John Arabadjis - Academia.edu (original) (raw)
Papers by John Arabadjis
arXiv (Cornell University), Feb 18, 2002
Chandra observations of lensing galaxy clusters have now provided accurate dark matter profiles f... more Chandra observations of lensing galaxy clusters have now provided accurate dark matter profiles for several objects in which the intracluster medium is likely to be in hydrostatic equilibrium. We discuss Chandra observations of the mass profile of one such cluster, EMSS1358+6245. We find no evidence for flattening of the mass density profile at radii greater than 50 h −1 50 kpc. This result, and similar findings from Chandra observations of other clusters, appear to rule out models in of dark matter self-interaction proposed to explain the flat cores of low-surface brightness galaxies.
Extracting the three-dimensional structure of the interstellar medium (ISM) from its absorption s... more Extracting the three-dimensional structure of the interstellar medium (ISM) from its absorption signature upon targets at different distances is not a simple task. Utilizing the appropriate tracer for each ISM component is essential, as is employing a robust algorithm for recovering D structure from 2D observations. We believe we have met successfully with these challenges, and present several maps of the local ISM. These maps are generated by assembling column density sets for a number of observationally-independent tracers, and performing a maximum entropy reconstruction to each to recover its underlying 3D structure. The components traced include baryons, atomic hydrogen, molecular hydrogen, gas-phase neutral sodium, and interstellar dust grains. From these maps it is possible to characterize the relative amounts of each component of the ISM as a function of position in the local Galactic disk.
X-ray observations of extragalactic sources are corrected for Galactic absorption, often under th... more X-ray observations of extragalactic sources are corrected for Galactic absorption, often under the assumption that the Galactic column is a constant value that can be measured from 21 cm observations. However, the Galactic column can vary on small scales and this can have an important effect in the absorption correction of extended sources, such as clusters of galaxies. By not including variations in the HI column, systematic residuals appear in spectral fits to extended sources that are manifest as either excess absorption or excess emission. We discuss the degree to which that this can lead to a very soft emission component in galaxy clusters, as has been claimed.
Chandra observations of lensing galaxy clusters have provided accurate dark matter profiles for s... more Chandra observations of lensing galaxy clusters have provided accurate dark matter profiles for some objects, clear evidence for departure from hydrostatic equilibrium in the intracluster plasma of others, and indications that the X-ray surface brightness distribution need not follow mass. We provide examples of these phenomena and discuss their implications for dark matter distributions in cluster cores, and for models of cluster merging. We find evidence for large departures from hydrostatic equilibrium (with total pressure exceeding thermal pressure by a factors of 2-3) in the cores (r < 100 kpc) of several clusters which show no evidence of strong shocks, and which appear to in hydrostatic equilibrium at larger radii. The spatial resolution of Chandra images of clusters exceeds that of existing merger simulations.
The structure of dark matter halos holds clues to the identity of the constituent dark matter par... more The structure of dark matter halos holds clues to the identity of the constituent dark matter particles. For example, if the particle species is self-interacting (Spergel and Steinhardt 2000), energy exchange between particles will alter the distribution of dark matter on scales that can be probed with the current generation of X-ray telescopes. We have assembled a sample of galaxy clusters from the archive of the Chandra X-ray Observatory. Using the deprojection and modelling techniques described in Arabadjis, Bautz, and Garmire (2002), we calculate several structural parameters for each each cluster and compare them to the results of recent large-scale structure simulations to constrain the dark matter self-interaction cross section.
We present projected temperature, electron density, plasma pressure, and entropy maps of the gala... more We present projected temperature, electron density, plasma pressure, and entropy maps of the galaxy cluster Abell 1689 derived from Chandra ACIS observations using three different techniques. The morphology of the pressure and entropy maps suggests that the overall dynamical state of the cluster is consistent with hydrostatic equilibrium, although we confirm the presence of a hot substructure about 170 kpc (projected) from the cluster center. We provide an estimate its effect on the measurement of the mass profile.
Chandra observations of lensing galaxy clusters have provided accurate dark matter profiles for s... more Chandra observations of lensing galaxy clusters have provided accurate dark matter profiles for some objects, clear evidence for departure from hydrostatic equilibrium in the intra-cluster plasma of others, and indications that the X-ray surface brightness distribution need not follow mass. I provide examples of these phenomena and discuss their implications for dark matter distributions in cluster cores, for models of
The Journal of Portfolio Management, 2014
Greater financial integration and central bank policy initiatives in major developed markets have... more Greater financial integration and central bank policy initiatives in major developed markets have led to an increase in cross-asset return correlations, highlighting the interest in broad measures of market-wide sentiment. Using an extensive array of institutional behavioral metrics across asset classes from State Street Associates, we find evidence that suggests marketwide sentiment varies with, and can be forecasted by, broad aggregates across many indicators of institutional investor flows. The large number and wide breadth across assets of these institutional flow measures encourages aggregation into a more manageable set of elements. To this end, we condense this broad information set into what we call a Multi-Asset Sentiment Score (MASS), a concise measure of behavior that captures trading sentiment using State Street Associates' broad information set.
With distant halo stars we can now map the outer Galactic halo, where previously only satellite g... more With distant halo stars we can now map the outer Galactic halo, where previously only satellite galaxies and a few globular clusters were known. This is crucial for an understanding of the manner and timescale of the assembly of the Milky Way's stellar halo and the total mass and shape of its dark halo. Our previous work has proved that
We present a mass profile of the fossil group NGC 1132. Using a deprojection analysis of Chandra ... more We present a mass profile of the fossil group NGC 1132. Using a deprojection analysis of Chandra imaging spectroscopy we derive plasma temperature and density profiles, and under the assumption of hydrostatic equilibrium we calculate the total gravitating mass. We examine the validity of the hydrostatic hypothesis using maps of the projected temperature, electron density, pressure, and entropy of the group plasma, and estimate the effect of departures from equilibrium on the measurment of its mass.
We present a spectral analysis of the central X-ray emission for a sample of galaxy clusters obse... more We present a spectral analysis of the central X-ray emission for a sample of galaxy clusters observed with Chandra. We constrain the quantity of a second cospatial temperature component using Markov Chain Monte Carlo sampling and discuss the implications for our understanding of cooling flows.
One of the main components of interstellar gas is the warm ionized material, with an average valu... more One of the main components of interstellar gas is the warm ionized material, with an average value of N_HII/N_HI =~ 0.37 , although the value in the Solar vicinity is not well known. We can determine N_HII along sight lines out of the Galaxy by measuring the X-ray absorption column toward extragalactic objects, since the soft X-ray opacity depends mainly upon the column of both neutral and warm ionized material (mainly through the sum of the HeI+HeII columns). For the extragalactic absorption targets, we use the isothermal parts of galaxy clusters. At low 21 cm column densities, (N_HI,21cm = 1 to 5 *E(20) cm(-2) ) there is a tight correlation between N_HI,21cm and that determined from X-ray absorption. However, the mean value of N_H,X/N_H,21cm = 0.85 (for H/He = 0.10), which is smaller than 1.0 and therefore unphysical; for the mean Galactic N_HII/N_HI ratio, it would be 1.23. An N_H,X/N_H,21cm of 1.0 can be achieved for a He/H ratio near 0.09, a value inferred from emission line st...
Through our detection of distant halo stars, we are now well placed to map the regions of the Gal... more Through our detection of distant halo stars, we are now well placed to map the regions of the Galactic halo where previously only satellite galaxies and a few globular clusters were known. Mapping this region is crucial for answering questions like: How and over what timescales was the Milky Way's stellar halo assembled? What is the total mass and shape of its dark halo? The Sagittarius dwarf has demonstrated that at least some of the stellar halo was accreted. But, HOW MUCH of the halo was accreted? Our previous efforts have proven that the Washington photometric system, in conjuction with spectroscopy, is capable of efficiently and unambiguously identifying halo stars out to 100 kpc or more. We require followup spectroscopy to map velocity substructure, which is more likely visible in the outer halo because of the long dynamical timescales, and to identify the rare objects in the extreme outer halo which will constrain the shape and size of its dark halo. We are applying for ...
The Journal of Portfolio Management, 2011
Do institutional investors care about past losses? If so, how do they frame the past to inform su... more Do institutional investors care about past losses? If so, how do they frame the past to inform subsequent decisions and what causes panic trading? The ability of investors to compartmentalize their losses and remain rational on their current investment decisions is an axiom of classical theory. Human nature suggests that we should place limits on this rationality and allow for the possibility that gain and losses affect future activity. This limited rationality allows not only for investors to be affected by their memory of past
The Astrophysical Journal, 2003
Observations of extragalactic objects need to be corrected for Galactic absorption and this is of... more Observations of extragalactic objects need to be corrected for Galactic absorption and this is often accomplished by using the measured 21 cm HI column. However, within the beam of the radio telescope there are variations in the HI column that can have important effects in interpreting absorption line studies and X-ray spectra at the softest energies. We examine the HI and DIRBE/IRAS data for lines of sight out of the Galaxy, which show evidence for HI variations in of up to a factor of three in 1 • fields. Column density enhancements would preferentially absorb soft X-rays in spatially extended objects and we find evidence for this effect in the ROSAT PSPC observations of two bright clusters of galaxies, Abell 119 and Abell 2142. For clusters of galaxies, the failure to include column density fluctuations will lead to systematically incorrect fits to the X-ray data in the sense that there will appear to be a very soft X-ray excess. This may be one cause of the soft X-ray excess in clusters, since the magnitude of the effect is comparable to the observed values.
The Astrophysical Journal, 1999
An extremely soft X-ray excess throughout galaxy clusters has been claimed as a new feature of th... more An extremely soft X-ray excess throughout galaxy clusters has been claimed as a new feature of these sytems, with important physical implications. We have reexamined this feature in the five clusters for which it has been discussed, using the most recent X-ray absorption cross sections, X-ray data processing techniques, and a consistent set of HI data. For the Virgo cluster, we find that the spectrum can be fit with a single-temperature thermal plasma and with an X-ray absorption column that is not significantly different than the Galactic HI column. The result for Abell 1367, Abell 1656 (Coma), Abell 1795, and Abell 2199 is similar in that the difference between the X-ray absorption column and the Galactic HI column is less than 3σ for He/H = 0.09, and for He/H = 0.10 only one cluster location leads to a Galactic HI column more than 3σ above the X-ray absorption column (Coma, with one location with a 3.6σ difference). We conclude that there is no strong evidence for the extremely soft X-ray excess in galaxy clusters.
The Astrophysical Journal, 2000
A study of the cores of galaxy clusters with the Einstein SSS indicated the presence of absorbing... more A study of the cores of galaxy clusters with the Einstein SSS indicated the presence of absorbing material corresponding to 10 12 M ⊙ of cold cluster gas, possibly resulting from cooling flows. Since this amount of cold gas is not confirmed by observations at other wavelengths, we examined whether this excess absorption is present in the ROSAT PSPC observations of 20 bright galaxy clusters. For 3/4 of the clusters, successful spectral fits were obtained with absorption due only to the Galaxy, and therefore no extra absorption is needed within the clusters, in disagreement with the results from the Einstein SSS data for some of the same clusters. For 1/4 of the clusters, none of our spectral fits was acceptable, suggesting a more complicated cluster medium than the two-temperature and cooling flow models considered here. However, even for these clusters, substantial excess absorption is not indicated.
The Astrophysical Journal, 1999
We study the column densities of neutral atomic, molecular, and warm ionized Galactic gas through... more We study the column densities of neutral atomic, molecular, and warm ionized Galactic gas through their continuous absorption of extragalactic X-ray spectra at |b| > 25 •. For N H,21cm < 5 × 10 20 cm −2 , there is an extremely tight relationship between N H,21cm and the X-ray absorption column, N H,x , with a mean ratio along 26 lines of sight of N H,x /N H,21cm = 0.972 ± 0.022. This is significantly less than the anticpated ratio of 1.23, which would occur if He were half He I and half He II in the warm ionized component. We suggest that the ionized component out of the plane is highly ionized, with He being mainly He II and He III. In the limiting case that H is entirely HI, we place an upper limit on the He abundance in the ISM of He/H≤ 0.103. At column densities N H,x > 5 × 10 20 cm −2 , which occurs at our lower latitudes, the X-ray absorption column N H,x is nearly double N H,21cm. This excess column cannot be due to the warm ionized component, even if He were entirely He I, so it must be due to a molecular component. This result implies that for lines of sight out of the plane with |b| ∼ 30 • , molecular gas is common and with a column density comprable to N H,21cm. This work bears upon the far infrared background, since a warm ionized component, anticorrelated with N H,21cm , might produce such a background. Not only is such an anticorrelation absent, but if the dust is destroyed in the warm ionized gas, the far infrared background may be slightly larger than that deduced by Puget et al. (1996).
The Astrophysical Journal, 2000
We have developed a technique to map the three-dimensional structure of the local interstellar me... more We have developed a technique to map the three-dimensional structure of the local interstellar medium using a maximum entropy reconstruction technique. A set of column densities N to stars of known distance can in principle be used to recover a three-dimensional density field n, since the two quantities are related by simple geometry through the equation N = C • n, where C is a matrix characterizing the stellar spatial distribution. In practice, however, there is an infinte number of solutions to this equation. We use a maximum entropy reconstruction algorithm to find the density field containing the least information which is consistent with the observations. The solution obtained with this technique is, in some sense, the model containing the minimum structure. We apply the algorithm to several simulated data sets to demonstrate its feasibility and success at recovering "real" density contrasts. This technique can be applied to any set of column densities whose end points are specified. In a subsequent paper we shall describe the application of this method to a set of stellar color excesses to derive a map of the dust distribution, and to soft X-ray absorption columns to hot stars to derive a map of the total density of the interstellar medium.
arXiv (Cornell University), Feb 18, 2002
Chandra observations of lensing galaxy clusters have now provided accurate dark matter profiles f... more Chandra observations of lensing galaxy clusters have now provided accurate dark matter profiles for several objects in which the intracluster medium is likely to be in hydrostatic equilibrium. We discuss Chandra observations of the mass profile of one such cluster, EMSS1358+6245. We find no evidence for flattening of the mass density profile at radii greater than 50 h −1 50 kpc. This result, and similar findings from Chandra observations of other clusters, appear to rule out models in of dark matter self-interaction proposed to explain the flat cores of low-surface brightness galaxies.
Extracting the three-dimensional structure of the interstellar medium (ISM) from its absorption s... more Extracting the three-dimensional structure of the interstellar medium (ISM) from its absorption signature upon targets at different distances is not a simple task. Utilizing the appropriate tracer for each ISM component is essential, as is employing a robust algorithm for recovering D structure from 2D observations. We believe we have met successfully with these challenges, and present several maps of the local ISM. These maps are generated by assembling column density sets for a number of observationally-independent tracers, and performing a maximum entropy reconstruction to each to recover its underlying 3D structure. The components traced include baryons, atomic hydrogen, molecular hydrogen, gas-phase neutral sodium, and interstellar dust grains. From these maps it is possible to characterize the relative amounts of each component of the ISM as a function of position in the local Galactic disk.
X-ray observations of extragalactic sources are corrected for Galactic absorption, often under th... more X-ray observations of extragalactic sources are corrected for Galactic absorption, often under the assumption that the Galactic column is a constant value that can be measured from 21 cm observations. However, the Galactic column can vary on small scales and this can have an important effect in the absorption correction of extended sources, such as clusters of galaxies. By not including variations in the HI column, systematic residuals appear in spectral fits to extended sources that are manifest as either excess absorption or excess emission. We discuss the degree to which that this can lead to a very soft emission component in galaxy clusters, as has been claimed.
Chandra observations of lensing galaxy clusters have provided accurate dark matter profiles for s... more Chandra observations of lensing galaxy clusters have provided accurate dark matter profiles for some objects, clear evidence for departure from hydrostatic equilibrium in the intracluster plasma of others, and indications that the X-ray surface brightness distribution need not follow mass. We provide examples of these phenomena and discuss their implications for dark matter distributions in cluster cores, and for models of cluster merging. We find evidence for large departures from hydrostatic equilibrium (with total pressure exceeding thermal pressure by a factors of 2-3) in the cores (r < 100 kpc) of several clusters which show no evidence of strong shocks, and which appear to in hydrostatic equilibrium at larger radii. The spatial resolution of Chandra images of clusters exceeds that of existing merger simulations.
The structure of dark matter halos holds clues to the identity of the constituent dark matter par... more The structure of dark matter halos holds clues to the identity of the constituent dark matter particles. For example, if the particle species is self-interacting (Spergel and Steinhardt 2000), energy exchange between particles will alter the distribution of dark matter on scales that can be probed with the current generation of X-ray telescopes. We have assembled a sample of galaxy clusters from the archive of the Chandra X-ray Observatory. Using the deprojection and modelling techniques described in Arabadjis, Bautz, and Garmire (2002), we calculate several structural parameters for each each cluster and compare them to the results of recent large-scale structure simulations to constrain the dark matter self-interaction cross section.
We present projected temperature, electron density, plasma pressure, and entropy maps of the gala... more We present projected temperature, electron density, plasma pressure, and entropy maps of the galaxy cluster Abell 1689 derived from Chandra ACIS observations using three different techniques. The morphology of the pressure and entropy maps suggests that the overall dynamical state of the cluster is consistent with hydrostatic equilibrium, although we confirm the presence of a hot substructure about 170 kpc (projected) from the cluster center. We provide an estimate its effect on the measurement of the mass profile.
Chandra observations of lensing galaxy clusters have provided accurate dark matter profiles for s... more Chandra observations of lensing galaxy clusters have provided accurate dark matter profiles for some objects, clear evidence for departure from hydrostatic equilibrium in the intra-cluster plasma of others, and indications that the X-ray surface brightness distribution need not follow mass. I provide examples of these phenomena and discuss their implications for dark matter distributions in cluster cores, for models of
The Journal of Portfolio Management, 2014
Greater financial integration and central bank policy initiatives in major developed markets have... more Greater financial integration and central bank policy initiatives in major developed markets have led to an increase in cross-asset return correlations, highlighting the interest in broad measures of market-wide sentiment. Using an extensive array of institutional behavioral metrics across asset classes from State Street Associates, we find evidence that suggests marketwide sentiment varies with, and can be forecasted by, broad aggregates across many indicators of institutional investor flows. The large number and wide breadth across assets of these institutional flow measures encourages aggregation into a more manageable set of elements. To this end, we condense this broad information set into what we call a Multi-Asset Sentiment Score (MASS), a concise measure of behavior that captures trading sentiment using State Street Associates' broad information set.
With distant halo stars we can now map the outer Galactic halo, where previously only satellite g... more With distant halo stars we can now map the outer Galactic halo, where previously only satellite galaxies and a few globular clusters were known. This is crucial for an understanding of the manner and timescale of the assembly of the Milky Way's stellar halo and the total mass and shape of its dark halo. Our previous work has proved that
We present a mass profile of the fossil group NGC 1132. Using a deprojection analysis of Chandra ... more We present a mass profile of the fossil group NGC 1132. Using a deprojection analysis of Chandra imaging spectroscopy we derive plasma temperature and density profiles, and under the assumption of hydrostatic equilibrium we calculate the total gravitating mass. We examine the validity of the hydrostatic hypothesis using maps of the projected temperature, electron density, pressure, and entropy of the group plasma, and estimate the effect of departures from equilibrium on the measurment of its mass.
We present a spectral analysis of the central X-ray emission for a sample of galaxy clusters obse... more We present a spectral analysis of the central X-ray emission for a sample of galaxy clusters observed with Chandra. We constrain the quantity of a second cospatial temperature component using Markov Chain Monte Carlo sampling and discuss the implications for our understanding of cooling flows.
One of the main components of interstellar gas is the warm ionized material, with an average valu... more One of the main components of interstellar gas is the warm ionized material, with an average value of N_HII/N_HI =~ 0.37 , although the value in the Solar vicinity is not well known. We can determine N_HII along sight lines out of the Galaxy by measuring the X-ray absorption column toward extragalactic objects, since the soft X-ray opacity depends mainly upon the column of both neutral and warm ionized material (mainly through the sum of the HeI+HeII columns). For the extragalactic absorption targets, we use the isothermal parts of galaxy clusters. At low 21 cm column densities, (N_HI,21cm = 1 to 5 *E(20) cm(-2) ) there is a tight correlation between N_HI,21cm and that determined from X-ray absorption. However, the mean value of N_H,X/N_H,21cm = 0.85 (for H/He = 0.10), which is smaller than 1.0 and therefore unphysical; for the mean Galactic N_HII/N_HI ratio, it would be 1.23. An N_H,X/N_H,21cm of 1.0 can be achieved for a He/H ratio near 0.09, a value inferred from emission line st...
Through our detection of distant halo stars, we are now well placed to map the regions of the Gal... more Through our detection of distant halo stars, we are now well placed to map the regions of the Galactic halo where previously only satellite galaxies and a few globular clusters were known. Mapping this region is crucial for answering questions like: How and over what timescales was the Milky Way's stellar halo assembled? What is the total mass and shape of its dark halo? The Sagittarius dwarf has demonstrated that at least some of the stellar halo was accreted. But, HOW MUCH of the halo was accreted? Our previous efforts have proven that the Washington photometric system, in conjuction with spectroscopy, is capable of efficiently and unambiguously identifying halo stars out to 100 kpc or more. We require followup spectroscopy to map velocity substructure, which is more likely visible in the outer halo because of the long dynamical timescales, and to identify the rare objects in the extreme outer halo which will constrain the shape and size of its dark halo. We are applying for ...
The Journal of Portfolio Management, 2011
Do institutional investors care about past losses? If so, how do they frame the past to inform su... more Do institutional investors care about past losses? If so, how do they frame the past to inform subsequent decisions and what causes panic trading? The ability of investors to compartmentalize their losses and remain rational on their current investment decisions is an axiom of classical theory. Human nature suggests that we should place limits on this rationality and allow for the possibility that gain and losses affect future activity. This limited rationality allows not only for investors to be affected by their memory of past
The Astrophysical Journal, 2003
Observations of extragalactic objects need to be corrected for Galactic absorption and this is of... more Observations of extragalactic objects need to be corrected for Galactic absorption and this is often accomplished by using the measured 21 cm HI column. However, within the beam of the radio telescope there are variations in the HI column that can have important effects in interpreting absorption line studies and X-ray spectra at the softest energies. We examine the HI and DIRBE/IRAS data for lines of sight out of the Galaxy, which show evidence for HI variations in of up to a factor of three in 1 • fields. Column density enhancements would preferentially absorb soft X-rays in spatially extended objects and we find evidence for this effect in the ROSAT PSPC observations of two bright clusters of galaxies, Abell 119 and Abell 2142. For clusters of galaxies, the failure to include column density fluctuations will lead to systematically incorrect fits to the X-ray data in the sense that there will appear to be a very soft X-ray excess. This may be one cause of the soft X-ray excess in clusters, since the magnitude of the effect is comparable to the observed values.
The Astrophysical Journal, 1999
An extremely soft X-ray excess throughout galaxy clusters has been claimed as a new feature of th... more An extremely soft X-ray excess throughout galaxy clusters has been claimed as a new feature of these sytems, with important physical implications. We have reexamined this feature in the five clusters for which it has been discussed, using the most recent X-ray absorption cross sections, X-ray data processing techniques, and a consistent set of HI data. For the Virgo cluster, we find that the spectrum can be fit with a single-temperature thermal plasma and with an X-ray absorption column that is not significantly different than the Galactic HI column. The result for Abell 1367, Abell 1656 (Coma), Abell 1795, and Abell 2199 is similar in that the difference between the X-ray absorption column and the Galactic HI column is less than 3σ for He/H = 0.09, and for He/H = 0.10 only one cluster location leads to a Galactic HI column more than 3σ above the X-ray absorption column (Coma, with one location with a 3.6σ difference). We conclude that there is no strong evidence for the extremely soft X-ray excess in galaxy clusters.
The Astrophysical Journal, 2000
A study of the cores of galaxy clusters with the Einstein SSS indicated the presence of absorbing... more A study of the cores of galaxy clusters with the Einstein SSS indicated the presence of absorbing material corresponding to 10 12 M ⊙ of cold cluster gas, possibly resulting from cooling flows. Since this amount of cold gas is not confirmed by observations at other wavelengths, we examined whether this excess absorption is present in the ROSAT PSPC observations of 20 bright galaxy clusters. For 3/4 of the clusters, successful spectral fits were obtained with absorption due only to the Galaxy, and therefore no extra absorption is needed within the clusters, in disagreement with the results from the Einstein SSS data for some of the same clusters. For 1/4 of the clusters, none of our spectral fits was acceptable, suggesting a more complicated cluster medium than the two-temperature and cooling flow models considered here. However, even for these clusters, substantial excess absorption is not indicated.
The Astrophysical Journal, 1999
We study the column densities of neutral atomic, molecular, and warm ionized Galactic gas through... more We study the column densities of neutral atomic, molecular, and warm ionized Galactic gas through their continuous absorption of extragalactic X-ray spectra at |b| > 25 •. For N H,21cm < 5 × 10 20 cm −2 , there is an extremely tight relationship between N H,21cm and the X-ray absorption column, N H,x , with a mean ratio along 26 lines of sight of N H,x /N H,21cm = 0.972 ± 0.022. This is significantly less than the anticpated ratio of 1.23, which would occur if He were half He I and half He II in the warm ionized component. We suggest that the ionized component out of the plane is highly ionized, with He being mainly He II and He III. In the limiting case that H is entirely HI, we place an upper limit on the He abundance in the ISM of He/H≤ 0.103. At column densities N H,x > 5 × 10 20 cm −2 , which occurs at our lower latitudes, the X-ray absorption column N H,x is nearly double N H,21cm. This excess column cannot be due to the warm ionized component, even if He were entirely He I, so it must be due to a molecular component. This result implies that for lines of sight out of the plane with |b| ∼ 30 • , molecular gas is common and with a column density comprable to N H,21cm. This work bears upon the far infrared background, since a warm ionized component, anticorrelated with N H,21cm , might produce such a background. Not only is such an anticorrelation absent, but if the dust is destroyed in the warm ionized gas, the far infrared background may be slightly larger than that deduced by Puget et al. (1996).
The Astrophysical Journal, 2000
We have developed a technique to map the three-dimensional structure of the local interstellar me... more We have developed a technique to map the three-dimensional structure of the local interstellar medium using a maximum entropy reconstruction technique. A set of column densities N to stars of known distance can in principle be used to recover a three-dimensional density field n, since the two quantities are related by simple geometry through the equation N = C • n, where C is a matrix characterizing the stellar spatial distribution. In practice, however, there is an infinte number of solutions to this equation. We use a maximum entropy reconstruction algorithm to find the density field containing the least information which is consistent with the observations. The solution obtained with this technique is, in some sense, the model containing the minimum structure. We apply the algorithm to several simulated data sets to demonstrate its feasibility and success at recovering "real" density contrasts. This technique can be applied to any set of column densities whose end points are specified. In a subsequent paper we shall describe the application of this method to a set of stellar color excesses to derive a map of the dust distribution, and to soft X-ray absorption columns to hot stars to derive a map of the total density of the interstellar medium.