Imaging the Heart of Our Galaxy

New radio images of the center of the Milky Way are providing an unprecedented view of the structure and processes occurring in the Galactic center.

Sgr A structure

JVLA images of Sgr A at 5.5 GHz. The large-scale, bright ring structure is Sgr A East, a supernova remnant. The mini-spiral structure along the lower-right edge of the ring is Sgr A West, and Sgr A* is located near the center of the mini-spiral structure. Click for a closer look! [Zhao et al. 2016]

Improved Radio View

A recent study led by Jun-Hui Zhao (Harvard-Smithsonian Center for Astrophysics) presents new images of the Galactic center using the Jansky Very Large Array (JVLA) at 5.5 GHz. The images center on the radio-bright zone at the core of our galaxy, with the field of view covering the central 13’ of the Milky Way — equivalent to a physical size of ~100 light-years.

Due to recent hardware and software improvements in the VLA, these images are much deeper than any previously obtained of the Galactic center, reaching an unprecedented 100,000:1 dynamic range. Not only do these observations provide a detailed view of previously known structures within the Sagittarius A radio complex in the Milky Way’s heart, but they also reveal new features that can help us understand the processes that formed this bright complex.

Features in Sagittarius A

Sgr A consists of three main components nested within each other: the supernova remnant Sgr A East, the mini-spiral structure Sgr A West (located off-center within the Sgr A East structure), and the compact radio source Sgr A* (located near the center of the mini-spiral). Sgr A* is the supermassive black hole that resides at the very center of the Milky Way.

The newest JVLA images reveal numerous filamentary sources that trace out two radio lobes, oriented nearly perpendicular to the Galactic plane and ~50 light-years in size. These are smaller radio counterparts to the enormous (on the scale of 30,000 light-years!) gamma-ray Fermi bubbles that have been observed to extend from the Galactic center. The bipolar radio structures appear to be due to winds emanating from Sgr A* itself, from a central cluster of massive stars, or from a combination of the two.

Top: Superposition of the JVLA image of Sgr A (blue) and a molecular line image taken with the SMA (red) that shows Sgr A*’s circumnuclear disk. Bottom left: Molecular emission is shown in contours, and the Sigma Front is traced by blue lines. Bottom right: The authors’ geometrical model for the supernova explosion and resulting emission. [Adapted from Zhao et al. 2016]

Top: superposition of the JVLA image of Sgr A (blue) and a molecular line image (red) showing Sgr A*’s circumnuclear disk. Bottom left: molecular emission is shown in contours, and the Sigma Front is traced by blue lines. Bottom right: a geometrical model for the supernova explosion and resulting emission. [Zhao et al. 2016]

Supernova Structures

The outermost shape of Sgr A East — which looks like an elliptical ring — is thought to be an expanding spherical shell from a past supernova explosion, appearing as an ellipse because of our angle of view. In the newest JVLA images, Zhao and collaborators identify a new structure inside of the ring that they term the “Sigma Front”.

The authors argue that this emission front — which is shaped like the capital Greek letter sigma — may be the reflection of the supernova blast wave bouncing off of the dense, clumpy circumnuclear molecular disk around Sgr A* (which encircles the mini-spiral, but isn’t visible in radio wavelengths). Under this assumption, they use the Sigma Front to constrain the geometry of the supernova explosion.

These new JVLA images contain a wealth of information in their detail, and analysis is only just beginning. Further examination of these images will continue to help us learn about the activity at the heart of our galaxy.

Citation

Jun-Hui Zhao et al 2016 ApJ 817 171. doi:10.3847/0004-637X/817/2/171

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