An official website of the United States government
HII regions are the archetypical tracers of high-mass star formation. Because of their high luminosities, they can be seen across the entire Galactic disk from mid-infrared to radio wavelengths. A uniformly sensitive survey of Galactic HII regions across the disk would allow us to constrain the properties of Galactic structure and star formation. We have cataloged over 8000 HII regions and candidates in the WISE Catalog of Galactic HII Regions (astro.phys.wvu.edu/wise), but only 2000 of these are confirmed HII regions. The work is ongoing, but from our survey completeness limits and population synthesis modeling, we predict there are nearly 10,000 HII regions in the Milky Way created by a central star of type B2 or earlier. A population of especially interesting HII regions trace the Outer Scutum-Centaurus spiral arm (OSC), the most distant molecular spiral arm in the Milky Way. These regions represent star formation at low densities and low metallicities, similar to the conditions in galaxies like the Large Magellanic Cloud or a much younger Milky Way. To date, we have detected high-mass star formation at 17 locations in the OSC, with the most distant source at 23.5 kpc from the Sun and 17 kpc from the Galactic Center. They have molecular cloud masses up to 105 Msol and central stellar types as early as O4. By comparing molecular and stellar masses, we can begin to put constraints on the star formation efficiency of these distant outer Galaxy sources. We map the ionized gas using the Very Large Array at X-band in the D-configuration. We map the 13CO, HCN, and HCO+ molecular gas emission using the Argus array on the Green Bank Telescope, producing individual 5 arcmin maps with 8 arcsec resolution and 0.5 K sensitivity in 20 minutes.
more »
« less
A high pitch angle structure in the Sagittarius Arm
Context. In spiral galaxies, star formation tends to trace features of the spiral pattern, including arms, spurs, feathers, and branches. However, in our own Milky Way, it has been challenging to connect individual star-forming regions to their larger Galactic environment owing to our perspective from within the disk. One feature in nearly all modern models of the Milky Way is the Sagittarius Arm, located inward of the Sun with a pitch angle of ∼12°. Aims. We map the 3D locations and velocities of star-forming regions in a segment of the Sagittarius Arm using young stellar objects (YSOs) from the Spitzer /IRAC Candidate YSO (SPICY) catalog to compare their distribution to models of the arm. Methods. Distances and velocities for these objects are derived from Gaia EDR3 astrometry and molecular line surveys. We infer parallaxes and proper motions for spatially clustered groups of YSOs and estimate their radial velocities from the velocities of spatially associated molecular clouds. Results. We identify 25 star-forming regions in the Galactic longitude range ℓ ∼ 4. ° 0–18. ° 5 arranged in a narrow, ∼1 kpc long linear structure with a high pitch angle of ψ = 56° and a high aspect ratio of ∼7:1. This structure includes massive star-forming regions such as M8, M16, M17, and M20. The motions in the structure are remarkably coherent, with velocities in the direction of Galactic rotation of | V φ |≈240 ± 3 km s −1 (slightly higher than average) and slight drifts inward ( V R ≈ −4.3 km s −1 ) and in the negative Z direction ( V Z ≈ −2.9 km s −1 ). The rotational shear experienced by the structure is ΔΩ = 4.6 km s −1 kpc −1 . Conclusions. The observed 56° pitch angle is remarkably high for a segment of the Sagittarius Arm. We discuss possible interpretations of this feature as a substructure within the lower pitch angle Sagittarius Arm, as a spur, or as an isolated structure.
more »
« less
- PAR ID:
- 10313513
- Date Published:
- Journal Name:
- Astronomy & Astrophysics
- Volume:
- 651
- ISSN:
- 0004-6361
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
The Outer Scutum-Centaurus spiral arm (OSC) is the outermost molecular spiral arm in the Galaxy and contains the most distant known high-mass star formation regions in the Milky Way. HII regions are the archetypical tracers of high-mass star formation, and because of their high luminosities, they can be seen across the entire Galactic disk from mid-infrared to radio wavelengths. We have detected HII regions at nearly 20 locations in the OSC, as far as 23.5 kpc from the Sun and 15 kpc from the Galactic center on the far side of the Galactic center. The far outer Galaxy has lower metallicity than the more inner regions of the Milky Way, with 12 + log(O/H) = 8.29 at the OSC versus 8.9 and 8.54 at the Galactic Center and the Solar neighborhood, respectively. Coupled with lower gas densities, star formation in the OSC could be similar to that of a much younger Milky Way or galaxies like the Large Magellanic Cloud. We find large reservoirs of diffuse and dense molecular gas (13CO, HCO+, HCN) in the OSC with the Argus array on the Green Bank Telescope (up to 105 Solar masses). We are also able to estimate the central ionizing sources from Very Large Array continuum observations, showing central stellar types as early as O4. Combined, these observations allow us to study chemical abundances and star formation efficiencies on the outer edge of the Milky Way, putting constraints on star formation properties towards the edge of the Galaxy’s molecular disk.more » « less
-
The Outer Scutum-Centaurus spiral arm (OSC) is the outermost molecular spiral arm in the Galaxy and contains the most distant known high-mass star formation regions in the Milky Way. HII regions are the archetypical tracers of high-mass star formation, and because of their high luminosities, they can be seen across the entire Galactic disk from mid-infrared to radio wavelengths. We have detected HII regions at nearly 20 locations in the OSC, as far as 23.5 kpc from the Sun and 15 kpc from the Galactic center on the far side of the Galactic center. The far outer Galaxy has lower metallicity than the more inner regions of the Milky Way, with 12 + log(O/H) = 8.29 at the OSC versus 8.9 and 8.54 at the Galactic Center and the Solar neighborhood, respectively. Coupled with lower gas densities, star formation in the OSC could be similar to that of a much younger Milky Way or galaxies like the Large Magellanic Cloud. We find large reservoirs of diffuse and dense molecular gas (13CO, HCO+, HCN) in the OSC with the Argus array on the Green Bank Telescope (up to 105 Solar masses). We are also able to estimate the central ionizing sources from Very Large Array continuum observations, showing central stellar types as early as O4. Combined, these observations allow us to study chemical abundances and star formation efficiencies on the outer edge of the Milky Way, putting constraints on star formation properties towards the edge of the Galaxy's molecular disk.more » « less
-
There is relatively little known about Galactic star formation in the outer edges of the Milky Way, particularly in the Outer Scutum-Centaurus spiral arm (OSC). Lying about 15 kpc from the center of the Galaxy, the OSC was discovered in 2011 and is the most distant molecular spiral arm of the Milky Way. The OSC warps up to 4 degrees above the Galactic plane and as a result, has been excluded from the scope of many surveys of the Galactic plane, typically confined to a single degree above or below the plane. The goal of our study is to identify radio continuum from HII regions in the OSC in order to better understand the population of high-mass star formation regions in the outer Galaxy. We observed 12 HII Regions in the OSC using the Very Large Array at 10 GHz. Of our 12 targets, 7 are re-observations of undetected sources from Armentrout et al. (2017). The remaining 5 targets are sources without previously observed 10 GHz radio continuum data. We identify 10 GHz radio continuum associated with 7 of our OSC HII region targets for the first time. Assuming one dominant ionizing source per HII region, we assign spectral types from O9 to O5.5 for these sources, depending on their distance and continuum intensity. The remaining 5 nondetections represent lower-mass (B-type) star-forming regions below the sensitivity limit of our survey. These regions represent very high-mass star formation on the outer edge of the Galaxy, where densities and metallicities might be more similar to that of a much younger Milky Way or lower mass galaxies like the Magellanic Clouds.more » « less
-
null (Ed.)Luminous hot stars ( M K s ≲ 0 mag and T eff ≳ 8000 K) dominate the stellar energy input to the interstellar medium throughout cosmological time, are used as laboratories to test theories of stellar evolution and multiplicity, and serve as luminous tracers of star formation in the Milky Way and other galaxies. Massive stars occupy well-defined loci in colour–colour and colour–magnitude spaces, enabling selection based on the combination of Gaia EDR3 astrometry and photometry and 2MASS photometry, even in the presence of substantive dust extinction. In this paper we devise an all-sky sample of such luminous OBA-type stars, which was designed to be complete rather than very pure, providing targets for spectroscopic follow-up with the SDSS-V survey. To estimate the purity and completeness of our catalogue, we derive stellar parameters for the stars in common with LAMOST DR6 and we compare the sample to other O and B-type star catalogues. We estimate ‘astro-kinematic’ distances by combining parallaxes and proper motions with a model for the expected velocity and density distribution of young stars; we show that this adds useful constraints on the distances and therefore luminosities of the stars. With these distances we map the spatial distribution of a more stringently selected subsample across the Galactic disc, and find it to be highly structured, with distinct over- and under-densities. The most evident over-densities can be associated with the presumed spiral arms of the Milky Way, in particular the Sagittarius-Carina and Scutum-Centaurus arms. Yet, the spatial picture of the Milky Way’s young disc structure emerging in this study is complex, and suggests that most young stars in our Galaxy ( t age < t dyn ) are not neatly organised into distinct spiral arms. The combination of the comprehensive spectroscopy to come from SDSS-V (yielding velocities, ages, etc.) with future Gaia data releases will be crucial in order to reveal the dynamical nature of the spiral arms themselves.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1051/0004-6361/202141198
