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The ISM

The ISM. (Diffuse Emission in Galaxies). K.D.Kuntz The Henry A. Rowland Department of Physics and Astronomy The Johns Hopkins University. KU-4/2007. Introduction. {. Population Studies. Due to single stars. Large Scale Emission (multiple degrees or larger).

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The ISM

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  1. The ISM (Diffuse Emission in Galaxies) K.D.Kuntz The Henry A. Rowland Department of Physics and Astronomy The Johns Hopkins University KU-4/2007 Introduction

  2. { Population Studies Due to single stars Large Scale Emission (multiple degrees or larger) Xray Emission from Galaxies Point Sources HMXB LMXB Diffuse Emission SNR SFR Bubbles Super-Bubbles Un ID’d Multiple SW & SNE XAS-7/2007 Introduction

  3. Outline • Diffuse Emission in the Milky Way • Large Scale Emission (LHB, Galactic Halo, Etc.) • Super Bubbles • Star-Forming Regions • Diffuse Emission in Other Galaxies • Global Correlations XAS-7/2007 Introduction

  4. Fundamentals of X-ray Astronomy • X-ray Absorption Cross-Section is E-2/3 • Lower energy photons more readily absorbed • One can see further at higher energies XAS-7/2007 Introduction

  5. Fundamentals of X-ray Astronomy • Typical hot ISM T~106K →emission below 0.7 keV • Most parts of the hot ISM hidden from us • Most of what we see at low E is local! XAS-7/2007 Introduction

  6. Diversion I • A high Galactic latitude, bulk of sources are AGN • Deep exposures reveal more & fainter sources XAS-7/2007 LHB

  7. Diversion I • Does ΣAGN=total X-ray surface brightness? Yes, but…answer depends upon the energy. XAS-7/2007 LHB

  8. A Model of the Sky • Consider a map of the absorbing column due to neutral and molecular gas in the Milky Way. • If all of the emission is due to AGN, what does the sky look like? XAS-7/2007 LHB

  9. A Model of the Sky Absorption ¼ keV em. XAS-7/2007 LHB

  10. A Model of the Sky Model Observed R12= ¼ keV XAS-7/2007 LHB

  11. The Observed Sky ¾ keV ¼ keV XAS-7/2007 LHB

  12. The Observed Sky • At ¼ keV and ¾ keV there is diffuse local emission • Not identifiable with any single SN, SNR, or SFR • Distribution in ¼ keV and ¾ keV bands is different • (not just an effect of absorption) • Distance to emission (and thus its mass) is unknown. XAS-7/2007 LHB

  13. Diversion II • Assume: • Uniform local emission • A screen of absorption • Uniform distant emission XAS-7/2007 LHB

  14. Diversion II • If τ=σNH varies with position • Anticorrelation of τ and surface brightness • By fitting anticorrelation • Can determine • IL and ID XAS-7/2007 LHB

  15. LHB • From shadows, determine ID&IL in many directions • Given distances to clouds → upper limit on size of local emitting region → lower limit on distance to distant emission XAS-7/2007 LHB

  16. LHB • Assuming thermal emission • ROSAT band values → local emission has T~106K • Upper limit on size of region → crossing time~106 years → n, T, L are relatively uniform • Therefore IL proportional to R XAS-7/2007 LHB

  17. LHB The Local Hot Bubble: • Irregular region filled with T=106K emission • Inside a larger(?) region (Local Cavity) • With below average density of neutral material • Controversy: contamination by very local emission • The heliosphere is X-ray bright • Accounts for some unknown fraction of “LHB” emission • Some truly LHB emission does exist XAS-7/2007 LHB

  18. LHB The Local Hot Bubble: • At T~106K, emission is so low-energy that we can’t see similar bubbles further away than a few 100 pc. • Could be a major component of galactic X-ray emission • Bulk of emission below Chandra/XMM bandpass XAS-7/2007 LHB

  19. Galactic Halo At high Galactic latitude the ID component is the halo. • In some directions the halo emission is substantial ¼ keV XAS-7/2007 Halo

  20. 0.25 keV I100 Galactic Halo At high Galactic latitude the ID component is the halo. • In some directions the halo emission is substantial XAS-7/2007 Halo

  21. Galactic Halo At high Galactic latitude the ID component is the halo. • Distant emission at ¼ keV is mottled NGP SGP Loop I Superbubble XAS-7/2007 Halo

  22. Galactic Halo At high Galactic latitude the ID component is the halo. • Emission at ¾ keV is smoother than at ¼ keV ¼ keV ¾ keV XAS-7/2007 Halo

  23. Galactic Halo • Distance to emission is unknown • ¼ keV emission at z>300pc • ¾ keV emission beyond Magellanic Clouds? • ROSAT: Two thermal components: • Soft ( ¼ keV component) kT~0.09, T~106K • Since patchy possibly due to Galactic chimneys • Hard ( ¾ keV component) kT~0.25, T~3×106K • Could be Galactic halo or Local Group or Extragalactic (low z) • XMM: Temperature of hard component spatially variable. XAS-7/2007 Halo

  24. Galactic Disk Emission In this image of the Galactic anticenter at ¾ keV, • The disk absorption is less than expected if all emission were extragalactic • There must by X-ray emission due to the Galactic disk • Must “exactly” balance the absorption • Stars can provide some fraction • Source population unclear • Spatial distribution is unclear (due to emission and abs. being intermixed along LOS.) • kT~0.6 keV ¾ keV XAS-7/2007 Disk

  25. Galactic Ridge Emission Hard (kT~7.0 keV) Emission along the Galactic plane: • Truly diffuse emission or unresolved point sources? • Seemingly diffuse component has same distribution as the near IR → distribution is stellar → due to point sources • Identification of point source population problematic(?) • Spectrum of emission is problematic(?) XAS-7/2007 Ridge

  26. Star-Forming Regions From pop. synthesis models, expected source of X-rays: • O star winds >>> B star winds • O star SNe ~ B star SNe (depends on age) From R.Chen XAS-7/2007 SFR

  27. ~30 OB stars ~5 OB stars Star-Forming Regions M17 Shows plenty of diffuse emission Observed SFRs (without SNe): While Orion does not. XAS-7/2007 SFR

  28. Star-Forming Regions 30 Doradus Somewhat older SFR: What a mess! HII SNR XAS-7/2007 SFR

  29. Miscellaneous Loop I SuperBubble Galactic Bulge XAS-7/2007 Ridge

  30. Milky Way Summary Soft Halo kT~0.09 keV SuperBubble kT~0.25 keV Gal. Ridge kT~7.0 keV Disk kT~0.6 keV SNR LHB kT~0.09 Gal. Bulge kT~0.35 keV Hard Halo kT~0.2 keV SFR kT~0.2,0.6 XAS-7/2007 Summary

  31. Milky Way Summary What is the filling factor? • Is it a pervasive hot medium with embedded cool clouds? • Is it restricted to hot bubbles in a pervasive cool medium? XAS-7/2007 Summary

  32. NGC 4631 NGC 5746 Other Galaxies Star Formation Galactic Potential Two Opposed Views Dominates is Late-Type Spirals Only the most massive galaxies XAS-7/2007 Summary

  33. Other Galaxies Global Correlations among X, B, H, 12µm, FIR, 6cm: • Many correlations depend on morphology Reflecting differences in SFR, bulge vs. disk, AGN prob. • 12µm, FIR, 6cm correlated (star formation) • Small dust grains, Large dust grains, thermal free-free • Linear correlations • B, H correlated (stellar populations, IMF) • Young massive stars, All ages of low mass stars • Non-linear correlations • But B,H not correlated well with 12µm,FIR,6cm XAS-7/2007 Correlation

  34. Other Galaxies How is X correlated? • Early-type galaxies – X and B weakly correlated • Non-linear correlation • Late-type galaxies – X and FIR strongly correlated • LX/LB correlated w/ L60/L100 correlated with SFR • X, FIR, 6cm all have linear correlations XAS-7/2007 Correlation

  35. Other Galaxies }B Why? • Point sources: • HMXB: end products of massive star formation • Short-lived, appear shortly after star formation • LMXB: end products of low mass star formation • Long lived, appear long after star formation • Diffuse emission: • Halo: determined by mass of the system • Star formation: stellar winds & SNR • Other components • AGN: }B/H }H }FIR }? XAS-7/2007 Correlation

  36. Other Galaxies What about the individual components X? • Point source emission correlated non-linearly with B • Roughly consistent with expectations • Diffuse emission only weakly correlated with B • Complicated since X-ray/FIR emission is immediate while the B-band emission lasts for as long as B stars. XAS-7/2007 Correlation

  37. Other Galaxies Bottom line: • X-ray emission from early-type galaxies dominated(?) by point source populations with possible contribution from hydrostatic halo and other, minor diffuse sources. • X-ray emission from late-type galaxies dominated by star formation/hot ISM • Concentrating on ISM → concentrating on late-type disks XAS-7/2007 Correlation

  38. Diffuse Emission in Sc-Sd Spatial distribution of the X-ray follows that of UV XAS-7/2007 Correlation

  39. Diffuse Emission in Sc-Sd X-ray surface brightness correlated with UV • Trend may not be universal XAS-7/2007 Correlation

  40. Diffuse Emission in Sc-Sd Spectrum fit by two thermal components: • kT~0.2 keV and kT~0.6 keV XAS-7/2007 Correlation

  41. Diffuse Emission in Sc-Sd • Most late type galaxies have similar 2 temperature structure • Ratio of emission strengths may vary with Hubble type. XAS-7/2007 Correlation

  42. Diffuse Emission in Sc-Sd Why? • Inflection points of the cooling curve? • Similar to temperatures measured in Star-forming regions • Characteristic temperatures of two different emission processes? • Lack of spectral resolution • Continuous E.M. distributions always produce kT=(0.2,0.6) at Chandra/XMM resolution Just delays the question Stars (kT~.6)? XAS-7/2007 Correlation

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