A Council of Giants

Monthly Notices of the Royal Astronomical Society, May 2014

Distances and near-infrared luminosities of the brightest galaxies in the Local Volume have been re-evaluated in order to gain a fully homogeneous collection of data for elucidating the framework of the Local Sheet and its relevance to Local Group evolution. It is demonstrated that the Local Sheet is both geometrically and dynamically distinct from the Local Supercluster and that the evolution of the Sheet and Local Group were probably interconnected. The Sheet is inclined by 8° with respect to the Local Supercluster, and the dispersion of giant members about the mid-plane is only 230 kpc. A ‘Council of Giants’ with a radius of 3.75 Mpc encompasses the Local Group, demarcating a clear upper limit to the realm of influence of the Local Group. The only two giant elliptical galaxies in the Sheet sit on opposite sides of the Council, raising the possibility that they have somehow shepherded the evolution of the Local Group. The position vector of the Andromeda galaxy with respect to the Milky Way deviates only 11° from the Sheet plane and only 11° from the projected axis of the ellipticals. The Local Group appears to be moving away from a ridge in the potential surface of the Council on a path parallel to the elliptical axis. Spin directions of the giants in the Council are distributed over the sky in a pattern which is very different from that of giants beyond, possibly in reaction to the central mass asymmetry that developed into the Local Group. By matching matter densities of Group and Council giants, the edge of the volume of space most likely to have contributed to the development of the Local Group is shown to be very close to where gravitational forces from the Local Group and the Council balance. The boundary specification reveals that the Local Sheet formed out of a density perturbation of very low amplitude (∼10 per cent), but that normal matter was incorporated into galaxies with relatively high efficiency (∼40 per cent). It appears that the development of the giants of the Local Sheet was guided by a pre-existing flattened framework of matter.

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A Council of Giants

Marshall L. McCall 0 0 Department of Physics and Astronomy, York University , Toronto , Ontario L3T 3R1, Canada A B S T R A C T Distances and near-infrared luminosities of the brightest galaxies in the Local Volume have been re-evaluated in order to gain a fully homogeneous collection of data for elucidating the framework of the Local Sheet and its relevance to Local Group evolution. It is demonstrated that the Local Sheet is both geometrically and dynamically distinct from the Local Supercluster and that the evolution of the Sheet and Local Group were probably interconnected. The Sheet is inclined by 8 with respect to the Local Supercluster, and the dispersion of giant members about the mid-plane is only 230 kpc. A 'Council of Giants' with a radius of 3.75 Mpc encompasses the Local Group, demarcating a clear upper limit to the realm of influence of the Local Group. The only two giant elliptical galaxies in the Sheet sit on opposite sides of the Council, raising the possibility that they have somehow shepherded the evolution of the Local Group. The position vector of the Andromeda galaxy with respect to the Milky Way deviates only 11 from the Sheet plane and only 11 from the projected axis of the ellipticals. The Local Group appears to be moving away from a ridge in the potential surface of the Council on a path parallel to the elliptical axis. Spin directions of the giants in the Council are distributed over the sky in a pattern which is very different from that of giants beyond, possibly in reaction to the central mass asymmetry that developed into the Local Group. By matching matter densities of Group and Council giants, the edge of the volume of space most likely to have contributed to the development of the Local Group is shown to be very close to where gravitational forces from the Local Group and the Council balance. The boundary specification reveals that the Local Sheet formed out of a density perturbation of very low amplitude (10 per cent), but that normal matter was incorporated into galaxies with relatively high efficiency (40 per cent). It appears that the development of the giants of the Local Sheet was guided by a pre-existing flattened framework of matter. - in the rendition initiating this work (the Local Volume Catalog, or LVC Karachentsev et al. 2004) contains all known galaxies either with distances less than 10 Mpc or with radial velocities less than 550 km s1 with respect to the Local Group (a Hubble flow distance of 7.7 Mpc). Within the Local Volume, the Milky Way, Andromeda, and the smaller companions which comprise the Local Group reside in a layer of galaxies, mostly dwarfs, which has an apparent thickness of about 1.5 Mpc (Schmidt & Boller 1992b; Peebles 1993; Peebles et al. 2001; Karachentsev et al. 2004; Karachentsev 2005; Tully et al. 2008; Fingerhut 2012). At various times, the layer has been referred to as the Local Cloud (de Vaucouleurs 1975), the Coma-Sculptor Cloud (Tully & Fisher 1987), the local plane (Peebles 1993), the local filament (Klypin et al. 2003), the Local pancake (Karachentsev et al. 2004), and the Local Sheet (Peebles et al. 2001; Tully et al. 2008; Peebles & Nusser 2010). At a certain level, it is the proximate manifestation of the Local Supercluster, whose existence was in fact established in part using the most luminous members of the layer (de Vaucouleurs 1953). However, models of the local velocity field seem to require that the Local Group be housed in a flattened body of galaxies distinct from the Local Supercluster (Klypin et al. 2003). Indeed, it has been argued that the supergalactic arrangement of nearby groups in the plane of the sky is evidence for such a body (de Vaucouleurs 1975). Also, peculiar velocities of galaxies show a sharp discontinuity at a distance of about 7 Mpc (Tully et al. 2008). Studies of local structure are traditionally anchored to the supergalactic coordinate system, but whether or not this is the appropriate framework to adopt has not been examined thoroughly. Any local flattened structure distinct from the Local Supercluster ought to be traced most reliably by its most luminous members, because they pinpoint the location of the largest concentrations of dark matter. Consequently, to isolate such a structure and elucidate its character, this paper focuses on carefully mapping the distribution and properties of luminous spiral and elliptical galaxies in the Local Volume. The framework is vital for guiding studies of the dwarf population locally (Fingerhut 2012), results from which will be presented separately. 2 S A M P L E A sample of luminous galaxies in the Local Volume was constructed primarily from the LVC. For a galaxy to be included in the sample, it was required that the tabulated absolute magnitude in B be equal to or brighter than 18.0. The adopted luminosity cut-off is fully 3 mag brighter than the median absolute magnitude of galaxies near the edge of the LVC (8 to 10 Mpc). This means that LVC selection c (...truncated)


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Marshall L. McCall. A Council of Giants, Monthly Notices of the Royal Astronomical Society, 2014, pp. 405-426, 440/1, DOI: 10.1093/mnras/stu199