Dwarf Spheroidal Satellite Of M31 Research Articles

Elemental Abundances in M31: The Kinematics and Chemical Evolution of Dwarf Spheroidal Satellite Galaxies*

Abstract We present deep spectroscopy from Keck/DEIMOS of Andromeda I, III, V, VII, and X, all of which are dwarf spheroidal satellites of M31. The sample includes 256 spectroscopic members across all five dSphs. We confirm previous measurements of the velocity dispersions and dynamical masses, and we provide upper limits on bulk rotation. Our measurements confirm that M31 satellites obey the same relation between stellar mass and stellar metallicity as Milky Way (MW) satellites and other dwarf galaxies in the Local Group. The metallicity distributions show trends with stellar mass that are similar to those of MW satellites, including evidence in massive satellites for external influence, like pre-enrichment or gas accretion. We present the first measurements of individual element ratios, like [Si/Fe], in the M31 system, as well as measurements of the average [α/Fe] ratio. The trends of [α/Fe] with [Fe/H] also follow the same galaxy mass–dependent patterns as MW satellites. Less massive galaxies have more steeply declining slopes of [α/Fe] that begin at lower [Fe/H]. Finally, we compare the chemical evolution of M31 satellites to M31's Giant Stellar Stream and smooth halo. The properties of the M31 system support the theoretical prediction that the inner halo is composed primarily of massive galaxies that were accreted early. As a result, the inner halo exhibits higher [Fe/H] and [α/Fe] than surviving satellite galaxies.

The major merger origin of the Andromeda II kinematics

AbstractProlate rotation (i.e. rotation around the long axis) has been reported for two Local Group dwarf galaxies: Andromeda II, a dwarf spheroidal satellite of M31, and Phoenix, a transition type dwarf galaxy. The prolate rotation may be an exceptional indicator of a past major merger between dwarf galaxies. We showed that this type of rotation cannot be obtained in the tidal stirring scenario, in which the satellite is transformed from disky to spheroidal by tidal forces of the host galaxy. However, we successfully reproduced the observed Andromeda II kinematics in controlled, self-consistent simulations of mergers between equal-mass disky dwarf galaxies on a radial or close-to-radial orbit. In simulations including gas dynamics, star formation and ram pressure stripping, we are able to reproduce more of the observed properties of Andromeda II: the unusual rotation, the bimodal star formation history and the spatial distribution of the two stellar populations, as well as the lack of gas. We support this scenario by demonstrating the merger origin of prolate rotation in the cosmological context for sufficiently resolved galaxies in the Illustris large-scale cosmological hydrodynamical simulation.

Modeling the kinematics and star formation history of Andromeda II by a gas-rich major merger of dwarf galaxies

AbstractWe present an evolutionary model for the origin of Andromeda II, a dwarf spheroidal satellite of M31. The model is an extension of the scenario proposed by Łokas et al. (2014) involving a major merger between two gas-rich disky dwarf galaxies.

Andromeda II as a merger remnant

Abstract Using N-body simulations, we study the origin of prolate rotation observed in the kinematic data for Andromeda II (And II), a dwarf spheroidal satellite of M31. We propose an evolutionary model for the origin of And II involving a merger between two discy dwarf galaxies with different disc scalelengths. The dwarfs are placed on a radial orbit towards each other with their angular momenta inclined by 90 deg. The merger remnant forms a stable triaxial galaxy with rotation only around the longest axis whose origin is naturally explained as due to the symmetry of the initial configuration and the conservation of angular momentum components along the direction of the merger. The stars originating from the two dwarfs show significantly different surface density profiles while having very similar kinematics as required by the data. We also study an alternative scenario for the formation of And II, via tidal stirring of a discy dwarf. While intrinsic rotation occurs naturally in this model as a remnant of the initial rotation of the disc, it is mostly around the shortest axis of the stellar component. We conclude that the velocity distribution in And II is much more naturally explained by a scenario involving a past merger.

Hα Survey of low-mass satellites of the neighboring galaxies M31 and M81

Images have been obtained at the 6-m telescope at the Special Astrophysical Observatory (SAO) of the Russian Academy of Sciences in the H-alpha line and in the continuum for 20 dwarf spheroidal satellites of M31: And XI-And XXX, plus the distant Globular cluster Bol 520. Their star formation rates (FR) are estimated using the H-alpha flux and the ultraviolet FUV flux measured with the GALEX space telescope. Most of the dSph satellites of M31 have extremely low star formation rates with a characteristic upper limit of SFR~5x10^{-7}. We have made similar estimates of SFR from the H-alpha and FUV fluxes for 13 galaxies with low surface brightness recently discovered in the neighborhood of M81. Eleven of them are physical satellites of M81 with typical SFR< 5x10^{-5}. The median stellar masses of these satellites of M31 and M81 are 0.9 and 1.9 million solar masses, respectively. Our H-alpha observations place a 2-3 times stricter limit on the value of SFR than the data from the GALEX satellite, with a substantially higher (3-5 times) angular resolution.

PAndAS’ PROGENY: EXTENDING THE M31 DWARF GALAXY CABAL

We present the discovery of five new dwarf galaxies, Andromeda XXIII-XXVII, located in the outer halo of M31. These galaxies were found in the second year of data from the Pan-Andromeda Archaeological Survey (PAndAS) of the M31/M33 subgroup. This survey now provides an almost complete panoramic view of the M31 halo out to an average projected radius of ~150 kpc. The metal-poor stellar density map for this whole region serves, not only as an illustration of the discovery space for satellite galaxies, but also gives a birds-eye view of the ongoing assembly process of an L* disk galaxy. Four of the new dwarfs appear as well-defined spatial over-densities of stars lying on the expected locus of metal-poor red giant branch stars at the distance of M31. The fifth over-density, And XXVII, is embedded in an extensive stream of such stars and is possibly the remnant of a strong tidal disruption event. All five satellites have metallicities and luminosities typical of dwarf spheroidal galaxies and continue the trend whereby the brighter dwarf spheroidal satellites of M31 generally have much larger half-light radii than their Milky Way counterparts. With an extended sample of M31 satellite galaxies we also revisit the spatial distribution of this population and find that, within the current limits of the PAndAS survey, the surface density of satellites is essentially constant out to 150 kpc. This corresponds to a radial density distribution of satellites varying as 1/r, a result seemingly in conflict with the predictions of cosmological simulations. (Abridged)

Rotation of halo populations in the Milky Way and M31

We search for signs of rotation in the subsystems of the Milky Way and M31 that are defined by their satellite galaxies, their globular cluster populations, and their BHB stars. A set of simple distribution functions are introduced to describe anisotropic and rotating stellar populations embedded in dark haloes of approximate Navarro-Frenk-White form. The BHB stars in the Milky Way halo exhibit a dichotomy between a prograde rotating, comparatively metal-rich component ([Fe/H] > -2) and a retrograde rotating, comparatively metal-poor ([Fe/H] < -2) component. The prograde metal-rich population may be associated with the accretion of a massive satellite (~ 10^9 solar masses). The metal-poor population may characterise the primordial stellar halo and the net retrograde rotation could then reflect an underestimate in our adopted local standard of rest circular velocity Theta_0. If Theta_0 is ~ 240 km/s then the metal-poor component has no rotation and there is a net prograde rotation signal of ~45 km/s in the metal-rich component. There is reasonable evidence that the Milky Way globular cluster and satellite galaxy systems are rotating with <v_phi> ~50 km/s and <v_phi> ~ 40 km/s respectively. Furthermore, a stronger signal is found for the satellite galaxies when the angular momentum vector of the satellites is inclined with respect to the normal of the disc. The dwarf spheroidal satellites of M31 exhibit prograde rotation relative to the M31 disc with <v_phi> ~ 40 km/s. We postulate that this group of dwarf spheroidals may share a common origin. We also find strong evidence for systemic rotation in the globular clusters of M31 particularly for the most metal-rich.

Andromeda X, a New Dwarf Spheroidal Satellite of M31: Photometry

We report the discovery of Andromeda X, a new dwarf spheroidal satellite of M31, based on stellar photometry from the Sloan Digital Sky Survey (SDSS). Using follow-up imaging data we have estimated its distance and other physical properties. We find that Andromeda X has a dereddened central surface brightness of mu_V,0 ~ 26.7 mag arcsec^-2 and a total apparent magnitude of V_tot ~ 16.1, which at the derived distance modulus, (m - M)_0 ~ 24.12 - 24.34, yields an absolute magnitude of M_V ~ -8.1 +/- 0.5; these values are quite comparable to those of Andromeda IX, a previously-discovered low luminosity M31 satellite. The discoveries of Andromeda IX and Andromeda X suggest that such extremely faint satellites may be plentiful in the Local Group.

A Keck DEIMOS Kinematic Study of Andromeda IX: Dark Matter on the Smallest Galactic Scales

We present the results of a kinematic survey of the dwarf spheroidal satellite of M31, And-IX, which appears to be the lowest surface brightness and also the faintest galaxy (M_V = -8.3) found to date. Using Keck/DEIMOS spectroscopic data, we have measured its velocity relative to M31, its velocity dispersion, and its metallicity. It exhibits a significant velocity dispersion sigma_v = 6.8+3.0-2.0km/s, which coupled with the low luminosity implies a very high mass to V-band light ratio, M/L ~ 93+120-50msun/lsun (M/L > 17msun/lsun at 99% confidence). Unless strong tidal forces have perturbed this system, this smallest of galaxies is a highly dark matter dominated system.

Andromeda IX: A New Dwarf Spheroidal Satellite of M31

We report the discovery of a new dwarf spheroidal satellite of M31, Andromeda IX, based on resolved stellar photometry from the Sloan Digital Sky Survey (SDSS). Using both SDSS and public archival data we have estimated its distance and other physical properties, and compared these to the properties of a previously known dwarf spheroidal companion, Andromeda V, also observed by SDSS. Andromeda IX is the lowest surface brightness galaxy found to date (mu_{V} ~ 26.8 mag arcsec^-2), and at the distance we estimate from the position of the tip of Andromeda IX's red giant branch, (m - M)_{0} ~ 24.5 (805 kpc), Andromeda IX would also be the faintest galaxy known (M_{V} ~ -8.3).