Sloshing in its cD halo: MUSE kinematics of the central galaxy NGC 3311 in the Hydra I cluster

Abstract

Context. Early-type galaxies (ETGs) show a strong size evolution with redshift. This evolution is explained by fast "in-situ" star formation at high-z followed by a late mass assembly mostly driven by minor mergers that deposit stars primarily in the outer halo. Aims. We aim to identify the main structural components of the Hydra I cD galaxy NGC 3311 to investigate the connection between the central galaxy and the surrounding stellar halo. Methods. We produce maps of the line-of-sight velocity distribution (LOSVD) moments from a mosaic of MUSE pointings covering NGC 3311 out to 25 kpc. Combining deep photometric and spectroscopic data, we model the LOSVD maps using a finite mixture distribution, including four non-concentric components that are nearly isothermal spheroids, with different line-of-sight systemic velocities V, velocity dispersions σ, and small (constant) values of the higher order Gauss-Hermite moments h 3 and h 4 . Results. The kinemetry analysis indicates that NGC 3311 is classified as a slow rotator, although the galaxy shows a line-of-sight velocity gradient along the photometric major axis. The comparison of the correlations between h 3 and h 4 with V/σ with simulated galaxies indicates that NGC 3311 assembled mainly through dry mergers. The σ profile rises to 400 km s -1 at 20 kpc, a significant fraction (0.55) of the Hydra I cluster velocity dispersion, indicating that stars there were stripped from progenitors orbiting in the cluster core. The finite mixture distribution modeling supports three inner components related to the central galaxy and a fourth component with large effective radius (51 kpc) and velocity dispersion (327 km s -1 ) consistent with a cD envelope. We find that the cD envelope is offset from the center of NGC 3311 both spatially (8.6 kpc) and in velocity (∆V = 204 kms −1 ), but coincide with the cluster core X-ray isophotes and the mean velocity of core galaxies. Also, the envelope contributes to the broad wings of the LOSVD measured by large h 4 values within 10 kpc. Conclusions. The cD envelope of NGC 3311 is dynamically associated with the cluster core, which in Hydra I is in addition displaced from the cluster center, presumably due to a recent subcluster merger.

Spectroscopic decomposition of the galaxy and halo of the cD galaxyNGC 3311

“…While this effect would have little impact on the decomposed spectra if the halo stellar populations are uniform throughout, the presence of substructures within the halo due to accreted material would affect the spectra. Fits to imaging data by Arnaboldi et al (2012) and Barbosa et al (2018) have revealed three distinct substructures within the field-of-view of the MUSE data, which are attributed to tidal tails. As a result, if the stellar populations of these substructures are significantly different to that of the halo, it is likely that the decomposed halo spectrum obtained from this data will be biased towards these properties.…”

“…It must be noted at this point that the models we use for NGC 3311 in this study assume that the halo is smooth and symmetric both inside and outside of the available field of view. In reality however, Arnaboldi et al (2012) and Barbosa et al (2018) found that the halo is asymmetric, with an excess of light towards the north east of the central galaxy due to the halo lying off-centre and the presence of what that they identified as a tidal stream from an in falling galaxy. This effect can be seen in Fig.…”

“…Previous works have modelled NGC 3311 as a single Sérsic profile, such as Arnaboldi et al (2012) and Vasterberg et al (1991) who found effective radii of 270 arcsec (n = 5, V-band) and 80 arcsec (n = 4, r-band) respectively. However, recent fits to V-band photometry of NGC 3311 by Barbosa et al (2018) over a larger field of view found a best fit using a four-component model-three components to fit the galaxy and a fourth to model the halo, in addition to modelling NGC 3309 and a neighbouring dwarf galaxy to reduce their effects on the fits. Multi-component fits to cD galaxies have also been found to best characterise their light profiles in other recent works (e.g.…”

Structure of Brightest Cluster Galaxies and Intracluster Light

“…A different approach is to consider stellar velocities. It is motivated by the rising radial velocity dispersion profiles that approach the cluster dispersion, i.e., the relative velocities of the cluster galaxies (Ventimiglia et al 2010;Toledo et al 2011;Arnaboldi et al 2012;Melnick et al 2012;Murphy et al 2014;Bender et al 2015; Barbosa et al 2018). The ICL is hereby the dynamically hot component that is kinematically controlled by the gravitational potential of the whole cluster, i.e., unbound from the BCG.…”

The Hydra I cluster core

“…One of the nearest cD-galaxies is NGC 3311, the central galaxy in the Hydra I galaxy cluster. Recent publications on NGC 3311 are mainly from our group (Ventimiglia et al 2008(Ventimiglia et al , 2010(Ventimiglia et al , 2011Coccato et al 2011;Misgeld et al 2011;Richtler et al 2011;Arnaboldi et al 2012; Barbosa et al 2016Barbosa et al , 2018. Photometric and kinematical studies using the galaxy light, planetary nebulae (PNe) and globular clusters (GCs) clearly demonstrate the close connection of the inner galaxy with its host cluster.…”