We present the discovery of a very hot gas phase of the Milky Way circumgalactic medium (CGM) at T ≈ 10 7 K, using deep XMM-Newton RGS observations of 1ES 1553+113. The hot gas, coexisting with a warm-hot phase at T ≈ 10 6 K is α−enhanced, with [O/Fe] = 0.9 +0.7 −0.3 , indicating core-collapse supernovae enrichment. Additionally we find [Ne/O] and [N/O] = 0.7 +1.6 −0.2 , such that N/Ne is consistent with solar. Along with the enrichment by AGB stars and core-collapse supernovae, this indicates that some Oxygen has depleted onto dust and/or transited to cooler gas phase(s). These results may affect previous baryonic and metallic mass estimations of the warm-hot and hot CGM from the observations of Oxygen emission and absorption. Our results provide insights on the heating, mixing and chemical enrichment of the Milky Way CGM, and provide inputs to theoretical models of galaxy evolution.

We present a deep XMM-Newton observation of the Galactic halo emission in the direction of the blazar 1ES 1553+113. In order to extract the Galactic halo component from the diffuse soft X-ray emission spectrum, accurately modeling the foreground components is crucial. Here we present complex modeling of the foregrounds with unprecedented details. A careful analysis of the spectrum yields two temperature components of the halo gas (T em 1 = 10 6.32 K, T em 2 = 10 6.82 K). We find that these temperatures obtained from the emission spectrum are not consistent with those from the absorption spectrum (T ab 1 = 10 6.11 K, T ab 2 = 10 7.06 K), unlike the previous studies that found only one temperature component of the Milky Way circumgalactic medium. This provides us with interesting insights into the nature of emitting and absorbing systems. We discuss several possibilities objectively, and conclude that most likely we are observing multiple (3 to 4) discrete temperatures between 10 5.5 K and 10 7 K.

We report a ≈3.6σ detection of the warm-hot, massive, extended circumgalactic medium (CGM) around an L starburst spiral galaxy NGC 3221, using deep Suzaku observations. The temperature of the gas is ≈ 10 6.1 K, comparable to that of the Milky Way CGM. The spatial extent of the gas is at least ≈ 150 kpc. For a β-model of density profile with solar abundance, the central emission measure is EM o ≈ 3 ± 1 × 10 −5 cm −6 kpc and the central electron density is n eo ≈ 3.8 ± 0.6 × 10 −4 cm −3 , with a slope of β ≈ 0.56. We investigate a range of β values, and find that the details of the density profile do not change our results significantly. The mass of the warm-hot gas, assuming MW-type metallicity of 0.3 Z is 22 ± 3 × 10 10 M . This is the most massive baryon component of the galaxy and can account for the missing baryons in NGC 3221. Ours is the first detection of an extended CGM around an L spiral galaxy, where the baryon fraction f b ≈ 0.14 ± 0.04 is consistent with the cosmological mean value. We also investigated the missing metals problem in conjunction with the missing baryons problem and conclude that metals are likely to be preferentially expelled from the galaxy. We further investigate the thermodynamics of the hot gaseous halo combining the physical properties of the galactic disk and the CGM. We find that the CGM can be heated and enriched with metals by the starburst-driven feedback. However, some of the outflowing gas is likely to leave the galaxy, and some is likely to precipitate back onto the disk, providing fuel for the next generation of star-formation.

We analyzed Suzaku and Chandra observations of the soft diffuse X-ray background toward four sight lines with the goal of characterizing the X-ray emission from the Milky Way circumgalactic medium (CGM). We identified two thermal components of the CGM, one at a uniform temperature of kT = 0.176 ± 0.008 keV and the other at temperatures in the range kT = 0.65–0.90 keV. The uniform lower-temperature component is consistent with the Galaxy’s virial temperature (∼106 K). The temperatures of the hotter components are similar to that recently discovered (∼107 K) in the sight line to blazar 1ES 1553+113, passing close to the Fermi bubble. Alternatively, the spectra can be described by just one lower-temperature component with supersolar neon abundance, once again similar to that found in the 1ES 1553+113 sight line. The additional hot component or the overabundance of Ne is required at a significance of >4σ, but we cannot distinguish between the two possibilities. These results show that the supervirial temperature gas or an enhanced Ne abundance in the warm-hot gas in the CGM is widespread, and these are not necessarily related to the Fermi bubble.

We show how dense compact discrete shells of circumstellar gas immediately outside the red supergiants affect the optical light curves of type II-P/II-L SNe taking the example of SN 2013ej. The earlier efforts in the literature had used an artificial circumstellar medium (CSM) stitched to the surface of an evolved star which had not gone through a phase of late-stage heavy mass loss, which in essence, is the source of the CSM to begin with. In contrast we allow enhanced mass loss rate from the modeled star during the 16 O and 28Si burning stages and construct the CSM from the resulting mass-loss history in a selfconsistent way. Once such evolved pre-SN stars are exploded, we find that the models with early interaction between the shock and the dense CSM reproduce the light curves far better than those without that mass loss and hence having no dense, nearby CSM. The required explosion energy for the progenitors with a dense CSM is reduced by almost a factor of two compared to those without the CSM. Our model, with a more realistic CSM profile and presupernova and explosion parameters, fits observed data much better throughout the rise, plateau and radioactive tail phases compared to previous studies. This points to an intermediate class of supernovae between type II-P/II-L and type II-n SNe with the characteristics of simultaneous UV and optical peak, slow decline after peak and a longer plateau.

Most of the baryonic mass in the circumgalactic medium (CGM) of a spiral galaxy is believed to be warm-hot, with temperature around 106 K. The narrow O vi absorption lines probe a somewhat cooler component at , but broad O vi absorbers have the potential to probe the hotter CGM. Here we present 376 ks Chandra LETG observations of a carefully selected galaxy in which the presence of broad O vi together with the non-detection of Lyα was indicative of hot gas. The strongest line expected to be present at ≈106 K is O vii λ21.602. There is a hint of an absorption line at the redshifted wavelength, but the line is not detected with better than 2σ significance. A physical model, taking into account strengths of several other lines, provides better constraints. Our best-fit absorber model has and . These parameters are consistent with the hot plasma model based on UV observations; other O vi models of cooler gas phases are ruled out at better than 99% confidence. Thus we have suggestive, but not conclusive evidence for the broad O vi absorber probing the hot gas from the shallow observations of this pilot program. About 800 ks of XMM-Newton observations will detect the expected absorption lines of O vii and O viii unequivocally. Future missions like XRISM, Arcus, and Athena will revolutionize the CGM science.

Using Suzaku data, we had found a 3.4σ evidence for the X-ray emitting warm–hot circumgalactic medium (CGM) in the L ⋆ galaxy NGC 3221. Here we present XMM-Newton data and outline an efficient, rigorous, and well-defined method to extract the faint CGM signal. We confirm the CGM detection at 4σ significance within 30–200 kpc of the galaxy. We claim with 99.62% confidence that the CGM is extended beyond 150 kpc. The average temperature of the CGM is K, but it is not isothermal. We find suggestive evidence for a declining temperature gradient out to 125 kpc and for supervirial temperature within 100 kpc. While a supervirial temperature component has been detected in the Milky Way CGM, this is the first time a temperature gradient has been observed in the warm–hot CGM of any spiral galaxy. The emission measure profile is well-fit with either a β-model or a constant-density profile. Deeper data are required to constrain the temperature and density profiles. We also confirm the Suzaku result that the warm–hot CGM is one of the most massive baryon components of NGC 3221 and can account for the missing galactic baryons.

For the first time, we present the simultaneous detection and characterization of three distinct phases at >105 K in z = 0 absorption using deep Chandra observations toward Mrk 421. The extraordinarily high signal-to-noise ratio (≥60) of the spectra has allowed us to detect a hot phase of the Milky Way circumgalactic medium (CGM) at 3.2 − 0.5 + 1.5 × 107 K coexisting with a warm-hot phase at 1.5 ± 0.1 × 106 K and a warm phase at 3.0 ± 0.4 × 105 K. The warm-hot phase is at the virial temperature of the Galaxy, and the warm phase may have cooled from the warm-hot phase, but the supervirial hot phase remains a mystery. We find that [C/O] in the warm and warm-hot phases, [Mg/O] in the warm-hot phase, and [Ne/O] in the hot phase are supersolar, and the hot and the warm-hot phases are α-enhanced. Nonthermal line broadening is evident in the warm-hot and the hot phases, and it dominates the total line broadening. Our results indicate that the >105 K CGM is a complex ecosystem. It provides insights on the thermal and chemical history of the Milky Way CGM and theories of galaxy evolution.