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Register a new userAtomic and Ionized Microstructures in the Diffuse Interstellar Medium
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Snezana Stanimirovic
Publication date
2018
fields
Physics
and research's language is
English
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It has been known for half a century that the interstellar medium (ISM) of our Galaxy is structured on scales as small as a few hundred km, more than 10 orders of magnitude smaller than typical ISM structures and energy input scales. In this review we focus on neutral and ionized structures on spatial scales of a few to ~10^4 Astronomical Units (AU) which appear to be highly overpressured, as these have the most important role in the dynamics and energy balance of interstellar gas: the Tiny Scale Atomic Structure (TSAS) and Extreme Scattering Events (ESEs) as the most over-pressured example of the Tiny Scale Ionized Structures (TSIS). We review observational results and highlight key physical processes at AU scales. We present evidence for and against microstructures as part of a universal turbulent cascade and as discrete structures, and review their association with supernova remnants, the Local Bubble, and bright stars. We suggest a number of observational and theoretical programs that could clarify the nature of AU structures. TSAS and TSIS probe spatial scales in the range of what is expected for turbulent dissipation scales, therefore are of key importance for constraining exotic and not-well understood physical processes which have implications for many areas of astrophysics. The emerging picture is one in which a magnetized, turbulent cascade, driven hard by a local energy source and acting jointly with phenomena such as thermal instability, is the source of these microstructures.
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Observing the interstellar medium (ISM) in $z gtrsim 6$ quasars host galaxies is essential for understanding the co-evolution between the supermassive black holes and their hosts. To probe the gas physical conditions and search for imprints of Active Galactic Nuclei (AGN) on the ISM, we report ALMA observations of the $rm [N II]_{122 mu m}$ and $rm [O I]_{146 mu m}$ lines and the underlying continuum from the $z=6.003$ quasar SDSS J231038.88+185519.7. Together with previous $rm [C II]_{158 mu m}$ and $rm [O III]_{88 mu m}$ observations, we use the ratios of these fine-structure lines to probe the ISM properties. Similar to other high-$z$ systems, this object exhibits a $rm [C II]_{158 mu m}$/$rm [O I]_{146 mu m}$ ratio comparable to the lowest values found in local (Ultra) luminous infrared galaxies, suggesting a warmer and denser gas component compared to typical local systems. The $rm [O III]_{88 mu m}$/$rm [O I]_{146 mu m}$ ratio is lower than that of other local and high-$z$ systems, indicating a smaller ionized gas fraction in this quasar. The $rm [O III]_{88 mu m}$/$rm [N II]_{122 mu m}$ ratio is comparable to that of local systems, and suggests a metallicity of $Z/Z_{odot}$=1.5$-$2.1. Based on the $rm [N II]_{122 mu m}$ detection, we estimate that $17%$ of the $rm [C II]_{158 mu m}$ emission is associated with ionized gas. The $rm [N II]_{122 mu m}$ line shows a flux deficit comparable to local systems. The $rm [O I]_{146 mu m}$ line, with a $rm [O I]_{146 mu m}$/FIR ratio $ge 2times$ than expected from the local relation, indicates no $rm [O I]_{rm 146 mu m}$ deficit. The low $rm [C II]_{158 mu m}$/$rm [O I]_{146 mu m}$ ratio, together with the high $rm [O I]_{146 mu m}$/FIR ratio in J2310+1855, reveals that the warm and dense gas is likely a result of AGN heating to the ISM.
We present a comprehensive analysis of interstellar absorption lines seen in moderately-high resolution, high signal-to-noise ratio optical spectra of SN 2014J in M82. Our observations were acquired over the course of six nights, covering the period from ~6 days before to ~30 days after the supernova reached its maximum B-band brightness. We examine complex absorption from Na I, Ca II, K I, Ca I, CH+, CH, and CN, arising primarily from diffuse gas in the interstellar medium (ISM) of M82. We detect Li I absorption over a range in velocity consistent with that exhibited by the strongest Na I and K I components associated with M82; this is the first detection of interstellar Li in a galaxy outside of the Local Group. There are no significant temporal variations in the absorption-line profiles over the 37 days sampled by our observations. The relative abundances of the various interstellar species detected reveal that the ISM of M82 probed by SN 2014J consists of a mixture of diffuse atomic and molecular clouds characterized by a wide range of physical/environmental conditions. Decreasing N(Na I)/N(Ca II) ratios and increasing N(Ca I)/N(K I) ratios with increasing velocity are indicative of reduced depletion in the higher-velocity material. Significant component-to-component scatter in the N(Na I)/N(Ca II) and N(Ca I)/N(Ca II) ratios may be due to variations in the local ionization conditions. An apparent anti-correlation between the N(CH+)/N(CH) and N(Ca I)/N(Ca II) ratios can be understood in terms of an opposite dependence on gas density and radiation field strength, while the overall high CH+ abundance may be indicative of enhanced turbulence in the ISM of M82. The Li abundance also seems to be enhanced in M82, which supports the conclusions of recent gamma-ray emission studies that the cosmic-ray acceleration processes are greatly enhanced in this starburst galaxy.
We discuss the absorption due to various constituents of the interstellar medium of M82 seen in moderately high resolution, high signal-to-noise ratio optical spectra of SN 2014J. Complex absorption from M82 is seen, at velocities 45 $le$ $v_{rm LSR}$ $le$ 260 km s$^{-1}$, for Na I, K I, Ca I, Ca II, CH, CH$^+$, and CN; many of the diffuse interstellar bands (DIBs) are also detected. Comparisons of the column densities of the atomic and molecular species and the equivalent widths of the DIBs reveal both similarities and differences in relative abundances, compared to trends seen in the ISM of our Galaxy and the Magellanic Clouds. Of the ten relatively strong DIBs considered here, six (including $lambda$5780.5) have strengths within $pm$20% of the mean values seen in the local Galactic ISM, for comparable N(K I); two are weaker by 20--45% and two (including $lambda$5797.1) are stronger by 25--40%. Weaker than expected DIBs [relative to N(K I), N(Na I), and E(B-V)] in some Galactic sight lines and toward several other extragalactic supernovae appear to be associated with strong CN absorption and/or significant molecular fractions. While the N(CH)/N(K I) and N(CN)/N(CH) ratios seen toward SN 2014J are similar to those found in the local Galactic ISM, the combination of high N(CH$^+$)/N(CH) and high W(5797.1)/W(5780.5) ratios has not been seen elsewhere. The centroids of many of the M82 DIBs are shifted, relative to the envelope of the K I profile -- likely due to component-to-component variations in W(DIB)/N(K I) that may reflect the molecular content of the individual components. We compare estimates for the host galaxy reddening E(B-V) and visual extinction A$_{rm V}$ derived from the various interstellar species with the values estimated from optical and near-IR photometry of SN 2014J.
With the use of the data from archives, we studied the correlations between the equivalent widths of four diffuse interstellar bands (4430$r{A}$, 5780$r{A}$, 5797$r{A}$, 6284$r{A}$) and properties of the target stars (colour excess values, distances and Galactic coordinates). Many different plots of the diffuse interstellar bands and their maps were produced and further analysed. There appears to be a structure in the plot of equivalent widths of 5780$r{A}$ DIB (and 6284$r{A}$ DIB) against the Galactic $x$-coordinate. The structure is well defined below $sim150$ m$r{A}$ and within $|x|<250$ pc, peaking around $x=170$ pc. We argue that the origin of this structure is not a statistical fluctuation. Splitting the data in the Galactic longitude into several subregions improves or lowers the well known linear relation between the equivalent widths and the colour excess, which was expected. However, some of the lines of sight display drastically different behaviour. The region within $150^circ<l<200^circ$ shows scatter in the correlation plots with the colour excess for all of the four bands with correlation coefficients $textrm{R}<0.58$. We suspect that the variation of physical conditions in the nearby molecular clouds could be responsible. Finally, the area $250^circ<l<300^circ$ displays (from the statistical point of view) significantly lower values of equivalent widths than the other regions -- this tells us that there is either a significant underabundance of carriers (when compared with the other regions) or that this has to be a result of an observational bias.
We map the distribution and properties of the Milky Ways interstellar medium as traced by diffuse interstellar bands (DIBs) detected in near-infrared stellar spectra from the SDSS-III/APOGEE survey. Focusing exclusively on the strongest DIB in the H-band, at ~1.527 microns, we present a projected map of the DIB absorption field in the Galactic plane, using a set of about 60,000 sightlines that reach up to 15 kpc from the Sun and probe up to 30 magnitudes of visual extinction. The strength of this DIB is linearly correlated with dust reddening over three orders of magnitude in both DIB equivalent width (W_DIB) and extinction, with a power law index of 1.01 +/- 0.01, a mean relationship of W_DIB/A_V = 0.1 Angstrom mag^-1, and a dispersion of ~0.05 Angstrom mag^-1 at extinctions characteristic of the Galactic midplane. These properties establish this DIB as a powerful, independent probe of dust extinction over a wide range of A_V values. The subset of about 14,000 robustly detected DIB features have an exponential W_DIB distribution. We empirically determine the intrinsic rest wavelength of this transition to be lambda_0 = 15,272.42 Angstrom, and then calculate absolute radial velocities of the carrier, which display the kinematical signature of the rotating Galactic disk. We probe the DIB carrier distribution in three dimensions and show that it can be characterized by an exponential disk model with a scaleheight of about 100 pc and a scalelength of about 5 kpc. Finally, we show that the DIB distribution also traces large-scale Galactic structures, including the central long bar and the warp of the outer disk.
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