Recent Progress in Understanding X-ray Emission From Early-type Galaxies: The Hot Gas Component Jimmy A. Irwin University of Michigan X-rays From Nearby Galaxies - September 7, 2007

Introduction Origin of hot gas and metals in ellipticals - stellar mass loss + accretion of gas during/after formation - metal-enriched by Type Ia/II supernovae Temperature: kT ~ keV Mass ~ up to M  Metallicity: ~solar for gas-rich systems Major outstanding problems: L X vs. L opt relation metallicity of hot gas

L X vs. L opt Relation factor of ~ dispersion in relation (e.g, Trinchieri & Fabbiano 1985; Brown & Bregman 1998; Irwin & Sarazin 1998; Beuing et al. 1999; O’Sullivan et al. 2001) X-ray bright galaxies - gas dominated X-ray faint galaxies - LMXB dominated O’Sullivan, Forbes, & Ponman 2001 L X  L opt

X-ray Bright vs. X-ray Faint NGC4494: L X,total (ROSAT) - 3 x ergs s -1 L X,gas (Chandra) - 4 x ergs s -1 NGC4636 (M V = -21.3)NGC4494 (M V = -21.5)

Cause of L X vs. L opt Dispersion Environmental? ram pressure stripping: L X  - cluster environment ICM pressure confinement: L X  “stifling” Internal? - variation in Type Ia supernovae-driven winds - variation in depth of dark matter gravitational potential Low L X /L opt in low density environments. High and low L X /L opt in high density environments. (e.g., Brown & Bregman 2000)

Are We Studying “Typical” Galaxies? Popular targets such as NGC4472, NGC4636, NGC1399, etc., are attractive because of their high X-ray count rates -- but are they typical of elliptical galaxies in general? Of the 34 galaxies in the Brown & Bregman (1998) optically- complete sample of elliptical galaxies, over half have their X-ray emission dominated by X-ray binaries. Smaller (fainter) galaxies that failed to make the Brown & Bregman (1998) sample are predominantly gas-poor.

Metallicity of the Hot Gas: History Initial studies with ASCA led to the “metallicity problem” (Arimoto et al. 1997) : hot gas: ~20% solar, stars:  solar - spectral codes (Raymond-Smith vs. MEKAL vs. APEC) - calibration - treatment of LMXB component - meteoric vs. photspheric abundances - temperature gradients (“Fe bias” - e.g., Buote 2000) Chandra/XMM-Newton studies of high L X /L opt galaxies find ~solar abundances (e.g., Xu et al. 2002; Matsushita et al. 2003; Kim & Fabbiano 2004; Humphrey & Buote 2006). Best spectra show non-solar abundance ratios.

Metallicity of Hot Gas in Ellipticals Abundance ratios indicate a Type Ia vs.Type II supernovae ratio of ~70% / ~30% (see also Humphrey & Buote 2006) MOS+PN for NGC4472, NGC4649, and NGC1399 O/Fe ~ Mg/Fe ~ Ni/Fe ~ 3 - 7

Metallicity of Gas in Low L X /L OPT Galaxies For the few X-ray faint ellipticals for which the metallicity is reported in the literature, highly sub-solar values were found: NGC4697: 7% solar (Sarazin, Irwin, & Bregman 2001) NGC1291: 13% solar (Irwin, Sarazin, & Bregman 2002) NGC4494, NGC3585, NGC5322 : <10% solar (O’Sullivan & Ponman 2004) NGC1553 : ~15% (Humphrey & Buote 2006) Potential Problems: poor statistics stronger relative LMXB contribution elements fixed at solar ratios

Low Metallicity Gas in NGC4697? 174 ksec Chandra 40 ksec XMM-Newton  2 = 0.95/616 d.o.f kT = 0.36 ± 0.05 keV, O = 0.13 ± 0.09 solar, Fe = 0.20 ± 0.11 solar

NGC4697 vs. X-ray Bright Galaxies

Bulge of M31 Old, metal-rich stellar population kT ~ 0.3 keV (Shirey et al. 2001; Takahashi et al. 2004) L X /L opt comparable to X-ray faint ellipticals 38 ksec Chandra  2 = 1.06/238 d.o.f 2 arcmin region

Bulge of M31 kT = 0.31 ± 0.02 keV O = 0.08 ± 0.06 solar Ne = 0.12 ± 0.06 solar Mg = 0.14 ± 0.07 solar Fe = 0.21 ± 0.09 solar Total keV flux Source: 85% Background: 15% Source keV flux Gas: 85% LMXBs:15%

Source of Low Metallicity Gas How are both L X /L opt and low metallicity achieved? One solution: ongoing accretion of pristine gas surrounding galaxies dilutes to subsolar metallicities observational evidence: extended HI structures observed around some ellipticals and S0s (Oosterloo et al. 2007) NGC4697: ~10 8 M   few x 10 8 M  of accreted pristine gas Larger, X-ray bright ellipticals: ~10 10 M   dilution ineffective

Conclusions Spread in L X,gas /L opt of early-type galaxies not well- understood, but Chandra/XMM-Newton making progress - environment vs. internal galactic properties. We know the most about high L X,gas /L opt galaxies, but these galaxies don’t necessarily represent a majority of early-type systems. Low L X,gas /L opt galaxies appear to have very low metallicities, possibly indicating significant dilution from accreted pristine gas.

Stellar Source for Soft Component? NGC4697 L X,gas (0.5-2 keV): 2.1 x ergs s -1 L K : 9.1 x L  (David et al. 2006) L X,gas (0.5-2 keV)/L K : 2.3 x ergs s -1 L  -1 (similar for M31) M32 (Revnivtsev et al. 2007) L X,stellar (0.5-2 keV)/L K : 4.1 x ergs s -1 L  -1 Gas component of NGC x larger than expected stellar contribution.

Stellar Source for Soft Component? M31 (diffuse) L X (0.5-2 keV): 1.7 x ergs s -1 L X (2-7 keV): 5.3 x ergs s -1 L X (0.5-2 keV)/ L X (2-7 keV) = 3.2 M32 (Revnivtsev et al. 2007) L X,stellar (0.5-2 keV)/L K : 4.1 x ergs s -1 L  -1 L X,stellar (2-7 keV)/L K : 2.7 x ergs s -1 L  -1 L X,stellar (0.5-2 keV)/ L X,stellar (2-7 keV) = 1.5

Metallicity of Hot Gas in Ellipticals For high X-ray count galaxies, solar elemental ratios are clearly ruled out, and give inaccurate abundances. fixed at solar ratios  2 = 2.27/665 d.o.f for Z = 3.04 solar