Leo V: A Companion of a Companion of the Milky Way Galaxy

We report the discovery of a new Milky Way dwarf spheroidal galaxy in the constellation of Leo identified in data from the Sloan Digital Sky Survey. Leo V lies at a distance of about 180 kpc, and is separated by about 3 degrees from another recent discovery, Leo IV. We present follow-up imaging from the Isaac Newton Telescope and spectroscopy from the Hectochelle fiber spectrograph at the Multiple Mirror Telescope. Leo V's heliocentric velocity is 173.4 km/s, which is offset by about 40 km/s from that of Leo IV. A simple interpretation of the kinematic data is that both objects may lie on the same stream, though the implied orbit is only modestly eccentric (e = 0.2)Comment: Submitted to ApJ (Letters

The contribution of microlensing surveys to the distance scale

In the early nineties several teams started large scale systematic surveys of the Magellanic Clouds and the Galactic Bulge to search for microlensing effects. As a by product, these groups have created enormous time-series databases of photometric measurements of stars with a temporal sampling duration and accuracy which are unprecedented. They provide the opportunity to test the accuracy of primary distance indicators, such as Cepheids, RRLyrae stars, the detached eclipsing binaries, or the luminosity of the red clump. We will review the contribution of the microlensing surveys to the understanding of the physics of the primary distance indicators, recent differential studies and direct distance determinations to the Magellanic Clouds and the Galactic Bulge.Comment: Invited review article to appear in: `Post-Hipparcos Cosmic Candles', A. Heck & F. Caputo (Eds), Kluwer Academic Publ., Dordrecht, in press. 21 pages; uses Kluwer's crckapb.sty LaTeX style file, enclose

Gravitational detection of a low-mass dark satellite at cosmological distance

The mass-function of dwarf satellite galaxies that are observed around Local Group galaxies substantially differs from simulations based on cold dark matter: the simulations predict many more dwarf galaxies than are seen. The Local Group, however, may be anomalous in this regard. A massive dark satellite in an early-type lens galaxy at z = 0.222 was recently found using a new method based on gravitational lensing, suggesting that the mass fraction contained in substructure could be higher than is predicted from simulations. The lack of very low mass detections, however, prohibited any constraint on their mass function. Here we report the presence of a 1.9 +/- 0.1 x 10^8 M_sun dark satellite in the Einstein-ring system JVAS B1938+666 at z = 0.881, where M_sun denotes solar mass. This satellite galaxy has a mass similar to the Sagittarius galaxy, which is a satellite of the Milky Way. We determine the logarithmic slope of the mass function for substructure beyond the local Universe to be alpha = 1.1^+0.6_-0.4, with an average mass-fraction of f = 3.3^+3.6_-1.8 %, by combining data on both of these recently discovered galaxies. Our results are consistent with the predictions from cold dark matter simulations at the 95 per cent confidence level, and therefore agree with the view that galaxies formed hierarchically in a Universe composed of cold dark matter.Comment: 25 pages, 7 figures, accepted for publication in Nature (19 January 2012

The population of variable stars in M54 (NGC6715)

We present new B, V and I CCD time-series photometry for 177 variable stars in a 13'X 13' field centered on the globular cluster M54 (lying at the center of the Sagittarius dwarf spheroidal galaxy), 94 of which are newly identified variables. The total sample is composed of 2 anomalous Cepheids, 144 RR Lyrae stars (108 RR0 and 36 RR1), 3 SX Phoenicis, 7 eclipsing binaries (5 W UMA and 2 Algol binaries), 3 variables of uncertain classification and 18 long-period variables. The large majority of the RR Lyrae variables likely belong to M54. Ephemerides are provided for all the observed short-period variables. The pulsational properties of the M54 RR Lyrae variables are close to those of Oosterhoff I clusters, but a significant number of long-period ab type RR Lyrae are present. We use the observed properties of the RR Lyrae to estimate the reddening and the distance modulus of M54, E(B-V)=0.16 +/- 0.02 and (m-M)_0=17.13 +/- 0.11, respectively, in excellent agreement with the most recent estimates. The metallicity has been estimated for a subset of 47 RR Lyrae stars, with especially good quality light curves, from the Fourier parameters of the V light curve. The derived metallicity distribution has a symmetric bell shape, with a mean of =-1.65 and a standard deviation sigma=0.16 dex. Seven stars have been identified as likely belonging to the Sagittarius galaxy, based on their too high or too low metallicity. This evidence, if confirmed, might suggest that old stars in this galaxy span a wide range of metallicities.Comment: 15 pages, 11 figures, accepted for publication by MNRA

The kinematic footprints of five stellar streams in Andromeda's halo

(abridged) We present a spectroscopic analysis of five stellar streams (`A', `B', `Cr', `Cp' and `D') as well as the extended star cluster, EC4, which lies within streamC, all discovered in the halo of M31 from our CFHT/MegaCam survey. These spectroscopic results were initially serendipitous, making use of our existing observations from the DEep Imaging Multi-Object Spectrograph mounted on the Keck II telescope, and thereby emphasizing the ubiquity of tidal streams that account for ~70% of the M31 halo stars in the targeted fields. Subsequent spectroscopy was then procured in streamCr/p and streamD to trace the velocity gradient along the streams. For the cluster EC4, candidate member stars with average [Fe/H]~-1.4 (Fe/H_spec=-1.6), are found at v_{hel}=-285 km/s suggesting it could be related to streamCp. No similarly obvious cold kinematic candidate is found for streamD, although candidates are proposed in both of two spectroscopic pointings along the stream (both at -400 km/s). Spectroscopy near the edge of streamB suggests a likely kinematic detection, while a candidate kinematic detection of streamA is found (plausibly associated to M33 rather than M31). The low dispersion of the streams in kinematics, physical thickness, and metallicity makes it hard to reconcile with a scenario whereby these stream structures as an ensemble are related to the giant southern stream. We conclude that the M31 stellar halo is largely made up of multiple kinematically cold streams.Comment: 19 pages, 12 figures, accepted in MNRAS. High resolution version, with fig10 here: http://www.ast.cam.ac.uk/~schapman/streams.pd

A Keck/DEIMOS spectroscopic survey of the faint M31 satellites And IX, And XI, And XII, and And XIII

We present the first spectroscopic analysis of the faint M31 satellite galaxies, AndXI and AndXIII, and a reanalysis of existing spectroscopic data for two further faint companions, And IX and AndXII. By combining data obtained using the DEIMOS spectrograph mounted on the Keck II telescope with deep photometry from the Suprime-Cam instrument on Subaru, we have calculated global properties for the dwarfs, such as systemic velocities, metallicites and half-light radii.We find each dwarf to be very metal poor ([Fe/H] -2 both photometrically and spectroscopically, from their stacked spectrum), and as such, they continue to follow the luminosity-metallicity relationship established with brighter dwarfs. We are unable to resolve a dispersion for And XI due to small sample size and low S/N, but we set a one sigma upper limit of sigma-v <5 km/s. For And IX, And XII and And XIII we resolve velocity dispersions of v=4.5 (+3.4,-3.2), 2.6(+5.1,-2.6) and 9.7(+8.9,-4.5) km/s, and derive masses within the half light radii of 6.2(+5.3,-5.1)x10^6 Msun, 2.4 (+6.5,-2.4)x10^6 Msun and 1.1(+1.4,-0.7)x10^7 Msun respectively. We discuss each satellite in the context of the Mateo relations for dwarf spheroidal galaxies, and the Universal halo profiles established for Milky Way dwarfs (Walker et al. 2009). For both galaxies, this sees them fall below the Universal halo profiles of Walker et al. (2009). When combined with the findings of McConnachie & Irwin (2006a), which reveal that the M31 satellites are twice as extended (in terms of both half-light and tidal radii) as their Milky Way counterparts, these results suggest that the satellite population of the Andromeda system could inhabit halos that are significantly different from those of the Milky Way in terms of their central densities (abridged).Comment: 26 pages, 18 figures, MNRAS submitte

Tidal Dwarf Galaxies at Intermediate Redshifts

We present the first attempt at measuring the production rate of tidal dwarf galaxies (TDGs) and estimating their contribution to the overall dwarf population. Using HST/ACS deep imaging data from GOODS and GEMS surveys in conjunction with photometric redshifts from COMBO-17 survey, we performed a morphological analysis for a sample of merging/interacting galaxies in the Extended Chandra Deep Field South and identified tidal dwarf candidates in the rest-frame optical bands. We estimated a production rate about 1.4 {\times} 10^{-5} per Gyr per comoving volume for long-lived TDGs with stellar mass 3 {\times} 10^{8-9} solar mass at 0.5<z<1.1. Together with galaxy merger rates and TDG survival rate from the literature, our results suggest that only a marginal fraction (less than 10%) of dwarf galaxies in the local universe could be tidally-originated. TDGs in our sample are on average bluer than their host galaxies in the optical. Stellar population modelling of optical to near-infrared spectral energy distributions (SEDs) for two TDGs favors a burst component with age 400/200 Myr and stellar mass 40%/26% of the total, indicating that a young stellar population newly formed in TDGs. This is consistent with the episodic star formation histories found for nearby TDGs.Comment: 9 pages, 5 figures, Accepted for publication in Astrophysics & Space Scienc

Local-Group tests of dark-matter Concordance Cosmology: Towards a new paradigm for structure formation

(abridged) Predictions of the Concordance Cosmological Model (CCM) of the structures in the environment of large spiral galaxies are compared with observed properties of Local Group galaxies. Five new most probably irreconcilable problems are uncovered. However, the Local Group properties provide hints that may lead to a solution of the above problems The DoS and bulge--satellite correlation suggest that dissipational events forming bulges are related to the processes forming phase-space correlated satellite populations. Such events are well known to occur since in galaxy encounters energy and angular momentum are expelled in the form of tidal tails, which can fragment to form populations of tidal-dwarf galaxies (TDGs) and associated star clusters. If Local Group satellite galaxies are to be interpreted as TDGs then the sub-structure predictions of CCM are internally in conflict. All findings thus suggest that the CCM does not account for the Local Group observations and that therefore existing as well as new viable alternatives have to be further explored. These are discussed and natural solutions for the above problems emerge.Comment: A and A, in press, 25 pages, 9 figures; new version contains minor text adjustments for conformity with the published version and additional minor changes resulting from reader's feedback. The speculation on a dark force has been added. Also, the Fritz Zwicky Paradox is now included to agree with the published versio

Search for 511 keV Emission in Satellite Galaxies of the Milky Way with INTEGRAL/SPI

Reproduced with permission from Astronomy & Astrophysics. © 2018 ESO.The positron annihilation gamma-ray signal in the Milky Way (MW) shows a puzzling morphology: a very bright bulge and a very low surface-brightness disk. A coherent explanation of the positron origin, propagation through the Galaxy and subsequent annihilation in the interstellar medium has not yet been found. Tentative explanations involve positrons from radioactivity, X-ray binaries, and dark matter (DM). Dwarf satellite galaxies (DSGs) are believed to be DM-dominated and hence promising candidates in the search for 511 keV emission as a result of DM annihilation into electron-positron pairs. The goal of this study is to constrain possible 511 keV gamma-ray signals from 39 DSGs of the MW and to test the annihilating DM scenario. We use the spectrometer SPI on INTEGRAL to extract individual spectra for the studied objects. As the diffuse galactic emission dominates the signal, the large scale morphology of the MW has been modelled accordingly and was included in a maximum likelihood analysis. Alternatively, a distance-weighted stacked spectrum has been determined. Only Reticulum II (Ret II) shows a 3.1 sigma signal. Five other sources show tentative 2 sigma signals. The mass-to-511-keV-luminosity-ratio shows a marginal trend towards higher values for intrinsically brighter objects, opposite to the V band mass-to-light-ratio, which is generally used to uncover DM in DSGs. All derived flux values are above the level implied by a DM interpretation of the MW bulge signal. The signal from Ret II is unlikely to be related to a DM origin alone, otherwise, the MW bulge would be about 100 times brighter than what is seen. Ret II is exceptional considering the DSG sample, and rather points to enhanced recent star formation activity, if its origins are similar to processes in the MW. Understanding this emission may provide further clues regarding the origin of the annihilation emission in the MW.Peer reviewe

Dark Matter and Fundamental Physics with the Cherenkov Telescope Array

The Cherenkov Telescope Array (CTA) is a project for a next-generation observatory for very high energy (GeV-TeV) ground-based gamma-ray astronomy, currently in its design phase, and foreseen to be operative a few years from now. Several tens of telescopes of 2-3 different sizes, distributed over a large area, will allow for a sensitivity about a factor 10 better than current instruments such as H.E.S.S, MAGIC and VERITAS, an energy coverage from a few tens of GeV to several tens of TeV, and a field of view of up to 10 deg. In the following study, we investigate the prospects for CTA to study several science questions that influence our current knowledge of fundamental physics. Based on conservative assumptions for the performance of the different CTA telescope configurations, we employ a Monte Carlo based approach to evaluate the prospects for detection. First, we discuss CTA prospects for cold dark matter searches, following different observational strategies: in dwarf satellite galaxies of the Milky Way, in the region close to the Galactic Centre, and in clusters of galaxies. The possible search for spatial signatures, facilitated by the larger field of view of CTA, is also discussed. Next we consider searches for axion-like particles which, besides being possible candidates for dark matter may also explain the unexpectedly low absorption by extragalactic background light of gamma rays from very distant blazars. Simulated light-curves of flaring sources are also used to determine the sensitivity to violations of Lorentz Invariance by detection of the possible delay between the arrival times of photons at different energies. Finally, we mention searches for other exotic physics with CTA.Comment: (31 pages, Accepted for publication in Astroparticle Physics