Halo Metallicity in NGC 891An X-ray/UV Perspective

Edmund Hodges-KluckJoel 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 Z Low 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, 109 M HI halo (Oosterloo+07) could not have

cooled from the 3x108 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

Phot

on F

lux

(arb

itrar

y)

Energy (keV)0.3 1.0 2.0

Energy (keV)0.3 1.0 2.0

0.01

0.1

1.0

10

100kT = 0.5 keV NH = 1021 cm2

Z = 0.1 Zʘ Z = 1.0 Zʘ

• 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. 2003 indicates < 2-3 Mʘ/yr• Observed vs. expected scale height

—Hobs measured assuming hydrostatic equilibrium—Hexp from cooling time

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

0.000 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.0080.00E+000

5.00E-004

1.00E-003

1.50E-003

2.00E-003

2.50E-003

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

Bkg.

Sub

. Rin

g Co

unt R

ate

[cts

/s/p

ixel

]

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