AChandraSurvey of Early‐Type Galaxies. I. Metal Enrichment in the Interstellar Medium

Abstract

We present the first in a series of papers studying with Chandra the X-ray properties of a sample of 28 early-type galaxies which span ∼3 orders of magnitude in X-ray luminosity (L X ). We report emission-weighted Fe abundance (Z Fe ) constraints and, for many of the galaxies, abundance constraints for key elements such as O, Ne, Mg, Si, S and Ni. We find no evidence of the very subsolar Z Fe historically reported, confirming a trend in recent X-ray observations of bright galaxies and groups, nor do we find any correlation between Z Fe and luminosity. Except in one case we do not find evidence for a multi-phase interstellar medium (ISM), indicating that multi-temperature fits required in previous ASCA analysis arose due to the strong temperature gradients which we are able to resolve with Chandra. We compare the stellar Z Fe , estimated from simple stellar population model fits, to that of the hot gas. Excepting one possible outlier we find no evidence that the gas is substantially more metal-poor than the stars and, in a few systems, Z Fe is higher in the ISM. In general, however, the two components exhibit similar metallicities, which is inconsistent with both galactic wind models and recent hierarchical chemical enrichment simulations. Adopting standard SNIa and SNII metal yields our abundance ratio constraints imply 66±11% of the Fe within the ISM was produced in SNIa, which is remarkably similar to the Solar neighbourhood, and implies similar enrichment histories for the cold ISM in a spiral and the hot ISM in elliptical galaxies. Although these values are sensitive to the considerable systematic uncertainty in the supernova yields, they are also in very good agreement with observations of more massive systems. These results indicate a remarkable degree of homology in the enrichment process operating from cluster scales to low-to-intermediate L X galaxies. In addition the data uniformly exhibit the low Z O /Z Mg abundance ratios which have been reported in the centres of clusters, groups and some galaxies. This is inconsistent with the standard calculations of metal production in SNII and may indicate an additional source of α-element enrichment, such as Population III hypernovae.

Abundance ratios in the hot ISM of elliptical galaxies

“…O'Connel 1976), this is not true as far as the hot gas abundances are concerned. This is indeed what is observed by Humphrey & Buote (2006) and predicted by wind models (Pipino et al 2005).…”

“…Humphrey & Buote 2006). This Fe abundance is similar to the average abundance of the stars in the same galaxies, as inferred from absorption lines in optical stellar spectra (Humphrey & Buote 2006), whose observed stellar pattern (e.g. Graves et al 2007) is well reproduced by the most recent monolithic formation models (e.g.…”

“…Overall, one can estimate that the contribution of SNIa to the metal enrichment of the hot ISM is quite important, being probably more than 60% (e.g. Humphrey & Buote 2006). This Fe abundance is similar to the average abundance of the stars in the same galaxies, as inferred from absorption lines in optical stellar spectra (Humphrey & Buote 2006), whose observed stellar pattern (e.g.…”

“…However, this is what is observed (e.g. Humphrey & Buote 2006). The models (1, 2, 3, and 4), which we introduced to address the theoretical Fe discrepancy, reduce the [Fe/H] ratio in the ISM with time without changing the average abundance in stars, hence reconciling the models with the observations.…”

“…Either some dilution/differential ejection or Fe-only dust models help in achieving closer agreement with observables once a suitable modification to the SNIa yield relative to the standard W7 model is adopted. The 2D explosion model C-DDT is the most suitable candidate for reproducing both the ISM [Fe/H] (Humphrey & Buote 2006;Thomas et al 2005), hence all models 1, 2, 3, and 4 are consistent to within 1σ as far as the Fe abundance is concerned. These models are consistent with the observationally inferred stellar [Mg/Fe] ratio to within 2σ 8 , the closest match being given by model 2.…”