In this paper, we report the discovery of 27 low-surface brightness galaxies, of which 12 are candidates for ultra-diffuse galaxies (UDG) in the Hydra I cluster, based on deep observations taken as ...part of the VST Early-type Galaxy Survey (VEGAS). This first sample of UDG candidates in the Hydra I cluster represents an important step in our project that aims to enlarge the number of confirmed UDGs and, through study of statistically relevant samples, constrain the nature and formation of UDGs. This study presents the main properties of this class of galaxies in the Hydra I cluster. For all UDGs, we analysed the light and colour distribution, and we provide a census of the globular cluster (GC) systems around them. Given the limitations of a reliable GC selection based on two relatively close optical bands only, we find that half of the UDG candidates have a total GC population consistent with zero. Of the other half, two galaxies have a total population larger than zero at 2
σ
level. We estimate the stellar mass, the total number of GCs, and the GC specific frequency (
S
N
). Most of the candidates span a range of stellar masses of 10
7
− 10
8
M
⊙
. Based on the GC population of these newly discovered UDGs, we conclude that most of these galaxies have a standard or low dark matter content, with a halo mass of ≤10
10
M
⊙
.
Context.
Early-type galaxies (ETGs) are found to follow a wide variety of merger and accretion histories in cosmological simulations.
Aims.
We characterize the photometric and kinematic properties of ...simulated ETG stellar halos, and compare them to the observations.
Methods.
We selected a sample of 1114 ETGs in the TNG100 simulation and 80 in the higher-resolution TNG50. These ETGs span a stellar mass range of 10
10.3
− 10
12
M
⊙
and they were selected within the range of
g
−
r
colour and
λ
-ellipticity diagram populated by observed ETGs. We determined photometric parameters, intrinsic shapes, and kinematic observables in their extended stellar halos. We compared the results with central IFU kinematics and ePN.S planetary nebula velocity fields at large radii, studying the variation in kinematics from center to halo, and connecting it to a change in the intrinsic shape of the galaxies.
Results.
We find that the simulated galaxy sample reproduces the diversity of kinematic properties observed in ETG halos. Simulated fast rotators (FRs) divide almost evenly in one third having flat
λ
profiles and high halo rotational support, a third with gently decreasing profiles, and another third with low halo rotation. However, the peak of rotation occurs at larger
R
than in observed ETG samples. Slow rotators (SRs) tend to have increased rotation in the outskirts, with half of them exceeding
λ
= 0.2. For
M
*
> 10
11.5
M
⊙
halo rotation is unimportant. A similar variety of properties is found for the stellar halo intrinsic shapes. Rotational support and shape are deeply related: the kinematic transition to lower rotational support is accompanied by a change towards rounder intrinsic shape. Triaxiality in the halos of FRs increases outwards and with stellar mass. Simulated SRs have relatively constant triaxiality profiles.
Conclusions.
Simulated stellar halos show a large variety of structural properties, with quantitative but no clear qualitative differences between FRs and SRs. At the same stellar mass, stellar halo properties show a more gradual transition and significant overlap between the two families, despite the clear bimodality in the central regions. This is in agreement with observations of extended photometry and kinematics.
Aims. We present kinematic and photometric evidence for an accretion event in the halo of the cD galaxy M 87 in the last Gyr. Methods. Using velocities for ~300 planetary nebulas (PNs) in the M 87 ...halo, we identify a chevron-like substructure in the PN phase-space. We implement a probabilistic Gaussian mixture model to identify PNs that belong to the chevron. From analysis of deep V-band images of M 87, we find that the region with the highest density of chevron PNs is a crown-shaped substructure in the light. Results. We assign a total of NPN,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 20 kpc × 60 kpc region around 70 kpc in radius. In this region, the substructure 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±1.0×109 L⊙ ,V, a colour of (B − V) = 0.76±0.05, and a stellar mass of M = 6.4±2.3×109 M⊙. Conclusions. The accretion of this progenitor galaxy has caused an important modification of the outer halo of M 87 in the last Gyr. This result provides strong evidence that the galaxy’s cD halo is growing through the accretion of smaller galaxies as predicted by hierarchical galaxy evolution models.
In this paper we introduce a new method for analysing Milky Way phase-space which allows us to reveal the imprint left by the Milky Way bar and spiral arms on the stars with full phase-space data in
...Gaia
Data Release 2. The unprecedented quality and extended spatial coverage of these data allowed us to discover six prominent stellar density structures in the disc to a distance of 5 kpc from the Sun. Four of these structures correspond to the spiral arms detected previously in the gas and young stars (Scutum-Centaurus, Sagittarius, Local, and Perseus). The remaining two are associated with the main resonances of the Milky Way bar where corotation is placed at around 6.2 kpc and the outer Lindblad resonance beyond the solar radius, at around 9 kpc. For the first time we provide evidence of the imprint left by spiral arms and resonances in the stellar densities not relying on a specific tracer, through enhancing the signatures left by these asymmetries. Our method offers new avenues for studying how the stellar populations in our Galaxy are shaped.
Aims. We present a kinematic study of a sample of 298 planetary nebulas (PNs) in the outer halo of the central Virgo galaxy M 87 (NGC 4486). The line-of-sight velocities of these PNs are used to ...identify subcomponents, to measure the angular momentum content of the main M 87 halo, and to constrain the orbital distribution of the stars at these large radii. Methods. We use Gaussian mixture modelling to statistically separate distinct velocity components and identify the M 87 smooth halo component, its unrelaxed substructures, and the intra-cluster (IC) PNs. We compute probability weighted velocity and velocity dispersion maps for the smooth halo, and its specific angular momentum profile (λR) and velocity dispersion profile. Results. The classification of the PNs into smooth halo and ICPNs is supported by their different PN luminosity functions. Based on a Kolmogorov–Smirnov (K–S) test, we conclude that the ICPN line-of-sight velocity distribution (LOSVD) is consistent with the LOSVD of the galaxies in Virgo subcluster A. The surface density profile of the ICPNS at 100 kpc radii has a shallow logarithmic slope, −αICL ≃ −0.8, dominating the light at the largest radii. Previous B − V colour and resolved star metallicity data indicate masses for the ICPN progenitor galaxies of a few ×108 M⊙. The angular momentum-related λR profile for the smooth halo remains below 0.1, in the slow rotator regime, out to 135 kpc average ellipse radius (170 kpc major axis distance). Combining the PN velocity dispersion measurements for the M 87 halo with literature data in the central 15 kpc, we obtain a complete velocity dispersion profile out to Ravg = 135 kpc. The σhalo profile decreases from the central 400 km s−1 to about 270 km s−1 at 2–10 kpc, then rises again to ≃300 ± 50 km s−1 at 50–70 kpc, to finally decrease sharply to σhalo ∼ 100 km s−1 at Ravg = 135 kpc. The steeply decreasing outer σhalo profile and the surface density profile of the smooth halo can be reconciled with the circular velocity curve inferred from assuming hydrostatic equilibrium for the hot X-ray gas. Because this rises to νc,X ∼ km s−1 at 200 kpc, the orbit distribution of the smooth M 87 halo is required to change strongly from approximately isotropic within Ravg ∼ 60 kpc to very radially anisotropic at the largest distances probed. Conclusions. The extended LOSVD of the PNs in the M 87 halo allows the identification of several subcomponents: the ICPNs, the “crown” accretion event, and the smooth M 87 halo. In galaxies like M 87, the presence of these subcomponents needs to be taken into account to avoid systematic biases in estimating the total enclosed mass. The dynamical structure inferred from the velocity dispersion profile indicates that the smooth halo of M 87 steepens beyond Ravg = 60 kpc and becomes strongly radially anisotropic, and that the velocity dispersion profile is consistent with the X-ray circular velocity curve at these radii without non-thermal pressure effects.
Context.
Massive early-type galaxies are believed to be the end result of an extended mass accretion history. The stars formed in situ very early on in the initial phase of the mass assembly might ...have originated from an extremely intense and rapid burst of star formation. These stars may still be found within the cores of such galaxies at
z
= 0, depending on their accretion and merger histories.
Aims.
We wish to investigate the presence of a surviving high-
z
compact progenitor component in the brightest galaxy of the Hydra I cluster, NGC 3311, by mapping its 2D kinematics and stellar population out to 2 effective radii. Our goal is to understand the formation of its several structural components and trace their mass assembly back in time.
Methods.
We combined MUSE observations, a customized and extended version of the state-of-the-art EMILES single stellar population models, and a newly developed parametric fully Bayesian framework to model the observed spectra using full-spectrum fitting.
Results.
We present 2D maps and radial profiles of the stellar velocity dispersion, age, total metallicity,
α
-element, sodium abundance (Na/Fe), and the initial mass function (IMF) slope. All properties have significant gradients, confirming the existence of multiple structural components, also including a “blue spot” characterized by younger and more metal-rich stars. We find that the component dominating the light budget of NGC 3311 within
R
≲ 2.0 kpc is the surviving
z
= 0 analog of a high-
z
compact core. This concentrated structure has a relatively small velocity dispersion (
σ
*
≈ 180 km s
−1
), is very old (ages ≳ 11 Gyr), metal-rich (Z/H ∼ 0.2 and Na/Fe ∼ 0.4), and has a bottom-heavy IMF (with slope Γ
b
∼ 2.4). In the outer region, instead, the line-of-sight velocity distribution becomes increasingly broad, and the stars are younger. They are also more metal and sodium poor but are richer in
α
-elements. The low-mass end of the IMF slope becomes Chabrier-like with increasing galactocentric distance.
Conclusions.
The existence of multiple structural components in NGC 3311 from photometry, kinematics, and stellar populations confirms the predictions from the two-phase formation scenario for NGC 3311, according to which a first very short, high-
z
star-formation episode formed a compact stellar structure in its core, which then grew in size by the extended mass assembly of relatively massive satellites. Interestingly, the outer stellar population has an overabundant
α
/Fe, most likely because NGC 3311, located at the center of the galaxy cluster, accreted stars from rapidly quenched satellites.
Context.
The stellar initial mass function (IMF) seems to be variable and not universal, contrary to what has been argued in the literature over the last three decades. Several relations of the ...low-mass end of the IMF slope with other stellar population, photometrical, and kinematical parameters of massive early-type galaxies (ETGs) have been proposed, but consensus on the factual cause of the observed variations has not yet been reached.
Aims.
We investigate the relationship between the IMF and other stellar population parameters in NGC 3311, the central galaxy of the Hydra I cluster. NGC 3311 is a unique laboratory, characterized by old and metal-rich stars, that is similar to other massive ETGs for which the IMF slope has been measured to be bottom-heavy (i.e., dwarf-rich); however, it has unusual stellar velocity dispersion and
α
/Fe profiles, both of which increase with radius.
Methods.
We use the spatially resolved stellar population parameters (age, total metallicity, and
α
/Fe) that were derived in a
forthcoming paper (Barbosa et al. 2020) – via the full-spectrum fitting of high signal-to-noise MUSE observations – to compare the IMF slope in the central part of NGC 3311 (
R
≲ 16 kpc) against other stellar parameters, with the goal of assessing their relations and dependencies.
Results.
For NGC 3311, we unambiguously invalidate the previously observed direct correlation between the IMF slope and the local stellar velocity dispersion, confirming some doubts that had been raised in the literature. This relation may simply arise as a spatial coincidence between the region with the largest stellar velocity dispersion and the region where the oldest in situ population is found and dominates the light. We also show robust evidence that the proposed IMF−metallicity relation is contaminated by the degeneracy between these two parameters. We do confirm that the stellar content in the innermost region of NGC 3311 follows a bottom-heavy IMF, in line with other literature results. The tightest correlations we found are those between stellar age and the IMF and between the galactocentric radius and the IMF.
Conclusions.
The variation of the IMF at its low-mass end is not due to kinematical, dynamical, or global properties in NGC 3311. We speculate instead that the IMF might be dwarf-dominated in the “red nuggets” that formed through a very short and intense star formation episode at high redshifts (
z
> 2) when the Universe was denser and richer in gas, and which then ended up being the central cores of today’s giant ellipticals.
We have compared the halo kinematics traced by globular clusters (GCs) and planetary nebulae (PNe) for two elliptical galaxies in the Fornax and Virgo clusters NGC 1399 and NGC 4649, and for the ...merger remnant NGC 5128 (Centaurus A). We find differences in the rotational properties of the PN, red GC and blue GC systems in all these three galaxies. NGC 1399 PNe and GCs show line of sight velocity distributions in specific regions that are significantly different, based on Kolmogorov-Smirnov tests. The PN system shows multispin components, with nearly opposite direction of rotation in the inner and the outer parts. The GCs velocity field is not point symmetric in the outer regions of the galaxy, indicating that the system has not reached dynamical equilibrium yet. In NGC 4649 PNe, red and blue GCs have different rotation axes and rotational velocities. Finally, in NGC 5128 both PNe and GCs deviate from equilibrium in the outer regions of the galaxy, and in the inner regions the PN system is rotationally supported, whereas the GC system is dominated by velocity dispersion. The observed different kinematic properties, including deviations from point symmetry, between PNe and GCs suggest that these systems are accreted at different times by the host galaxy, and the most recent accretion took place only few Gyr ago. We discuss two scenarios which may explain some of these differences: (i) tidal stripping of loosely bound GCs, and ii) multiple accretion of low luminosity and dwarf galaxies. Because these two mechanisms affect mostly the GC system, differences with the PNe kinematics can be expected.
Stellar halos in early-type galaxies (ETGs) are shaped by their accretion and merger histories. We use a sample of 1114 ETGs in the TNG100 simulation of the IllustrisTNG suite with stellar masses ...1010.3 ≤ M*/M⊙ ≤ 1012, selected at z = 0 within the range of the g − r colour and λ-ellipticity diagram populated by observed ETGs. We study how the rotational support and intrinsic shapes of the stellar halos depend on the fraction of stars accreted, both overall and separately, by major, minor, and mini mergers. Accretion histories in TNG100 ETGs as well as the final radial distributions of ex-situ stars fexsitu(R) relative to in-situ (“accretion classes”) strongly correlate with stellar mass. Low-mass galaxies have characteristic peaked rotation profiles and near-oblate shapes with rounder halos that are completely driven by the in-situ stars. At high fexsitu, major mergers decrease the in-situ peak in rotation velocity, flatten the V*/σ*(R) profiles, and increase the triaxiality of the stellar halos. Kinematic transition radii do not trace the transition between in-situ- and ex-situ-dominated regions, but for systems with M* > 1010.6 M⊙ the local rotational support of the stellar halos decreases with the local ex-situ fraction fexsitu(R) at fixed M*, and their triaxiality increases with fexsitu(R). These correlations between rotational support, intrinsic shapes, and local fexsitu are followed by fast and slow rotators alike with a continuous and overlapping sequence of properties, but slow rotators are concentrated at the high-fexsitu end dominated by dry major mergers. We find that in ∼20% of high-mass ETGs, the central regions are dominated by stars from a high-redshift compact progenitor. Merger events dynamically couple stars and dark matter: in high-mass galaxies and at large radii where fexsitu ≳ 0.5, both components tend to have similar intrinsic shapes and rotational support, and nearly aligned principal axes and spin directions. Based on these results we suggest that extended photometry and kinematics of massive ETGs (M* > 1010.6 M⊙) can be used to estimate the local fraction of ex-situ stars, and to approximate the intrinsic shapes and rotational support of the co-spatial dark matter component.
Context. The formation of intracluster light and extended halos around brightest cluster galaxies is closely related to morphological transformation, tidal stripping, and the disruption of galaxies ...in clusters. Aims. Here we look for observational evidence to characterize these processes, by studying the morphology and kinematics of the diffuse light in the core of the Hydra I cluster. Methods. With V-band surface photometry, we derive the structural parameters (Sersic index n, effective radius Re, b/a, and major axis position angle PA) of the two giant elliptical galaxies NGC 3311 and NGC 3309 in the cluster core. We construct a two-dimensional photometric model, and investigate the diffuse light structures in the residual image after subtracting the two-dimensional model. We also analyze deep long-slit spectra, and establish a link between the structures in the light distribution, the absorption-line kinematics, and the line-of-sight velocity distributions of nearby galaxies and planetary nebulae (PNs). Results. The central galaxy NGC 3311 is surrounded by an extended, symmetric outer halo with n = 10 and an additional, off-centered envelope whose centroid is shifted by about 50′′ to the north-east. Its luminosity LV = 1.2 × 1010 (±6.0 × 108) L⊙ corresponds to ~50% of the luminosity of the symmetric halo in the same region (~15% of its entire luminosity). The velocity dispersion of the halo rises to cluster core values, ~400−500 km s-1, for R > 20′′. On the basis of measured PN velocities, at least part of the off-centered envelope consists of high-velocity accreted stars. We also discover two tidal streams in the cluster center, emerging from the dwarf galaxy HCC 026 and the S0 galaxy HCC 007. The HCC 026 stream is redshifted by ~ 1200 km s-1 with respect to NGC 3311 (VN3311 ≃ 3800 km s-1), as for HCC 026 itself, a fraction of PNs in the off-centered envelope, and several other dwarf galaxies nearby. The stars in one of the HCC 026 tails are known to be consistent with the low-metallicity population of HCC 026, and our photometry shows that this galaxy is almost dissolved by the tidal field. The tidal stream around HCC 007 extends over at least ~110 kpc, is fairly thick, and is brighter on the side of the asymmetric outer halo of NGC 3311, which it may join. Its luminosity is several 109 L⊙, similar to the luminosity of the stripped-down galaxy HCC 007. The redshift of the stream is determined from a few PN velocities and is similar to those of both HCC 007 and HCC 026. Conclusions. An entire group of small galaxies is currently falling through the core of the Hydra I cluster; these galaxies have already been partially dissolved by the strong tidal field. Their light is being added to the outer halo and intracluster light around the cD galaxy NGC 3311. The Hydra I cluster provides a vivid example of the morphological transformation and tidal dissolution of galaxies in clusters.
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