Galaxy Cluster Gas Motions and Astro-H: Predictions and Challenges from Simulations
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Astro-H observations of galaxy clusters have the potential to measure gas motions from turbulence and sloshing. Simulations show that while Astro-H may not be able to distinguish between different turbulence dissipation scales or spectral indices, it could distinguish between different injection scales with certain pointing configurations. Observations of sloshing gas may measure line shifts and widths, but distinguishing between broadening from sloshing versus turbulence will be difficult, limiting constraints on viscosity. Accounting for photon scattering from the cluster center is also important to avoid biases in line shift measurements. Overall, better spatial resolution provided by future missions will be needed to fully characterize gas motions in clusters.
Galaxy Cluster Gas Motions and Astro-H: Predictions and Challenges from Simulations
- 1. GALAXY CLUSTER GAS MOTIONS AND ASTRO-H: PREDICTIONS AND CHALLENGES FROM SIMULATIONS John ZuHone, MIT Kavli Institute with Aurora Simionescu (ISAS/JAXA), Eric Miller (MIT), Mark Bautz (MIT), Maxim Markevitch (NASA/GSFC), and Irina Zhuravleva (Stanford)
- 2. ASTRO-H MISSION • To be launched early next year • Soft X-ray Spectrometer (SXS): • 3’x3’ FOV • ~5 eV spectral resolution • ~1’ spatial PSF
- 3. VELOCITY BROADENING 6.4 6.5 6.6 0.1110 normalizedcountss−1keV−1 Energy (keV) data and folded model sekiya 6−Jun−2011 12:36 vturb = 0 km/s vturb = 100 km/s vturb = 300 km/s vturb = 1000 km/s 6.7 w xyz j k e po Perseus simulated spectrum (wabs＊bapec) Astro-H cluster white paper, arXiv:1412.1176 He-like Fe line at E0 ≈ 6.7 keV
- 4. Coma 200 kpc Turbulence A2319 200 kpc Sloshing Astro-H cluster white paper, arXiv:1412.1176
- 5. Inertial Range Dissipation Scale Injection Scale P(k) ~ k-α k P(k) Homogeneous, isotropic turbulence
- 6. (Zhuravleva et al 2012) second-order structure function velocity autocorrelation power spectrum proportionality of 2D and 3D power spectra
- 9. MODEL FOR COMA THERMAL PROPERTIES Isothermal β-model: •β = 2/3 •nc = 3×10-3 cm-3 •rc = 300 kpc •T = 8 keV 1. Generate density, velocity fields 2. Project along z-axis for SB, line shift, width 3. Reblock images to 1.5’ resolution (SXS is 3’x3’) KINEMATIC PROPERTIES •Gaussian random field: •homogeneous, isotropic •power-law spectrum with upper and lower cutoffs
- 13. ERRORS • “cosmic variance errors” • Natural variations in the velocity field • 100 realizations of the velocity field taken from the power spectrum • Decrease with more baselines (more pointings spaced out at distance scales we want to resolve)
- 14. “strip” “big cross” “small cross” “fill” “checkerboard”
- 15. COSMIC VARIANCE
- 16. ERRORS • “measurement errors” • Statistical errors on line shift and width from Poisson statistics (increase with increasing line width, decrease with increasing exposure) • Systematic errors on the line shift from gain uncertainty • For most configurations, we assume ~100 ks per pointing, except “checkerboard”, for which we assume ~55 ks per pointing • Assume errors are normally distributed and add them to the map
- 17. BIAS CORRECTION
- 20. VARYING SPECTRAL INDEX Curves are essentially indistinguishable Kraichnan (MHD) Kolmogorov Burgers (shocks)
- 22. 1-σ confidence limits on power spectrum normalization and injection scale fits to structure function and average line width
- 23. WHAT SHOULD WE DO? “big cross” “checkerboard” ~500 ks total exposure (~100 ks per pointing) ~500 ks total exposure (~55 ks per pointing)
- 24. SLOSHING CORES FLASH simulations from ZuHone, Markevitch, & Johnson 2010 Gas motions of M ~ 0.3-0.5, several hundred km/s
- 25. • Can we detect these sloshing motions with Astro-H? • If so, what effect will these motions have on the shift and shape of spectral lines? • Can we use this spectral analysis to tell us something about microphysics?
- 39. DIFFERENT MOTIONS, SAME SHAPE (SOMETIMES)
- 40. SYNTHETIC OBSERVATIONS ZuHone et al 2014, arXiv:1407.1783 SIMX http://www.youtube.com/watch?v=fUMq6rmNshc http://yt-project.org http://hea-www.harvard.edu/simx/ + PHOX http://www.mpa-garching.mpg.de/~kdolag/Phox/ (Biffi et al 2012, 2013, MNRAS) event lists: RA, Dec, E Astro-H event files
- 41. 0.01 0.1 1 countssu−1d 2 4 6 8 −0.2 0 0.2 residuals Energy (keV) 0 2 8 17 30 48 68 93 122 154 190 SXI SXS
- 42. FITTING SPECTRA:Y-AXIS Model 1: Single-T APEC model, w/ thermal broadening only exposure time = 200 ks 0.01 0.1 0.02 0.05 0.2 countss!1 6.2 6.3 6.4 6.5 6.6 6.7 !0.1 0 0.1 residuals Energy (keV) Model 1μ = 291 +9 -12 km/skeV-1
- 43. FITTING SPECTRA:Y-AXIS Model 2: Single-T APEC model, w/ thermal and velocity broadening exposure time = 200 ks 0.01 0.1 0.02 0.05 0.2 countss!1 6.2 6.3 6.4 6.5 6.6 6.7 !0.1 0 0.1 residuals Energy (keV) Model 2 μ = 224 +15 -18 km/s σ = 276 +14 -15 km/skeV-1
- 44. FITTING SPECTRA:Y-AXIS Two single-T APEC models, w/ thermal broadening only exposure time = 200 ks 0.01 0.1 0.02 0.05 0.2 countss!1 6.2 6.3 6.4 6.5 6.6 6.7 !0.1 0 0.1 residuals Energy (keV) Model 3μ1 = 377 +13 -10 km/s μ2 = -51 +16 -20 km/skeV-1
- 45. ✓ ✓✖ (ish) What’s wrong with the line shifts?
- 46. REASON 1: NEED A BETTER MODEL
- 47. REASON 2: PSF SCATTERING
- 48. ✓ ✓ Correct for PSF Scattering
- 50. SUMMARY • What motions will Astro-H measure in galaxy clusters? • Turbulence in Coma: • Can’t distinguish between dissipation scales, spectral indices (for sane turbulence models) • Can distinguish between injection scales, but only for certain spatial configurations of pointings (“big cross” or “checkerboard”) • Gas sloshing: • Will be able to measure line shifts and widths from sloshing motions • It will be difficult to distinguished line broadening from sloshing motions vs. turbulent motions— constraints on viscosity are probably limited • Need to worry about contamination of photons from cluster center—can bias line shifts significantly • What do we need? Better spatial resolution! (Athena, SMART-X, etc.)