Hot and Diffuse Clouds near the Galactic Center Probed by Metastable H3+

By T. Oka, T. Geballe, M. Goto, T. Usuda, B. McCall. 2005

181 citations recorded by the source corpus.

Using an absorption line from the metastable (J, K) = (3, 3) level of H together with other lines of H and CO observed along several sight lines, we have discovered a vast amount of high-temperature (T ~ 250 K) and low-density (n ~ 100 cm-3) gas with a large velocity dispersion in the central molecular zone (CMZ) of the Galaxy, i.e., within 200 pc of the center. Approximately three-fourths of the H along the line of sight to the brightest source we observed, the Quintuplet object GCS 3-2, is inferred to be in the CMZ, with the remaining H located in intervening spiral arms. About half of the H in the CMZ has velocities near ~-100 km s-1, indicating that it is associated with the 180 pc radius expanding molecular ring, which approximately forms the outer boundary of the CMZ. The other half, with velocities of ~-50 and ~0 km s-1, is probably closer to the center. CO is not very abundant in these clouds. Hot and diffuse gas in which the (3, 3) level is populated was not detected toward several dense clouds and diffuse clouds in the Galactic disk where large column densities of colder H have been reported previously. Thus, the newly discovered environment appears to be unique to the CMZ. The large observed H column densities in the CMZ suggest an ionization rate much higher than in the diffuse interstellar medium in the Galactic disk. Our finding that the H in the CMZ is almost entirely in diffuse clouds indicates that the reported volume filling factor (f ≥ 0.1) for n ≥ 104 cm-3 clouds in the CMZ is an overestimate by at least an order of magnitude.

Subfields: Astronomy and Astrophysics, Atmospheric Science, Spectroscopy

View Paper: https://www.semanticscholar.org/paper/2d8a96cc25700be789e65cdadb9947368c6e0804

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Citations in the corpus

STAR FORMATION IN THE CENTRAL 400 PC OF THE MILKY WAY: EVIDENCE FOR A POPULATION OF MASSIVE YOUNG STELLAR OBJECTS (2009, background): “…the distribution of molecular clouds is asymmetric in the so-called “central molecular zone” showing 2/3 of the molecular gas on the positive longitude side of the Galactic center region (Bania 1977; Bally et al. 1988; Oka et al. 2005; Tsuboi et al. 1999; Martin et al. 2004; Morris & Serabyn 1996).”
Cool outflows in galaxies and their implications (2020, unknown): “(e.g. Oka et al. 2005; Indriolo et al. 2007).”
Spatial distribution of interstellar gas in the innermost 3 kpc of our galaxy (2007, result): “This view of a two-component molecular gas in the GC region has found support in several subsequent studies relying on other tracer molecules (e.g., RodŕıguezFernández et al. 2001; Oka et al. 2005; Magnani et al. 2006).”
HERSCHEL SURVEY OF GALACTIC OH+, H2O+, AND H3O+: PROBING THE MOLECULAR HYDROGEN FRACTION AND COSMIC-RAY IONIZATION RATE (2014, background): “…the Galactic center (e.g., Monje et al. 2011; Sonnentrucker et al. 2013; Schilke et al. 2010, 2014), but other studies have shown that the entire velocity range under consideration also contains absorption from gas in the CMZ (e.g., Oka et al. 2005; Geballe & Oka 2010; Goto et al. 2011, 2014).”
A 100 pc ELLIPTICAL AND TWISTED RING OF COLD AND DENSE MOLECULAR CLOUDS REVEALED BY HERSCHEL AROUND THE GALACTIC CENTER (2011, background): “…dust temperatures of about 19 K, 70 to 75% arises from the HI phase with dust temperatures of 17 to 22 K, and less than 10% is emitted by warm dust with T > 29 K associated with the extended HII and warm (100 to 300 K) molecular phases traced by H+3 absorption (Oka et al. 2005, Goto et al. 2008).”
INVESTIGATING THE COSMIC-RAY IONIZATION RATE IN THE GALACTIC DIFFUSE INTERSTELLAR MEDIUM THROUGH OBSERVATIONS OF H+3 (2011, methodology): “…(McCall et al. 1998, 2002, 2003; Indriolo et al. 2007, 2010; Crabtree et al. 2011), and toward the Galactic Center (Geballe et al. 1999; Goto et al. 2002, 2008, 2011; Oka et al. 2005; Geballe & Oka 2010) can be and have been used to constrain the ionization rate in each of these environments.”
Observational Signatures of Particle Acceleration in Supernova Remnants (2012, result): “This is also supported, as noted by Yusef-Zadeh et al. (2007), by the observation of diffuse low-frequency radio emission in the central regions (LaRosa et al., 2005) and the high estimates of the ionization rate compared to the values obtained in the Galactic disk (Oka et al., 2005).”
The Nine Lives of Cosmic Rays in Galaxies (2015, background): “The H+3 observations revealed large column densities and surprisingly high abundances in excited rotational states (Goto et al. 2011, Oka et al. 2005) at radial velocities distinct from those of the dominant giant molecular clouds; this implies the existence of an interesting component of dilute,…”