The SAMI Galaxy Survey: stellar population and structural trends across the Fundamental Plane

We study the Fundamental Plane (FP) for a volume- and luminosity-limited sample of 560 early-type galaxies from the SAMI survey. Using r −band sizes and luminosities from new Multi-Gaussian Expansion (MGE) photometric measurements, and treating luminosity as the dependent variable, the FP has coefficients a = 1.294 ± 0.039, b = 0.912 ± 0.025, and zero-point c = 7.067 ± 0.078. We leverage the high signal-to-noise of SAMI integral field spectroscopy, to determine how structural and stellar-population observables affect the scatter about the FP. The FP residuals correlate most strongly (8σ significance) with luminosity-weighted simple-stellar-population (SSP) age. In contrast, the structural observables surface mass density, rotation-to-dispersion ratio, Sérsic index and projected shape all show little or no significant correlation. We connect the FP residuals to the empirical relation between age (or stellar mass-to-light ratio ϒ⋆ ) and surface mass density, the best predictor of SSP age amongst parameters based on FP observables. We show that the FP residuals (anti-)correlate with the residuals of the relation between surface density and ϒ⋆ . This correlation implies that part of the FP scatter is due to the broad age and ϒ⋆ distribution at any given surface mass density. Using virial mass and ϒ⋆ we construct a simulated FP and compare it to the observed FP. We find that, while the empirical relations between observed stellar population relations and FP observables are responsible for most (75 per cent) of the FP scatter, on their own they do not explain the observed tilt of the FP away from the virial plane.

Measuring the surface mass density ellipticity of redMaPPer galaxy clusters using weak-lensing

In this work we study the shape of the projected surface mass density distribution of galaxy clusters using weak-lensing stacking techniques. In particular, we constrain the average aligned component of the projected ellipticity, ε, for a sample of redMaPPer clusters (0.1 ≤ z < 0.4). We consider six different proxies for the cluster orientation and measure ε for three ranges of projected distances from the cluster centres. The mass distribution in the inner region (up to 700 kpc) is better traced by the cluster galaxies with a higher membership probability, while the outer region (from 700 kpc up to 5 Mpc) is better traced by the inclusion of less probable galaxy cluster members. The fitted ellipticity in the inner region is ε = 0.21 ± 0.04, in agreement with previous estimates. We also study the relation between ε and the cluster mean redshift and richness. By splitting the sample in two redshift ranges according to the median redshift, we obtain larger ε values for clusters at higher redshifts, consistent with the expectation from simulations. In addition, we obtain higher ellipticity values in the outer region of clusters at low redshifts. We discuss several systematic effects that might affect the measured lensing ellipticities and their relation to the derived ellipticity of the mass distribution.

Resolving the Disc–Halo Degeneracy – II: NGC 6946

ABSTRACT The mass-to-light ratio (M/L) is a key parameter in decomposing galactic rotation curves into contributions from the baryonic components and the dark halo of a galaxy. One direct observational method to determine the disc M/L is by calculating the surface mass density of the disc from the stellar vertical velocity dispersion and the scale height of the disc. Usually, the scale height is obtained from near-IR studies of edge-on galaxies and pertains to the older, kinematically hotter stars in the disc, while the vertical velocity dispersion of stars is measured in the optical band and refers to stars of all ages (up to ∼10 Gyr) and velocity dispersions. This mismatch between the scale height and the velocity dispersion can lead to underestimates of the disc surface density and a misleading conclusion of the submaximality of galaxy discs. In this paper, we present the study of the stellar velocity dispersion of the disc galaxy NGC 6946 using integrated star light and individual planetary nebulae as dynamical tracers. We demonstrate the presence of two kinematically distinct populations of tracers that contribute to the total stellar velocity dispersion. Thus, we are able to use the dispersion and the scale height of the same dynamical population to derive the surface mass density of the disc over a radial extent. We find the disc of NGC 6946 to be closer to maximal with the baryonic component contributing most of the radial gravitational field in the inner parts of the galaxy (Vmax(bar) = 0.76(±0.14)Vmax).

Rotation Curve of the Milky Way and the Dark Matter Density

We review the current status of the study of rotation curve (RC) of the Milky Way, and present a unified RC from the Galactic Center to the galacto-centric distance of about 100 kpc. The RC is used to directly calculate the distribution of the surface mass density (SMD). We then propose a method to derive the distribution of dark matter (DM) density in the in the Milky Way using the SMD distribution. The best-fit dark halo profile yielded a local DM density of ρ ⊙ = 0.36 ± 0.02 GeV cm − 3 . We also review the estimations of the local DM density in the last decade, and show that the value is converging to a value at ρ ⊙ = 0.39 ± 0.09 GeV cm − 3 .

Setting the scene for BUFFALO: a study of the matter distribution in the HFF galaxy cluster MACS J0416.1−2403 and its parallel field

ABSTRACT In the context of the Beyond Ultradeep Frontier Fields And Legacy Observations (BUFFALO) survey, we present a new analysis of the merging galaxy cluster MACS J0416.1−2403 (z = 0.397) and its parallel field using Hubble Frontier Fields (HFF) data. We measure the surface mass density from a weak-lensing analysis and characterize the overall matter distribution in both the cluster and parallel fields. The surface mass distribution derived for the parallel field shows clumpy overdensities connected by filament-like structures elongated in the direction of the cluster core. We also characterize the X-ray emission in the parallel field and compare it with the lensing mass distribution. We identify five mass peaks at the >5σ level over the two fields, four of them being in the cluster one. Three of them are located close to galaxy overdensities and one is also close to an excess in the X-ray emission. Nevertheless, two of them have neither optical nor X-ray counterpart and are located close to the edges of the field of view, thus further studies are needed to confirm them as substructures. Finally, we compare our results with the predicted subhalo distribution of one of the Hydrangea/C-EAGLE simulated cluster. Significant differences are obtained suggesting the simulated cluster is at a more advanced evolutionary stage than MACS J0416.1−2403. Our results anticipate the upcoming BUFFALO observations that will link the two HFF fields, extending further the HST coverage.

The signature of primordial black holes in the dark matter halos of galaxies

Aims. The aim of this paper is to investigate the claim that stars in the lensing galaxy of a gravitationally lensed quasar system can always account for the observed microlensing of the individual quasar images. Methods. A small sample of gravitationally lensed quasar systems was chosen where the quasar images appear to lie on the fringe of the stellar distribution of the lensing galaxy. As with most quasar systems, all the individual quasar images were observed to be microlensed. The surface brightness of the lensing galaxy at the positions of the quasar images was measured from Hubble Space Telescope frames, and converted to stellar surface mass density. The surface density of smoothly distributed dark matter at the image positions was obtained from lensing models of the quasar systems and applied to the stellar surface mass density to give the optical depth to microlensing. This was then used to assess the probability that the stars in the lensing galaxy could be responsible for the observed microlensing. The results were supported by microlensing simulations of the star fields around the quasar images combined with values of convergence and shear from the lensing models. Results. Taken together, the probability that all the observed microlensing is due to stars was found to be ∼3 × 10−4. Errors resulting from the surface brightness measurement, the mass-to-light ratio, and the contribution of the dark matter halo do not significantly affect this result. Conclusions. It is argued that the most plausible candidates for the microlenses are primordial black holes, either in the dark matter halos of the lensing galaxies, or more generally distributed along the lines of sight to the quasars.

Weak lensing analysis of codex clusters using dark energy camera legacy survey: mass–richness relation

ABSTRACT We present the weak-lensing analysis of 279 CODEX clusters using imaging data from 4200 deg2 of the DECam Legacy Survey (DECaLS) Data Release 3. The cluster sample results from a joint selection in X-ray, optical richness in the range 20 ≤ λ < 110, and redshift in the range 0.1 ≤ z ≤ 0.2. We model the cluster mass (M200c) and the richness relation with the expression $\left\langle M_{\rm 200c} | \lambda \right\rangle \propto M_{0} \, (\lambda / 40)^{F_{\lambda }}$. By measuring the CODEX cluster sample as an individual cluster, we obtain the best-fitting values, $M_{0} = 3.24^{+0.29}_{-0.27} \times 10^{14} \text{M}_{\odot }$, and $F_{\lambda } = 1.00 ^{+0.22}_{-0.22}$ for the richness scaling index, consistent with a power-law relation. Moreover, we separate the cluster sample into three richness groups; λ = 20–30, 30–50, and 50–110, and measure the stacked excess surface mass density profile in each group. The results show that both methods are consistent. In addition, we find an excellent agreement between our weak lensing based scaling relation and the relation obtained with dynamical masses estimated from cluster member velocity dispersions measured by the SDSS-IV/SPIDERS team. This suggests that the cluster dynamical equilibrium assumption involved in the dynamical mass estimates is statistically robust for a large sample of clusters.

Does black hole growth depend fundamentally on host-galaxy compactness?

ABSTRACT Possible connections between central black hole (BH) growth and host-galaxy compactness have been found observationally, which may provide insight into BH–galaxy coevolution: compact galaxies might have large amounts of gas in their centres due to their high mass-to-size ratios, and simulations predict that high central gas density can boost BH accretion. However, it is not yet clear if BH growth is fundamentally related to the compactness of the host galaxy, due to observational degeneracies between compactness, stellar mass (M⋆) and star formation rate (SFR). To break these degeneracies, we carry out systematic partial-correlation studies to investigate the dependence of sample-averaged BH accretion rate ($\rm \overline{BHAR}$) on the compactness of host galaxies, represented by the surface-mass density, Σe, or the projected central surface-mass density within 1 kpc, Σ1. We utilize 8842 galaxies with H &lt; 24.5 in the five CANDELS fields at z = 0.5–3. We find that $\rm \overline{BHAR}$ does not significantly depend on compactness when controlling for SFR or M⋆ among bulge-dominated galaxies and galaxies that are not dominated by bulges, respectively. However, when testing is confined to star-forming galaxies at z = 0.5–1.5, we find that the $\rm \overline{BHAR}$–Σ1 relation is not simply a secondary manifestation of a primary $\rm \overline{BHAR}$–M⋆ relation, which may indicate a link between BH growth and the gas density within the central 1 kpc of galaxies.

IMF radial gradients in most massive early-type galaxies

ABSTRACT Using new long-slit spectroscopy obtained with X-Shooter at ESO-VLT, we study, for the first time, radial gradients of optical and near-infrared initial mass function (IMF)-sensitive features in a representative sample of galaxies at the very high mass end of the galaxy population. The sample consists of seven early-type galaxies (ETGs) at z ∼ 0.05, with central velocity dispersion in the range 300 ≲ σ ≲ 350 km s−1. Using state-of-the-art stellar population synthesis models, we fit a number of spectral indices, from different chemical species (including TiO and Na indices), to constrain the IMF slope (i.e. the fraction of low-mass stars), as a function of galactocentric distance, over a radial range out to ∼4 kpc. ETGs in our sample show a significant correlation of IMF slope and surface mass density. The bottom-heavy population (i.e. an excess of low-mass stars in the IMF) is confined to central galaxy regions with surface mass density above $\rm \sim 10^{10}\, M_\odot \, kpc^{-2}$, or, alternatively, within a characteristic radius of ∼2 kpc. Radial distance, in physical units, and surface mass density are the best correlators to IMF variations, with respect to other dynamical (e.g. velocity dispersion) and stellar population (e.g. metallicity) properties. Our results for the most massive galaxies suggest that there is no single parameter that fully explains variations in the stellar IMF, but IMF radial profiles at z ∼ 0 rather result from the complex formation and mass accretion history of galaxy inner and outer regions.

Satellite galaxies in the Illustris-1 simulation: anisotropic locations around relatively isolated hosts

ABSTRACT We investigate the locations of satellite galaxies in the z = 0 redshift slice of the hydrodynamical Illustris-1 simulation. As expected from previous work, the satellites are distributed anisotropically in the plane of the sky, with a preference for being located near the major axes of their hosts. Due to misalignment of mass and light within the hosts, the degree of anisotropy is considerably less when satellite locations are measured with respect to the hosts’ stellar surface mass density than when they are measured with respect to the hosts’ dark matter surface mass density. When measured with respect to the hosts’ dark matter surface mass density, the mean satellite location depends strongly on host stellar mass and luminosity, with the satellites of the faintest, least massive hosts showing the greatest anisotropy. When measured with respect to the hosts’ stellar surface mass density, the mean satellite location is essentially independent of host stellar mass and luminosity. In addition, the satellite locations are largely insensitive to the amount of stellar mass used to define the hosts’ stellar surface mass density, as long as at least 50–70 per cent of the hosts’ total stellar mass is used. The satellite locations are dependent upon the stellar masses of the satellites, with the most massive satellites having the most anisotropic distributions.