General models of Einstein gravity with a non-Newtonian weak-field limit

We investigate Einstein theories of gravity, coupled to a scalar field \vphi and point-like matter, which are characterized by a scalar field-dependent matter coupling function e^{H(\vphi)}. We show that under mild constraints on the form of the potential for the scalar field, there are a broad class of Einstein-like gravity models -characterized by the asymptotic behavior of H- which allow for a non-Newtonian weak-field limit with the gravitational potential behaving for large distances as ln r. The Newtonian term GM/r appears only as sub-leading. We point out that this behavior is also shared by gravity models described by f(R) Lagrangians. The relevance of our results for the building of infrared modified theories of gravity and for modified Newtonian dynamics is also discussed.Comment: 9 page

The Local Velocity Anomaly

There is a velocity discontinuity at about 7 Mpc between the galaxies of the Local Sheet that are moving together with low internal velocity dispersion and the adjacent structures. The Local Sheet bounds the Local Void. The Local Sheet is determined to have a peculiar velocity of 260 km/s away from the center of the void. In order for this large velocity to be generated by an absence of gravity, the Local Void must be at least 45 Mpc in diameter and be very empty.Comment: Invited review, "Galaxies in the Local Volume", Sydney, 8-13 July, 2007. eds. B. Koribalski & H. Jerjen, Astrophys. & Space Sci. Proceed. 10 pages with 7 figure

"Dark energy" in the Local Void

The unexpected discovery of the accelerated cosmic expansion in 1998 has filled the Universe with the embarrassing presence of an unidentified "dark energy", or cosmological constant, devoid of any physical meaning. While this standard cosmology seems to work well at the global level, improved knowledge of the kinematics and other properties of our extragalactic neighborhood indicates the need for a better theory. We investigate whether the recently suggested repulsive-gravity scenario can account for some of the features that are unexplained by the standard model. Through simple dynamical considerations, we find that the Local Void could host an amount of antimatter ($\sim5\times10^{15}\,M_\odot$) roughly equivalent to the mass of a typical supercluster, thus restoring the matter-antimatter symmetry. The antigravity field produced by this "dark repulsor" can explain the anomalous motion of the Local Sheet away from the Local Void, as well as several other properties of nearby galaxies that seem to require void evacuation and structure formation much faster than expected from the standard model. At the global cosmological level, gravitational repulsion from antimatter hidden in voids can provide more than enough potential energy to drive both the cosmic expansion and its acceleration, with no need for an initial "explosion" and dark energy. Moreover, the discrete distribution of these dark repulsors, in contrast to the uniformly permeating dark energy, can also explain dark flows and other recently observed excessive inhomogeneities and anisotropies of the Universe.Comment: 6 pages, accepted as a Letter to the Editor by Astrophysics and Space Scienc

The braided jets in the spiral galaxy NGC 4258

We have used the Hawaii Imaging Fabry-Perot Interferometer to synthesize 44,000 Hα and [N II] emission-line profiles at velocity resolution 68 km s-1 FWHM across the disk of the nearby Seyfert/LINER galaxy NGC 4258, including the prominent "four-branched jet" that extends to 10 and 5 kpc radii in the radio continuum and optical emission lines, respectively. A long-slit spectrum, which includes the emission lines Hα, [N II] λλ6548, 6583 and [S II] λλ6717, 6731, has also been obtained along the jets. These data are used for a comprehensive kinematic study of the galaxy. The disk velocity field is best fitted with a model that incorporates elliptical, bar-forced streaming motions, as suggested by the H I and CO kinematics. Velocity dispersions along the SE jet average 80 km s-1, compared to 40 km s-1 in the H II regions. This broadening arises from a helical, braided structure of three intertwined plasma streams, as inferred from both the spatial and kinematic structure of the jet. The emission-line profiles split into two distinct velocity systems with separations 300 ± 30 km s-1 within 1.7 kpc of the nucleus along the midaxis of the braid pattern. Gaussian decomposition of the line profiles show that the internal velocity dispersion of each helical strand is ≈100 km s-1. We observe two cycles of the triple-braid pattern, with wavelength ≈1.4 kpc and side-to-side amplitude ≈400 pc. If the braided streams represent ballistic motion of gas ejected from orbiting objects, the outflow velocity is ≈2000 km s-1. The masses and separations of the compact objects would be ≈3 × 106 M⊙ and ≈6 pc (0″.2), respectively. However, the emission-line velocity field of the SE jet suggests that the gas moves along the helices, perhaps as a result of fluid instabilities at the interface between the jet and the interstellar medium or of motion along magnetic flux tubes. We show that the NNW jet follows a clear channel between molecular cloud complexes, and exhibits several emission-line "hot spots" and bends where it interacts with the surrounding molecular gas. High-velocity gas in the SE and NW radio "plateaus" probably results from earlier ejections of gas from the nucleus in these directions, with the jet nozzle having precessed to its present orientation. We find that the jets have total ionized mass 1.5 × 106 M⊙ (1 cm-3/ne) and [N II] λ6583/Hα ratios consistent with shock excita-tion. An Einstein HRI image reveals X-ray emission from the nucleus and SE jet. We show that the jet's X-ray emission may be interpreted as thermal bremsstrahlung from hot, shocked jet gas

Imaging spectrophotometry of ionized gas in NGC 1068. I. Kinematics of the narrow-line region

We have used an imaging Fabry-Perot interferometer and CCD at the University of Hawaii 2.2 m telescope to synthesize [N II] λλ6548, 6583 emission-line profiles with (<1″, ∼140 km s-1) FWHM (spatial, velocity) resolution across the inner 1′ diameter of the nearby Seyfert galaxy NGC 1068. The stack of monochromatic images spatially resolved the high-velocity gas (∼6″ radius and ∼3000 km s-1 line widths) and has been extensively analyzed for kinematic and photometric content. Our profiles agree well with previous long-slit work, but their complete spatial coverage has now allowed us to constrain the gas volume distributions. We find that the narrow-line region (NLR) is distributed in a thick (FWHM ∼3″.3 = 230 pc) center-darkened, line-emitting cylinder that envelopes the collimated radio jet. The cylinder is composed of three distinct kinematic subsystems, which we discuss. 1. High-velocity gas emits 75% of the total dereddened narrow-line region (NLR) [N II] flux (corresponding to a mass of ∼38,000 [104.5 cm-3/ne] M⊙, with ne the mean electron density). A kinematic model consistent with the data is used to argue that this gas fills the cylinder with two families of nested cones of increasing opening angle in the NE and SW quadrants. The maximum opening angle may be as large as 150°. The axial inclination to the line of sight in the NE is 75°, so the cylinder axis is inclined ∼45° to the disk plane. The brightest high-velocity flux in the NE is concentrated in the conical shell with opening angle ∼82°. The shell is thick, with Gaussian dispersion ∼3″.3 = 230 pc FWHM perpendicular to the cone axis. The emitting filaments are optically thick, with Av ∼ 1.3 mag of internal extinction. Line-of-sight velocities are approximately proportional to r0.6 (with r the cylindrical radius from the jet axis), and so are roughly those of a constant-speed conical outflow. Intrinsic gas velocities deproject to ∼1500 km s-1 with respect to systemic velocity, and we infer an average mass loss of ∼0.15 [104.5 cm-3/ne] M⊙ yr-1. The large extranuclear line widths in this NLR apparently arise from geometrical projection and spatial averaging of an asymmetric, large-scale flow, with no evidence for intrinsic "turbulence" in the outflowing filaments. The filaments are too massive (≥10-2 M⊙) to be broad-line region (BLR) clouds blown to resolvable radii. We argue instead that molecular clouds with average density ∼102.5 cm-3 are forced by the stellar bar (prominent in near-IR images) from 10″ radius into the inner NLR, where they are exposed to a high-speed (0.1c) nuclear wind whose kinetic luminosity is similar to the ionizing luminosity of the nucleus. Fragments stripped by fluid instabilities are crushed to densities of ∼103-105 cm-3 during their wind acceleration, but they are stable to conductive thermal evaporation in the hot shocked wind for masses and velocities derived from our spectra. 2. Gas near systemic velocity is concentrated in a narrow kinematic feature brightened along the boundaries of the NE radio lobe. It accounts for 8% of the total NLR [N II] flux (corresponding to ∼1.2 ×10 5 [103 cm-3/ne] M⊙), but negligible kinetic energy. This component is also apparent in Ha images and so represents a mass enhancement rather than an excitation effect. Its deprojected velocity deviates by less than 175 km s-1 from the bar-forced flow field defined by gas in the large-scale disk. The only significant velocity deviation (80 km s-1 along the line of sight) is localized in an expanding "ripple" around the base of the NE lobe at ∼3″ radius, with no associated flux enhancement. 3. Nine percent of the total NLR [N II] flux (corresponding to ∼1000 [105 cm-3/ne] M⊙) is emitted by a component which dominates within 1″ radius and extends 2″ along ∼100° P.A. Its centroid is blueshifted by ∼270 km s-1 from systemic, and it has line width ∼600 km s-1 (i.e., typical of a Seyfert 2 galaxy). The brightest flux from this component apparently arises in spatially unresolved kinematic substructure that is associated with the nuclear triple radio-source (∼0″.7 extent). Our highest resolution images show a tight correlation between the brightest line emission and the radio source, in agreement with speckle line imaging

Catalog of Nearby Isolated Galaxies in the Volume z<0.01

We present a catalog of 520 most isolated nearby galaxies with radial velocities V_LG<3500 km/s covering the entire sky. This population of "space orphans" makes up 4.8% among 10900 galaxies with measured radial velocities. We describe the isolation criterion used to select our sample, called the "Local Orphan Galaxies" (LOG), and discuss their basic optical and HI properties. A half of the LOG catalog is occupied by the Sdm, Im and Ir morphological type galaxies without a bulge. The median ratio M_gas/M_star in the LOG galaxies exceeds 1. The distribution of the catalog galaxies on the sky looks uniform with some signatures of a weak clustering on the scale of about 0.5 Mpc. The LOG galaxies are located in the regions where the mean local density of matter is approximately 50 times lower than the mean global density. We indicate a number of LOG galaxies with distorted structures, which may be the consequence of interaction of isolated galaxies with massive dark objects

Broadening of Spectral Lines due to Dynamic Multiple Scattering and the Tully-Fisher Relation

The frequency shift of spectral lines is most often explained by the Doppler Effect in terms of relative motion, whereas the Doppler broadening of a particular line mainly depends on the absolute temperature. The Wolf effect on the other hand deals with the correlation induced spectral change and explains both the broadening and shift of the spectral lines. In this framework a relation between the width of the spectral line is related to the redshift z for the line and hence with the distance. For smaller values of z a relation similar to the Tully-Fisher relation can be obtained and for larger values of z a more general relation can be constructed. The derivation of this kind of relation based on dynamic multiple scattering theory may play a significant role in explaining the overall spectra of quasi stellar objects. We emphasize that this mechanism is not applicable for nearby galaxies, $z \leq 1$.Comment: 18 pages, 5 figures, revised Version has been submitted to Physical Review A. (2nd author's affiliation corrected

A search for ultra-compact dwarf galaxies in the NGC 1023 group of galaxies

We present a photometric search for UCD candidates in the nearby galaxy group NGC 1023 (d=11 Mpc) -- the poorest environment searched for UCDs yet --, based on wide field imaging with CFHT. After photometric and morphological selection, we obtain a sample of 21 UCD candidates with -12<M_V<-11 mag, if located at NGC 1023's distance. From spectroscopy taken at Calar Alto observatory, we identify the UCD candidate in closest projection to NGC 1023 as an emission line background galaxy. Our photometric data show that in the NGC 1023 group, the mass spectrum of analogs to Fornax/Virgo UCD is restricted to about 1/4 of the maximum Fornax/Virgo UCD mass. More spectroscopy is needed to further constrain the mass range of UCDs in this galaxy group.Comment: 5 pages, 4 figures, to appear in the proceedings of ESO Astrophysics Symposia: "Groups of Galaxies in the Nearby Universe", eds. I. Saviane, V. Ivanov, J. Borissov

Testing quantised inertia on galactic scales

Galaxies and galaxy clusters have rotational velocities apparently too fast to allow them to be gravitationally bound by their visible matter. This has been attributed to the presence of invisible (dark) matter, but so far this has not been directly detected. Here, it is shown that a new model that modifies inertial mass by assuming it is caused by Unruh radiation, which is subject to a Hubble-scale (Theta) Casimir effect predicts the rotational velocity (v) to be: v^4=2GMc^2/Theta (the Tully-Fisher relation) where G is the gravitational constant, M is the baryonic mass and c is the speed of light. The model predicts the outer rotational velocity of dwarf and disk galaxies, and galaxy clusters, within error bars, without dark matter or adjustable parameters, and makes a prediction that local accelerations should remain above 2c^2/Theta at a galaxy's edge.Comment: 7 pages, 1 figure. Accepted for publication in Astrophysics and Space Science on 27/7/201

Constraints on Galileon-induced precessions from solar system orbital motions

We use latest data from solar system planetary orbital motions to put constraints on some Galileon-induced precessional effects. Due to the Vainshtein mechanism, the Galileon-type spherically symmetric field of a monopole induces a small, screened correction proprtional to \sqrt{r} to its usual r^-1 Newtonian potential which causes a secular precession of the pericenter of a test particle. In the case of our solar system, latest data from Mars allow to constrain the magnitude of such an interaction down to \alpha <= 0.3 level. Another Galileon-type effect which might impact solar system dynamics is due to an unscreened constant gradient induced by the peculiar motion of the Galaxy. The magnitude of such an effect, depending on the different gravitational binding energies of the Sun and the planets, is \xi <= 0.004 from the latest bounds on the supplementary perihelion precession of Saturn.Comment: LaTex2e, 11 pages, 1 table, no figures, 35 references. To appear in Journal of Cosmology and Astroparticle Physics (JCAP