized, high velocity clouds surrounding the Galaxy.

Galactic corona

QSOS

Galactic tide

10.48550/arxiv.astro-ph/0411211

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HIGH-VELOCITY CLOUDS IN THE NEARBY SPIRAL GALAXY M 83

The Astrophysical Journal (2009)

Eric D. Miller Joel N. Bregman Bart P. Wakker

We present deep HI 21-cm and optical observations of the face-on spiral galaxy M 83 obtained as part of a project to search for high-velocity clouds (HVCs) in nearby galaxies. Anomalous-velocity neutral gas is detected toward M 83, with 5.6x10^7 Msolar of HI contained in a disk rotating 40-50 km/s more slowly in projection than the bulk of the gas. We interpret this as a vertically extended thick disk of neutral material, containing 5.5% of the total HI within the central 8 kpc. Using an automated source detection algorithm to search for small-scale HI emission features, we find eight distinct, anomalous-velocity HI clouds with masses ranging from 7x10^5 to 1.5x10^7 Msolar and velocities differing by up to 200 km/s compared to the HI disk. Large on-disk structures are coincident with the optical spiral arms, while unresolved off-disk clouds contain no diffuse optical emission down to a limit of 27 r' mag per square arcsec. The diversity of the thick HI disk and larger clouds suggests the influence of multiple formation mechanisms, with a galactic fountain responsible for the slowly-rotating disk and on-disk discrete clouds, and tidal effects responsible for off-disk cloud production. The mass and kinetic energy of the HI clouds are consistent with the mass exchange rate predicted by the galactic fountain model. If the HVC population in M 83 is similar to that in our own Galaxy, then the Galactic HVCs must be distributed within a radius of less than 25 kpc.

10.1088/0004-637x/692/1/470

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VizieR Online Data Catalog: AGN data and absorption-line measurements (Richter+, 2016)

yCat (2016)

P. Richter Bart P. Wakker C. Fechner Peter Herenz Thor Tepper-García

Line (geometry)

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Circumgalactic Gas at Its Extreme: Tidal Gas Streams around the Whale Galaxy NGC 4631 Explored with HST/COS

The Astrophysical Journal (2018)

P. Richter B. Winkel Bart P. Wakker N. M. Pingel Andrew J. Fox

Abstract We present a detailed analysis of the absorption properties of one of the tidal gas streams around the “Whale” galaxy NGC 4631 in the direction of the quasar 2MASS J12421031+3214268. Our study is based on ultraviolet spectral data obtained with the Cosmic Origins Spectrograph (COS) on board the Hubble Space Telescope ( HST ) and 21cm-data from the HALOGAS project and the Green Bank Telescope (GBT). We detect strong H i Ly α absorption in the velocity range +550 to +800 km s −1 related to gas from a NGC 4631 tidal stream known as Spur 2. We measure a column density of log ( N (H i /cm −2 )) = 18.68 ± 0.15, indicating that the quasar sightline traces the outer boundary of Spur 2 as seen in the 21 cm data. Metal absorption in Spur 2 is detected in the lines of O i , C ii , Si ii , and Si iii in a complex absorption pattern that reflects the multiphase nature of the gas. We find that the average neutral gas fraction in Spur 2 toward 2MASS J12421031+3214268 is only 14%. This implies that ionized gas dominates the total mass of Spur 2, which then may comprise more than 10 9 M ☉ . No significant depletion of Si is observed, showing that Spur 2 does not contain significant amounts of dust. From the measured O i /H i column density ratio, we determine an α abundance in Spur 2 of solar ([ α /H] = −0.90±0.16), which is substantially lower than what is observed in the NGC 4631 disk. The low metallicity and low dust content suggest that Spur 2 represents metal-deficient gas stripped off a gas-rich satellite galaxy during a recent encounter with NGC 4631.

10.3847/1538-4357/aae838

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The Kinematics of the Milky Way's Biconical Nuclear Wind

AAS (2016)

Andrew J. Fox Rongmon Bordoloi E. B. Jenkins Blair D. Savage Svea Hernández

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