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Register a new userIn Search of an Interface between Warm and Hot Gas within the Local Bubble
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Edward B. Jenkins n
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We have examined UV spectra recorded by the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope for three stars, HD32309, 41 Ari, and $eta$~Tel, that are located well inside the boundary of the Local Hot Bubble in our search for absorption features of Si IV, C IV, and N V that could reveal the presence of an interface between the local warm ($Tsim 7000$ K) neutral medium and a more distant hot ($Tsim 10^6$ K) interstellar medium. In all cases, we failed to detect such ions. Our most meaningful upper limit is that for log N(C IV)< 11.86 toward HD32309, which is below the expectation for a sight line that penetrates either a conductive/evaporative interface or a turbulent mixing layer. We offer conjectures on the reasons for these negative results in terms of either a suppression of a conductive layer caused by the shielding of the local cloud by other clouds, which may make it more difficult for us to sense discrete absorption features from gases at intermediate temperatures, or by the presence of a tangential magnetic field at most locations on the surface of the local cloud.
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In the mapping of the local ISM it is of some interest to know where the first indications of the boundary of the Local Bubble can be measured. The Hipparcos distances combined to B-V photometry and some sort of spectral classification permit mapping of the spatial extinction distribution. Photometry is available for almost the complete Hipparcos sample and Michigan Classification is available for brighter stars south of delta = +5 deg (1900). For the northern and fainter stars spectral types, e.g. the HD types, are given but a luminosity class is often missing. The B-V photometry and the parallax do, however, permit a dwarf/giant separation due to the value of the slope of the reddening vector compared to the gradient of the main sequence in a color magnitude diagram, in the form: B-V vs. M_V+A_V = V+5(1+log(pi)), together with the rather shallow extinction present in the Hipparcos sample. We present the distribution of median A_V(l, b) for stars with Hipparcos 2 distances less than 55 pc. The northern part of the first and second quadrant has most extinction, up to 0.2 mag and the southern part of the third and fourth quadrant the slightest extinction, 0.05 mag. The boundary of the extinction minimum appears rather coherent on an angular resolution of a few degrees
We search the Planck data for a thermal Sunyaev-Zeldovich (tSZ) signal due to gas filaments between pairs of Luminous Red Galaxies (LRGs) taken from the Sloan Digital Sky Survey Data Release 12 (SDSS/DR12). We identify $sim$260,000 LRG pairs in the DR12 catalog that lie within 6-10 $h^{-1} mathrm{Mpc}$ of each other in tangential direction and within 6 $h^{-1} mathrm{Mpc}$ in radial direction. We stack pairs by rotating and scaling the angular positions of each LRG so they lie on a common reference frame, then we subtract a circularly symmetric halo from each member of the pair to search for a residual signal between the pair members. We find a statistically significant (5.3$sigma$) signal between LRG pairs in the stacked data with a magnitude $Delta y = (1.31 pm 0.25) times 10^{-8}$. The uncertainty is estimated from two Monte Carlo null tests which also establish the reliability of our analysis. Assuming a simple, isothermal, cylindrical filament model of electron over-density with a radial density profile proportional to $r_c/r$ (as determined from simulations), where $r$ is the perpendicular distance from the cylinder axis and $r_c$ is the core radius of the density profile, we constrain the product of over-density and filament temperature to be $delta_c times (T_{rm e}/10^7 , {rm K}) times (r_c/0.5h^{-1} , {rm Mpc}) = 2.7 pm 0.5$. To our knowledge, this is the first detection of filamentary gas at over-densities typical of cosmological large-scale structure. We compare our result to the BAHAMAS suite of cosmological hydrodynamic simulations (McCarthy et al. 2017) and find a slightly lower, but marginally consistent Comptonization excess, $Delta y = (0.84 pm 0.24) times 10^{-8}$.
DXL (Diffuse X-rays from the Local Galaxy) is a sounding rocket mission designed to quantify and characterize the contribution of Solar Wind Charge eXchange (SWCX) to the Diffuse X-ray Background and study the properties of the Local Hot Bubble (LHB). Based on the results from the DXL mission, we quantified and removed the contribution of SWCX to the diffuse X-ray background measured by the ROSAT All Sky Survey (RASS). The cleaned maps were used to investigate the physical properties of the LHB. Assuming thermal ionization equilibrium, we measured a highly uniform temperature distributed around kT=0.097 keV+/-0.013 keV (FWHM)+/-0.006 keV (systematic). We also generated a thermal emission measure map and used it to characterize the three-dimensional (3D) structure of the LHB which we found to be in good agreement with the structure of the local cavity measured from dust and gas.
Thermally-broadened Lya absorbers (BLAs) offer an alternate method to using highly-ionized metal absorbers (OVI, OVII, etc.) to probe the warm-hot intergalactic medium (WHIM, T=10^5-10^7 K). Until now, WHIM surveys via BLAs have been no less ambiguous than those via far-UV and X-ray metal-ion probes. Detecting these weak, broad features requires background sources with a well-characterized far-UV continuum and data of very high quality. However, a recent HST/COS observation of the z=0.03 blazar Mrk421 allows us to perform a metal-independent search for WHIM gas with unprecedented precision. The data have high signal-to-noise (S/N~50 per ~20 km/s resolution element) and the smooth, power-law blazar spectrum allows a fully-parametric continuum model. We analyze the Mrk421 sight line for BLA absorbers, particularly for counterparts to the proposed OVII WHIM systems reported by Nicastro et al. (2005a,b) based on Chandra/LETG observations. We derive the Lya profiles predicted by the X-ray observations. The signal-to-noise ratio of the COS data is high (S/N~25 per pixel), but much higher S/N can be obtained by binning the data to widths characteristic of the expected BLA profiles. With this technique, we are sensitive to WHIM gas over a large (N_H, T) parameter range in the Mrk421 sight line. We rule out the claimed Nicastro et al. OVII detections at their nominal temperatures (T~1-2x10^6 K) and metallicities (Z=0.1 Z_sun) at >2 sigma level. However, WHIM gas at higher temperatures and/or higher metallicities is consistent with our COS non-detections.
We report the Chandra detection of OVII Kalpha absorption at z=0 in the direction of the z=0.03 Seyfert 1 galaxy Mkn 279. The high velocity cloud Complex C lies along this line of sight, with HI 21-cm emission and OVI 1032AA absorption both observed at velocities of ~ -150 km/s relative to the local standard of rest. We present an improved method for placing limits on the Doppler parameter and column density of a medium when only one unresolved line can be measured; this method is applied to the OVII absorption seen here, indicating that the OVII Doppler parameter is inconsistent with that of any low-velocity (Galactic thick disk) or high-velocity OVI (OVI_HV) component. Direct association of the OVII with the OVI_HV is further ruled out by the high temperatures required to produce the observed OVI_HV/OVII ratio and the significant velocity difference between the OVII and OVI_HV lines. If the OVII absorption is associated with a very broad, undetected OVI component, then the absorption must be broadened by primarily nonthermal processes. The large velocity dispersion and possible slight redshift of the OVII absorption (as well as limits on the absorbers temperature and density) may be indicative of a local intergalactic medium origin, though absorption from a hot, low-density Galactic corona cannot be ruled out.
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