First Results from HaloSat – A CubeSat to Study the Hot Galactic Halo

HaloSat is the first CubeSat for astrophysics funded by NASA\u27s Science Mission Directorate and is designed to map soft X-ray oxygen line emission across the sky in order to constrain the mass and spatial distribution of hot gas in the Milky Way. HaloSat will help determine if hot halos with temperatures near a million degrees bound to galaxies make a significant contribution to the cosmological budget of the normal matter (baryons). HaloSat was deployed from the International Space Station in July 2018 and began routine science operations in October 2018. We describe the on-orbit performance including calibration of the X-ray detectors and initial scientific results including an observation of a halo field and an observation of solar wind charge exchange emission from the helium-focusing cone

The Orion OB Association as a Generator for the Hot Circumgalactic Medium

Feedback from star formation may play a key role in energizing the hot, diffuse, X-ray emitting circumgalactic medium (CGM). We observed the diffuse hot gas on the interior of the Orion–Eridanus Superbubble (OES) produced by feedback from the Orion OB association. Using HaloSat, a CubeSat X-ray observatory, we cover the majority of the OES using 11 HaloSat fields, each with a 10° diameter. We find the gas is well described by two thermal plasma components. There are regions of enhanced emission measure (EM) that coincide with the Eridanus X-ray Enhancement and the Orion OB association. Individual field temperatures are statistically consistent with the weighted average of all of the OES fields: a warm temperature k T _w = 0.17 ± 0.02 keV and a hot temperature k T _h = 0.79 ± 0.12 keV. The gas is overpressured in comparison with typical interstellar medium pressures, and the rate of energy injected by Orion OB1 can sufficiently power growth of the superbubble. The gas’s radiative cooling timescale (∼30 Myr) is long in comparison with the rate of hot gas production. The temperatures and EMs of the gas agree with properties of the bulk CGM elsewhere in the Milky Way. If we take the OES as a typical superbubble, these factors together suggest that the hot CGM is energized by star formation activity

An X-ray Emission Study of the Milky Way Halo's Clumpy Distribution

International audienceSurrounding the Milky Way (MW) is a region of hot X-ray emitting gas, commonly referred to as the halo or the circumgalactic medium (CGM). The CGM has significant influence on the star formation history and evolution of the MW. Gas likely enters and exits the CGM from both the MW disk and from outside the MW, but the balance of these processes is not known. The HaloSat CubeSat has completed an all-sky survey of soft X-ray emission which is predominately produced by million-degree Kelvin gas in the CGM. A previous study of the HaloSat data mapped the distribution of CGM emission in the southern MW halo (30 degrees galactic latitude), which was also found to exhibit a similarly clumpy distribution. This strongly implies that this emission is dominated by gas ejected from the MW disk by feedback processes such as supernovae or OB stars. The HaloSat observations were also analyzed for any evidence of an additional hotter gas component in the CGM

An X-ray Emission Study of the Milky Way Halo's Clumpy Distribution

International audienceSurrounding the Milky Way (MW) is a region of hot X-ray emitting gas, commonly referred to as the halo or the circumgalactic medium (CGM). The CGM has significant influence on the star formation history and evolution of the MW. Gas likely enters and exits the CGM from both the MW disk and from outside the MW, but the balance of these processes is not known. The HaloSat CubeSat has completed an all-sky survey of soft X-ray emission which is predominately produced by million-degree Kelvin gas in the CGM. A previous study of the HaloSat data mapped the distribution of CGM emission in the southern MW halo (30 degrees galactic latitude), which was also found to exhibit a similarly clumpy distribution. This strongly implies that this emission is dominated by gas ejected from the MW disk by feedback processes such as supernovae or OB stars. The HaloSat observations were also analyzed for any evidence of an additional hotter gas component in the CGM

Widespread Detection of Two Components in the Hot Circumgalactic Medium of the Milky Way

International audienceSurrounding the Milky Way (MW) is the circumgalactic medium (CGM), an extended reservoir of hot gas that has significant implications for the evolution of the MW. We used the HaloSat all-sky survey to study the CGM's soft X-ray emission in order to better define its distribution and structure. We extend a previous HaloSat study of the southern CGM (Galactic latitude b 30°) and find evidence that at least two hot gas model components at different temperatures are required to produce the observed emission. The cooler component has a typical temperature of kT ∼0.18 keV, while the hotter component has a typical temperature of kT ∼0.7 keV. The emission measure in both the warm and hot components has a wide range (∼0.005-0.03, and ∼0.0005-0.004 cm −6 pc, respectively), indicating that the CGM is clumpy. A patch of relatively consistent CGM was found in the north, allowing for the CGM spectrum to be studied in finer detail using a stacked spectrum. The stacked spectrum is well described with a model including two hot gas components at temperatures of kT = 0.166 ± 0.005 keV and kT =-+ 0.69 0.05 0.04 keV. As an alternative to adding a hot component, a neon-enhanced single-temperature model of the CGM was also tested and found to have worse fit statistics and poor residuals