arXiv subjects

Dhanesh Krishnarao

Publications and source records attributed to Dhanesh Krishnarao.

At least 19 recordsLinked to original sources

arXiv2025

New Interpretation for the Orientation of the LMC's Gaseous Arms B and E using ULLYSES

The Large Magellanic Cloud (LMC) experiences disruption from tidal and ram-pressure forces as it travels through the halo of the Milky Way. In this project, we combine radio emission-line observations from the GASS and GASKAP surveys with UV absorption-line observations from the HST Ultraviolet Legacy Library of Young Stars as Essential Standards (ULLYSES) program to trace the material in front of the LMC. Along our 8 stellar sightlines near 30 Doradus, we observe gaseous structures likely associated with two arm-like features flowing in and around the LMC's disk. We detect the nearside gas in neutral, low, and medium ionization species. The lower-ionization species likely undergo both thermal and non-thermal broadening while the moderately-ionized phase is influenced by more non-thermal processes. The total integrated column density of AlIII decreases with increasing angular offset from 30 Doradus, with sightlines within 0.25 degrees containing more moderately ionized gas. We demonstrate from a Gaussian decomposition technique on the HI emission that both arms likely trace an additional 1.0 degree in Galactic longitude toward the 30 Doradus region than previously predicted. We constrain the orientation of the arms by suggesting that they likely converge around (l,b) =(280.5$^\circ$, -31.2$^\circ$) and at least partially cross in front of the LMC. Our observations are consistent with two competing origins of the arms: 1) outflowing material is swept back by tidal and ram-pressure forces or 2) tidally stripped inflows fuel the ongoing stellar activity inside the LMC. Future studies are needed to distinguish between these scenarios.

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arXiv2025

A three-dimensional, multi-wavelength view and time-dependent analysis of the Milky Way's local ionized gas

This work is the continuation of a series attempting to characterize the local warm ionized medium through both static and time dependent simulations. We build upon our three dimensional, observationally-derived simulation of the local photoionized interstellar medium - based on static photoionization simulations constrained by 3D dust maps - to include metals required to predict collisionally excited optical and infrared emission lines, providing the first all-sky prediction of a series of lines including [SII] 6716$\mathring{A}$, [NII] 6584$\mathring{A}$ and [OIII] 5007$\mathring{A}$. While these predictions only include O-star photoionization under ionization equilibrium, we also carry out a suite of radiation-hydrodynamics simulations including time-dependent metal ionization and the effects of supernova feedback to highlight missing features in our predicted skies. We use the simulations to estimate the very local (1 $\rm kpc^{2}$) Galactic star formation rate, finding a rate of 370 $\rm M_{\odot}~Myr^{-1}~kpc^{-2}$ provides the best match between the observationally-derived and ab-initio simulations. This is approximately a factor of four lower than previous estimates for the star formation rate required to support an observed layer of high-altitude diffuse ionized gas, possibly suggesting a `bursty' star formation history in the region surrounding the Sun. We also investigate the effects of O-star environments on their ability to ionize large volumes of diffuse ionized gas, and find it is likely ionized by a small number of luminous O-stars located in regions where the leakage of their Lyman continuum photons can produce the vast volumes of ionized gas observed in the midplane and at high galactic altitudes.

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arXiv2025

Ultraviolet Properties of Multi-phase Gas Toward the Inner Galaxy

We present a systematic study of the multi-phase interstellar gas in the Inner Galaxy using HST/STIS absorption spectroscopy of 16 massive stars located at spectroscopic distances between 1.3 and 10 kpc in the region $-30^\circ\lesssim l \lesssim+30^\circ$ and $-15^\circ\lesssim b \lesssim+15^\circ$. These sight lines probe gas above and below the Sagittarius Carina, Scutum Crux-Centaurus, Norma, and Near 3 kpc spiral arms in a range of $z$-height from 0 to 1.5 kpc. Along the 16 sight lines, we measure velocity centroids for 800 UV absorption-line components across multiple gas phases (molecular CO, neutral, low ion, and high ion). We find that 619/800 components have velocities that are consistent with a simple model of co-rotation with the disk, indicating that multi-phase gas with disk-like kinematics extends at least 1 kpc into the halo. We present a database of absorption-line parameters that can be used for kinematic modeling of gas flows into and out of the Galactic disk.

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arXiv2025

The H$\alpha$ sky in three dimensions

We combine parallax distances to nearby O stars with parsec-scale resolution three-dimensional dust maps of the local region of the Milky Way (within 1.25 kpc of the Sun) to simulate the transfer of Lyman continuum photons through the interstellar medium. Assuming a fixed gas-to-dust ratio, we determine the density of ionized gas, electron temperature, and H$\alpha$ emissivity throughout the local Milky Way. There is good morphological agreement between the predicted and observed H$\alpha$ all-sky map of the Wisconsin H$\alpha$ Mapper. We find that our simulation underproduces the observed H$\alpha$ emission while overestimating the sizes of HII regions, and we discuss ways in which agreement between simulations and observations may be improved. Of the total ionizing luminosity of $5.84 \times 10^{50}~{\rm photons~s^{-1}}$, 15% is absorbed by dust, 64% ionizes "classical'' HII regions, 11% ionizes the diffuse warm ionized medium, and 10% escapes the simulation volume. We find that 18% of the high altitude ($|b| > 30^{\circ}$) H$\alpha$ arises from dust-scattered rather than direct emission. These initial results provide an impressive validation of the three-dimensional dust maps and O-star parallaxes, opening a new frontier for studying the ionized ISM's structure and energetics in three dimensions.

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arXiv2025

The Gaseous Blowout of the 30 Doradus Starburst Region in the LMC

Widespread galactic winds emanate from the Large Magellanic Cloud (LMC), with the 30 Doradus starburst region generating the fastest and most concentrated gas flows. We report on the gas distribution, kinematics, and ionization conditions of the near-side outflow along 8 down-the-barrel sightlines using UV absorption-line observations from the HST's ULLYSES program for this region along with H I 21-cm observations from the GASS and GASKAP surveys. We find that within 1.7 degrees from the center of 30 Doradus, the wind reaches maximum speeds of $100-150\,\text{km}\,\text{s}^{-1}$ from the LMC's disk. The total integrated column densities of low-ions (O I, Si II, and Fe II) in the blueshifted wind, up to $v_{\rm LSR}=150\,\text{km}\,\text{s}^{-1}$, are highest near the center and decline radially outward. We estimate an outflow mass of $M_{\rm outflow,\,Si II}\approx(5.7-8.6)\,\times 10^{5} M_{\odot}$, outflow rate of $\dot{M}_{\rm outflow}\gtrsim0.02 M_{\odot}\,\text{yr}^{-1}$, and mass loading factor of $\eta\gtrsim0.10$ within 0.52 degrees from the center of 30 Doradus. The observed ion ratios$-$together with photoionization modeling$-$reveal that this wind is roughly $40-97\%$ photoionized. The metallicities and dust depletion patterns of the high-velocity absorbers at $v_{\rm LSR}\approx+120\,\text{km}\,\text{s}^{-1}$ can be explained by either a foreground Milky Way (MW) halo cloud or an outflow from the LMC. For the high-ions, Si IV and C IV are broader and kinematically offset from the low-ions, suggesting turbulent mixing layers (TMLs) existing in the wind. Finally, our hydrodynamical simulations of the Magellanic Clouds (MCs) and MW system suggest that the Magellanic Corona can protect the LMC winds from the ram-pressure forces exerted by the MW's halo.

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arXiv2024

Time-dependent metal ionization and the persistence of collisionally excited emission lines in the diffuse ionized gas of star forming galaxies

We extend our time-dependent hydrogen ionization simulations of diffuse ionized gas to include metals important for collisional cooling and diagnostic emission lines. The combination of heating from supernovae and time-dependent collisional and photoionization from midplane OB stars produces emission line intensities (and emission line ratios) that follow the trends observed in the Milky Way and other edge-on galaxies. The long recombination times in low density gas result in persistent large volumes of ions with high ionization potentials, such as O III and Ne III. In particular, the vertically extended layers of Ne III in our time-dependent simulations result in [Ne III] 15$\mu$m/[Ne II] 12$\mu$m emission line ratios in agreement with observations of the edge-on galaxy NGC 891. Simulations adopting ionization equilibrium do not allow for the persistence of ions with high ionization states and therefore cannot reproduce the observed emission lines from low density gas at high altitudes.

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arXiv2024

The Truncated Circumgalactic Medium of the Large Magellanic Cloud

The Large Magellanic Cloud (LMC) is the nearest massive galaxy to the Milky Way. Its circumgalactic medium is complex and multi-phase, containing both stripped HI structures like the Magellanic Stream and Bridge, and a diffuse warm corona seen in high-ion absorption. We analyze 28 AGN sightlines passing within 35 kpc of the LMC with archival HST/COS spectra to characterize the cool (T\approx10^4$ K) gas in the LMC CGM, via new measurements of UV absorption in six low ions (OI, FeII, SiII, AlII, SII, and NiII) and one intermediate ion (SiIII). We show that a declining column-density profile is present in all seven ions, with the low-ion profiles having a steeper slope than the high-ion profiles in CIV and SiIV reported by Krishnarao et al. 2022. Crucially, absorption at the LMC systemic velocity is only detected (in all ions) out to 17 kpc. Beyond this distance, the gas has a lower velocity and is associated with the Magellanic Stream. These results demonstrate that the LMC's CGM is composed of two distinct components: a compact inner halo extending to 17 kpc, and a more extended stripped region associated with the Stream. The compactness and truncation of the LMC's inner CGM agree with recent simulations of ram-pressure stripping of the LMC by the Milky Way's extended corona.

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arXiv2024

The SDSS-V Local Volume Mapper (LVM): Scientific Motivation and Project Overview

We present the Sloan Digital Sky Survey V (SDSS-V) Local Volume Mapper (LVM). The LVM is an integral-field spectroscopic survey of the Milky Way, Magellanic Clouds, and of a sample of local volume galaxies, connecting resolved pc-scale individual sources of feedback to kpc-scale ionized interstellar medium (ISM) properties. The 4-year survey covers the southern Milky Way disk at spatial resolutions of 0.05 to 1 pc, the Magellanic Clouds at 10 pc resolution, and nearby large galaxies at larger scales totaling $>4300$ square degrees of sky, and more than 55M spectra. It utilizes a new facility of alt-alt mounted siderostats feeding 16 cm refractive telescopes, lenslet-coupled fiber-optics, and spectrographs covering 3600-9800A at R ~ 4000. The ultra-wide field IFU has a diameter of 0.5 degrees with 1801 hexagonally packed fibers of 35.3 arcsec apertures. The siderostats allow for a completely stationary fiber system, avoiding instability of the line spread function seen in traditional fiber feeds. Scientifically, LVM resolves the regions where energy, momentum, and chemical elements are injected into the ISM at the scale of gas clouds, while simultaneously charting where energy is being dissipated (via cooling, shocks, turbulence, bulk flows, etc.) to global scales. This combined local and global view enables us to constrain physical processes regulating how stellar feedback operates and couples to galactic kinematics and disk-scale structures, such as the bar and spiral arms, as well as gas in- and out-flows.

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arXiv2024

The persistence of high altitude non-equilibrium diffuse ionized gas in simulations of star forming galaxies

Widespread, high altitude, diffuse ionized gas with scale heights of around a kiloparsec is observed in the Milky Way and other star forming galaxies. Numerical radiation-magnetohydrodynamic simulations of a supernova-driven turbulent interstellar medium show that gas can be driven to high altitudes above the galactic midplane, but the degree of ionization is often less than inferred from observations. For computational expediency, ionizing radiation from massive stars is often included as a post-processing step assuming ionization equilibrium. We extend our simulations of a Milky Way-like interstellar medium to include the combined effect of supernovae and photoionization feedback from midplane OB stars and a population of hot evolved low mass stars. The diffuse ionized gas has densities below 0.1 ${\rm cm^{-3}}$, so recombination timescales can exceed millions of years. Our simulations now follow the time-dependent ionization and recombination of low density gas. The long recombination timescales result in diffuse ionized gas that persists at large altitudes long after the deaths of massive stars that produce the vast majority of the ionized gas. The diffuse ionized gas does not exhibit the large variability inherent in simulations that adopt ionization equilibrium. The vertical distribution of neutral and ionized gas is close to what is observed in the Milky Way. The volume filling factor of ionized gas increases with altitude resulting in the scale height of free electrons being larger than that inferred from H$α$ emission, thus reconciling the observations of ionized gas made in H$α$ and from pulsar dispersion measurements.

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arXiv2023

The Eighteenth Data Release of the Sloan Digital Sky Surveys: Targeting and First Spectra from SDSS-V

The eighteenth data release of the Sloan Digital Sky Surveys (SDSS) is the first one for SDSS-V, the fifth generation of the survey. SDSS-V comprises three primary scientific programs, or "Mappers": Milky Way Mapper (MWM), Black Hole Mapper (BHM), and Local Volume Mapper (LVM). This data release contains extensive targeting information for the two multi-object spectroscopy programs (MWM and BHM), including input catalogs and selection functions for their numerous scientific objectives. We describe the production of the targeting databases and their calibration- and scientifically-focused components. DR18 also includes ~25,000 new SDSS spectra and supplemental information for X-ray sources identified by eROSITA in its eFEDS field. We present updates to some of the SDSS software pipelines and preview changes anticipated for DR19. We also describe three value-added catalogs (VACs) based on SDSS-IV data that have been published since DR17, and one VAC based on the SDSS-V data in the eFEDS field.

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arXiv2022

Caught in the Act: A Metal-Rich High-Velocity Cloud in the Inner Galaxy

We characterize the chemical and physical conditions in an outflowing high-velocity cloud in the inner Galaxy. We report a super-solar metallicity of [O/H] = $+0.36\pm0.12$ for the high-velocity cloud at $v_\mathrm{LSR}$ = 125.6 km s$^{-1}$ toward the star HD 156359 ($l$ = 328.$^{\circ}$7, $b$ = $-$14.$^{\circ}$5, $d$ = 9 kpc, $z$ = $-$2.3 kpc). Using archival observations from FUSE, HST STIS, and ESO FEROS we measure high-velocity absorption in H I, O I, C II, N II, Si II, Ca II, Si III, Fe III, C IV, Si IV, N V, and O VI. We measure a low H I column density of log $N$(H I) = $15.54\pm0.05$ in the HVC from multiple unsaturated H I Lyman series lines in the FUSE data. We determine a low dust depletion level in the HVC from the relative strength of silicon, iron, and calcium absorption relative to oxygen, with [Si/O]=$-0.33\pm0.14$, [Fe/O]=$-0.30\pm0.20$, and [Ca/O] =$-0.56\pm0.16$. Analysis of the high-ion absorption using collisional ionization models indicates that the hot plasma is multi-phase, with the C IV and Si IV tracing 10$^{4.9}$ K gas and N V and O VI tracing 10$^{5.4}$ K gas. The cloud's metallicity, dust content, kinematics, and close proximity to the disk are all consistent with a Galactic wind origin. As the HD 156359 line of sight probes the inner Galaxy, the HVC appears to be a young cloud caught in the act of being entrained in a multi-phase Galactic outflow and driven out into the halo.

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arXiv2022

Observations of a Magellanic Corona

The Large and Small Magellanic Clouds (LMC/SMC) are the closest major satellite galaxies of the Milky Way. They are likely on their first passage on an infalling orbit towards our Galaxy (Besla et al. 2007) and trace the ongoing dynamics of the Local Group (D'Onghia & Fox 2016). Recent measurements of a high mass for the LMC (M_halo = 10^(11.1-11.4) solar masses; Penarrubia et al. 2016, Erkal et al. 2018, 2019, Kallivayalil et al. 2018) imply the LMC should host a Magellanic Corona: a collisionally ionized, warm-hot gaseous halo at the virial temperature (10^(5.3-5.5) K) initially extending out to the virial radius (100-130 kpc). Such a Corona would have shaped the formation of the Magellanic Stream (Lucchini et al. 2020), a tidal gas structure extending over 200 degrees across the sky (D'Onghia & Fox 2016, Besla et al. 2012, Nidever et al. 2010) that is bringing in metal poor gas to the Milky Way (Fox et al. 2014). No observational evidence for such an extended Corona has been published previously, with detections of highly ionized gas only reported in directions directly toward the LMC, where winds from the LMC disk may dominate (deBoer & Savage 1980, Wakker et al. 1998). Here we show evidence for this Magellanic Corona with a potential direct detection in highly ionized oxygen (O^+5), and indirectly via triply-ionized carbon and silicon, seen in ultraviolet absorption toward background quasars. We find that the Magellanic Corona is part of a pervasive multiphase Magellanic circumgalactic medium (CGM) seen in many ionization states with a declining projected radial profile out to at least 35 kpc from the LMC and a total ionized CGM mass of log_10(M_HII;CGM/solar masses) = 9.1 +/- 0.2. The evidence for the Magellanic Corona is a crucial step forward in characterizing the Magellanic Group and its nested evolution with the Local Group.

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arXiv2022

Photometric Signature of Ultra-Harmonic Resonances in Barred Galaxies

Bars may induce morphological features, such as rings, through their resonances. Previous studies suggested that the presence of 'dark-gaps', or regions of a galaxy where the difference between the surface brightness along the bar major axis and along the bar minor axis are maximal, can be attributed to the location of bar corotation. Here, using GALAKOS, a high-resolution N-body simulation of a barred galaxy, we test this photometric method's ability to identify the bar corotation resonance. Contrary to previous work, our results indicate that 'dark-gaps' are a clear sign of the location of the 4:1 ultra-harmonic resonance instead of bar corotation. Measurements of the bar corotation can indirectly be inferred using kinematic information, e.g., by measuring the shape of the rotation curve. We demonstrate our concept on a sample of 578 face-on barred galaxies with both imaging and integral field observations and find the sample likely consists primarily of fast bars.

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arXiv2021

Molecular Gas within the Milky Way's Nuclear Wind

We report the first direct detection of molecular hydrogen associated with the Galactic nuclear wind. The Far-Ultraviolet Spectroscopic Explorer spectrum of LS 4825, a B1 Ib-II star at $l,b$ = 1.67$^{\circ}$,$-$6.63$^{\circ}$ lying $d$ = 9.9$^{+1.4}_{-0.8}$ kpc from the Sun, $\sim$1 kpc below the Galactic plane near the Galactic Center, shows two high-velocity H$_2$ components at $v_\mathrm{LSR}$ = $-79$ and $-108$ km s$^{-1}$. In contrast, the FUSE spectrum of the nearby ($\sim$0.6$^{\circ}$ away) foreground star HD 167402 at $d$=4.9$^{+0.8}_{-0.7}$ kpc reveals no H$_2$ absorption at these velocities. Over 60 lines of H$_2$ from rotational levels $J$ = 0 to 5 are identified in the high-velocity clouds. For the $v_\mathrm{LSR}$ = $-79$ km s$^{-1}$ cloud we measure total log $N$(H$_2$) $\geq$ 16.75 cm$^{-2}$, molecular fraction $f_\mathrm{H_2}$ $\geq$ 0.8%, and $T_{01}$ $\geq$ 97 and $T_{25}$ $\leq$ 439 K for the ground- and excited-state rotational excitation temperatures. At $v_\mathrm{LSR}$ = $-108$ km s$^{-1}$, we measure log $N$(H$_2$) = 16.13 $\pm$ 0.10 cm$^{-2}$, $f_\mathrm{H_2}$ $\geq$ 0.5%, and $T_{01}$ = 77$^{+34}_{-18}$ and $T_{25}$ = 1092$^{+149}_{-117}$ K, for which the excited state ortho- to para-H$_2$ is 1.0$^{+0.3}_{-0.1}$, much less than the equilibrium value of 3 expected for gas at this temperature. This non-equilibrium ratio suggests that the $-108$ km s$^{-1}$ cloud has been recently excited and has not yet had time to equilibrate. As the LS 4825 sight line passes close by a tilted section of the Galactic disk, we propose that we are probing a boundary region where the nuclear wind is removing gas from the disk.

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arXiv2021

Galaxy Zoo: 3D -- Crowd-sourced Bar, Spiral and Foreground Star Masks for MaNGA Target Galaxies

The challenge of consistent identification of internal structure in galaxies - in particular disc galaxy components like spiral arms, bars, and bulges - has hindered our ability to study the physical impact of such structure across large samples. In this paper we present Galaxy Zoo: 3D (GZ: 3D) a crowdsourcing project built on the Zooniverse platform which we used to create spatial pixel (spaxel) maps that identify galaxy centres, foreground stars, galactic bars and spiral arms for 29831 galaxies which were potential targets of the MaNGA survey (Mapping Nearby Galaxies at Apache Point Observatory, part of the fourth phase of the Sloan Digital Sky Surveys or SDSS-IV), including nearly all of the 10,010 galaxies ultimately observed. Our crowd-sourced visual identification of asymmetric, internal structures provides valuable insight on the evolutionary role of non-axisymmetric processes that is otherwise lost when MaNGA data cubes are azimuthally averaged. We present the publicly available GZ:3D catalog alongside validation tests and example use cases. These data may in the future provide a useful training set for automated identification of spiral arm features. As an illustration, we use the spiral masks in a sample of 825 galaxies to measure the enhancement of star formation spatially linked to spiral arms, which we measure to be a factor of three over the background disc, and how this enhancement increases with radius.

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arXiv2020

Are the Milky Way and Andromeda unusual? A comparison with Milky Way and Andromeda Analogs

Our Milky Way provides a unique test case for galaxy evolution models, thanks to our privileged position within the Milky Way's disc. This position also complicates comparisons between the Milky Way and external galaxies, due to our inability to observe the Milky Way from an external point of view. Milky Way analog galaxies offer us a chance to bridge this divide by providing the external perspective that we otherwise lack. However, over-precise definitions of "analog" yield little-to-no galaxies, so it is vital to understand which selection criteria produce the most meaningful analog samples. To address this, we compare the properties of complementary samples of Milky Way analogs selected using different criteria. We find the Milky Way to be within 1$σ$ of its analogs in terms of star-formation rate and bulge-to-total ratio in most cases, but we find larger offsets between the Milky Way and its analogs in terms of disc scale length; this suggests that scale length must be included in analog selections in addition to other criteria if the most accurate analogs are to be selected. We also apply our methodology to the neighbouring Andromeda galaxy. We find analogs selected on the basis of strong morphological features to display much higher star-formation rates than Andromeda, and we also find analogs selected on Andromeda's star-formation rate to over-predict Andromeda's bulge extent. This suggests both structure and star-formation rate should be considered when selecting the most stringent Andromeda analogs.

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arXiv2020

Discovery of High-Velocity H$α$ Emission in the Direction of the Fermi Bubble

Wisconsin H-Alpha Mapper (WHAM) observations reveal high-velocity and [NII]$\lambda6584$ emission lines in the same direction and velocity as ultraviolet absorption-line features that have been previously associated with the biconical gamma-ray lobes known as the Fermi Bubbles. We measure an extinction-corrected intensity of $I_{\textrm{H}α}=0.84^{+0.10}_{-0.09}$ Rayleigh for emission with line center $v_\textrm{LSR}=-221\pm3~\textrm{km}~\textrm{s}^{-1}$, corresponding to an emission measure of $EM = 2.00^{+0.64}_{-0.63}~\textrm{cm}^{-6}~\textrm{pc}$. This emission arises at the same velocity as Hubble Space Telescope/Cosmic Origins Spectrograph observations of ultraviolet absorption features detected in the PDS 456 quasar sight line that passes through the northern Bubble near $l = 10^\circ.4, b = +11^\circ.2$. We estimate the total column density of ionized gas in this velocity component to be $N(H^{+}) = \left(3.28 \pm 0.33\right) \times 10^{18}~\textrm{cm}^{-2}$. The comparison of ionized gas emission and absorption yields an estimate for the characteristic density of $n_{e,c} = 1.8 \pm 0.6~\textrm{cm}^{-3}$ and a characteristic length of $L_{c} =0.56 \pm 0.21~\textrm{pc}$ assuming $30\%$ solar metallicity. For a temperature of $T_{e}=8500^{+2700}_{-2600}$ K---consistent with the measured line widths and [NII]/H$α$ line ratio---the gas has a thermal pressure of $p/k = 32,000^{+15,000}_{-14,000}~\textrm{cm}^{-3}~\textrm{K}$. Assuming the gas is $\sim 6.5$ kpc distant, the derived density and pressure appear to be anomalously high for gas $\sim 1.3$ kpc above the Galactic midplane. The large thermal pressure is comparable to both a hot halo or Fermi Bubble model, but suggest that the H$α$ arises in an overpressurized zone.

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arXiv2020

Discovery of diffuse optical emission lines from the inner Galaxy: Evidence for LI(N)ER-like gas

Optical emission lines are used to categorize galaxies into three groups according to their dominant central radiation source: active galactic nuclei, star formation, or low-ionization (nuclear) emission regions [LI(N)ERs] that may trace ionizing radiation from older stellar populations. Using the Wisconsin H-Alpha Mapper, we detect optical line emission in low-extinction windows within eight degrees of Galactic Center. The emission is associated with the 1.5-kiloparsec-radius "Tilted Disk" of neutral gas. We modify a model of this disk and find that the hydrogen gas observed is at least $48\%$ ionized. The ratio [NII] $λ$6584 $\overset{\lower.5em\circ}{\mathrm{A}}$/H$α$ $λ$6563 $\overset{\lower.5em\circ}{\mathrm{A}}$ increases from 0.3 to 2.5 with Galactocentric radius; [OIII] $λ$5007 $\overset{\lower.5em\circ}{\mathrm{A}}$ and H$β$ $λ$4861 $\overset{\lower.5em\circ}{\mathrm{A}}$ are also sometimes detected. The line ratios for most Tilted Disk sightlines are characteristic of LI(N)ER galaxies.

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