Absorption signatures of warm-hot gas at low redshift: NeVIII

At z<1 a large fraction of the baryons is thought to reside in diffuse gas that has been shock-heated to high temperatures (log T=5-6). Absorption by the 770.41, 780.32 {\AA} doublet of NeVIII in quasar spectra represents a unique tool to study this elusive warm-hot phase. We have developed an analytic model for the properties of NeVIII absorbers that allows for an inhomogeneous metal distribution. Our model agrees with the predictions of a simulation from the OWLS project indicating that the average line-of-sight metal-filling fraction within the absorbing gas is low (c_L~0.1). Most of the NeVIII in our model is produced in low-density, collisionally ionized gas (log n=-6 to -4 cm^-3, log T=5-6). Strong NeVIII absorbers (log N(NeVIII)>14), like those recently detected by HST/COS, are found to arise in higher density gas (log n>-4, log T=5.75). NeVIII cloudlets harbour only 1 per cent of the cosmic baryon budget. The baryon content of the surrounding gas (which has similar densities and temperatures as the NeVIII cloudlets) is a factor 1/c_L higher. We conclude that NeVIII absorbers are robust probes of shock-heated diffuse gas, but that spectra with signal-to-noise ratios S/N>100 would be required to detect the bulk of the baryons in warm-hot gas.Comment: 19 pages, 10 figures; MNRAS in press; minor changes with respect to v

The Magellanic Stream: break up and accretion onto the hot Galactic corona

The Magellanic HI Stream (~2x10^9 Msun [d/55 kpc]^2) encircling the Galaxy at a distance 'd' is arguably the most important tracer of what happens to gas accreting onto a disk galaxy. Recent observations reveal that the Stream's mass is in fact dominated (3:1) by its ionised component. Here we revisit the origin of the mysterious H-alpha recombination emission observed along much of its length that is overly bright (~150-200 milli-Rayleigh) for the known Galactic ultraviolet background (~20-40 mR / [d/55 kpc]^2). In an earlier model, we proposed that a slow shock cascade was operating along the Stream due to its interaction with the extended Galactic hot corona. We find that, for a smooth coronal density profile, this model can explain the bright H-alpha emission if the coronal density satisfies 2 < (n / 10^{-4} cm^{-3}) < 4 at d = 55 kpc. But in view of updated parameters for the Galactic halo and mounting evidence that most of the Stream must lie far beyond the Magellanic Clouds (d>55 kpc), we revisit the shock cascade model in detail. At lower densities, the HI gas is broken down by the shock cascade but mostly mixes with the hot corona without significant recombination. At higher densities, the hot coronal mass (including the other baryonic components) exceeds the baryon budget of the Galaxy. If the H-alpha emission arises from the shock cascade, the upper limit on the smooth coronal density constrains the Stream's mean distance to < 75 kpc. If, as some models indicate, the Stream is even further out, either the shock cascade is operating in a regime where the corona is substantially mass-loaded with recent gas debris, or an entirely different ionization mechanism is responsible.Comment: Significant expansion of the parameter space explored in response to referee's comments. ApJ accepte

An HST/COS legacy survey of intervening SiIII absorption in the extended gaseous halos of low-redshift galaxies

Doubly ionized silicon (SiIII) is a powerful tracer of diffuse ionized gas inside and outside of galaxies. It can be observed in the local Universe in ultraviolet (UV) absorption against bright extragalactic background sources. We here present an extensive study of intervening SiIII-selected absorbers and their relation to the circumgalactic medium (CGM) of galaxies at low redshift (z<=0.1), based on the analysis of UV absorption spectra along 303 extragalactic lines of sight obtained with the Cosmic Origins Spectrograph (COS) on board the Hubble Space Telescope (HST). Along a total redshift path of Dz=24 we identify 69 intervening SiIII systems that all show associated absorption from other low and high ions. We derive a bias-corrected number density of dN/dz(SiIII)=2.5 for absorbers with column densities log N(SiIII)>12.2. We develop a geometrical model for the absorption-cross section of the CGM around the local galaxy population and find excellent agreement between the model predictions and the observations. We further compare redshifts and positions of the absorbers with that of ~64,000 galaxies using archival galaxy-survey data. For the majority of the absorbers we identify possible host galaxies within 300 km/s of the absorbers and derive impact parameters rho<200 kpc, demonstrating that the spatial distributions of SiIII absorbers and galaxies are highly correlated. Our study indicates that the majority of SiIII-selected absorbers in our sample trace the CGM of nearby galaxies within their virial radii at a typical covering fraction of ~70 per cent. From a detailed ionization model we estimate that diffuse gas in the CGM around galaxies, as traced by SiIII, contains substantially more baryonic mass than their neutral interstellar medium.Comment: 32 pages, 17 figures; final version accepted for publication in A&

The rapid onset of stellar bars in the baryon-dominated centers of disk galaxies

Recent observations of high-redshift galactic disks ($z\approx 1-3$) show a strong negative trend in the dark matter fraction $f_{DM}$ with increasing baryonic surface density. For this to be true, the inner baryons must dominate over dark matter in early massive galaxies, as observed in the Milky Way today. If disks are dominant at early times, we show that stellar bars form promptly within these disks, leading to a high bar fraction at early times. New JWST observations provide the best evidence to date for mature stellar bars in this redshift range. The disk mass fraction $f_{disk}$ within $R_s=2.2 R_{disk}$ is the dominant factor in determining how rapidly a bar forms. Using 3D hydro simulations of halo-disk-bulge galaxies, we confirm the "Fujii relation" for the exponential dependence of the bar formation time $\tau_{bar}$ as a function of $f_{disk}$. For $f_{disk} > 0.3$, the bar formation time declines exponentially fast with increasing $f_{disk}$. This relation is a challenge to simulators - barred models with inadequate resolution fall off this curve. Instead of Fujii's arbitrary threshold for when a bar forms, for the first time, we exploit the exponential growth timescale associated with a positive feedback cycle as the bar emerges from the underlying disk. A modified, mass-dependent trend is observed for halos relevant to systems at cosmic noon ($10.5 < \log M_{halo} < 12$), where the bar onset is slower for higher mass halos at a fixed $f_{disk}$. If baryons dominate over dark matter within $R \approx R_s$, we predict that a high fraction of bars will be found in high-redshift disks long before $z = 1$. Due to its widespread use in simulations, we investigate the Efstathiou-Lake-Negroponte criterion for bar instability: this sub-optimal parameter is inversely related to $f_{disk}$, with a secondary dependence on $M_{halo}$.Comment: 27 pages, 8 figures, 1 table - Astrophysical Journal, accepted (9 March 2023

Prospective dark matter annihilation signals from the Sagittarius Dwarf Spheroidal

The Sagittarius Dwarf Spheroidal galaxy (Sgr) is investigated as a target for dark matter (DM) annihilation searches utilizing J-factor distributions calculated directly from a high-resolution hydrodynamic simulation of the infall and tidal disruption of Sgr around the Milky Way. In contrast to past studies, the simulation incorporates DM, stellar and gaseous components for both the Milky Way and the Sgr progenitor galaxy. The simulated distributions account for significant tidal disruption affecting the DM density profile. Our estimate of the J-factor value for Sgr, JSgr = 1.48 × 1010 M2☉ kpc−5 (6.46 × 1016 GeV cm−5), is significantly lower than found in prior studies. This value, while formally a lower limit, is likely close to the true J-factor value for Sgr. It implies a DM cross-section incompatibly large in comparison with existing constraints would be required to attribute recently observed gamma-ray emission from Sgr to DM annihilation. We also calculate a J-factor value using a NFW profile fitted to the simulated DM density distribution to facilitate comparison with past studies. This NFW J-factor value supports the conclusion that most past studies have overestimated the dark matter density of Sgr on small scales. This, together with the fact that the Sgr has recently been shown to emit gamma-rays of astrophysical origin, complicate the use of Sgr in indirect DM detection searches

Magnetized High Velocity Clouds in the Galactic Halo: A New Distance Constraint

High velocity gas that does not conform to Galactic rotation is observed throughout the Galaxy's halo. One component of this gas, H i high velocity clouds (HVCs), have attracted attention since their discovery in the 1960s and remain controversial in terms of their origins, largely due to the lack of reliable distance estimates. The recent discovery of enhanced magnetic fields toward HVCs has encouraged us to explore their connection to cloud evolution, kinematics, and survival as they fall through the magnetized Galactic halo. For a reasonable model of the halo magnetic field, most infalling clouds see transverse rather than radial field lines. We find that significant compression (and thereby amplification) of the ambient magnetic field occurs in front of the cloud and in the tail of material stripped from the cloud. The compressed transverse field attenuates hydrodynamical instabilities. This delays cloud destruction, though not indefinitely. The observed ${\boldsymbol{B}}$ field compression is related to the cloud's distance from the Galactic plane. As a result, the observed rotation measure provides useful distance information on a cloud's location.A.G. and T.T. G. acknowledge financial support from the Australian Research Council (ARC) through an Australian Laureate Fellowship awarded to J.B.H. N.M.G. acknowledges the support of the ARC through Future Fellowship FT150100024

The influence of phytochemical composition and resulting sensory attributes on preference for salad rocket (Eruca sativa) accessions by consumers of varying TAS2R38 diplotype

Seven accessions of Eruca sativa (“salad rocket”) were subjected to a randomised consumer assessment. Liking of appearance and taste attributes were analysed, as well as perceptions of bitterness, hotness, pepperiness and sweetness. Consumers were genotyped for TAS2R38 status to determine if liking is influenced by perception of bitter compounds such as glucosinolates (GSLs) and isothiocyanates (ITCs). Responses were combined with previously published data relating to phytochemical content and sensory data in Principal Component Analysis to determine compounds influencing liking/perceptions. Hotness, not bitterness, is the main attribute on which consumers base their liking of rocket. Some consumers rejected rocket based on GSL/ITC concentrations, whereas some preferred hotness. Bitter perception did not significantly influence liking of accessions, despite PAV/PAV ‘supertasters’ scoring higher for this attribute. High sugar-GSL/ITC ratios significantly reduce perceptions of hotness and bitterness for some consumers. Importantly the GSL glucoraphanin does not impart significant influence on liking or perception traits

The Nature of the Warm/Hot Intergalactic Medium I. Numerical Methods, Convergence, and OVI Absorption

We perform a series of cosmological simulations using Enzo, an Eulerian adaptive-mesh refinement, N-body + hydrodynamical code, applied to study the warm/hot intergalactic medium. The WHIM may be an important component of the baryons missing observationally at low redshift. We investigate the dependence of the global star formation rate and mass fraction in various baryonic phases on spatial resolution and methods of incorporating stellar feedback. Although both resolution and feedback significantly affect the total mass in the WHIM, all of our simulations find that the WHIM fraction peaks at z ~ 0.5, declining to 35-40% at z = 0. We construct samples of synthetic OVI absorption lines from our highest-resolution simulations, using several models of oxygen ionization balance. Models that include both collisional ionization and photoionization provide excellent fits to the observed number density of absorbers per unit redshift over the full range of column densities (10^13 cm-2 <= N_OVI <= 10^15 cm^-2). Models that include only collisional ionization provide better fits for high column density absorbers (N_OVI > 10^14 cm^-2). The distribution of OVI in density and temperature exhibits two populations: one at T ~ 10^5.5 K (collisionally ionized, 55% of total OVI) and one at T ~ 10^4.5 K (photoionized, 37%) with the remainder located in dense gas near galaxies. While not a perfect tracer of hot gas, OVI provides an important tool for a WHIM baryon census.Comment: 22 pages, 21 figures, emulateapj, accepted for publication in Ap