Stellar populations and substructure in brightest cluster galaxy halos

The build-up of the cD halo of M87 - evidence for accretion in the last Gyr

Longobardi A., Arnaboldi M., Gerhard O., Mihos J. C., 2015a, A&A, 579, L3

We present kinematic and photometric evidence for an accretion event in the halo of the cD galaxy M87 in the last Gyr. Using velocities for ~300 planetary nebulas (PNs) in the M87 halo, we identify a chevron-like substructure in the PN phase-space. We implement a probabilistic Gaussian mixture model to identify the PNs that belong to the chevron. From analysis of deep V-band images of M87, we find that the region with the highest density of PNs associated to the chevron, is a crown-shaped substructure in the optical light. We assign a total of N_(PN,sub)=54 to the substructure, which extends over ~50 kpc along the major axis where we also observe radial variations of the ellipticity profile and a colour gradient. The substructure has highest surface brightness in a 20kpc x 60kpc region around 70 kpc in radius. In this region, it causes an increase in surface brightness by >60%. The accretion event is consistent with a progenitor galaxy with a V-band luminosity of L=2.8\pm1.0 x 10^9 L_(sun,V), a colour of (B-V)=0.76\pm0.05, and a stellar mass of M=6.4\pm2.3 x 10^9 M_sun. The accretion of this progenitor galaxy has caused an important modification of the outer halo of M87 in the last Gyr. By itself it is strong evidence that the galaxy's cD halo is growing through the accretion of smaller galaxies as predicted by hierarchical galaxy evolution models.

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The outer regions of the giant Virgo galaxy M87 II. Kinematic separation of stellar halo and intracluster light

Longobardi A., Arnaboldi M., Gerhard O., Hanuschik R., 2015b, A&A, 579, A135

We present a spectroscopic study of 287 Planetary Nebulas (PNs) in a total area of ~0.4 deg^2 around the BCG M87 in Virgo A. With these data we can distinguish the stellar halo from the co-spatial intracluster light (ICL). PNs were identified from their narrow and symmetric redshifted lambda 5007\4959 Angstrom [OIII] emission lines, and the absence of significant continuum. We implement a robust technique to measure the halo velocity dispersion from the projected phase-space to identify PNs associated with the M87 halo and ICL. The velocity distribution of the spectroscopically confirmed PNs is bimodal, containing a narrow component centred on the systemic velocity of the BCG and an off-centred broader component, that we identify as halo and ICL, respectively. Halo and ICPN have different spatial distributions: the halo PNs follow the galaxy's light, whereas the ICPNs are characterised by a shallower power-law profile. The composite PN number density profile shows the superposition of different PN populations associated with the M87 halo and the ICL, characterised by different PN alpha-parameters, the ICL contributing ~3 times more PNs per unit light. Down to m_5007=28.8, the M87 halo PN luminosity function (PNLF) has a steeper slope towards faint magnitudes than the IC PNLF, and both are steeper than the standard PNLF for the M31 bulge. Moreover, the IC PNLF has a dip at ~1-1.5 mag fainter than the bright cutoff, reminiscent of the PNLFs of systems with extended star formation history. The M87 halo and the Virgo ICL are dynamically distinct components with different density profiles and velocity distribution. The different alpha values and PNLF shapes of the halo and ICL indicate distinct parent stellar populations, consistent with the existence of a gradient towards bluer colours at large radii. These results reflect the hierarchical build-up of the Virgo cluster.

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The planetary nebula population in the halo of M87

Longobardi A.,Arnaboldi M.,Gerhard O.,Coccato L.,Okamura S.,Freeman K. C., 2013, A&A, 558, 42

Over the past years we conducted an observational campain with the Planetary Nebulae Spectrograph (Douglas et al. 2002) aimed to measure the radial velocities of PNe in the halos of ETGs (see Figure 1). In our first official data release (Coccato et al. 2009) we combined absorption line data and PNe radial velocity measurements in 16 ETGs. Our analysis showed that: i) PNe are good tracers of the mean stellar population kinematics, as their kinematics and number density agrees with the stellar absorption line kinematics and surface brightness; ii) outer halos have more complex radial profiles of the lR parameter (a proxy for the angular momentum, Emsellem et al. 2007) than observed within 1 Re. Interestingly, in the halo, some fast rotators have declining lR radial profiles, almost reaching the slow rotator regime, while some slow rotators have slowly increasing lR profiles, which reach the fast rotator regime (see Figure 2); iii) the velocity dispersion profiles fall into two groups, with part of the galaxies characterized by slowly decreasing profiles and the remainder having steeply falling profiles; iv) the halo kinematics are correlated with other galaxy properties, such as luminosity, shape, total stellar mass, V/s, and number of PNe per unit luminosity, with a clear distinction between fast and slow rotators.

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Distinct core and halo stellar population in the bright coma cluster galaxy NGC 4889

Coccato, L., Gerhard, O., Arnaboldi, M. 2010,  MNRAS,  407,  L26

We construct radial profiles of line strength indices along the major axis of NGC 4889 by combining literature data for the central regions and new deep spectroscopic data for the halo regions. We then derive age, metallicity and alpha-enhancement radial profiles and their gradients using Single Stellar Population models by Thomas et al. (2003).   This represents the most spatially extended dataset with both stellar kinematics and line strength indices for a brightest cluster galaxy.
We observe a different population content and gradient between the central regions of the galaxy (R<18 kpc) and the outer halo (R>18 kpc). The inner ~18 kpc (~1.2 Re) of NGC 4889 are characterized by a strong [Z/H] gradient and a nearly constant values of [alpha/Fe]. The outer regions (18 kpc < R < 60 kpc) are characterized by a constant metallicity content strong negative gradient in the abundance ratio and older ages.
These data indicate that the central parts of NGC 4889 and its halo have undergone different formation mechanisms. Data in the center indicate a short star formation timescale, where the stars formed outside-in, reminiscent of a quasi-monolithic dissipative collapse. On the contrary, the data in the halo suggest that it was accreted from shredded satellite galaxies, as suggested also by numerical simulations, over the central galaxy that was already in place.
Our measurements are also consistent with recent results on the size evolution of bright ETGs with redshift, i.e. at high redshifts ETGs are smaller and more compact than ETGs of similar mass at z = 0. Their effective radius evolves as Re ~ (1+z)-1.3 (van Dokkum et al. 2010). Scaling the present Re of NGC 4889 with this relation would predict Re = 6.2 kpc, at z = 1, which is consistent with the half light radius measured if considering the central regions of the galaxy only, on the assumption that outer regions of NGC 4889 were accreted later, at z < 1. Our finding for NGC 4889 suggests that we may have found local stellar population signatures of the observed ETG size evolution.

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