arXiv subjects

Nicholas Scott

Publications and source records attributed to Nicholas Scott.

At least 55 records · Page 3Linked to original sources

arXiv2016

The SAMI galaxy survey: Galaxy Interactions and Kinematic Anomalies in Abell 119

Galaxy mergers are important events that can determine the fate of a galaxy by changing its morphology, star-formation activity and mass growth. Merger systems have commonly been identified from their disturbed morphologies, and we now can employ Integral Field Spectroscopy to detect and analyze the impact of mergers on stellar kinematics as well. We visually classified galaxy morphology using deep images ($μ_{\rm r} = 28\,\rm mag\,\, arcsec^{-2}$) taken by the Blanco 4-m telescope at the Cerro Tololo Inter-American Observatory. In this paper we investigate 63 bright ($M_{\rm r}<-19.3$) spectroscopically-selected galaxies in Abell 119; of which 53 are early type and 20 galaxies show a disturbed morphology by visual inspection. A misalignment between the major axes in the photometric image and the kinematic map is conspicuous in morphologically-disturbed galaxies. Our sample is dominated by early-type galaxies, yet it shows a surprisingly tight Tully-Fisher relation except for the morphologically-disturbed galaxies which show large deviations. Three out of the eight slow rotators in our sample are morphology disturbed. The visually-selected morphologically-disturbed galaxies are generally more asymmetric, visually as well as kinematically. Our findings suggest that galaxy interactions, including mergers and perhaps fly-bys, play an important role in determining the orientation and magnitude of galaxy's angular momentum.

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arXiv2016

The Atlas3D project -- XXXI. Nuclear radio emission in nearby early-type galaxies

We present the results of a high-resolution, 5 GHz, Karl G. Jansky Very Large Array study of the nuclear radio emission in a representative subset of the Atlas3D survey of early-type galaxies (ETGs). We find that 51 +/- 4% of the ETGs in our sample contain nuclear radio emission with luminosities as low as 10^18 W/Hz. Most of the nuclear radio sources have compact (< 25-110 pc) morphologies, although < 10% display multi-component core+jet or extended jet/lobe structures. Based on the radio continuum properties, as well as optical emission line diagnostics and the nuclear X-ray properties, we conclude that the majority of the central 5 GHz sources detected in the Atlas3D galaxies are associated with the presence of an active galactic nucleus (AGN). However, even at sub-arcsecond spatial resolution, the nuclear radio emission in some cases appears to arise from low-level nuclear star formation rather than an AGN, particularly when molecular gas and a young central stellar population is present. This is in contrast to popular assumptions in the literature that the presence of a compact, unresolved, nuclear radio continuum source universally signifies the presence of an AGN. Additionally, we examine the relationships between the 5 GHz luminosity and various galaxy properties including the molecular gas mass and - for the first time - the global kinematic state. We discuss implications for the growth, triggering, and fueling of radio AGNs, as well as AGN-driven feedback in the continued evolution of nearby ETGs.

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arXiv2016

A normal supermassive black hole in NGC 1277

The identification of galaxies with `overly massive' black holes requires two measurements: a black hole mass (M_bh) and a host spheroid mass (M_sph,*). Here we provide our measurements for NGC 1277. Our structural decomposition reveals that NGC 1277 is dominated by a `classical' spheroid with a Sersic index n=5.3, a half-light radius R_e=2.1 kpc, and a stellar mass of 2.7x10^{11} M_sun (using M_*/L_V=11.65, Martin-Navarro et al.). This mass is an order of magnitude greater than originally reported. Using the latest (M_bh)-n, (M_bh)-(M_sph,*) and (M_bh)-sigma relations, the expected black hole mass is respectively (0.57^{+1.29}_{-0.40})x10^9 M_sun, (1.58^{+4.04}_{-1.13})x10^9 M_sun, and (2.27^{+4.04}_{-1.44})x10^9 M_sun (using sigma=300 km/s) for which the `sphere-of-influence' is 0".31. Our new kinematical maps obtained from laser guide star assisted, adaptive optics on the Keck I Telescope dramatically reaffirm the presence of the inner, nearly edge-on, disk seen in this galaxy's image. We also report that this produces a large velocity shear (~400 km/s) across the inner 0".2 (70 pc) plus elevated values of sqrt{sigma^2+(V_rot)^2} across the inner 3".8x0".6 region of the galaxy. Our new multi-Gaussian expansion (MGE) models and Jeans Anisotropic MGE (JAM) analysis struggled to match this extended component. Our optimal black hole mass, albeit a probable upper limit because of the disk, is 1.2x10^9 M_sun (M/L_V=12.3). This is an order of magnitude smaller than originally reported and 4 times smaller than recently reported. It gives an M_bh/M_sph,* ratio of 0.45% in agreement with the median (~0.5%) and range (0.1 to 5.0%) observed in non-dwarf, early-type galaxies. This result highlights the need for caution with inner disks.

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arXiv2015

The SAMI Pilot Survey: The Fundamental and Mass Planes in Three Low-Redshift Clusters

Using new integral field observations of 106 galaxies in three nearby clusters we investigate how the intrinsic scatter of the Fundamental Plane depends on the way in which the velocity dispersion and effective radius are measured. Our spatially resolved spectroscopy, combined with a cluster sample with negligible relative distance errors allows us to derive a Fundamental Plane with minimal systematic uncertainties. From the apertures we tested, we find that velocity dispersions measured within a circular aperture with radius equal to one effective radius minimises the intrinsic scatter of the Fundamental Plane. Using simple yet powerful Jeans dynamical models we determine dynamical masses for our galaxies. Replacing luminosity in the Fundamental Plane with dynamical mass, we demonstrate that the resulting Mass Plane has further reduced scatter, consistent with zero intrinsic scatter. Using these dynamical models we also find evidence for a possibly non-linear relationship between dynamical mass-to-light ratio and velocity dispersion.

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arXiv2015

The Atlas3D Project - XXX. Star formation histories and stellar population scaling relations of early-type galaxies

We present the stellar population content of early-type galaxies from the Atlas3D survey. Using spectra integrated within apertures covering up to one effective radius, we apply two methods: one based on measuring line-strength indices and applying single stellar population (SSP) models to derive SSP-equivalent values of stellar age, metallicity, and alpha enhancement; and one based on spectral fitting to derive non-parametric star-formation histories, mass-weighted average values of age, metallicity, and half-mass formation timescales. Using homogeneously derived effective radii and dynamically-determined galaxy masses, we present the distribution of stellar population parameters on the Mass Plane (M_JAM, Sigma_e, R_maj), showing that at fixed mass, compact early-type galaxies are on average older, more metal-rich, and more alpha-enhanced than their larger counterparts. From non-parametric star-formation histories, we find that the duration of star formation is systematically more extended in lower mass objects. Assuming that our sample represents most of the stellar content of today's local Universe, approximately 50% of all stars formed within the first 2 Gyr following the big bang. Most of these stars reside today in the most massive galaxies (>10^10.5 M_sun), which themselves formed 90% of their stars by z~2. The lower-mass objects, in contrast, have formed barely half their stars in this time interval. Stellar population properties are independent of environment over two orders of magnitude in local density, varying only with galaxy mass. In the highest-density regions of our volume (dominated by the Virgo cluster), galaxies are older, alpha-enhanced and have shorter star-formation histories with respect to lower density regions.

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arXiv2014

The (black hole)-bulge mass scaling relation at low masses

Several recent papers have reported on the occurrence of active galactic nuclei (AGN) containing under-massive black holes relative to a linear scaling relation between black hole mass (M_bh) and host spheroid stellar mass (M_sph,*). Dramatic revisions to the M_bh-M_sph,* and M_bh-L_sph relations, based on samples containing predominantly inactive galaxies, have however recently identified a new steeper relation at M_bh < (2-10)x10^8 M_Sun, roughly corresponding to M_sph,* < (0.3-1)x10^{11} M_Sun. We show that this steeper, quadratic-like M_bh-M_sph,* relation defined by the Sersic galaxies, i.e. galaxies without partially depleted cores, roughly tracks the apparent offset of the AGN having 10^5 < M_bh/M_Sun < 0.5x10^8. That is, these AGN are not randomly offset with low black hole masses, but also follow a steeper (non-linear) relation. As noted by Busch et al., confirmation or rejection of a possible AGN offset from the steeper M_bh-M_sph,* relation defined by the Sersic galaxies will benefit from improved stellar mass-to-light ratios for the spheroids hosting these AGN. Several implications for formation theories are noted. Furthermore, reasons for possible under- and over-massive black holes, the potential existence of intermediate mass black holes (<10^5 M_Sun), and the new steep (black hole)--(nuclear star cluster) relation, M_bh ~ (M_nc)^{2.7+/-0.7}, are also discussed.

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arXiv2014

Super-massive black hole mass scaling relations

Using black hole masses which span 10^5 to 10^(10) solar masses, the distribution of galaxies in the (host spheroid stellar mass)-(black hole mass) diagram is shown to be strongly bent. While the core-Sersic galaxies follow a near-linear relation, having a mean M_(bh)/M_(sph) mass ratio of ~0.5%, the Sersic galaxies follow a near-quadratic relation: M_bh~M_sph^(2.22+\-0.58). This is not due to offset pseudobulges, but is instead an expected result arising from the long-known bend in the M_(sph)-sigma relation and the log-linear M_(bh)-sigma relation.

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arXiv2014

The Atlas3D project -- XXIX. The new look of early-type galaxies and surrounding fields disclosed by extremely deep optical images

Galactic archeology based on star counts is instrumental to reconstruct the past mass assembly of Local Group galaxies. The development of new observing techniques and data-reduction, coupled with the use of sensitive large field of view cameras, now allows us to pursue this technique in more distant galaxies exploiting their diffuse low surface brightness (LSB) light. As part of the Atlas3D project, we have obtained with the MegaCam camera at the Canada-France Hawaii Telescope extremely deep, multi--band, images of nearby early-type galaxies. We present here a catalog of 92 galaxies from the Atlas3D sample, that are located in low to medium density environments. The observing strategy and data reduction pipeline, that achieve a gain of several magnitudes in the limiting surface brightness with respect to classical imaging surveys, are presented. The size and depth of the survey is compared to other recent deep imaging projects. The paper highlights the capability of LSB--optimized surveys at detecting new prominent structures that change the apparent morphology of galaxies. The intrinsic limitations of deep imaging observations are also discussed, among those, the contamination of the stellar halos of galaxies by extended ghost reflections, and the cirrus emission from Galactic dust. The detection and systematic census of fine structures that trace the present and past mass assembly of ETGs is one of the prime goals of the project. We provide specific examples of each type of observed structures -- tidal tails, stellar streams and shells --, and explain how they were identified and classified. We give an overview of the initial results. The detailed statistical analysis will be presented in future papers.

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arXiv2014

Connection between dynamically derived IMF normalisation and stellar population parameters

We report on empirical trends between the dynamically determined stellar initial mass function (IMF) and stellar population properties for a complete, volume-limited sample of 260 early-type galaxies from the Atlas3D project. We study trends between our dynamically-derived IMF normalisation and absorption line strengths, and interpret these via single stellar population- (SSP-) equivalent ages, abundance ratios (measured as [alpha/Fe]), and total metallicity, [Z/H]. We find that old and alpha-enhanced galaxies tend to have on average heavier (Salpeter-like) mass normalisation of the IMF, but stellar population does not appear to be a good predictor of the IMF, with a large range of normalisation at a given population parameter. As a result, we find weak IMF-[alpha/Fe] and IMF-age correlations, and no significant IMF-[Z/H] correlation. The observed trends appear significantly weaker than those reported in studies that measure the IMF normalisation via low-mass star demographics inferred through stellar spectral analysis.

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arXiv2014

The Atlas3D project - XXIV. The intrinsic shape distribution of early-type galaxies

We use the Atlas3D sample to perform a study of the intrinsic shapes of early-type galaxies, taking advantage of the available combined photometric and kinematic data. Based on our ellipticity measurements from the Sloan Digital Sky Survey Data Release 7, and additional imaging from the Isaac Newton Telescope, we first invert the shape distribution of fast and slow rotators under the assumption of axisymmetry. The so-obtained intrinsic shape distribution for the fast rotators can be described with a Gaussian with a mean flattening of q=0.25 and standard deviation sigma_q = 0.14, and an additional tail towards rounder shapes. The slow rotators are much rounder, and are well described with a Gaussian with mean q = 0.63 and sigma_q =0.09. We then checked that our results were consistent when applying a different and independent method to obtain intrinsic shape distributions, by fitting the observed ellipticity distributions directly using Gaussian parametrisations for the intrinsic axis ratios. Although both fast and slow rotators are identified as early-type galaxies in morphological studies, and in many previous shape studies are therefore grouped together, their shape distributions are significantly different, hinting at different formation scenarios. The intrinsic shape distribution of the fast rotators shows similarities with the spiral galaxy population. Including the observed kinematic misalignment in our intrinsic shape study shows that the fast rotators are predominantly axisymmetric, with only very little room for triaxiality. For the slow rotators though there are very strong indications that they are (mildly) triaxial.

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arXiv2014

The Supermassive Black Hole Mass - Spheroid Stellar Mass Relation for Sérsic and Core-Sérsic Galaxies

We have examined the relationship between supermassive black hole mass (M$_{BH}$) and the stellar mass of the host spheroid (M$_{sph,*}$) for a sample of 75 nearby galaxies. To derive the spheroid stellar masses we used improved 2MASS K$_s$-band photometry from the ARCHANGEL photometry pipeline. Dividing our sample into core-Sérsic and Sérsic galaxies, we find that they are described by very different M$_{BH}$-M$_{sph,*}$ relations. For core-Sérsic galaxies - which are typically massive and luminous, with M$_{BH} \gtrsim 2x10^8 M_\odot$ - we find M$_{BH} \propto M_{sph,*}^{(0.97 \pm 0.14)}$, consistent with other literature relations. However, for the Sérsic galaxies - with typically lower masses, M$_{sph,*} \lesssim 3x10^10 M_\odot$ - we find M$_BH \propto M_{sph,*}^{(2.22 \pm 0.58)}$, a dramatically steeper slope that differs by more than 2 standard deviations. This relation confirms that, for Sérsic galaxies, M$_{BH}$ is not a constant fraction of M$_{sph,*}$. Sérsic galaxies can grow via the accretion of gas which fuels both star formation and the central black hole, as well as through merging. Their black hole grows significantly more rapidly than their host spheroid, prior to growth by dry merging events that produce core-Sérsic galaxies, where the black hole and spheroid grow in lock step. We have additionally compared our Sérsic M$_{BH}$-M$_{sph,*}$ relation with the corresponding relation for nuclear star clusters, confirming that the two classes of central massive object follow significantly different scaling relations.

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arXiv2014

The SAMI Pilot Survey: The Kinematic Morphology-Density Relation in Abell 85, Abell 168 and Abell 2399

We examine the kinematic morphology of early-type galaxies (ETGs) in three galaxy clusters Abell 85, 168 and 2399. Using data from the Sydney-AAO Multi-object Integral field spectrograph (SAMI) we measured spatially-resolved kinematics for 79 ETGs in these clusters. We calculate $λ_{R}$, a proxy for the projected specific stellar angular momentum, for each galaxy and classify the 79 ETGs in our samples as fast or slow rotators. We calculate the fraction of slow rotators in the ETG populations ($f_{SR}$) of the clusters to be $0.21\pm0.08$, $0.08\pm0.08$ and $0.12\pm0.06$ for Abell 85, 168 and 2399 respectively, with an overall fraction of $0.15\pm0.04$. These numbers are broadly consistent with the values found in the literature, confirming recent work asserting that the fraction of slow rotators in the ETG population is constant across many orders of magnitude in global environment. We examine the distribution of kinematic classes in each cluster as a function of environment using the projected density of galaxies: the kinematic morphology-density relation. We find that in Abell 85 $f_{SR}$ increases in higher density regions but in Abell 168 and Abell 2399 this trend is not seen. We examine the differences between the individual clusters to explain this. In addition, we find slow rotators on the outskirts of two of the clusters studied, Abell 85 and 2399. These galaxies reside in intermediate to low density regions and have clearly not formed at the centre of a cluster environment. We hypothesise that they formed at the centres of groups and are falling into the clusters for the first time.

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arXiv2014

The ATLAS3D Project - XXVIII. Dynamically-driven star formation suppression in early-type galaxies

We present measurements of the star formation rate (SFR) in the early-type galaxies (ETGs) of the ATLAS3D sample, based on Wide-field Infrared Survey Explorer (WISE) 22um and Galaxy Evolution Explorer (GALEX) far-ultraviolet emission. We combine these with gas masses estimated from 12CO and HI data in order to investigate the star formation efficiency (SFE) in a larger sample of ETGs than previously available. We first recalibrate (based on WISE data) the relation between old stellar populations (traced at Ks-band) and 22um luminosity, allowing us to remove the contribution of 22um emission from circumstellar dust. We then go on to investigate the position of ETGs on the Kennicutt-Schmidt (KS) relation. Molecular gas-rich ETGs have comparable star formation surface densities to normal spiral galaxy centres, but they lie systematically offset from the KS relation, having lower star formation efficiencies by a factor of ~2.5 (in agreement with other authors). This effect is driven by galaxies where a substantial fraction of the molecular material is in the rising part of the rotation curve, and shear is high. We show here for the first time that although the number of stars formed per unit gas mass per unit time is lower in ETGs, it seems that the amount of stars formed per free-fall time is approximately constant. The scatter around this dynamical relation still correlates with galaxy properties such as the shape of the potential in the inner regions. This leads us to suggest that dynamical properties (such as shear or the global stability of the gas) may be important second parameters that regulate star formation and cause much of the scatter around star-formation relations.

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arXiv2014

Distribution of Slow and Fast Rotators in the Fornax Cluster

We present integral field spectroscopy of 10 early-type galaxies in the nearby, low-mass, Fornax cluster, from which we derive spatially resolved stellar kinematics. Based on the morphologies of their stellar velocity maps we classify 2/10 galaxies as slow rotators, with the remaining 8 galaxies fast rotators. Supplementing our integral field observations with morphological and kinematic data from the literature, we analyse the `kinematic' type of all 30 galaxies in the Fornax cluster brighter than M_K = -21.5 mag (M_* ~ 6 x 10^9 M_sun). Our sample's slow rotator fraction within one virial radius is 7(^+4_-6) per cent. 13(^+8_-6} per cent of the early-type galaxies are slow rotators, consistent with the observed fraction in other galaxy aggregates. The fraction of slow rotators in Fornax varies with cluster-centric radius, rising to 16(^+11_-8) per cent of all kinematic types within the central 0.2 virial radii, from 0 per cent in the cluster outskirts. We find that, even in mass-matched samples of slow and fast rotators, slow rotators are found preferentially at higher projected environmental density than fast rotators. This demonstrates that dynamical friction alone cannot be responsible for the differing distributions of slow and fast rotators. For dynamical friction to play a significant role, slow rotators must reside in higher mass sub-halos than fast rotators and/or form in the centres of groups before being accreted on to the cluster.

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arXiv2014

The Atlas3D project - XXVI. HI discs in real and simulated fast and slow rotators

One quarter of all nearby early-type galaxies (ETGs) outside Virgo host a disc/ring of HI with size from a few to tens of kpc and mass up to ~1e+9 solar masses. Here we investigate whether this HI is related to the presence of a stellar disc within the host making use of the classification of ETGs in fast and slow rotators (FR/SR). We find a large diversity of HI masses and morphologies within both families. Surprisingly, SRs are detected as often, host as much HI and have a similar rate of HI discs/rings as FRs. Accretion of HI is therefore not always linked to the growth of an inner stellar disc. The weak relation between HI and stellar disc is confirmed by their frequent kinematical misalignment in FRs, including cases of polar and counterrotating gas. In SRs the HI is usually polar. This complex picture highlights a diversity of ETG formation histories which may be lost in the relative simplicity of their inner structure and emerges when studying their outer regions. We find that LCDM hydrodynamical simulations have difficulties reproducing the HI properties of ETGs. The gas discs formed in simulations are either too massive or too small depending on the star formation feedback implementation. Kinematical misalignments match the observations only qualitatively. The main point of conflict is that nearly all simulated FRs and a large fraction of all simulated SRs host corotating HI. This establishes the HI properties of ETGs as a novel challenge to simulations.

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arXiv2013

NGC1266 as a Local Candidate for Rapid Cessation of Star Formation

We present new Spectrographic Areal Unit for Research on Optical Nebulae (SAURON) integral-field spectroscopy and Swift Ultraviolet Optical Telescope (UVOT) observations of molecular outflow host galaxy NGC 1266 that indicate NGC 1266 has experienced a rapid cessation of star formation. Both the SAURON maps of stellar population age and the Swift UVOT observations demonstrate the presence of young ($< 1$ Gyr) stellar populations within the central 1 kpc, while existing Combined Array for Research in Millimeter-wave Astronomy (CARMA) CO(1--0) maps indicate that the sites of current star formation are constrained to the inner few hundred parsecs of the galaxy only. The optical spectrum of NGC 1266 from Moustakas & Kennicutt (2006) reveal a characteristic post-starburst (K+A) stellar population and Davis et al. (2012) confirm that ionized gas emission in the system originate from a shock. Galaxies with K+A spectra and shock-like ionized gas line ratios may comprise an important, overlooked segment of the post-starburst population, containing exactly those objects in which the AGN is actively expelling the star-forming material. While AGN activity is not the likely driver of the post-starburst event that occurred 500 Myr ago, the faint spiral structure seen in the Hubble Space Telescope (HST) Wide-field Camera 3 (WFC3) Y-, J- and H-band imaging seems to point to the possibility of gravitational torques being the culprit. If the molecular gas were driven into the center at the same time as the larger scale galaxy disk underwent quenching, the AGN might be able to sustain the presence of molecular gas for $\gtrsim 1$ Gyr by cyclically injecting turbulent kinetic energy into the dense molecular gas via a radio jet, inhibiting star formation.

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arXiv2013

The ATLAS3D Project -- XXIII. Angular momentum and nuclear surface brightness profiles

[Abridged and Edited] We investigate nuclear light profiles in 135 ATLAS3D galaxies for which the Hubble Space Telescope (HST) imaging is available and compare them to the large scale kinematics obtained with the SAURON integral-field spectrograph. Specific angular momentum, lambda_R, correlates with the shape of nuclear light profiles, where cores are typically found in slow rotators and core-less galaxies are fast rotators. Cores are found only in massive galaxies and only in systems with the stellar mass M>8x10^10 Msun. Based on our sample, we, however, see no evidence for a bimodal distribution of nuclear slopes. The best predictor for finding a core is based on the stellar velocity dispersion within an effective radius, sigma_e, and specific angular momentum, where cores are found for lambda_R<0.25 and sigma_e>160 km/s. We estimate that only about 10% of nearby early-type galaxies contain cores. Furthermore, we show that there is a genuine population of fast rotators with cores. We also show that core fast rotators are morphologically, kinematically and dynamically different from core slow rotators. The cores of fast rotators could harbour black holes of similar masses to those in core slow rotators, but typically more massive than those found in core-less fast rotators. Core-less galaxies, and especially core-less fast rotators, are under-luminous in the diffuse X-ray emission, but the presence of a core does not imply high X-ray luminosities. We postulate a possible population of core-less galaxies among slow rotators, which can not be explained as face-on discs, but comprise a genuine sub-population of slow rotators. These galaxies are typically less massive and flatter than core slow rotators, and show evidence for dynamical cold structures and exponential photometric components. We discuss possible processes for formation of cores and their subsequent preservation.

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arXiv2013

The Atlas3D project - XX. Mass-size and mass-sigma distributions of early-type galaxies: bulge fraction drives kinematics, mass-to-light ratio, molecular gas fraction and stellar initial mass function

We study the (M,sigma) and (M,Re) projections of the thin Mass Plane (MP) (M,sigma,Re) which describes the distribution of the galaxy population. The distribution of galaxy properties on the MP is characterized by: (i) a zone of exclusion described by two power-laws joined by a break at M 3e10 Msun. This results in a break in the mean M-sigma relation with Msigma^2.3 and M sigma^4.7 at small and large sigma respectively; (ii) a mass M 2e11 Msun which separates a population dominated by flat fast rotator with disks and spiral galaxies at lower masses, from one dominated by quite round slow rotators at larger masses; (iii) below that mass the distribution of ETGs properties tends to be constant along lines of constant sigma; (iv) it forms a parallel sequence with the distribution of spiral galaxies; (v) at even lower masses, the distribution of fast rotator ETGs and late spirals naturally extends to that of dwarf ETGs (Sph) and dwarf irregulars (Im) respectively. We show via dynamical models that sigma traces the bulge fraction, which drives the observed trends in M/L, Hbeta, colour, IMF and molecular gas fraction. We interpret this as due to a combination of two main effects: (i) an increase of the bulge fraction which increases sigma and greatly enhances the likelihood for a galaxy to have its star formation quenched, and (ii) dry merging, increasing galaxy mass along lines of nearly constant sigma, while leaving the population unchanged. [Abriged]

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