Special Session: The extreme ISM in the inner 200 pc of the Galaxy
Event date
-
Location
Online
Event Type
Meeting

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The inner galaxy provides a unique environment to study both interstellar medium processes in extreme states and the conditions of active galactic nuclei. The central 200pc of the Milky Way shows high densities, radiation field, cosmic ray fluxes as well as strong magnetic fields. These effects may explain the high rate and special characteristics of galactic center star formation. Energetic events originating in the vicinity of the central black hole shapes the kinematics and structure, especially out of the plane. Several new data sets and observatories have recently given improved insights into the gas, dust, and magnetic field, including SOFIA, ALMA, and MeerKat observations. We will be having a Special Session to explore this new data on the exciting characteristics of the extreme ISM in the central 200 pc, with the following invited talks:

Shuo Zhang (Bard College): Giant Molecular Clouds: Story Tellers of the Galactic Center's History in the Past Few Hundred Years

Watch Shuo Zhang (Bard College) video using this link

Mark Morris (UCLA): Probing the Remarkable Galactic Center Magnetic Field with HAWC+

Watch Mark Morris (UCLA) video using this link

Perry Hatchfield (University of Connecticut): What "IGNITES" Star Formation in the Central Molecular Zone? A multi-wavelength exploration of early stages of star formation throughout the Galactic Center

Watch Perry Hatchfield (University of Connecticut) video using this link

Andrew Harris (University of Maryland): SOFIA-upGREAT imaging spectroscopy of the [C II] 150um fine structure line of the SgrB region in the Galactic center

Watch Andrew Harris (University of Maryland) video using this link

Peter Schilke (University of Cologne): Using Hydrides to determine the state of the ISM

Watch Peter Schilke (University of Cologne) video using this link

Shuo Zhang (Bard College): Giant Molecular Clouds: Story Tellers of the Galactic Center's History in the Past Few Hundred Years
Bard College, Annandale-on-Hudson, NY
The central molecular zone extending up to ~300 pc from the Galactic center harbors the densest and most massive molecular clouds of the Milky Way, containing about 5-10% of the total molecular gas mass of the entire galaxy. Two decades of X-ray observations have revealed that these giant molecular clouds exhibit time-varying Fe Kα line emission and hard X-ray continuum emission. This phenomena can be best explained by the X-ray reflection scenario, where cold molecular gas reflect incoming X-rays. Past Sgr A* X-ray outbursts are the mostly likely source that illuminated the central molecular zone. Therefore, X-ray emission from molecular clouds at different distances from the Galactic center can be used to reconstruct Sgr A*'s activity history in the past a few hundred years. In this talk, I will review the X-ray observation efforts on the central molecular zone and also present the newest results on the molecular clouds' time variability in the 20-year-long baseline.

Mark Morris (UCLA): Probing the Remarkable Galactic Center Magnetic Field with HAWC+
University of California, Los Angeles, Los Angeles, CA
The ever-growing capabilities of instrumentation across the electromagnetic spectrum from radio to X-rays have recently honed our view of the magnetic field in the central few hundred parsecs of the Galaxy. We now understand that the field is relatively strong compared to the rest of the Galaxy, it is highly ordered on large scales, and it has a substantial effect on the dynamics of the gas there. SOFIA is playing a key role in this era of discovery with the HAWC+ Far-IR Bolometer Camera and Polarimeter, which measures polarized thermal emission from magnetically aligned dust grains. HAWC+ has elucidated the magnetic field structure within dense regions of the central molecular zone of the Galaxy, including the circumnuclear disk orbiting the central supermassive black hole. A particularly revealing environment is the Sickle HII region, G0.18-0.04, and its associated molecular cloud, which lies at the intersection of the magnetic, nonthermal Radio Arc structure and the superbubble surrounding the Quintuplet cluster of massive, young stars. HAWC+ polarimetry of the Sickle shows in detail how the field has interacted with the HII region at and near the ionization front. The interaction has morphological consequences, depending on whether the field in the molecular cloud is perpendicular or parallel to the ionization front. The data also indicate clearly that the field within the dense gas of the Sickle cloud and HII region is perpendicular to the ambient large-scale inter-cloud field probed by radio continuum observations. At certain points, the two field systems appear to be undergoing a direct interaction, possibly creating the relativistic particles of the Radio Arc via magnetic field line reconnection.

Perry Hatchfield (University of Connecticut): What "IGNITES" Star Formation in the Central Molecular Zone? A multi-wavelength exploration of early stages of star formation throughout the Galactic Center
The Milky Way’s Central Molecular Zone (CMZ) provides a unique opportunity to explore the physics of star formation under extreme physical conditions unlike those found elsewhere in the Galaxy. Recent and upcoming surveys of the innermost few hundred parsecs of the Galaxy have provided us with the chance to explore both star forming and quiescent dense gas structures in the CMZ with a panoply of different wavelength observations, including those from SOFIA, Spitzer, Herschel, and the Submillimeter Array. I will present a preliminary look at the early onset of star formation in all dense CMZ clouds, complete to all possible ongoing massive star formation. I will introduce the IGNITES project, in collaboration with the FORCAST Galactic Center Program, to characterize the far infrared dust continuum emission from both young stellar objects and quiescent dense substructure throughout the CMZ. I will discuss the ongoing work and future plans to use data from FORCAST to expand our understanding of star formation in the context of the extreme dynamical environment of the Galactic Center.

1Northwestern Univ., Evanston, IL
Recent radio and X-ray observations show evidence of a large-scale 400pc structure away from the Galactic plane which has been interpreted as a relic of activity from a few million years ago. Notably, nearly all of the mysterious Galactic center magnetized filaments are housed within this large-scale structure suggesting a possible causal connection between the two. In addition there has recently been significant appreciation of the role of cosmic rays in driving a wind from the Galactic center region. In this talk, after a brief history of VLA discovery of radio filaments from the 80's, I will discuss an interpretation that ties together many aspects of recent measurements. In addition, I will present a new statistical study of the spectral index, magnetic field strength and size distribution of about 1000 nonthermal radio filaments.

Andrew Harris (University of Maryland): SOFIA-upGREAT imaging spectroscopy of the [C II] 150um fine structure line of the SgrB region in the Galactic center
Andrew Harris1, Rolf Guesten2, Miguel Angel Requena Torres1, Denise Riquelme2, GREAT Team3
1University of Maryland, College Park, College Park, MD, 2MPI Radioastronomie, Bonn, Germany, 3MPIfR/Uni Cologne, Bonn/Cologne, Germany
SOFIA-upGREAT spectroscopic imaging of the 158um [C II] spectral line explored a 67x45 pc field toward the Sgr B region in our Galactic center. Sgr B extends as a coherent structure spanning some 34 pc along the Galactic plane. Bright [C II] emission encompasses Sgr B1 (G0.5--0.0), the G0.60-0.01 H II region, and passes behind and beyond the luminous star forming cores toward Sgr B2 (G0.7--0.0). [C II], 70um, and 20cm emission share nearly identical spatial distributions. Combined with the lack of [C II] self-absorption, this indicates that these probes trace UV on the near surfaces of more extended clouds visible in CO isotopologues and 160um continuum. Stars from regions of local star formation likely dominate the UV field. Photodissociation regions and H II regions contribute similar amounts of [C II] flux. Sgr B is a major contributor to the entire Galactic center's [C II] luminosity, but the extreme star formation cores of Sgr B2 contribute negligible amounts to the total. Velocity fields and association with a narrow dust lane indicate that the cores may have been produced in a local cloud-cloud collision. The cores are likely local analogs of the intense star formation regions where ideas to explain the C+ deficit'' in ultra-luminous galaxies can be tested

Peter Schilke (University of Cologne): Using Hydrides to determine the state of the ISM
Peter Schilke1, David Neufeld2, Wonju Kim3, Arshia Jacob2, Dariusz Lis4
1University of Cologne, Koeln, Germany, 2Johns Hopkins University, Baltimore, MD, 3University of Cologne, Cologne, Germany, 4Jet Propulsion Laboratory, Pasadena, CA
The ISM consists of atomic and molecular gas, as well as radiation and cosmic rays. While some of these components can be, and have been for decades, detected and characterized by observing classical tracers such as atomic Hydrogen and molecules like CO, Herschel observations showed that a significant portion of the ISM is either missed entirely by the traditional tracers, or at least inadequately characterized. This statement concerns mostly the gas "in between" the purely molecular and atomic phases, and molecular, but CO-dark gas. Herschel has shown that these components as well as the cosmic ray flux can be characterized well by observation of hydrides. Those species, being fundamental chemical building blocks, are usually the first molecules that form and therefore reside in this atomic-molecular transition zone. Herschel produced some unexpected results, such as the detection of the ArH+ molecule, which is a tracer of almost pure atomic gas, much better than H itself. Being very light molecules, hydrides tend to have their ground state transitions at THz frequencies, outside the range accessible from the ground. Since the demise of Herschel, SOFIA is the only observatory that can carry the torch of hydride observations. In our Galaxy, most of these observations have been absorption measurements toward strong background continuum sources. This offers the advantage that column densities can be well described without uncertainties related to excitation conditions, but the disadvantage that only a few lines of sight are available. In the HyGAL SOFIA legacy project, we set out to characterize the state of the ISM using hydride observations. I will describe results from Herschel and SOFIA that can be used to derive ISM properties as a function of position in the Galaxy, including the Central Molecular Zone, and touch upon the problems encountered trying to assign the absorption to a specific location along the line of sight.