The Recent Growth History of the Fornax Cluster Derived from Simultaneous Sloshing and Gas Stripping: Simulating the Infall of NGC 1404

Abstract

We derive the recent growth history of the Fornax Cluster, in particular the recent infall of the giant elliptical galaxy NGC 1404. We show, using a simple cluster minor merger simulation tailored to Fornax and NGC 1404, that a second or more likely third encounter between the two reproduces all main merger features observed in both objects; we firmly exclude a first infall scenario. Our simulations reveal a consistent picture: NGC 1404 passed by NGC 1399 about 1.1 -1.3 Gyrs ago from the NE to the SW and is now almost at the point of its next encounter from the S. This scenario explains the sloshing patterns observed in Fornax -a prominent northern cold front and an inner southern cold front. This scenario also explains the truncated atmosphere, the gas stripping radius of NGC 1404, and its faint gas tail. Independent of the exact history, we can make a number of predictions. A detached bow shock south of NGC 1404 should exist which is a remnant of the galaxy's previous infall at a distance from NGC 1404 between 450 -750 kpc with an estimated Mach number between 1.3 and 1.5. The wake of NGC 1404 also lies S of the galaxy with enhanced turbulence and a slight enhancement in metallicity compared to the undisturbed regions of the cluster. SW of NGC 1404, there is likely evidence of old turbulence originating from the previous infall. No scenario predicts enhanced turbulence outside of the cold front north west of the cluster center.

A New Class of X-Ray Tails of Early-type Galaxies and Subclusters in Galaxy Clusters: Slingshot Tails versus Ram Pressure Stripped Tails

“…As the secondary slows toward apocenter, the previously leading bow shock continues to propagate outwards as the secondary turns around, hence the shock appears behind the secondary on the tail side, not leading it, and can be found at large distances behind the secondary (the shock can be located up to 1 Mpc behind the secondary). Such a detached bow shock is visually marked in Figure 2 and in the simulations of Sheardown et al (2018).…”

“…The mass of the primary cluster in this suite of simulations is set to 6 × 10 14 M . Sheardown et al (2018) work presents three tailored simulations of the infall of the elliptical galaxy NGC 1404 into the Fornax Cluster. As with ZuHone (2011), their simulations were run using FLASH, using a similar simulation design.…”

“…The first and second column show electron density and temperature slices for a ∼ 1:10 merger with a small impact parameter from the V1 simulation in Sheardown et al (2018). The third and fourth column likewise show electron density and temperature slices but for a 1:3 merger we ran for this paper (following the simulation design of Sheardown et al (2018)) using two idealized clusters with a pericenter distance of 330 kpc. The first row shows the secondary at pericenter with a typical ram pressure stripped tail.…”

“…It is worth noting that the stagnation point method (Vikhlinin et al 2001;Su et al 2017b) to determine a secondary's velocity from stagnation point pressure relies on the analogy of a (quasi-)stable flow past a blunt body. The merger stage prior and near pericenter passage would qualify for this, with columns (a) in Sheardown et al (2018), the same as the two left-hand columns of Figure 2. Each column shows an X-ray photon intensity projection in the orbital plane; a gas fraction slice of the secondary, showing the extent that the tail has been stripped and mixed with the ICM; a slice of the shear rate, showing the locations of strong shear flows; and finally a colormap of the flow field, overlaid with velocity vectors.…”

“…If the secondary has completed a second passage of the cluster center, the sloshing in the core will have evolved further, producing a prominent cold front on the opposite side of the cluster to the secondary. Further, the wake of the secondary could appear as a characteristic brightness edge in the primary, marking roughly the secondary's orbit (see Sheardown et al (2018)). However, a caveat to using the dynamical state of the cluster to help identify a slingshot tail is that it would only work with a simple cluster setup, i.e.…”