arXiv subjects

N. Lehner

Publications and source records attributed to N. Lehner.

At least 19 recordsLinked to original sources

arXiv2025

Prospects for studying million-degree gas in the Milky Way halo using the forbidden optical [FeX] and [FeXIV] intersystem lines

The Milky Way is surrounded by large amounts of hot gas at temperatures T>10^6 K, which represents a major baryon reservoir. We here explore the prospects of studying the hot coronal gas in Milky Way halo by analyzing the highly forbidden optical coronal lines of [FeX] and [FeXIV] in absorption against bright extragalactic background sources. We use a semi-analytic model of the Milky Way's coronal gas distribution together wih HESTIA simulations of the Local Group and observational constraints to predict the expected FeX and FeXIV column densities as well as the line shapes and strengths. We predict column densities of log N(FeX)=15.40 and log N(FeXIV)=15.23 in the Milky Way's hot halo and estimate that a minimum S/N of 50,000 (25,000) is required to detect [FeX] l6374.5 ([FeXIV] l5302.9) absorption at a 3sigma level. Using archical optical data from an original sample of 739 high resolution AGN spectra from VLT/UVES and KECK/HIRES, we generate a stacked composite spectrum to measure an upper limit for the column densities of FeX and FeXIV in the Milky Way's coronal gas. No [FeX] and [FeXIV] is detected in our composite spectrum, which achieves a maximum S/N= 1,240 near 5300 A. We derive 3sigma upper column-density limits of log N(FeX)<16.27 and log N(FeXIV)<15.85, in line with the above-mentioned predictions. While [FeX] and [FexIX] absorption is too weak to be detected with current optical data, we outline how up-coming extragalactic spectral surveys with millions of medium- to high-resolution optical spectra will provide the necessary sensitivity and spectral resolution to measure velocity-resolved [FeX] and [FeXIV] absorption in the Milky Way's coronal gas (and beyond). This gives the prospect of opening a new window for studying the dominant baryonic mass component of the Milky Way in the form of hot coronal gas via optical spectroscopy.

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arXiv2022

Intermediate- and high-velocity clouds in the Milky Way II: evidence for a Galactic fountain with collimated outflows and diffuse inflows

We model the kinematics of the high- and intermediate- velocity clouds (HVCs and IVCs) observed in absorption towards a sample of 55 Galactic halo stars with accurate distance measurements. We employ a simple model of a thick disc whose main free parameters are the gas azimuthal, radial and vertical velocities ($v_ϕ$, $v_{\rm R}$ and $v_{\rm z}$), and apply it to the data by fully accounting for the distribution of the observed features in the distance-velocity space. We find that at least two separate components are required to reproduce the data. A scenario where the HVCs and the IVCs are treated as distinct populations provides only a partial description of the data, which suggests that a pure velocity-based separation may give a biased vision of the gas physics at the Milky Way's disc-halo interface. Instead, the data are best described by a combination of an inflow and an outflow components, both characterised by rotation with $v_ϕ$ comparable to that of the disc and $v_{\rm z}$ of 50-100 km/s. Features associated with the inflow appear to be diffused across the sky, while those associated with the outflow are mostly confined within a bi-cone pointing towards ($l\!=\!220^{\circ}$, $b\!=\!+40^{\circ}$) and ($l\!=\!40^{\circ}$, $b\!=\!-40^{\circ}$). Our findings indicate that the lower ($|z|\!\lesssim\!10$ kpc) Galactic halo is populated by a mixture of diffuse inflowing gas and collimated outflowing material, which are likely manifestations of a galaxy-wide gas cycle triggered by stellar feedback, that is, the galactic fountain.

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arXiv2022

Intermediate- and high-velocity clouds in the Milky Way I: covering factors and vertical heights

Intermediate- and high-velocity clouds (IVCs, HVCs) are a potential source of fuel for star formation in the Milky Way (MW), but their origins and fates depend sensitively on their distances. We search for IVC and HVC in HST high-resolution ultraviolet spectra of 55 halo stars at vertical heights $|z| \gtrsim 1$ kpc. We show that IVCs ($40 \leq |v_{\rm LSR}| <90$ km/s) have a high detection rate - the covering factor, $f_c$ - that is about constant ($f_c =0.90\pm 0.04$) from $z=1.5$ to $14$ kpc, implying IVCs are essentially confined to $|z|\lesssim 1.5$ kpc. For the HVCs ($90 \leq |v_{\rm LSR}| \lesssim 170$ km/s), we find $f_c$ increases from $f_c \simeq 0.14\pm 0.10$ at $|z|\lesssim 2-3$ kpc to $f_c =0.60\pm 0.15$ at $5\lesssim |z|\lesssim 14$ kpc, the latter value being similar to that found towards QSOs. In contrast, the covering factor of very high-velocity clouds (VHVCs, $|v_{\rm LSR}|\gtrsim 170$ km/s) is $f_c<4\%$ in the stellar sample compared to 20\% in a QSO sample, implying these clouds must be at $d\gtrsim 10-15$ kpc ($|z|\gtrsim 10$ kpc). Gas clouds with $|v_{\rm LSR}|>40$ km/s at $|b|\gtrsim 15^\circ$ have therefore $|v_{\rm LSR}|$ decreasing with decreasing $|z|$. Assuming each feature originates from a single cloud, we derive scale-heights of $1.0 \pm 0.3$ and $2.8 \pm 0.3$ kpc for the IVCs and HVCs, respectively. Our findings provide support to the "rain" and galactic fountain models. In the latter scenario, VHVCs may mostly serve as fuel for the MW halo. In view of their locations and high covering factors, IVCs and HVCs are good candidates to sustain star formation in the MW.

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arXiv2021

The CGM$^2$ Survey: Circumgalactic O VI from dwarf to massive star-forming galaxies

We combine 126 new galaxy-O VI absorber pairs from the CGM$^2$ survey with 123 pairs drawn from the literature to examine the simultaneous dependence of the column density of O VI absorbers ($N_{\rm O VI}$) on galaxy stellar mass, star formation rate, and impact parameter. The combined sample consists of 249 galaxy-O VI absorber pairs covering $z=0$-$0.6$, with host galaxy stellar masses $M^*=10^{7.8}$-$10^{11.2}$ $M_\odot$ and galaxy-absorber impact parameters $R_\perp=0$-$400$ proper kiloparsecs. In this work, we focus on the variation of $N_{\rm O VI}$ with galaxy mass and impact parameter among the star-forming galaxies in the sample. We find that the average $N_{\rm O VI}$ within one virial radius of a star-forming galaxy is greatest for star-forming galaxies with $M^*=10^{9.2}$-$10^{10}$ $M_\odot$. Star-forming galaxies with $M^*$ between $10^{8}$ and $10^{11.2}$ $M_\odot$ can explain most O VI systems with column densities greater than 10$^{13.5}$ cm$^{-2}$. 60% of the O VI mass associated with a star-forming galaxy is found within one virial radius and 35% is found between one and two virial radii. In general, we find that some departure from hydrostatic equilibrium in the CGM is necessary to reproduce the observed O VI amount, galaxy mass dependence, and extent. Our measurements serve as a test set for CGM models over a broad range of host galaxy masses.

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arXiv2019

ESA Voyage 2050 white paper: A complete census of the gas phases in and around galaxies, far-UV spectropolarimetry as a prime tool for understanding galaxy evolution and star formation

(abridged) The far-UV wavelength range (912-2000A) provides access to atomic and molecular transitions of many species the interstellar medium (ISM), circumgalactic medium (CGM), and intergalactic medium, within phases spanning a wide range of ionization, density, temperature, and molecular gas fraction. Far-UV space telescopes have enabled detailed studies of the ISM in the Milky Way thanks to absorption features appearing in the UV spectra of hot stars and yielding fundamental insights into the composition and physical characteristics of all phases of the ISM along with the processes that influence them. However, we have yet to design a spectrometer able to observe the full UV domain at resolving power R>10^5 with a signal-to-noise ratio SNR>500. Such a resolution is necessary to resolve lines from both the cold molecular hydrogen and the warm metal ions with a turbulent velocity of about 1 km s-1, and to differentiate distinct velocity components. Future UV spectroscopic studies of the Milky Way ISM must revolutionize our understanding of the ISM as a dynamical, unstable, and magnetized medium, and rise to the challenge brought forward by current theories. Another interesting prospect is to transpose the same level of details that has been reached for the Milky Way to the ISM in external galaxies, in particular in metal-poor galaxies, where the ISM chemical composition, physical conditions, and topology change dramatically, with significant consequences on the star-formation properties. Finally, we need to be able to perform statistical analyses of background quasar lines of sight intersecting the CGM of galaxies at various redshifts and to comprehend the role of gas exchanges and flows for galaxy evolution.

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arXiv2019

Completing the Hydrogen Census in the Circumgalactic Medium at z~0

Over the past decade, Lyman-alpha and metal line absorption observations have established the ubiquity of a gas-rich circumgalactic medium (CGM) around star-forming galaxies at z~0.2 potentially tracing half of the missing baryonic mass within galaxy halos. Unfortunately, these observations only provide a statistical measure of the gas in the CGM and do not constrain the spatial distribution and kinematics of the gas. Furthermore, we have limited sensitivity to Lyman-alpha at z~0 with existing instruments. As such, we remain ignorant of how this gas may flow from the CGM onto the disks of galaxies where it can fuel ongoing star-formation in the present day. Fortunately, 21-cm HI observations with radio telescopes can map HI emission providing both spatial and kinematic information for the CGM in galaxies at z=0. Observations with phased array feeds, radio cameras, on single-dish telescopes yield unmatched surface brightness sensitivity and survey speed. These observations can complete the census of HI in the CGM below N(HI)<10^17 cm^-2 and constrain how gas accretion is proceeding in the local universe, particularly when used in concert with UV absorption line data.

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arXiv2018

New constraints on the nature and origin of the Leading Arm of the Magellanic Stream

We present a new precision measurement of gas-phase abundances of S, O, N, Si, Fe, P, Al, Ca as well as molecular hydrogen (H_2) in the Leading Arm (region II, LAII) of the Magellanic Stream (MS) towards the Seyfert galaxy NGC 3783. The results are based on high-quality archival ultraviolet/optical/radio data from various different instruments (HST/STIS, FUSE, AAT, GBT, GB140ft, ATCA). Our study updates previous results from lower-resolution data and provides for the first time a self-consistent component model of the complex multi-phase absorber, delivering important constraints on the nature and origin of LAII. We derive a uniform, moderate alpha abundance in the two main absorber groups at +245 and +190 km s^-1 of alpha/H = 0.30 pm 0.05 solar, a low nitrogen abundance of N/H = 0.05 pm 0.01 solar, and a high dust content with substantial dust depletion values for Si, Fe, Al, and Ca. These alpha, N, and dust abundances in LAII are similar to those observed in the Small Magellanic Cloud (SMC). From the analysis of the H_2 absorption, we determine a high thermal pressure of P/k = 1680 K cm^-3 in LAII, in line with the idea that LAII is located in the inner Milky Way halo at a z-height of <20 kpc where it hydrodynamically interacts with the ambient hot coronal gas. Our study supports a scenario, in which LAII stems from the break-up of a metal- and dust-enriched progenitor cloud that was recently (200-500 Myr ago) stripped from the SMC.

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arXiv2017

Revealing the Ionization Properties of the Magellanic Stream using Optical Emission

The Magellanic Stream, a gaseous tail that trails behind the Magellanic Clouds, could replenish the Milky Way with a tremendous amount of gas if it reaches the Galactic disk before it evaporates into the halo. To determine how the Magellanic Stream's properties change along its length, we have conducted an observational study of the H-alpha emission, along with other optical warm ionized gas tracers, toward 39 sight lines. Using the Wisconsin H-alpha Mapper telescope, we detect H-alpha emission brighter than 30 - 50 mR in 26 of our 39 sight lines. This H-alpha emission extends more than 2-degree away from the HI emission. By comparing H-alpha and [OI] intensities, we find that regions with log NHI = 19.5 - 20.0 are 16 - 67% ionized. Most of the H-alpha intensities along the Magellanic Stream are much higher than expected if the primary ionization source is photoionization from Magellanic Clouds, the Milky Way, and the extragalactic background. We find that the additional contribution from self ionization through a "shock cascade" that results as the Stream plows through the halo might be sufficient to reproduce the underlying level of H-alpha emission along the Stream. In the sparsely sampled region below the South Galactic Pole, there exists a subset of sight lines with uncharacteristically bright emission, which suggest that gas is being ionized further by an additional source that could be a linked to energetic processes associated with the Galactic center.

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arXiv2017

An HST/COS legacy survey of high-velocity ultraviolet absorption in the Milky Way's circumgalactic medium and the Local Group

To characterize the absorption properties of this circumgalactic medium (CGM) and its relation to the LG we present the so-far largest survey of metal absorption in Galactic high-velocity clouds (HVCs) using archival ultraviolet (UV) spectra of extragalactic background sources. The UV data are obtained with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope (HST) and are supplemented by 21 cm radio observations of neutral hydrogen. Along 270 sightlines we measure metal absorption in the lines of SiII, SiIII, CII, and CIV and associated HI 21 cm emission in HVCs in the velocity range |v_LSR|=100-500 km s^-1. With this unprecedented large HVC sample we were able to improve the statistics on HVC covering fractions, ionization conditions, small-scale structure, CGM mass, and inflow rate. For the first time, we determine robustly the angular two point correlation function of the high-velocity absorbers, systematically analyze antipodal sightlines on the celestial sphere, and compare the absorption characteristics with that of Damped Lyman alpha absorbers (DLAs) and constrained cosmological simulations of the LG. Our study demonstrates that the Milky Way CGM contains sufficient gaseous material to maintain the Galactic star-formation rate at its current level. We show that the CGM is composed of discrete gaseous structures that exhibit a large-scale kinematics together with small-scale variations in physical conditions. The Magellanic Stream clearly dominates both the cross section and mass flow of high-velocity gas in the Milky Way's CGM. The possible presence of high-velocity LG gas underlines the important role of the local cosmological environment in the large-scale gas-circulation processes in and around the Milky Way (abridged).

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arXiv2015

The First Data Release of the KODIAQ Survey

We present and make publicly available the first data release (DR1) of the Keck Observatory Database of Ionized Absorption toward Quasars (KODIAQ) survey. The KODIAQ survey is aimed at studying galactic and circumgalactic gas in absorption at high-redshift, with a focus on highly-ionized gas traced by OVI, using the HIRES spectrograph on the Keck-I telescope. KODIAQ DR1 consists of a fully-reduced sample of 170 quasars at 0.29 < z_em < 5.29 observed with HIRES at high resolution (36,000 <= R <= 103,000) between 2004 and 2012. DR1 contains 247 spectra available in continuum normalized form, representing a sum total exposure time of ~1.6 megaseconds. These co-added spectra arise from a total of 567 individual exposures of quasars taken from the Keck Observatory Archive (KOA) in raw form and uniformly processed using a HIRES data reduction package made available through the XIDL distribution. DR1 is publicly available to the community, housed as a higher level science product at the KOA. We will provide future data releases that make further QSOs, including those with pre-2004 observations taken with the previous-generation HIRES detectors.

astro-ph.CO↗

arXiv2013

The Bimodal Metallicity Distribution of the Cool Circumgalactic Medium at z<1

We assess the metal content of the cool (10^4 K) circumgalactic medium (CGM) about galaxies at z<1 using an H I-selected sample of 28 Lyman limit systems (LLS, defined here as absorbers with 16.2<log N(H I)<18.5) observed in absorption against background QSOs by the Cosmic Origins Spectrograph on-board the Hubble Space Telescope. The N(H I) selection avoids metallicity biases inherent in many previous studies of the low-redshift CGM. We compare the column densities of weakly ionized metal species (e.g., O II, Si II, Mg II) to N(H I) in the strongest H I component of each absorber. We find that the metallicity distribution of the LLS (and hence the cool CGM) is bimodal with metal-poor and metal-rich branches peaking at [X/H]=-1.6 and -0.3 (or about 2.5% and 50% solar metallicities). The cool CGM probed by these LLS is predominantly ionized. The metal-rich branch of the population likely traces winds, recycled outflows, and tidally stripped gas; the metal-poor branch has properties consistent with cold accretion streams thought to be a major source of fresh gas for star forming galaxies. Both branches have a nearly equal number of absorbers. Our results thus demonstrate there is a significant mass of previously-undiscovered cold metal-poor gas and confirm the presence of metal enriched gas in the CGM of z<1 galaxies.

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arXiv2012

High-velocity clouds as streams of ionized and neutral gas in the halo of the Milky Way

High-velocity clouds (HVC), fast-moving ionized and neutral gas clouds found at high galactic latitudes, may play an important role in the evolution of the Milky Way. The extent of this role depends sensitively on their distances and total sky covering factor. We search for HVC absorption in HST high resolution ultraviolet spectra of a carefully selected sample of 133 AGN using a range of atomic species in different ionization stages. This allows us to identify neutral, weakly ionized, or highly ionized HVCs over several decades in HI column densities. The sky covering factor of UV-selected HVCs with |v_LSR|>90 km/s is 68%+/-4% for the entire Galactic sky. We show that our survey is essentially complete, i.e., an undetected population of HVCs with extremely low N(H) (HI+HII) is unlikely to be important for the HVC mass budget. We confirm that the predominantly ionized HVCs contain at least as much mass as the traditional HI HVCs and show that large HI HVC complexes have generally ionized envelopes extending far from the HI contours. There are also large regions of the Galactic sky that are covered with ionized high-velocity gas with little HI emission nearby. We show that the covering factors of HVCs with 90<|v_LSR|<170 km/s drawn from the AGN and stellar samples are similar. This confirms that these HVCs are within 5-15 kpc of the sun. The covering factor of these HVCs drops with decreasing vertical height, which is consistent with HVCs being decelerated or disrupted as they fall to the Milky Way disk. The HVCs with |v_LSR|>170 km/s are largely associated with the Magellanic Stream at b<0 and its leading arm at b>0 as well as other large known HI complexes. Therefore there is no evidence in the Local Group that any galaxy shows a population of HVCs extending much farther away than 50 kpc from its host, except possibly for those tracing remnants of galaxy interaction.

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arXiv2011

The First Observations of Low-Redshift Damped Lyman-α Systems with the Cosmic Origins Spectrograph: Chemical Abundances and Affiliated Galaxies

We present Cosmic Origins Spectrograph (COS) measurements of metal abundances in eight 0.083<z<0.321 damped Lyman-α(DLA) and sub-damped Ly-α absorption systems serendipitously discovered in the COS-Halos survey. We find that these systems show a large range in metallicities, with -1.10<[Z/H]<0.31, similar to the spread found at higher redshifts. These low-redshift systems on average have subsolar metallicities, but do show a rise in metallicity over cosmic time when compared to higher-redshift systems. We find the average sub-DLA metallicity is higher than the average DLA metallicity at all redshifts. Nitrogen is underabundant with respect to α-group elements in all but perhaps one of the absorbers. In some cases, [N/α] is significantly below the lowest nitrogen measurements in nearby galaxies. Systems for which depletion patterns can be studied show little, if any, depletion, which is characteristic of Milky Way halo-type gas. We also identify affiliated galaxies for 3 of the sub-DLAs using spectra obtained from Keck/LRIS. None of these sub-DLAs arise in the stellar disks of luminous galaxies; instead, these absorbers may exist in galaxy halos at impact parameters ranging from 38 to 92 kpc. Multiple galaxies are present near two of the sub-DLAs, and galaxy interactions may play a role in the dispersal of the gas. Many of these low-redshift absorbers exhibit simple kinematics, but one sub-DLA has a complicated mix of at least 13 components spread over 150 km/s. We find three galaxies near this sub-DLA, which also suggests that galaxy interactions roil the gas. This study reinforces the view that DLAs have a variety of origins, and low-redshift studies are crucial for understanding absorber-galaxy connections.

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arXiv2011

Cos observations of metal line and broad lyman alpha absorption in the multi-phase o vi and ne viii system toward he 02226-4110

Observations of the QSO HE 0226-4110 (zem = 0.495) with the Cosmic Origins Spectrograph (COS) from 1134 to 1796 Å with a resolution of ~17 km s-1 and signal-to- noise (S/N) per resolution element of 20 to 40 are used to study the multi-phase absorption system at z = 0.20701 containing O VI and Ne VIII. The system was previously studied with lower S/N observations with FUSE and STIS. The COS observations provide more reliable measures of the H I and metal lines present in the system and reveal the clear presence of broad Lyman α (BLA) absorption with b = 72(+13, -6) km s-1 and logN(H I) = 13.87\pm0.08. Detecting BLAs associated with warm gas absorbers is crucial for determining the temperature, metallicity and total baryonic content of the absorbers. The BLA is probably recording the trace amount of thermally broadened H I in the collisionally ionized plasma with log T ~5.7 that also produces the O VI and Ne VIII absorption. The total hydrogen column in the collisionally ionized gas, logN(H) ~ 20.1, exceeds that in the cooler photoionized gas in the system by a factor of ~22. The oxygen abundance in the collisionally ionized gas is [O/H] = -0.89\pm0.08\pm0.07. The absorber probably occurs in the circumgalactic environment (halo) of a foreground L = 0.25L* disk galaxy with an impact parameter of 109h70-1 kpc identified by Mulchaey & Chen (2009).

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arXiv2011

A Multiphase Absorber Containing O VI and Broad H I Directly Tracing 10^6 K Plasma at Low-Redshift Toward HE 0153-4520

Observations of the QSO HE 0153-4520 (z-em = 0.450) with the Cosmic Origins Spectrograph (COS) from 1134 to 1796 A with a resolution of ~17 km/s and signal-to- noise per resolution element of 20 to 40 are used to study a multi-phase partial Lyman limit system (LLS) at z = 0.22601 tracing both cool and hot gas. FUSE observations of the Lyman limit break yield log N(H I) = 16.61(0.12, -0.17) The observed UV absorption lines of H I 1216 to 926, C III, C II, N III, N II, Si III, and Si II imply the existence of cool photoionized gas in the LLS with log U = -2.8\pm0.1 and log N(H) = 19.35\pm0.18, log n(H) = -2.9\pm0.2, log T = 4.27\pm0.02, log (P/k) = 1.75\pm0.17, and log L(kpc) = 0.70\pm0.25. The abundances are [X/H] = -0.8 (+0.3, -0.2) for N, Si and C but the result is sensitive to the assumed shape of the ionizing background radiation field. The multi-phase system has strong O VI and associated broad Ly α absorption (BLA) with log N(O VI) = 14.21\pm0.02, b(O VI) = 37\pm1 km/s, log N(H I) = 13.70(+0.05,-0.08), b(H I)=140 (+14, -16) km/s and b(H I)/b(O VI) = 3.9\pm0.4. The O VI does not arise in the cool photoionized gas of the LLS. The O VI and BLA imply the direct detection of thermally broadened absorption by hot gas with log T = 6.07 (+0.09, -0.12), [O/H] = -0.28 (+0.09, -0.08), and log N(H) = 20.41 (+0.13, -0.17). The absorber probably occurs in the circumgalactic environment (halo) of a foreground galaxy.

astro-ph.CO↗

arXiv2010

Fundamental Properties of the Highly Ionized Plasmas in the Milky Way

The cooling transition temperature gas in the interstellar medium (ISM), traced by the high ions, Si IV, C IV, N V, and O VI, helps to constrain the flow of energy from the hot ISM with T >10^6 K to the warm ISM with T< 2x10^4 K. We investigate the properties of this gas along the lines of sight to 38 stars in the Milky Way disk using 1.5-2.7 km/s resolution spectra of Si IV, C IV, and N V absorption from the Space Telescope Imaging Spectrograph (STIS), and 15 km/s resolution spectra of O VI absorption from the Far Ultraviolet Spectroscopic Explorer (FUSE). The absorption by Si IV and C IV exhibits broad and narrow components while only broad components are seen in N V and O VI. The narrow components imply gas with T<7x10^4 K and trace two distinct types of gas. The strong, saturated, and narrow Si IV and C IV components trace the gas associated with the vicinities of O-type stars and their supershells. The weaker narrow Si IV and C IV components trace gas in the general ISM that is photoionized by the EUV radiation from cooling hot gas or has radiatively cooled in a non-equilibrium manner from the transition temperature phase, but rarely the warm ionized medium (WIM) probed by Al III. The broad Si IV, C IV, N V, and O VI components trace collisionally ionized gas that is very likely undergoing a cooling transition from the hot ISM to the warm ISM. The cooling process possibly provides the regulation mechanism that produces N(C IV)/N(Si IV) = 3.9 +/- 1.9. The cooling process also produces absorption lines where the median and mean values of the line widths increase with the energy required to create the ion.

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arXiv2009

Origin(s) of the Highly Ionized High-Velocity Clouds Based on Their Distances

Previous HST and FUSE observations have revealed highly ionized high-velocity clouds (HVCs) or more generally low HI column HVCs along extragalactic sightlines over 70-90% of the sky. The distances of these HVCs have remained largely unknown hampering to distinguish a "Galactic" origin (e.g., outflow, inflow) from a "Local Group" origin (e.g., warm-hot intergalactic medium). We present the first detection of highly ionized HVCs in the Cosmic Origins Spectrograph (COS) spectrum of the early-type star HS1914+7134 (l = 103, b=+24) located in the outer region of the Galaxy at 14.9 kpc. Two HVCs are detected in absorption at v_LSR = -118 and -180 km/s in several species, including CIV, SiIV, SiIII, CII, SiII, OI, but HI 21-cm emission is only seen at -118 \km. Within 17 degrees of HS1914+7134, we found HVC absorption of low and high ions at similar velocities toward 5 extragalactic sight lines, suggesting that these HVCs are related. The component at -118 km/s is likely associated with the Outer Arm of the Milky Way. The highly ionized HVC at -180 km/s is an HVC plunging at high speed onto the thick disk of the Milky Way. This is the second detection of highly ionized HVCs toward Galactic stars, supporting a "Galactic" origin for at least some of these clouds.

astro-ph.GA↗

arXiv2009

Properties and Origin of the High-Velocity Gas Toward the Large Magellanic Cloud

In the spectra of 139 early-type Large Magellanic Cloud (LMC) stars observed with FUSE and with deep radio Parkes HI 21-cm observations along those stars, we search for and analyze the absorption and emission from high-velocity gas at +90 10^6 M_sun) HVC complex that is linked to stellar feedback occurring in a dwarf spiral galaxy.

astro-ph.GA↗