Evidence for massive warm-hot circumgalactic medium around NGC 3221

Abstract

We report a 3.4$\sigma$ detection of the warm-hot, massive, extended circumgalactic medium (CGM) around an L$^\star$ star-forming spiral galaxy NGC 3221, using deep Suzaku observations. The temperature of the gas is $10^{6.1}$ K, comparable to that of the Milky Way CGM. The spatial extent of the gas is at least $150$ kpc. For a $\beta$-model of density profile with solar abundance, the central emission measure is EM = $3\pm 1 \times 10^{-5}$ cm$^{-6}$ kpc and the central electron density is $n_{eo} = 4\pm 1 \times10^{-4}$ cm$^{-3}$, with a slope of $\beta = 0.56$. We investigate a range of $\beta$ values, and find that the details of the density profile do not change our results significantly. The mass of the warm-hot gas, assuming a metallicity of $\frac{1}{3}$ Z$_\odot$ is $16 \pm 3 \times 10^{10}$ M$_\odot$, being the most massive baryon component of NGC 3221. The baryon fraction is $f_b$ = 0.120 $\pm$ 0.036 (statistical) $^{+0.104}_{-0.048}$ (systematic), consistent with the cosmological mean value, closing the baryon budget of this galaxy. We also investigated the missing metals problem in conjunction with the missing baryons problem and conclude that metals are likely to be preferentially expelled from the galaxy. Ours is the first detection of an extended warm-hot CGM around an external L$^\star$ star-forming spiral galaxy, where the CGM likely accounts for the missing galactic baryons.