Halo Metallicity in NGC 891 An X-ray/UV Perspective Edmund Hodges-Kluck Joel Bregman University of Michigan

How do baryons cycle in and out of galaxies, and how does this impact galaxy evolution? – What happened to the missing baryons? – Are contemporary halos formed by infall from the IGM or galactic fountains?

High ZLow Z Metallicity is a distinguishing factor

QSO Absorption Lines – column density of halo metals Dust Extinction/Scattering/Emission – dust type depends on Z X-ray Emission – Z from simple thermal model How do we Measure Halo Metallicity?

Case Study: NGC 891 Edge-on Milky Way analog Nearby (10 Mpc) Bright X-ray halo Giant HI halo

Halo Metallicity from the X-rays Clean test possible in the outer halo seen in 100 ks with XMM-Newton

Halo Metallicity from the X-rays 1T fits to XMM+CXO spectra prefer Z < Z to 3σ (5σ joint fit) Hodges-Kluck+Bregman (2013)

Halo Metallicity from the UV UV photons leaking out of the disk scatter off dust grains in the halo with metallicity-dependent spectrum

Galex and Swift UVOT sensitivity limited by foreground fluctuations (sky background is low) Swift UVOT has persistent scattered-light artifacts, but these can be corrected by subtracting scaled templates in each filter

NGC 891

UV SED favors Milky Way- type dust over LMC or SMC dust, but need HST data to bracket UV bump (UVW1 filter has red leak)

QSO Absorption Lines – SOLAR (Bregman+2013) Dust Extinction/Scattering/Emission – SOLAR (Rand+2011, Hodges-Kluck+2013, in prep) X-ray Emission – SUBSOLAR (Hodges-Kluck+Bregman 2013) How do we Measure Halo Metallicity?

Hard to eject cold gas without ejecting hot gas, too But, 10 9 M HI halo (Oosterloo+07) could not have cooled from the 3x10 8 M hot component at current cooling rate of <0.5 M /yr Halo may not be in a steady state X-rays could trace steady hot accretion; cooler gas might be from a galactic fountain or wind fallback Different Components?

Halo metallicity distinguishes between infall and galactic fountain scenarios In NGC 891, cold gas seems to have solar metallicity, hot gas subsolar Hot, cold gas may have different origin New method in UV using Swift may be fruitful alternative to QSO absorption Summary

Photon Flux (arbitrary) Energy (keV) Energy (keV) kT = 0.5 keV N H = cm 2 Z = 0.1 Z ʘ Z = 1.0 Z ʘ Metallicity is a key indicator that is directly measurable in the X-rays At CCD resolution, good S/N needed to distinguish spectral shape

From a fitting perspective, low metallicity results from flux below 0.6 keV The absorbing column in the outer halo is constrained well by other observations

Low metallicity not likely caused by systematics: Unaccounted bkg Abundance table Absorption model Calibration issues

But, the data also admit a 2-T model where both components have solar metallicity. This is in the outer halo

Two reality checks favor accretion Limit on cooling rate from UV O VI from Otte et al indicates < 2-3 M ʘ /yr Observed vs. expected scale height H obs measured assuming hydrostatic equilibrium H exp from cooling time

Clearly, the scattered light can bias a search for extended emission as objects of interest are usually centered on the chip

Edge Bkg. Count Rate [cts/s/pixel] Bkg. Sub. Ring Count Rate [cts/s/pixel]

UV – r color is too blue in most cases to be produced by an old stellar halo population But, a stellar fountain may be possible