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Register a new userThe hot circumgalactic medium of the Milky Way: evidence for super-virial, virial, and sub-virial temperature, non-solar chemical composition, and non-thermal line broadening
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For the first time, we present the simultaneous detection and characterization of three distinct phases at $>10^5$ K in $z=0$ absorption, using deep $it{Chandra}$ observations toward Mrk 421. The extraordinarily high signal-to-noise ratio ($geqslant60$) of the spectra has allowed us to detect a $it{hot}$ phase of the Milky Way circumgalactic medium (CGM) at 3.2$^{+1.5}_{-0.5}times$ 10$^7$ K, coexisting with a $textit{warm-hot}$ phase at 1.5$pm$0.1$times$10$^6$ K and a $textit{warm}$ phase at 3.0$pm$0.4$times$10$^5$ K. The $textit{warm-hot}$ phase is at the virial temperature of the Galaxy, and the $textit{warm}$ phase may have cooled from the $textit{warm-hot}$ phase, but the super-virial $textit{hot}$ phase remains a mystery. We find that [C/O] in the $textit{warm}$ and $textit{warm-hot}$ phases, [Mg/O] in the $textit{warm-hot}$ phase and [Ne/O] in the $textit{hot}$ phase are super-solar, and the $textit{hot}$ and the $textit{warm-hot}$ phases are $alpha-$enhanced. Non-thermal line broadening is evident in the $textit{warm-hot}$ and the $textit{hot}$ phases and it dominates the total line broadening. Our results indicate that the $>10^5$ 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.
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We analyzed Suzaku and Chandra observations of the soft diffuse X-ray background toward four sightlines 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 $rm kT = 0.176pm0.008 ~keV$ and the other at temperatures ranging between $rm kT = 0.65-0.90~ keV$. The uniform lower temperature component is consistent with the Galaxys virial temperature ($ sim10^{6}~ K$). The temperatures of the hotter components are similar to that recently discovered ($rm sim 10^{7}~ K$; Das et al.) in the sightline to blazar 1ES1553+113, passing close to the Fermi bubble. Alternatively, the spectra can be described by just one lower-temperature component with super-solar Neon abundance, once again similar to that found in the 1ES1553+113 sightline. The additional hot component or the overabundance of Ne is required at a significance of $>4sigma$, but we cannot distinguish between the two possibilities. These results show that the super-virial 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 present joint Suzaku and Chandra observations of MKW4. With a global temperature of 1.6 keV, MKW4 is one of the smallest galaxy groups that have been mapped in X-rays out to the virial radius. We measure its gas properties from its center to the virial radius in the north, east, and northeast directions. Its entropy profile follows a power-law of $propto r^{1.1}$ between R$_{500}$ and R$_{200}$ in all directions, as expected from the purely gravitational structure formation model. The well-behaved entropy profiles at the outskirts of MKW4 disfavor the presence of gas clumping or thermal non-equilibrium between ions and electrons in this system. We measure an enclosed baryon fraction of 11% at R$_{200}$, remarkably smaller than the cosmic baryon fraction of 15%. We note that the enclosed gas fractions at R$_{200}$ are systematically smaller for groups than for clusters from existing studies in the literature. The low baryon fraction of galaxy groups, such as MKW4, suggests that their shallower gravitational potential well may make them more vulnerable to baryon losses due to AGN feedback or galactic winds. We find that the azimuthal scatter of various gas properties at the outskirts of MKW4 is significantly lower than in other systems, suggesting that MKW4 is a spherically symmetric and highly relaxed system.
We study the impact of cosmic rays (CRs) on the structure of virial shocks, using a large suite of high-resolution cosmological FIRE-2 simulations accounting for CR injection by supernovae. In massive ($M_{rm halo} gtrsim 10^{11},M_{odot}$), low-redshift ($zlesssim 1-2$) halos, which are expected to form hot halos with slowly-cooling gas in quasi-hydrostatic equilibrium (with a stable virial shock), our simulations without CRs do exhibit clear virial shocks. The cooler phase condensing out from inflows becomes pressure-confined to over-dense clumps, embedded in low-density, volume-filling hot gas whose cooling time is much longer than inflow time. The gas thus transitions sharply from cool free-falling inflow, to hot and thermal-pressure supported at approximately the virial radius ($approx R_{rm vir}$), and the shock is quasi-spherical. With CRs, we previously argued that halos in this particular mass and redshift range build up CR-pressure-dominated gaseous halos. Here, we show that when CR pressure dominates over thermal pressure, there is no significant virial shock. Instead, inflowing gas is gradually decelerated by the CR pressure gradient and the gas is relatively subsonic out to and even beyond $R_mathrm{vir}$. Rapid cooling also maintains sub-virial temperatures in the inflowing gas within $sim R_mathrm{vir}$.
The initial velocity dispersion of newborn stars is a major unconstrained aspect of star formation theory. Using near-infrared spectra obtained with the APOGEE spectrograph, we show that the velocity dispersion of young (1-2 Myr) stars in NGC 1333 is 0.92+/-0.12 km/s after correcting for measurement uncertainties and the effect of binaries. This velocity dispersion is consistent with the virial velocity of the region and the diffuse gas velocity dispersion, but significantly larger than the velocity dispersion of the dense, star-forming cores, which have a sub-virial velocity dispersion of 0.5 km/s. Since the NGC 1333 cluster is dynamically young and deeply embedded, this measurement provides a strong constraint on the initial velocity dispersion of newly-formed stars. We propose that the difference in velocity dispersion between stars and dense cores may be due to the influence of a 70 micro-Gauss magnetic field acting on the dense cores, or be the signature of a cluster with initial sub-structure undergoing global collapse.
Many young extra-galactic clusters have a measured velocity dispersion that is too high for the mass derived from their age and total luminosity, which has led to the suggestion that they are not in virial equilibrium. Most of these clusters are confined to a narrow age range centred around 10 Myr because of observational constraints. At this age the cluster light is dominated by luminous evolved stars, such as red supergiants, with initial masses of ~13-22 Msun for which (primordial) binarity is high. In this study we investigate to what extent the observed excess velocity dispersion is the result of the orbital motions of binaries. We demonstrate that estimates for the dynamical mass of young star clusters, derived from the observed velocity dispersion, exceed the photometric mass by up-to a factor of 10 and are consistent with a constant offset in the square of the velocity dispersion. This can be reproduced by models of virialised star clusters hosting a massive star population of which ~25 is in binaries, with typical mass ratios of ~0.6 and periods of ~1000 days. We conclude that binaries play a pivotal role in deriving the dynamical masses of young (~10 Myr) moderately massive and compact (<1e5 Msun; > 1 pc) star clusters.
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