Andromeda's Halo Research Articles

Chemo-dynamical substructure in the M31 inner halo globular clusters: further evidence for a recent accretion event

ABSTRACT Based upon a metallicity selection, we identify a significant subpopulation of the inner halo globular clusters in the Andromeda Galaxy, which we name the Dulais Structure. It is distinguished as a co-rotating group of 10–20 globular clusters that appear to be kinematically distinct from, and on average more metal-poor than, the majority of the inner halo population. Intriguingly, the orbital axis of this Dulais Structure is closely aligned with that of the younger accretion event recently identified using a subpopulation of globular clusters in the outer halo of Andromeda, and this is strongly suggestive of a causal relationship between the two. If this connection is confirmed, a natural explanation for the kinematics of the globular clusters in the Dulais Structure is that they trace the accretion of a substantial progenitor (∼1011 M⊙) into the halo of Andromeda during the last few billion years, which may have occurred as part of a larger group infall.

The PAndAS View of the Andromeda Satellite System. III. Dwarf Galaxy Detection Limits

We determine the detection limits of the search for dwarf galaxies in the Pan-Andromeda Archaeological Survey (PAndAS) using the algorithm developed by the PAndAS team. The recovery fractions of artificial dwarf galaxies are, as expected, a strong function of physical size and luminosity and, to a lesser extent, distance. We show that these recovery fractions vary strongly with location in the surveyed area because of varying levels of contamination from both the Milky Way foreground stars and the stellar halo of Andromeda. We therefore provide recovery fractions that are a function of size, luminosity, and location within the survey on a scale of ∼1 × 1 deg2 (or ∼14 × 14 kpc2). Overall, the effective surface brightness for a 50% detection rate ranges between 28 and 30 mag arcsec−2. This is in line with expectations for a search that relies on photometric data that are as deep as the PAndAS survey. The derived detection limits are an essential ingredient on the path to constraining the global properties of Andromeda’s system of satellite dwarf galaxies and, more broadly, to providing constraints on dwarf galaxy formation and evolution in a cosmological context.

Observing the Stellar Halo of Andromeda in Cosmological Simulations: The AURIGA2PANDAS Pipeline

Abstract We present a direct comparison of the Pan-Andromeda Archaeological Survey (PAndAS) observations of the stellar halo of M31 with the stellar halos of six galaxies from the Auriga simulations. We process the simulated halos through the Auriga2PAndAS pipeline and create PAndAS-like mocks that fold in all observational limitations of the survey data (foreground contamination from the Milky Way stars, incompleteness of the stellar catalogs, photometric uncertainties, etc.). This allows us to study the survey data and the mocks in the same way and generate directly comparable density maps and radial density profiles. We show that the simulations are overall compatible with the observations. Nevertheless, some systematic differences exist, such as a preponderance for metal-rich stars in the mocks. While these differences could suggest that M31 had a different accretion history or has a different mass compared with the simulated systems, it is more likely a consequence of an underquenching of the star formation history of galaxies, related to the resolution of the Auriga simulations. The direct comparison enabled by our approach offers avenues to improve our understanding of galaxy formation as they can help pinpoint the observable differences between observations and simulations. Ideally, this approach will be further developed through an application to other stellar halo simulations. To facilitate this step, we release the pipeline to generate the mocks, along with the six mocks presented and used in this contribution.

Subaru Hyper Suprime Cam Survey of the Andromeda Halo

AbstractWe present a progress report on our deep and wide-field imaging survey of the Andromeda halo with Hyper Suprime Cam (HSC) mounted on Subaru. HSC is the upgraded prime focus camera after Suprime-Cam, having a field of view of 1.77 square degree (1.5 degree in diameter), namely about 10 times larger than that of Suprime-Cam. This camera will thus offer us great opportunities to explore unique and legacy surveys for the Andromeda halo, as well as for other Galactic Archaeology science cases.

THE PROLATE DARK MATTER HALO OF THE ANDROMEDA GALAXY

We present new limits on the global shape of the dark matter halo in the Andromeda galaxy using and generalizing non-spherical mass models developed by Hayashi & Chiba and compare our results with theoretical predictions of cold dark matter (CDM) models. This is motivated by the fact that CDM models predict non-spherical virialized dark halos, which reflect the process of mass assembly in the galactic scale. Applying our models to the latest kinematic data of globular clusters and dwarf spheroidal galaxies in the Andromeda halo, we find that the most plausible cases for Andromeda yield a prolate shape for its dark halo, irrespective of assumed density profiles. We also find that this prolate dark halo in Andromeda is consistent with theoretical predictions in which the satellites are distributed anisotropically and preferentially located along major axes of their host halos. It is a reflection of the intimate connection between galactic dark matter halos and the cosmic web. Therefore, our result is profound in understanding internal dynamics of halo tracers in Andromeda, such as orbital evolutions of tidal stellar streams, which play important roles in extracting the abundance of CDM subhalos through their dynamical effects on stream structures.

The global dark halo structure of the Andromeda galaxy

AbstractWe set new limits on the global shape of the dark halo in the Andromeda galaxy based on axisymmetric mass models constructed by Hayashi & Chiba (2012). This is motivated by the fact that CDM models predict non-spherical virialized dark halos, which reflect the process of mass assembly in the galactic scale. Based on the application of our models to latest kinematical data of globular clusters and dwarf spheroidal galaxies in the Andromeda halo, we find that the most plausible cases for Andromeda yield not a spherical but a prolate shape for its dark halo. We also find that the prolate dark halo is consistent with theoretical predictions in which the satellites are distributed anisotropically and preferentially located along major axes of their galactic host halos. It is a reflection of the intimate connection between galactic dark matter halos and the cosmic web.

A spectroscopic survey of EC4, an extended cluster in Andromeda's halo

We present a spectroscopic survey of candidate red giant branch stars in the extended star cluster, EC4, discovered in the halo of M31 from our CFHT/MegaCam survey, overlapping the tidal streams, Stream Cp and Stream Cr. These observations used the DEep Imaging Multi-Object Spectrograph (DEIMOS) mounted on the Keck II telescope to obtain spectra around the CaII triplet region with ~1.3 Angstroms resolution. Six stars lying on the red giant branch within 2 core-radii of the centre of EC4 are found to have an average vr=-287.9^{+1.9}_{-2.4}km/s and velocity dispersion of 2.7^{+4.2}_{-2.7}km/s, taking instrumental errors into account. The resulting mass-to-light ratio for EC4 is M/L=6.7^{+15}_{-6.7}Msun/Lsun, a value that is consistent with a globular cluster within the 1 sigma errors we derive. From the summed spectra of our member stars, we find EC4 to be metal-poor, with [Fe/H]=-1.6+/-0.15. We discuss several formation and evolution scenarios which could account for our kinematic and metallicity constraints on EC4, and conclude that EC4 is most comparable with an extended globular cluster. We also compare the kinematics and metallicity of EC4 with Stream Cp and Stream Cr, and find that EC4 bears a striking resemblance to Stream Cp in terms of velocity, and that the two structures are identical in terms of both their spectroscopic and photometric metallicities. From this we conclude that EC4 is likely related to Stream Cp.

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.

The Extended Star Formation History of the Andromeda Spheroid at 35 kpc on the Minor Axis

Using the HST ACS, we have obtained deep optical images reaching well below the oldest main-sequence turnoff in fields on the southeast minor axis of the Andromeda galaxy, 35 kpc from the nucleus. These data probe the star formation history in the extended halo of Andromeda—that region beyond 30 kpc that appears both chemically and morphologically distinct from the metal-rich, highly disturbed inner spheroid. The present data, together with our previous data for fields at 11 and 21 kpc, do not show a simple trend toward older ages and lower metallicities, as one might expect for populations further removed from the obvious disturbances of the inner spheroid. Specifically, at 11, 21, and 35 kpc, the mean ages are 9.7, 11.0, and 10.5 Gyr, respectively, and the mean [Fe/H] values are –0.65, –0.87, and –0.98, respectively. In the best-fit model of the 35 kpc population, one-third of the stars are younger than 10 Gyr, whereas only ~10% of the stars are truly ancient and metal-poor. The extended halo thus exhibits clear evidence of its hierarchical assembly, and the contribution from any classical halo formed via early monolithic collapse must be small.

H I clouds in the proximity of M 33

Aims. Neutral hydrogen clouds are found in the Milky Way and Andromeda halo both as large complexes and smaller isolated clouds. Here we present a search for Hi clouds in the halo of M 33, the third spiral galaxy of the Local Group. Methods. We have used two complementary data sets: a 3° x 3° map of the area provided by the Arecibo Legacy Fast ALFA (ALFALFA) survey and deeper pointed observations carried out with the Arecibo telescope in two fields that permit sampling of the northeastern and southwestern edges of the HI disc. Results. The total amount of HI around M 33 detected by our survey is about 10 7 M ⊙ . At least 50% of this mass is made of Hi clouds that are related both in space and velocity to the galaxy. We discuss several scenarios for the origin of these clouds focusing on the two most interesting ones: (a) dark-matter dominated gaseous satellites, (b) debris from filaments flowing into M 33 from the intergalactic medium or generated by a previous interaction with M 31. Both scenarios seem to fit with the observed cloud properties. Some structures are found at anomalous velocities, particularly an extended Hi complex previously detected by Thilker et al. (2002). Even though the ALFALFA observations seem to indicate that this cloud is possibly connected to M 33 by a faint gas bridge, we cannot firmly establish its extragalactic nature or its relation to M 33. Conclusions. Taking into account that the clouds associated with M 33 are likely to be highly ionised by the extragalactic ultraviolet (UV) radiation, we predict that the total gas mass associated with them is >5 × 10 7 M ⊙ . If the gas is steadily falling towards the M 33 disc it can provide the fuel needed to sustain a current star formation rate of 0.5 M ⊙ yr -1 .

Microarcsecond astrometry in the Local Group

AbstractMeasuring the proper motions and geometric distances of galaxies within the Local Group is very important for our understanding of its history, present state and future. Currently, proper motion measurements using optical methods are limited only to the closest companions of the Milky Way. However, given that VLBI provides the best angular resolution in astronomy and phase-referencing techniques yield astrometric accuracies of ≈ 10 micro-arcseconds, measurements of proper motions and angular rotation rates of galaxies out to a distance of ~ 1 Mpc are feasible. This paper presents results of VLBI observations in regions of H2O maser activity of the Local Group galaxies M33 and IC 10. Two masing regions in M33 are on opposite sides of the galaxy. This allows a comparison of the angular rotation rate (as measured by the VLBI observations) with the known inclination and rotation speed of the Hi gas disk leading to a determination of a geometric distance of 730 ± 100 ± 135 kpc. The first error indicates the statistical error of the proper-motion measurements, while the second error is the systematic error of the rotation model. Within the errors, this distance is consistent with the most recent Cepheid distance to M33. Since all position measurements were made relative to an extragalactic background source, the proper motion of M33 has also been measured. This provides a three dimensional velocity vector of M33, showing that this galaxy is moving with a velocity of 190 ± 59 km s−1 relative to the Milky Way. For IC 10, we obtain a motion of 215 ± 42 km s−1 relative to the Milky Way. These measurements promise a new handle on dynamical models for the Local Group and the mass and dark matter halo of Andromeda and the Milky Way.

Stellar Cluster Fiducial Sequences with the Advanced Camera for Surveys

We present color-magnitude diagrams of five Galactic globular clusters and one Galactic open cluster spanning a wide range of metallicity (-2.1 [Fe/H] +0.3), as observed in the F606W (broad V) and F814W (I) bands with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope. These clusters are part of two large ACS programs measuring the star formation history in the Andromeda halo, tidal stream, and outer disk. In these programs, the clusters serve as empirical isochrones and as calibrators for the transformation of theoretical isochrones to the ACS bandpasses. To make these data more accessible to the community, for each cluster we provide a ridgeline tracing the stars on the main sequence, subgiant branch, and red giant branch, plus the locus of stars on the horizontal branch. In addition, we provide the transformation of the Victoria-Regina isochrones to the ACS bandpasses.

RR Lyrae Stars in the Andromeda Halo from Deep Imaging with the Advanced Camera for Surveys

We present a complete census of RR Lyrae stars in a halo field of the Andromeda galaxy. These deep observations, taken as part of a program to measure the star formation history in the halo, spanned a period of 41 days with sampling on a variety of timescales, enabling the identification of short- and long-period variables. Although the long-period variables cannot be fully characterized within the time span of this program, the enormous advance in sensitivity provided by the Advanced Camera for Surveys on the Hubble Space Telescope allows accurate characterization of the RR Lyrae population in this field. We find 29 RRab stars with a mean period of 0.594 days, 25 RRc stars with a mean period of 0.316 days, and one RRd star with a fundamental period of 0.473 days and a first-overtone period of 0.353 days. These 55 RR Lyrae stars imply a specific frequency SRR ≈ 5.6, which is large given the high mean metallicity of the halo, but not surprising given that these stars arise from the old, metal-poor tail of the distribution. This old population in the Andromeda halo cannot be clearly placed into one of the Oosterhoff types: the ratio of RRc/RRabc stars is within the range seen in Oosterhoff II globular clusters, the mean RRab period is in the gap between Oosterhoff types, and the mean RRc period is in the range seen in Oosterhoff I globular clusters. The periods of these RR Lyrae stars suggest a mean metallicity of [Fe/H] ≈ -1.6, while their brightness implies a distance modulus to Andromeda of 24.5 ± 0.1, in good agreement with the Cepheid distance.

Taking measure of the Andromeda halo: a kinematic analysis of the giant stream surrounding M31

We present a spectroscopic survey of the giant stellar stream found in the halo of the Andromeda galaxy. Taken with the DEIMOS multi-object spectrograph on the Keck2 telescope, these data display a narrow velocity dispersion of 11+/-3 km/s, with a steady radial velocity gradient of 245 km/s over the 125 kpc radial extent of the stream studied so far. This implies that the Andromeda galaxy possesses a substantial dark matter halo. We fit the orbit of the stream in different galaxy potential models. In a simple model with a composite bulge, disk and halo, where the halo follows a ``universal'' profile that is compressed by the formation of the baryonic components, we find that the kinematics of the stream require a total mass inside 125 kpc of 7.5^(+2.5)_(-1.3) x 10^{11} M_Sun, or more than 5.4 x 10^{11} M_Sun at the 99% confidence level. This is the first galaxy in which it has been possible to measure the halo mass distribution by such direct dynamical means over such a large distance range. The resulting orbit shows that if M32 or NGC 205 are connected with the stream, they must either trail or lag the densest region of the stream by more than 100 kpc. Furthermore, according to the best-fit orbit, the stream passes very close to M31, causing its demise as a coherent structure and producing a fan of stars that will pollute the inner halo, thereby confusing efforts to measure the properties of genuine halo populations. Our data show that several recently identified planetary nebulae, which have been proposed as evidence for the existence of a new companion of M31, are likely members of the Andromeda Stream.

The mass of the Andromeda galaxy

ABSTRA C T This paper argues that the Milky Way galaxy is probably the largest member of the Local Group. The evidence comes from estimates of the total mass of the Andromeda galaxy (M31) derived from the three-dimensional positions and radial velocities of its satellite galaxies, as well as the projected positions and radial velocities of its distant globular clusters and planetary nebulae. The available data set comprises 10 satellite galaxies, 17 distant globular clusters and nine halo planetary nebulae with radial velocities. We find that the halo of Andromeda has a mass of ,12:3 118 26 10 11 M(; together with a scalelength of ,90 kpc and a predominantly isotropic velocity distribution. For comparison, our earlier estimate for the Milky Way halo is 19 136 217 10 11 M(: Although the error bars are admittedly large, this suggests that the total mass of M31 is probably less than that of the Milky Way .W e verify the robustness of our results to changes in the modelling assumptions and to errors caused by the small size and incompleteness of the data set. Our surprising claim can be checked in several ways in the near future. The numbers of satellite galaxies, planetary nebulae and globular clusters with radial velocities can be increased by ground-based spectroscopy, while the proper motions of the companion galaxies and the unresolved cores of the globular clusters can be measured using the astrometric satellites Space Interferometry Mission (SIM) and Global Astrometric Interferometer for Astrophysics (GAIA). Using 100 globular clusters at projected radii 20 & R & 50 kpc with both radial velocities and proper motions, it will be possible to estimate the mass within 50 kpc to an accuracy of ,20 per cent. Measuring the proper motions of the companion galaxies with SIM and GAIA will reduce the uncertainty in the total mass caused by the small size of the data set to ,22 per cent.

Precessing Gamma jets, GRB and the twin rings around SN1987A

Gamma jets (GJ) at MeV energies may be produced in galactic halos by Inverse Compton Scattering (ICS) of collinear ultrarelativistic electron beams (at GeV energies) onto thermal photons. These electron jets are emitted by neutron star jets (NSJ) or black holes (BH) while the target thermal photons are generated by a nearby binary stellarlike companion (or by a relic accreting disk). The GJ precesses in a conical shape as a lighthouse (with a characteristic Keplerian binary period) forced by the NSJ magnetic lines interaction with the stellar companion ones. The sudden blazing of the GJ, once the observer is into the beam cone direction, is detected as a GRB. Our predicted GRB spectra fit the GRB observed ones; GRB luminosity, variability and counts are well explained if the GJ sources are located in an ”extended” and an ”evaporating” galactic halo as the one recently required in the fast running away NS model. Observed jets such as SS433, the Great Annihilator, superluminal sources such as the last GRS1758-258, GRS1915+105 are the ideal candidates for NSJ as well as GJ. The SGRs are the lower energy and wider beamed version of this GJ model so they are linked to classical GRBs. The last mysterious presence of the twin rings near SN1987A (as well as the ring traces near NGC6543) are the spectacular signature of such (double sided) conical jet precession projected onto the red giant relic spherical shell. The tiny but very puzzling offaxis nature of the SN1987A twin rings is due to the high eccentricity of the binary NSJ. The GRBs from the nearby Andromeda halo might be detected as soon as the instrumental sensitivity is increased.

Local anisotropy of gamma-bursts' angular distribution

Using COMPTON observatory data, we have found the existence of the local cluster of gamma-ray sources in the region of the celestial sphere corresponding to the halo of Andromeda (galaxy M31). It is in agreement with the giant dark matter halo (GDMH) model.