Cores and Cusps in the Dwarf Spheroidals

We consider the problem of determining the structure of the dark halo of nearby dwarf spheroidal galaxies (dSphs) from the spherical Jeans equations. Whether the dark halos are cusped or cored at the centre is an important strategic problem in modern astronomy. The observational data comprise the line-of-sight velocity dispersion of a luminous tracer population. We show that when such data are analysed to find the dark matter density with the spherical Poisson and Jeans equations, then the generic solution is a dark halo density that is cusped like an isothermal. Although milder cusps (like the Navarro-Frenk-White 1/r cusp and even cores are possible, they are not generic. Such solutions exist only if the anisotropy parameter beta and the logarithmic slope of the stellar density gamma satisfy the constraint gamma = 2 x beta at the centre or if the radial velocity dispersion falls to zero at the centre. This surprisingly strong statement is really a consequence of the assumption of spherical symmetry, and the consequent coordinate singularity at the origin. So, for example, a dSph with an exponential light profile can exist in Navarro-Frenk- White halo and have a flat velocity dispersion, but anisotropy in general drives the dark halo solution to an isothermal cusp. The identified cusp or core is therefore a consequence of the assumptions (particularly of spherical symmetry and isotropy), and not the data.Comment: MNRAS, in pres

Virial Sequences for Thick Discs and Haloes: Flattening and Global Anisotropy

The virial theorem prescribes the ratio of the globally-averaged equatorial to vertical velocity dispersion of a tracer population in spherical and flattened dark haloes. This gives sequences of physical models in the plane of global anisotropy and flattening. The tracer may have any density, though there are particularly simple results for power-laws and exponentials. We prove the flattening theorem: for a spheroidally stratified tracer density with axis ratio q in a dark density potential with axis ratio g, the ratio of globally averaged equatorial to vertical velocity dispersion depends only on q/g. As the stellar halo density and velocity dispersion of the Milky Way are accessible to observations, this provides a new method for measuring the flattening of the dark matter. If the kinematics of the local halo subdwarfs are representative, then the Milky Way's dark halo is oblate with a flattening in the potential of g ~ 0.85, corresponding to a flattening in the dark matter density of ~ 0.7. The fractional pressure excess for power-law populations is roughly proportional to both the ellipticity and the fall-off exponent. Given the same pressure excess, if the density profile of one stellar population declines more quickly than that of another, then it must be rounder. This implies that the dual halo structure claimed by Carollo et al. (2007) for the Galaxy, a flatter inner halo and a rounder outer halo, is inconsistent with the virial theorem. For the thick disc, we provide formulae for the virial sequences of double-exponential discs in logarithmic and Navarro-Frenk-White (NFW) haloes. There are good matches to the observational data on the flattening and anisotropy of the thick disc if the thin disc is exponential with a short scalelength ~ 2.6 kpc and normalisation of 56 solar masses per square parsec, together with a logarithmic dark halo.Comment: MNRAS, submitted, 13 pages, 7 figures, small changes to made to correspond to final accepted versio

The MOND Fundamental Plane

Modified Newtonian Dynamics (MOND) has been shown to be able to fit spiral galaxy rotation curves as well as giving a theoretical foundation for empirically determined scaling relations, such as the Tully - Fisher law, without the need for a dark matter halo. As a complementary analysis, one should investigate whether MOND can also reproduce the dynamics of early - type galaxies (ETGs) without dark matter. As a first step, we here show that MOND can indeed fit the observed central velocity dispersion $\sigma_0$ of a large sample of ETGs assuming a simple MOND interpolating functions and constant anisotropy. We also show that, under some assumptions on the luminosity dependence of the Sersic n parameter and the stellar M/L ratio, MOND predicts a fundamental plane for ETGs : a log - linear relation among the effective radius $R_{eff}$, $\sigma_0$ and the mean effective intensity $\langle I_e \rangle$. However, we predict a tilt between the observed and the MOND fundamental planes.Comment: 16 pages, 2 figures, 2 tables, accepted for publication on MNRA

Galaxy morphological classification in deep-wide surveys via unsupervised machine learning

Accepted versionGalaxy morphology is a fundamental quantity, that is essential not only for the full spectrum of galaxy-evolution studies, but also for a plethora of science in observational cosmology. While a rich literature exists on morphological-classification techniques, the unprecedented data volumes, coupled, in some cases, with the short cadences of forthcoming 'Big-Data' surveys (e.g. from the LSST), present novel challenges for this field. Large data volumes make such datasets intractable for visual inspection (even via massively-distributed platforms like Galaxy Zoo), while short cadences make it difficult to employ techniques like supervised machine-learning, since it may be impractical to repeatedly produce training sets on short timescales. Unsupervised machine learning, which does not require training sets, is ideally suited to the morphological analysis of new and forthcoming surveys. Here, we employ an algorithm that performs clustering of graph representations, in order to group image patches with similar visual properties and objects constructed from those patches, like galaxies. We implement the algorithm on the Hyper-Suprime-Cam Subaru-Strategic-Program Ultra-Deep survey, to autonomously reduce the galaxy population to a small number (160) of 'morphological clusters', populated by galaxies with similar morphologies, which are then benchmarked using visual inspection. The morphological classifications (which we release publicly) exhibit a high level of purity, and reproduce known trends in key galaxy properties as a function of morphological type at zPeer reviewe

The expansion field: The value of H_0

Any calibration of the present value of the Hubble constant requires recession velocities and distances of galaxies. While the conversion of observed velocities into true recession velocities has only a small effect on the result, the derivation of unbiased distances which rest on a solid zero point and cover a useful range of about 4-30 Mpc is crucial. A list of 279 such galaxy distances within v<2000 km/s is given which are derived from the tip of the red-giant branch (TRGB), from Cepheids, and from supernovae of type Ia (SNe Ia). Their random errors are not more than 0.15 mag as shown by intercomparison. They trace a linear expansion field within narrow margins from v=250 to at least 2000 km/s. Additional 62 distant SNe Ia confirm the linearity to at least 20,000 km/s. The dispersion about the Hubble line is dominated by random peculiar velocities, amounting locally to <100 km/s but increasing outwards. Due to the linearity of the expansion field the Hubble constant H_0 can be found at any distance >4.5 Mpc. RR Lyr star-calibrated TRGB distances of 78 galaxies above this limit give H_0=63.0+/-1.6 at an effective distance of 6 Mpc. They compensate the effect of peculiar motions by their large number. Support for this result comes from 28 independently calibrated Cepheids that give H_0=63.4+/-1.7 at 15 Mpc. This agrees also with the large-scale value of H_0=61.2+/-0.5 from the distant, Cepheid-calibrated SNe Ia. A mean value of H_0=62.3+/-1.3 is adopted. Because the value depends on two independent zero points of the distance scale its systematic error is estimated to be 6%. Typical errors of H_0 come from the use of a universal, yet unjustified P-L relation of Cepheids, the neglect of selection bias in magnitude-limited samples, or they are inherent to the adopted models.Comment: 44 pages, 4 figures, 6 tables, accepted for publication in the Astronony and Astrophysics Review 15

Variation of floral traits across a geographical framework of pollinator assemblages differing in functional, morphological and ethological features

The study of geographical variation of phenotypic floral traits provides a suitable background to understand the role of pollinator assemblages as drivers of floral diversity patterns. Here we present three plant-pollinator systems that differ in pollinator assemblages features. Monttea aphylla (Plantaginaceae), widespread in the Monte desert, offers simultaneously oils and nectar as reward to functionally distinct pollinators. Variation in floral rewards as a response to functionally diverse assemblages, as well as to other biotic and abiotic factors suggest that investment in the more expensive reward (oil) is promoted in communities where nectar foragers in addition to oil-collecting bees are present and stronger competition for the services of the more specialized pollinator should favor the evolution of narrower pollination niches. In communities where specialized foragers are the only available, competition for niches should be relaxed and investment in the more expensive reward would not be compensated with better services of pollinators. Calceolaria polyrhiza (Calceolariaceae) is widely distributed in the arid Patagonian steppe and in the temperate forests understory. Pollinators are either of two oil-collecting bees species which strongly differ in size. Through analyses of floral integration and covariation patterns we observed that plant-pollinator phenotypic matching across the geographical range is facilitated through variation in mechanical-fit related traits and their decoupling from variation on attraction-related traits, which are mainly affected by climatic and edaphic gradients. The decoupled geographical variation between floral modules across Patagonia is discussed as strategy that facilitates plant-pollinator phenotypic matching in this environmental heterogeneous and stressful area. Finally, phenotypic selection in the finch-pollinated Patagonian bush Anarthrophyllum desideratum (Fabaceae) showed that variation in floral traits was only partially explained by assemblage shifts; the finch species dominant in most populations seems to be locally idiosyncratic in the way it handles flowers to access nectar. As a consequence, pollen is carried on different parts of the head. Both, changes in pollinator assemblages and behavior across populations appear to complementarily account for the geographical variation in flower phenotype.Fil: Sérsic, AN. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales; Argentina.Fil: Sérsic, AN. Consejo Nacional de Investigaciones Científicas y Técnicas. Laboratorio de biología evolutiva-Biología floral; Argentina.Fil: Sérsic, AN. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto Multidisciplinario de Biología Vegetal; Argentina.Fil: Baranzelli, M. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales; Argentina.Fil: Baranzelli, M. Consejo Nacional de Investigaciones Científicas y Técnicas. Laboratorio de biología evolutiva-Biología floral; Argentina.Fil: Baranzelli, M. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto Multidisciplinario de Biología Vegetal; Argentina.Fil: Cosacov, A. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales; Argentina.Fil: Cosacov, A. Consejo Nacional de Investigaciones Científicas y Técnicas. Laboratorio de biología evolutiva-Biología floral; Argentina.Fil: Cosacov, A. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto Multidisciplinario de Biología Vegetal; Argentina.Fil: Ferreiro, G. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales; Argentina.Fil: Ferreiro, G. Consejo Nacional de Investigaciones Científicas y Técnicas. Laboratorio de biología evolutiva-Biología floral; Argentina.Fil: Ferreiro, G. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto Multidisciplinario de Biología Vegetal; Argentina.Fil: Paiaro, V. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales; Argentina.Fil: Paiaro, V. Consejo Nacional de Investigaciones Científicas y Técnicas. Laboratorio de biología evolutiva-Biología floral; Argentina.Fil: Paiaro, V. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto Multidisciplinario de Biología Vegetal; Argentina.Ecologí