Peering Through Our Dusty Galaxy

A suite of on-going and forthcoming visible and near-infrared wavelength surveys – various Sloan Digitial Sky Surveys, the Gaia  mission, the Large Synoptic Survey Telescope – is beginning to reshape our view of the Milky Way Galaxy. This revolution in our understanding of the Milky Way Galaxy is likely to continue well into the next decade. At visible wavelengths, dust obscuration and absorption e ects can be significant toward the Galactic plane, but these effects are not problematic at radio wavelengths, and there is a rich history of using radio observations to find objects deep within the Galaxy. Further, there has been a host of surveys of the Galaxy at wavelengths that do penetrate deep into the Galaxy, from the infrared to X-ray (e.g., with Spitzer , Herschel , Chandra , XMM-Newton, and Fermi ) that the VLASS will complement, allowing for a rich multiwavelength characterization of sources detected. VLASS naturally lends itself to finding a variety of Galactic radio sources. The panoply of Galactic science means that there is a large discovery space for serendipitous phenomena.

The following three topics will be further illuminated by the Galactic coverage afforded by the VLASS.

Compact Objects: VLASS can be used as a “finding survey” for rare classes of pulsars such as double neutron star (DNS) or pulsar-blackhole (PSR-BH) systems. A series of “filters” such as spectral index, polarization and compactness can be used to winnow the large number of sources detected to a feasible number on which to conduct a periodicity search using single dish telescopes. The 2 – 4 GHz frequency of VLASS makes it especially sensitive to rare systems that are likely to be deep in the Galactic plane and highly scattered.

Coronal Magnetic Activity on Cool Stars: The radio emission from nearby active stars provides a unique probe of accelerated particles and magnetic fields that occur in them, which is useful for a broader understanding of dynamo processes in stars, as well as the particle environment around those stars. The large magnetic field strengths now known to occur around some brown dwarfs were first detected through their e ect on cm-wavelength radio emission before the signatures were seen through Zeeman splitting of absorption lines at near infrared wavelengths. The stellar byproduct of exoplanet transit probes like Kepler  and TESS will yield information on key stellar parameters like rotation, white-light flaring, and asteroseismic constraints on stellar ages. Unlike other diagnostics of magnetic activity, such as coronal emission which displays a maximum value of LX= Lbol 10􀀀3 corresponding to the maximum amount of coronal heating a star is able to maintain, incoherent stellar radio emission usually shows no saturation effects. Thus, the level of radio emission can vary by orders of magnitude depending on the instantaneous particle acceleration events and plasma properties. VLASS will be able to detect ultracool dwarfs to 10–20 pc, active dwarf stars to a few tens of parsecs, and active binaries to slightly less than 2 kpc. In the nearest star forming regions (150–300 pc), VLASS sensitivity limits will probe stellar radio luminosities that enable studies of particle acceleration in young stellar objects at a range of evolutionary stages.

Star Formation and Evolution, Distant Thermal Sources, and Galactic Structure: Young, massive stars produce HII regions, while intermediate and low-mass stars end their lives by expelling their outer layers into the ISM, producing planetary nebulae (PNe). In each case, the hot central star ionizes surrounding material, which then yields free-free radio emission. Radio observations permit identifying these thermal Galactic sources because they are relatively unaffected by dust obscuration. Detecting distant thermal sources, such as HII regions and PNe, requires angular resolution and adequate brightness temperature sensitivity. Given that most HII regions and PNe exhibit compact sub-structure, an angular resolution of a few arcseconds is desirable. Expected brightness temperatures might be a few hundreds of degrees to 1000 K. Thus, arcsecond angular resolution and a brightness temperature sensitivity of order 10 K or better is sufficient to detect large numbers of distant thermal sources. By combining the VLASS with radio observations at other frequencies, and infrared observations, an expanded sample of HII regions and PNe throughout 75% of the Galactic disk would be obtained.