Abstract
Abstract The M49 group, residing outside the virial radius of the Virgo cluster, is falling onto the cluster from the south. We report results from deep XMM-Newton mosaic observations of M49. Its hot gas temperature is 0.8 keV at the group center and rises to 1.5 keV beyond the brightest group galaxy (BGG). The group gas extends to radii of ∼300 kpc to the north and south. The observations reveal a cold front ∼20 kpc north of the BGG center and an X-ray-bright stripped tail 70 kpc long and 10 kpc wide to the southwest of the BGG. We argue that the atmosphere of the infalling group was slowed by its encounter with the Virgo cluster gas, causing the BGG to move forward subsonically relative to the group gas. We measure declining temperature and metallicity gradients along the stripped tail. The tail gas can be traced back to the cooler and enriched gas uplifted from the BGG center by buoyant bubbles, implying that active galactic nucleus outbursts may have intensified the stripping process. We extrapolate to a virial radius of 740 kpc and derive a virial mass of 4.6 × 1013 M ⊙ for the M49 group. Its group atmosphere appears truncated and deficient when compared with isolated galaxy groups of similar temperatures. If M49 is on its first infall to Virgo, the infall region of a cluster could have profound impacts on galaxies and groups that are being accreted onto galaxy clusters. Alternatively, M49 may have already passed through Virgo once.
Highlights
Hierarchical structure formation is the cornerstone of modern cosmology
We present our study of M49 (NGC 4472), a galaxy group residing beyond the virial radius of a galaxy cluster, Virgo, the true frontier of cluster evolution
The astrophysical X-ray background (AXB) model contains a power-law model powCXB with a photon index fixed at Γ = 1.46 characterizing the cosmic X-ray background (CXB; De Luca & Molendi 2004), a thermal emission model apecMW with a temperature allowed to vary between 0.15 and 0.6 keV representing the Milky Way emission (McCammon et al 2002), and another thermal emission model apecLB with a temperature fixed at 0.11 keV for the Local Bubble emission
Summary
Hierarchical structure formation is the cornerstone of modern cosmology. As the most massive virialized systems in the universe, clusters of galaxies formed late, with the vast bulk of their dark matter, hot gas, and member galaxies assembled from z ∼ 0.5 (e.g., Boylan-Kolchin et al 2009). The infall region provides unique distinguishing power for models of the hierarchical assembly of dark matter halos and the growth of galaxy clusters. For each region of interest, we simultaneously fit the M49 and the FWC data with their NXB parameters linked (ratios of their bknpow normalizations are determined by simultaneously fitting spectra extracted from the unexposed corners of both the FWC and M49 observations). We compare the area-corrected count rates in the 6–12 keV energy band within the field of view (excluding the central 10′) and those in the unexposed corners of the detector. If this ratio is below 1.15, we consider the observation not contaminated by the residual soft proton flare (Molendi et al 2004). Its photon index is allowed to vary between 0.1 and 1.4 (Snowden & Kuntz 2014)
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