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  1. arXiv:2311.07661 (Published 2023-11-13)

    The evolution of galaxies and clusters at high spatial resolution with AXIS

    H. R. Russell et al.
    Comments: 29 pages, 18 figures; this white paper is part of a series commissioned for the AXIS Probe mission concept

    Stellar and black hole feedback heat and disperse surrounding cold gas clouds, launching gas flows off circumnuclear and galactic disks and producing a dynamic interstellar medium. On large scales bordering the cosmic web, feedback drives enriched gas out of galaxies and groups, seeding the intergalactic medium with heavy elements. In this way, feedback shapes galaxy evolution by shutting down star formation and ultimately curtailing the growth of structure after the peak at redshift 2-3. To understand the complex interplay between gravity and feedback, we must resolve both the key physics within galaxies and map the impact of these processes over large scales, out into the cosmic web. The Advanced X-ray Imaging Satellite (AXIS) is a proposed X-ray probe mission for the 2030s with arcsecond spatial resolution, large effective area, and low background. AXIS will untangle the interactions of winds, radiation, jets, and supernovae with the surrounding ISM across the wide range of mass scales and large volumes driving galaxy evolution and trace the establishment of feedback back to the main event at cosmic noon.

  2. arXiv:2307.02812 (Published 2023-07-06)

    Tracing the chemical footprint of shocks in AGN-host and starburst galaxies with ALMA multi-line molecular studies

    Ko-Yun Huang, Serena Viti
    Comments: Accepted for publication in Faraday Discussions 2023. 18 pages, 7 figures and 2 tables
    Categories: astro-ph.GA

    Multi-line molecular observations are an ideal tool for a systematic study of the physico-chemical processes in the Interstellar Medium (ISM), given the wide range of critical densities associated with different molecules and their transitions, and the dependencies of chemical reactions on the energy budget of the system. Recently high spatial resolution of typical shock tracers - SiO, HNCO, and CH3OH - have been studied in the potentially shocked regions in two nearby galaxies: NGC 1068 (an AGN-host galaxy) (Huang et al., Astron. Astrophys., 2022, 666, A102; Huang et al., in prep.) and NGC 253 (a starburst galaxy) (K.-Y. Huang et al., arXiv, 2023, preprint, arXiv:2303.12685, DOI: 10.48550/arXiv.2303.12685). This paper is dedicated to the comparative study of these two distinctively different galaxies, with the aim of determining the differences in their energetics and understanding large-scale shocks in dfferent types of galaxies.

  3. arXiv:2110.13929 (Published 2021-10-26)

    Mapping physical parameters in Orion KL at high spatial resolution

    Olivia H. Wilkins, P. Brandon Carroll, Geoffrey A. Blake
    Comments: 28 pages, 15 figures, to be published in ApJ
    Categories: astro-ph.GA

    The Orion Kleinmann-Low nebula (Orion KL) is notoriously complex and exhibits a range of physical and chemical components. We conducted high angular resolution (sub-arcsecond) observations of $^13$CH$_{3}$OH $\nu=0$ ($\sim$0.3$^{\prime\prime}$ and $\sim$0.7$^{\prime\prime}$) and CH$_3$CN $\nu_8=1$ ($\sim$0.2$^{\prime\prime}$ and $\sim$0.9$^{\prime\prime}$) line emission with the Atacama Large Millimeter/submillimeter Array (ALMA) to investigate Orion KL's structure on small spatial scales (${\le}350$ au). Gas kinematics, excitation temperatures, and column densities were derived from the molecular emission via a pixel-by-pixel spectral line fitting of the image cubes, enabling us to examine the small-scale variation of these parameters. Sub-regions of the Hot Core have a higher excitation temperature in a 0.2$^{\prime\prime}$ beam than a 0.9$^{\prime\prime}$ beam, indicative of possible internal sources of heating. Furthermore, the velocity field includes a bipolar ${\sim}7{-}8$ km s$^{-1}$ feature with a southeast-northwest orientation against the surrounding ${\sim}4{-}5$ km s$^{-1}$ velocity field, which may be due to an outflow. We also find evidence of a possible source of internal heating toward the Northwest Clump, since the excitation temperature there is higher in a smaller beam versus a larger beam. Finally, the region southwest of the Hot Core (Hot Core-SW) presents itself as a particularly heterogeneous region bridging the Hot Core and Compact Ridge. Additional studies to identify the (hidden) sources of luminosity and heating within Orion KL are necessary to better understand the nebula and its chemistry.

  4. arXiv:2103.03772 (Published 2021-03-05)

    FUV Variable Sources in M31

    Denis Leahy, Megan Buick, Joseph Postma, Cole Morgan
    Comments: 19 pages, 11 figures, 7 tables
    Categories: astro-ph.GA

    The Ultraviolet Imaging Telescope (UVIT) onboard the AstroSat observatory has imaged the Andromeda Galaxy (M31) from 2017 to 2019 in FUV and NUV with the high spatial resolution of ~1". The survey covered the large sky area of M31 with a set of observations (Fields) each 28 arcminutes in diameter. Field 1 was observed in two epochs with the F148W filter, separated by 1133 days (~3.10 years). The 6.4 kpc diameter Field 1 (at the distance of M31) includes a substantial part of the inner spiral arms of the galaxy. We identify UVIT sources in both epochs of Field 1 and obtain catalogs of sources that are variable in FUV at > 3 sigma and > 5 sigma confidence level. The fraction of FUV variable sources is higher for brighter sources, and the fraction is higher in the two main spiral arms compared to other areas. This is evidence that a significant fraction of the FUV variables are associated with hot young stars. Source counterparts are found for 42 of the 86 > 5 sigma FUV variables using existing catalogs. The counterparts include 10 star clusters, 6 HII regions, 5 regular or semiregular variables, 6 other variables and 6 nova or nova candidates. The UVIT FUV-NUV and FUV-FUV color-magnitude diagrams confirm the association of most of the FUV variables with hot young stars. A catalog of UVIT photometry for the variable sources is presented.

  5. arXiv:2101.04022 (Published 2021-01-11)

    A new distance to the Brick, the dense molecular cloud G0.253+0.016

    M. Zoccali, E. Valenti, F. Surot, O. A. Gonzalez, A. Renzini, A. Valenzuela Navarro
    Comments: 7 pages, 4 figures. Accepted for publication in MNRAS
    Categories: astro-ph.GA

    We analyse the near infrared colour magnitude diagram of a field including the giant molecular cloud G0.253+0.016 (a.k.a. The Brick) observed at high spatial resolution, with HAWK-I at the VLT. The distribution of red clump stars in a line of sight crossing the cloud, compared with that in a direction just beside it, and not crossing it, allow us to measure the distance of the cloud from the Sun to be 7.20, with a statistical uncertainty of +/-0.16 and a systematic error of +/-0.20 kpc. This is significantly closer than what is generally assumed, i.e., that the cloud belongs to the near side of the central molecular zone, at 60 pc from the Galactic center. This assumption was based on dynamical models of the central molecular zone, observationally constrained uniquely by the radial velocity of this and other clouds. Determining the true position of the Brick cloud is relevant because this is the densest cloud of the Galaxy not showing any ongoing star formation. This puts the cloud off by 1 order of magnitude from the Kennicutt-Schmidt relation between the density of the dense gas and the star formation rate. Several explanations have been proposed for this absence of star formation, most of them based on the dynamical evolution of this and other clouds, within the Galactic center region. Our result emphasizes the need to include constraints coming from stellar observations in the interpretation of our Galaxy central molecular zone.

  6. arXiv:1912.04615 (Published 2019-12-10)

    The Athena space X-ray Observatory and the astrophysics of hot plasma

    Didier Barret, Anne Decourchelle, Andy Fabian, Matteo Guainazzi, Kirpal Nandra, Randall Smith, Jan-Willem den Herder
    Comments: 12 pages, 9 figures, Proceedings of the XMM-Newton 2019 science workshop "Astrophysics of hot plasma in extended X -ray sources", accepted for publication in Astronomische Nachrichten

    The properties (temperature, density, chemical composition, velocity) of hot astrophysical plasma and the physical processes affecting them (heating/cooling, turbulence, shocks, acceleration) can be probed by high resolution X-ray spectroscopy, to be complemented by high spatial resolution imaging. The paper presents a status of the ESA's Advanced Telescope for High Energy Astrophysics (Athena) mission, particularly focusing on the science performance of its two focal plane instruments for the studies of extended X-ray sources: the Wide Field Imager (WFI) and the X-ray Integral Field Unit (X-IFU). This paper then provides a brief summary of the breakthroughs expected with Athena on the astrophysics of hot plasma, building on the vast heritage of the discoveries and revolutionary results obtained by Chandra and XMM-Newton in this field. As of November 12th, 2019, Athena successfully concluded its feasibility study, and has since then moved into the definition phase, with a launch date scheduled in the early 2030s.

  7. arXiv:1912.03683 (Published 2019-12-08)

    The colors and sizes of recently quenched galaxies: a result of compact starburst before quenching

    Po-Feng Wu et al.
    Comments: 16 pages, 8 figures, 1 table, accepted by ApJ
    Categories: astro-ph.GA

    We analyze the colors and sizes of 32 quiescent (UVJ-selected) galaxies with strong Balmer absorption ($\mbox{EW}(H\delta) \geq 4$\AA) at $z\sim0.8$ drawn from DR2 of the LEGA-C survey to test the hypothesis that these galaxies experienced compact, central starbursts before quenching. These recently quenched galaxies, usually referred to as post-starburst galaxies, span a wide range of colors and we find a clear correlation between color and half-light radius, such that bluer galaxies are smaller. We build simple toy models to explain this correlation: a normal star-forming disk plus a central, compact starburst component. Bursts with exponential decay timescale of $\sim$~100 Myr that produce $\sim10\%$ to more than 100\% of the pre-existing masses can reproduce the observed correlation. More significant bursts also produce bluer and smaller descendants. Our findings imply that when galaxies shut down star formation rapidly, they generally had experienced compact, starburst events and that the large, observed spread in sizes and colors mostly reflects a variety of burst strengths. Recently quenched galaxies should have younger stellar ages in the centers; multi-wavelength data with high spatial resolution are required to reveal the age gradient. Highly dissipative processes should be responsible for this type of formation history. While determining the mechanisms for individual galaxies is challenging, some recently quenched galaxies show signs of gravitational interactions, suggesting that mergers are likely an important mechanism in triggering the rapid shut-down of star-formation activities at $z\sim0.8$.

  8. arXiv:1901.06350 (Published 2019-01-18)

    The Milky Way like galaxy NGC 6384 and its nuclear star cluster at high NIR spatial resolution using LBT/ARGOS commissioning data

    Iskren Y. Georgiev et al.
    Comments: 12 pages (+9 appendix), 11 figures, Accepted in MNRAS
    Categories: astro-ph.GA

    We analyse high spatial resolution near infra-red (NIR) imaging of NGC6384, a Milky Way like galaxy, using ARGOS commissioning data at the Large Binocular Telescope (LBT). ARGOS provides a stable PSF$_{\rm FWHM}\!=\!0.2"\!-\!0.3"$ AO correction of the ground layer across the LUCI2 NIR camera $4'\!\times4'$ field by using six laser guide stars (three per telescope) and a natural guide star for tip-tilt sensing and guiding. Enabled by this high spatial resolution we analyse the structure of the nuclear star cluster (NSC) and the central kiloparsec of NGC6384. We find via 2D modelling that the NSC ($r_{\rm eff}\!\simeq\!10$pc) is surrounded by a small ($r_{\rm eff}\!\simeq\!100$pc) and a larger Sersi\'c ($r_{\rm eff}\!\simeq\!400$pc), all embedded within the NGC\,6384 large-scale boxy/X-shaped bulge and disk. This proof-of-concept study shows that with the high spatial resolution achieved by ground-layer AO we can push such analysis to distances previously only accessible from space. SED-fitting to the NIR and optical HST photometry allowed to leverage the age-metallicity-extinction degeneracies and derive the effective NSC properties of an young to old population mass ratio of $8\%$ with ${\cal M}_{\rm\star,old}\!\simeq\!3.5\times10^7M_\odot$, Age$_{\rm old,\ young}\!=\!10.9\pm1.3$Gyr and 226\,Myr $\pm62\%$, metallicity [M/H]$=\!-0.11\pm0.16$ and $0.33\pm39\%$dex, and $E(B\!-\!V)\!=\!0.63$ and 1.44mag.

  9. arXiv:1810.12251 (Published 2018-10-29)

    ALMA detects a radial disk wind in DG Tau

    M. Guedel et al.
    Comments: Astronomy and Astrophysics Letters, in press, 8 pages
    Categories: astro-ph.SR, astro-ph.GA

    Aims: We aim to use the high spatial resolution of the Atacama Large Millimeter/submillimeter Array (ALMA) to map the flow pattern of molecular gas near DG Tau and its disk, a young stellar object driving a jet and a molecular outflow. Methods: We use observations from ALMA in the J = 2 - 1 transition of 12CO, 13CO, and C18O to study the Keplerian disk of DG Tau and outflows that may be related to the disk and the jet. Results: We find a new wind component flowing radially at a steep angle (~25 deg from the vertical) above the disk with a velocity of ~ 3.1 km/s. It continues the trend of decreasing velocity for increasing distance from the jet axis ("onion-like velocity structure"). Conclusions: The new component is located close to the protostellar disk surface and may be related to photoevaporative winds.

  10. arXiv:1805.10296 (Published 2018-05-25)

    WG021416.37-210535.3, a quadruply lensed quasar in three public surveys

    Adriano Agnello
    Comments: Published in RNAAS, two pages, one figure
    Categories: astro-ph.GA, astro-ph.CO

    The Southern Hemisphere has just recently begun to be charted by wide-field surveys, with a sufficient depth and image quality to enable the discovery of strongly lensed quasars. The quadruply imaged quasar WG0214-2105 (r.a.=02:14:16.37, dec.=-21:05:35.3) is a previously unknown lens, with `blue' mid-IR colors and high UV deficit, found in the intersection of three survey footprints: the Dark Energy Survey public DR1 (DES, Abbott et al. 2018), The VST-ATLAS (Shanks et al. 2015) and Pan-STARRS (Chambers et al. 2016). Its discovery relied on high spatial resolution from the Gaia mission (Lindegren et al. 2016) and mid-IR color preselection in the WISE catalog (Wright et al. 2010).

  11. arXiv:1805.09911 (Published 2018-05-24)

    Star Formation Rate Distribution in the Galaxy NGC 1232

    Alexandre Araújo de Souza, Lucimara P. Martins, Alberto Rodríguez-Ardila, Luciano Fraga
    Comments: 13 pages + table (27 pages), 8 figures
    Journal: 2018, AJ, 155, 234
    Categories: astro-ph.GA

    NGC 1232 is a face-on spiral galaxy and a great laboratory for the study of star-formation due to its proximity. We obtained high spatial resolution H{\alpha} images of this galaxy, with adaptive optics, using the SAM instrument at the SOAR telescope, and used these images to study its H ii regions. These observations allowed us to produce the most complete H ii region catalog for it to date, with a total of 976 sources. This doubles the number of H ii regions previously found for this object. We used these data to construct the H ii luminosity function, and obtained a power-law index lower than the typical values found for Sc galaxies. This shallower slope is related to the presence of a significant number of high-luminosity H ii regions (logL>39 dex). We also constructed the size distribution function, verifying that, as for most galaxies, NGC 1232 follows an exponential law. We also used the H{\alpha} luminosity to calculate the star formation rate. An extremely interesting fact about this galaxy is that X-ray diffuse observations suggest that NGC 1232 recently suffered a collision with a dwarf galaxy. We found an absence of star formation around the region where the X-ray emission is more intense, which we interpret as a star formation quenching due to the collision. Along with that, we found an excess of star-forming regions in the northeast part of the galaxy, where the X-ray emission is less intense.

  12. arXiv:1711.01593 (Published 2017-11-05)

    VLBA polarimetric monitoring of 3C 111

    T. Beuchert et al.
    Comments: 19 pages, 22 figures, 2 tables, accepted for publication by A&A
    Categories: astro-ph.GA

    We aim to better understand the dynamics within relativistic magneto-hydrodynamical flows in the extreme environment and close vicinity of supermassive black holes. To do so, we analyze the peculiar radio galaxy 3C 111, for which long-term polarimetric observations are available. We make use of the high spatial resolution of the VLBA network and the MOJAVE monitoring program, which provides high data quality also for single sources and allows us to study jet dynamics on parsec scales in full polarization with an evenly sampled time-domain. We additionally consider data from the IRAM 30-m Telescope as well as the SMA. Jet properties such as the electric vectors, the (polarized) flux density, feature size, and brightness temperature, describe a complex evolution of the polarized jet. The electric vector position angles (EVPAs) of features traveling down the jet perform a large and smooth rotation of $\gtrsim 180^{\circ}$ across a distance of about 20 pc. In contrast, the EVPAs are strongly variable within the first parsecs of the jet. We find a tendency towards transverse EVPAs across the jet with a local anomaly of aligned vectors in between. The transverse extent of the flow decreases coincident with a jump in brightness temperature around where we observe the EVPAs to turn into alignment with the jet flow. Also the gradients of the feature size and particle density with distance steepen in that region. We interpret the propagating polarized features with shocks and the observed local anomalies with the interaction of these shocks with a recollimation shock of the underlying flow. Together with a sheared magnetic field, this shock-shock interaction can explain the large rotation of the EVPA. The superimposed variability of the EVPAs close to the core is likely related to a clumpy Faraday screen, which also contributes significantly to the observed EVPA rotation in that region.

  13. arXiv:1703.02395 (Published 2017-03-07)

    Science cases for a visible interferometer

    Philippe Stee et al.
    Comments: White Paper prospective. This book is dedicated to the memory of our colleague Olivier Chesneau who passed away at the age of 41

    High spatial resolution is the key for the understanding various astrophysical phenomena. But even with the future E-ELT, single dish instruments are limited to a spatial resolution of about 4 mas in the visible. For the closest objects within our Galaxy most of the stellar photosphere remains smaller than 1 mas. With the success of long baseline interferometry these limitations were soom overcome. Today low and high resolution interferometric instruments on the VLTI and CHARA offer an immense range of astrophysical studies. Combining more telescopes and moving to visible wavelengths broadens the science cases even more. With the idea of developing strong science cases for a future visible interferometer, we organized a science group around the following topics: pre-main sequence and main sequence stars, fundamental parameters, asteroseismology and classical pulsating stars, evolved stars, massive stars, active galactic nuclei (AGNs) and imaging techniques. A meeting was organized in January 15 \& 16, 2015 in Nice with the support of the Action Specific in Haute R\'esolution Angulaire (ASHRA), the Programme National en Physique Stellaire (PNPS), the Lagrange Laboratory and the Observatoire de la C\^ote d'Azur, in order to present these cases and to discuss them further for future visible interferometers. This White Paper presents the outcome of the exchanges. This book is dedicated to the memory of our colleague Olivier Chesneau who passed away at the age of 41.

  14. arXiv:1403.2266 (Published 2014-03-10)

    High Spatial Resolution of the Mid-Infrared Emission of Compton-Thick Seyfert 2 Galaxy Mrk3

    Dinalva A. Sales, D. Ruschel-Dutra, M. G. Pastoriza, R. Riffel, Cláudia Winge
    Comments: 11 pages, 6 figures
    Journal: MNRAS, 2014
    Categories: astro-ph.GA

    Mid-infrared (MIR) spectra observed with Gemini/Michelle were used to study the nuclear region of the Compton-thick Seyfert 2 (Sy 2) galaxy Mrk 3 at a spatial resolution of $\sim$200 pc. No polycyclic aromatic hydrocarbons (PAHs) emission bands were detected in the N-band spectrum of Mrk 3. However, intense [Ar III] 8.99 $\mu$m, [S IV] 10.5 $\mu$m and [Ne II] 12.8 $\mu$m ionic emission-lines, as well as silicate absorption feature at 9.7$\mu$m have been found in the nuclear extraction ($\sim$200 pc). We also present subarcsecond-resolution Michelle N-band image of Mrk 3 which resolves its circumnuclear region. This diffuse MIR emission shows up as a wings towards East-West direction closely aligned with the S-shaped of the Narrow Line Region (NLR) observed at optical [O III]$\lambda$5007\AA image with Hubble/FOC. The nuclear continuum spectrum can be well represented by a theoretical torus spectral energy distribution (SED), suggesting that the nucleus of Mrk 3 may host a dusty toroidal structure predicted by the unified model of active galactic nucleus (AGN). In addition, the hydrogen column density (N$_H\,=\,4.8^{+3.3}_{-3.1}\times\,10^{23}$ cm$^{-2}$) estimated with a torus model for Mrk 3 is consistent with the value derived from X-ray spectroscopy. The torus model geometry of Mrk 3 is similar to that of NGC 3281, both Compton-thick galaxies, confirmed through fitting the 9.7$\mu$m silicate band profile. This results might provide further evidence that the silicate-rich dust can be associated with the AGN torus and may also be responsible for the absorption observed at X-ray wavelengths in those galaxies.

  15. arXiv:1309.0433 (Published 2013-09-02)

    The B1 shock in the L1157 outflow as seen at high spatial resolution

    M. Benedettini et al.
    Comments: 13 pages, 12 figures
    Categories: astro-ph.GA

    We present high spatial resolution (750 AU at 250 pc) maps of the B1 shock in the blue lobe of the L1157 outflow in four lines: CS (3-2), CH3OH (3_K-2_K), HC3N (16-15) and p-H2CO (2_02-3_01). The combined analysis of the morphology and spectral profiles has shown that the highest velocity gas is confined in a few compact (~ 5 arcsec) bullets while the lowest velocity gas traces the wall of the gas cavity excavated by the shock expansion. A large velocity gradient model applied to the CS (3-2) and (2-1) lines provides an upper limit of 10^6 cm^-3 to the averaged gas density in B1 and a range of 5x10^3< n(H2)< 5x10^5 cm^-3 for the density of the high velocity bullets. The origin of the bullets is still uncertain: they could be the result of local instabilities produced by the interaction of the jet with the ambient medium or could be clump already present in the ambient medium that are excited and accelerated by the expanding outflow. The column densities of the observed species can be reproduced qualitatively by the presence in B1 of a C-type shock and only models where the gas reaches temperatures of at least 4000 K can reproduce the observed HC3N column density.

  16. arXiv:1205.1577 (Published 2012-05-08)

    Complex Organic Molecules at High Spatial Resolution Toward Orion-KL I: Spatial Scales

    Susanna L. Widicus Weaver, Douglas N. Friedel
    Comments: Accepted for publication in the Astrophysical Journal Supplement; Part 1 of a 2 paper series; 37 pages
    Categories: astro-ph.GA

    Here we present high spatial resolution (<1 arcsecond) observations of molecular emission in Orion-KL conducted using the Combined Array for Research in Millimeter-Wave Astronomy (CARMA). This work was motivated by recent millimeter continuum imaging studies of this region conducted at a similarly high spatial resolution, which revealed that the bulk of the emission arises from numerous compact sources, rather than the larger-scale extended structures typically associated with the Orion Hot Core and Compact Ridge. Given that the spatial extent of molecular emission greatly affects the determination of molecular abundances, it is important to determine the true spatial scale for complex molecules in this region. Additionally, it has recently been suggested that the relative spatial distributions of complex molecules in a source might give insight into the chemical mechanisms that drive complex chemistry in star-forming regions. In order to begin to address these issues, this study seeks to determine the spatial distributions of ethyl cyanide [C2H5CN], dimethyl ether [(CH3)2O], methyl formate [HCOOCH3], formic acid [HCOOH], acetone [(CH3)2CO], SiO, methanol [CH3OH], and methyl cyanide [CH3CN] in Orion-KL at \lambda = 3 mm. We find that for all observed molecules, the molecular emission arises from multiple components of the cloud that include a range of spatial scales and physical conditions. Here we present the results of these observations and discuss the implications for studies of complex molecules in star-forming regions.

  17. arXiv:1011.0710 (Published 2010-11-02)

    A High-Resolution Survey of HI Absorption toward the Central 200 pc of the Galactic Center

    Cornelia C. Lang, W. Miller Goss, Claudia Cyganowski, Kelsey I. Clubb
    Comments: 59 pages, including 41 figures; accepted for publication in Astrophysical Journal Supplement Series in December 2010
    Categories: astro-ph.GA

    We present an HI absorption survey of the central 250 pc of the Galaxy. Very Large Array (VLA) observations were made at 21 cm in the DnC and CnB configurations and have a resolution of ~15"(0.6 pc at the Galactic Center (GC) distance) and a velocity resolution of ~2.5 km/s. This study provides HI data with high spatial resolution, comparable with the many high resolution observations which have been made of GC sources over the past ten years. Here we present an overview of the HI absorption toward ~40 well-known continuum sources and a detailed comparison of the ionized, atomic and molecular components of the interstellar medium for the Sgr B, Radio Arc and Sgr C regions. In these well-known regions, the atomic gas appears to be closely correlated in both velocity and distribution to the ionized and molecular gas, indicating that it resides in photo-dissociation regions related to the HII regions in the GC. Toward the majority of the radio continuum sources, HI absorption by the 3-kpc arm is detected, constraining these sources to lie beyond a 5 kpc distance in the Galaxy.