arXiv subjects

Rodrigo A. Ibata

Publications and source records attributed to Rodrigo A. Ibata.

At least 55 records · Page 3Linked to original sources

arXiv2018

Phlegethon, a nearby $75°$-long retrograde stellar stream

We report the discovery of a $75°$ long stellar stream in Gaia DR2 catalog, found using the new STREAMFINDER algorithm. The structure is probably the remnant of a now fully disrupted globular cluster, lies $\approx 3.8 \, {\rm kpc}$ away from the Sun in the direction of the Galactic bulge, and possesses highly retrograde motion. We find that the system orbits close to the Galactic plane at Galactocentric distances between $4.9$ and $19.8 \, {\rm kpc}$. The discovery of this extended and extremely low surface brightness stream ($Σ_G\sim 34.3 \, {\rm mag \, arcsec^{-2}}$) with a mass of only $2580\pm140 \, {\rm\,M_\odot}$, demonstrates the power of the STREAMFINDER algorithm to detect even very nearby and ultra-faint structures. Due to its proximity and length we expect that Phlegethon will be a very useful probe of the Galactic acceleration field.

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arXiv2018

Ghostly Tributaries to the Milky Way: Charting the Halo's Stellar Streams with the Gaia DR2 catalogue

We present a panoramic map of the stellar streams of the Milky Way based upon astrometric and photometric measurements from the Gaia DR2 catalogue. In this first contribution, we concentrate on the halo at heliocentric distances beyond 5 kpc, and at Galactic latitudes $|b|>30°$, using the STREAMFINDER algorithm to detect structures along plausible orbits that are consistent with the Gaia proper motion measurements. We find a rich network of criss-crossing streams in the halo. Some of these structures were previously-known, several are new discoveries, but others are potentially artefacts of the Gaia scanning law and will require confirmation. With these initial discoveries, we are starting to unravel the complex formation of the halo of our Galaxy.

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arXiv2018

Pristine Dwarf-Galaxy Survey I: A detailed photometric and spectroscopic study of the very metal-poor Draco II satellite

We present a detailed study of the faint Milky Way satellite Draco II (Dra II) from deep CFHT/MegaCam broadband $g$ and $i$ photometry and narrow-band metallicity-sensitive CaHK observations, along with follow-up Keck II/DEIMOS multi-object spectroscopy. Forward modeling of the deep photometry allows us to refine the structural and photometric properties of Dra II: the distribution of stars in colour-magnitude space implies Dra II is old (13.5 $ \pm 0.5 $ Gyr), very metal poor, very faint ($ L_V = 180 ^{+124}_{-72} L_\odot $), and at a distance $d = 21.5 \pm 0.4$ kpc. The narrow-band, metallicity-sensitive CaHK Pristine photometry confirms this very low metallicity ([Fe/H]$ = -2.7 \pm 0.1$ dex). Even though our study benefits from a doubling of the spectroscopic sample size compared to previous investigations, the velocity dispersion of the system is still only marginally resolved ($σ_{vr}<5.9$ km s$^{-1}$ at the 95 per cent confidence level) and confirms that Dra II is a dynamically cold stellar system with a large recessional velocity ($\langle v_{r}\rangle = -342.5^{+1.1}_{-1.2} $ km s$^{-1}$). We further show that the spectroscopically confirmed members of Dra~II have a mean proper motion of $(μ_α^*,μ_δ)=(1.26 \pm 0.27,0.94 \pm 0.28) $ mas yr$^{-1}$ in the Gaia DR2 data, which translates to an orbit with a pericenter and an apocenter of $21.3 ^{+0.7}_{-1.0}$ and $153.8 ^{+56.7}_{-34.7}$ kpc, respectively. Taken altogether, these properties favour the scenario of Dra~II being a potentially disrupting dwarf galaxy. The low-significance extra-tidal features we map around the satellite tentatively support this scenario.

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arXiv2018

Galactic cartography with SkyMapper: I. Population sub-structure and the stellar number density of the inner halo

The stars within our Galactic halo presents a snapshot of its ongoing growth and evolution, probing galaxy formation directly. Here, we present our first analysis of the stellar halo from detailed maps of Blue Horizontal Branch (BHB) stars drawn from the SkyMapper Southern Sky Survey. To isolate candidate BHB stars from the overall population, we develop a machine-learning approach through the application of an Artificial Neural Network (ANN), resulting in a relatively pure sample of target stars. From this, we derive the absolute $u$ magnitude for the BHB sample to be $\sim2\ mag$, varying slightly with $(v-g)_0$ and $(u-v)_0$ colours. We examine the BHB number density distribution from 5272 candidate stars, deriving a double power-law with a break radius of $r_s = 11.8\pm0.3\ kpc$, and inner and outer slopes of $α_{in} = -2.5\pm0.1$ and $α_{out} = -4.5\pm0.3$ respectively. Through isochrone fitting of simulated BHB stars, we find a colour-age/metallicity correlation, with older/more metal-poor stars being bluer, and establish a parameter to indicate this age (or metallicity) variation. Using this, we construct the three-dimensional population distribution of BHB stars in the halo and identify significant substructure. Finally, in agreement with previous studies, we also identify a systemic age/metallicity shift spanning $\sim3\ kpc$ to $ \sim20\ kpc$ in galactocentric distance.

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arXiv2018

STREAMFINDER II: A possible fanning structure parallel to the GD-1 stream in Pan-STARRS1

STREAMFINDER is a new algorithm that we have built to detect stellar streams in an automated and systematic way in astrophysical datasets that possess any combination of positional and kinematic information. In Paper I, we introduced the methodology and the workings of our algorithm and showed that it is capable of detecting ultra-faint and distant halo stream structures containing as few as $\sim 15$ members ($Σ_{\rm G} \sim 33.6\, {\rm mag \, arcsec^{-2}}$) in the Gaia dataset. Here, we test the method with real proper motion data from the Pan-STARRS1 survey, and by selecting targets down to $r_{0}=18.5$ mag we show that it is able to detect the GD-1 stellar stream, whereas the structure remains below a useful detection limit when using a Matched Filter technique. The radial velocity solutions provided by STREAMFINDER for GD-1 candidate members are found to be in good agreement with observations. Furthermore, our algorithm detects a $\sim 40^°$ long structure approximately parallel to GD-1, and which fans out from it, possibly a sign of stream-fanning due to the triaxiality of the Galactic potential. This analysis shows the promise of this method for detecting and analysing stellar streams in the upcoming Gaia DR2 catalogue.

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arXiv2017

On the Origin of the Monoceros Ring - I: Kinematics, proper motions, and the nature of the progenitor

The Monoceros Ring (MRi) structure is an apparent stellar overdensity that has been postulated to entirely encircle the Galactic plane and has been variously described as being due to line-of-sight effects of the Galactic warp and flare or of extragalactic origin (via accretion). Despite being intensely scrutinised in the literature for more than a decade, no studies to-date have been able to definitively uncover its origins. Here we use $N$-body simulations and a genetic algorithm to explore the parameter space for the initial position, orbital parameters and, for the first time, the final location of a satellite progenitor. We fit our models to the latest Pan-STARRS data to determine whether an accretion scenario is capable of producing an in- Plane ring-like structure matching the known parameters of the MRi. Our simulations produce streams that closely match the location, proper motion and kinematics of the MRi structure. However, we are not able to reproduce the mass estimates from earlier studies based on Pan-STARRS data. Furthermore, in contrast with earlier studies our best-fit models are those for progenitors on retrograde orbits. If the MRi was produced by satellite accretion, we find that its progenitor has an initial mass upper limit of ~$10^{10}$M$_\odot$ and the remnant is likely located behind the Galactic bulge, making it diffcult to locate observationally. While our models produce realistic MRi-like structures we cannot definitively conclude that the MRi was produced by the accretion of a satellite galaxy.

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arXiv2017

The Lopsidedness of Satellite Galaxy Systems in $Λ$CDM simulations

The spatial distribution of satellite galaxies around pairs of galaxies in the Sloan Digital Sky Survey (SDSS) have been found to bulge significantly towards the respective partner. Highly anisotropic, planar distributions of satellite galaxies are in conflict with expectations derived from cosmological simulations. Does the lopsided distribution of satellite systems around host galaxy pairs constitute a similar challenge to the standard model of cosmology? We investigate whether such satellite distributions are present around stacked pairs of hosts extracted from the $Λ$CDM simulations Millennium-I, Millennium-II, ELVIS, and Illustris-1. By utilizing this set of simulations covering different volumes, resolutions, and physics, we implicitly test whether a lopsided signal exists for different ranges of satellite galaxy masses, and whether the inclusion of hydrodynamical effects produces significantly different results. All simulations display a lopsidedness similar to the observed situation. The signal is highly significant for simulations containing a sufficient number of hosts and resolved satellite galaxies (up to $5\,σ$ for Millennium-II). We find a projected signal that is up to twice as strong as that reported for the SDSS systems for certain opening angles ($\sim16\%$ more satellites in the direction between the pair than expected for uniform distributions). Considering that the SDSS signal is a lower limit owing to likely back- and foreground contamination, the $Λ$CDM simulations appear to be consistent with this particular empirical property of galaxy pairs.

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arXiv2017

A rogues gallery of Andromeda's dwarf galaxies I. A predominance of red horizontal branches

We present homogeneous, sub-horizontal branch photometry of twenty dwarf spheroidal satellite galaxies of M31 observed with the Hubble Space Telescope. Combining our new data for sixteen systems with archival data in the same filters for another four, we show that Andromeda dwarf spheroidal galaxies favor strikingly red horizontal branches or red clumps down to ~10^{4.2} Lsun (M_V ~ -5.8). The age-sensitivity of horizontal branch stars implies that a large fraction of the M31 dwarf galaxies have extended star formation histories (SFHs), and appear inconsistent with early star formation episodes that were rapidly shutdown. Systems fainter than ~10^{5.5} Lsun show the widest range in the ratios and morphologies of red and blue horizontal branches, indicative of both complex SFHs and a diversity in quenching timescales and/or mechanisms, which is qualitatively different from what is currently known for faint Milky Way (MW) satellites of comparable luminosities. Our findings bolster similar conclusions from recent deeper data for a handful of M31 dwarf galaxies. We discuss several sources for diversity of our data such as varying halo masses, patchy reionization, mergers/accretion, and the environmental influence of M31 and the Milky Way on the early evolution of their satellite populations. A detailed comparison between the histories of M31 and MW satellites would shed signifiant insight into the processes that drive the evolution of low-mass galaxies. Such a study will require imaging that reaches the oldest main sequence turnoffs for a significant number of M31 companions.

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arXiv2017

Stability of Satellite Planes in M31 II: Effects of the Dark Subhalo Population

The planar arrangement of nearly half the satellite galaxies of M31 has been a source of mystery and speculation since it was discovered. With a growing number of other host galaxies showing these satellite galaxy planes, their stability and longevity have become central to the debate on whether the presence of satellite planes are a natural consequence of prevailing cosmological models, or represent a challenge. Given the dependence of their stability on host halo shape, we look into how a galaxy plane's dark matter environment influences its longevity. An increased number of dark matter subhalos results in increased interactions that hasten the deterioration of an already-formed plane of satellite galaxies in spherical dark halos. The role of total dark matter mass fraction held in subhalos in dispersing a plane of galaxies present non trivial effects on plane longevity as well. But any misalignments of plane inclines to major axes of flattened dark matter halos lead to their lifetimes being reduced to < 3 Gyrs. Distributing > 40% of total dark mass in subhalos in the overall dark matter distribution results in a plane of satellite galaxies that is prone to change through the 5 Gyr integration time period.

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arXiv2017

Considerations on how to investigate planes of satellite galaxies

The existence of a spatially thin, kinematically coherent Disk of Satellites (DoS) around the Milky Way (MW), is a problem that often garners vivacious debate in the literature or at scientific meetings. One of the most recent incarnations of this wrangle occurred with two papers by Maji et al, who argued that these structures "maybe a misinterpretation of the data". These claims are in stark contrast to previous works. Motivated by this and other recent publications on this problem, we discuss necessary considerations to make, observational effects to consider, and pitfalls to avoid when investigating satellite galaxy planes such as the MW's DoS. In particular, we emphasize that conclusions need to have a statistical basis including a determination of the significance of satellite alignments, observational biases must not be ignored, and measurement errors (e.g. for proper motions) need to be considered. We discuss general problems faced by attempts to determine the dynamical stability of the DoS via orbit integrations of MW satellite galaxies, and demonstrate that to interpret simulations, it is helpful to compare them with a null case of isotropically distributed satellite positions and velocities. Based on these criteria, we find that the conclusions of Maji et al. do not hold up to scrutiny, and that their hydrodynamic cosmological simulation of a single host shows no evidence for a significant kinematic coherence among the simulated satellite galaxies, in contrast to the observed MW system.

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arXiv2017

Feeling the pull, a study of natural Galactic accelerometers. II: kinematics and mass of the delicate stellar stream of the Palomar 5 globular cluster

We present two spectroscopic surveys of the tidal stellar stream of the Palomar 5 globular cluster, undertaken with the VLT/FLAMES and AAT/AAOmega instruments. We use these data in conjunction with photometric data presented in the previous contribution in this series to classify the survey stars in terms of their probability of belonging to the Palomar 5 stellar stream. We find that high-probability candidates are only found in a very narrow spatial interval surrounding the locus of the stream on the sky. PanSTARRS RRLyrae stars in this region of sky are also distributed in a similar manner. The absence of significant "fanning" of this stellar stream confirms that Palomar 5 does not follow a chaotic orbit. Previous studies have found that Palomar 5 is largely devoid of low-mass stars, and we show that this is true also of the stellar populations along the trailing arm out to $6°$. Within this region, which contains 73\% of the detected stars, the population is statistically identical to the core, implying that the ejection of the low-mass stars occurred before the formation of the stream. We also present an updated structural model fit to the bound remnant, which yields a total mass of $4297\pm98 {\rm\,M_\odot}$ and a tidal radius $0.145\pm0.009$ kpc. We estimate the mass of the observed system including the stream to be $12200\pm400 {\rm\,M_\odot}$, and the initial mass to have been $\sim47000\pm1500 {\rm\,M_\odot}$. These observational constraints will be employed in our next study to model the dynamics of the system in detail.

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arXiv2017

Measuring the Sun's Motion with Stellar Streams

We present a method for measuring the Sun's motion using the proper motions of Galactic halo star streams. The method relies on the fact that the motion of the stars perpendicular to a stream from a low-mass progenitor is close to zero when viewed from a non-rotating frame at rest with respect to the Galaxy, and that the deviation from zero is due to the reflex motion of the observer. The procedure we implement here has the advantage of being independent of the Galactic mass distribution. We run a suite of simulations to test the algorithm we have developed, and find that we can recover the input Solar motion to good accuracy with data of the quality that will soon become available from the ESA/Gaia mission.

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arXiv2017

A novel JEAnS analysis of the Fornax dwarf using evolutionary algorithms: mass follows light with signs of an off-centre merger

Dwarf galaxies, among the most dark matter dominated structures of our universe, are excellent test-beds for dark matter theories. Unfortunately, mass modelling of these systems suffers from the well documented mass-velocity anisotropy degeneracy. For the case of spherically symmetric systems, we describe a method for non-parametric modelling of the radial and tangential velocity moments. The method is a numerical velocity anisotropy "inversion", with parametric mass models, where the radial velocity dispersion profile, $σ_{\mathrm{rr}}^2$ is modeled as a B-spline, and the optimization is a three step process that consists of: (i) an Evolutionary modelling to determine the mass model form and the best B-spline basis to represent $σ_{\mathrm{rr}}^2$; (ii) an optimization of the smoothing parameters; (iii) a Markov chain Monte Carlo analysis to determine the physical parameters. The mass-anisotropy degeneracy is reduced into mass model inference, irrespective of kinematics. We test our method using synthetic data. Our algorithm constructs the best kinematic profile and discriminates between competing dark matter models. We apply our method to the Fornax dwarf spheroidal galaxy. Using a King brightness profile and testing various dark matter mass models, our model inference favours a simple mass-follows-light system. We find that the anisotropy profile of Fornax is tangential ($β(r) < 0$) and we estimate a total mass of $M_{\text{tot}} = 1.613 ^{+0.050}_{-0.075} \times 10^8 \, \text{M}_{\odot}$, and a mass-to-light ratio of $Υ_V = 8.93 ^{+0.32}_{-0.47} \, (\text{M}_{\odot}/\text{L}_{\odot})$. The algorithm we present is a robust and computationally inexpensive method for non-parametric modelling of spherical clusters independent of the mass-anisotropy degeneracy.

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arXiv2017

The Pristine survey I: Mining the Galaxy for the most metal-poor stars

We present the Pristine survey, a new narrow-band photometric survey focused on the metallicity-sensitive Ca H & K lines and conducted in the northern hemisphere with the wide-field imager MegaCam on the Canada-France-Hawaii Telescope (CFHT). This paper reviews our overall survey strategy and discusses the data processing and metallicity calibration. Additionally we review the application of these data to the main aims of the survey, which are to gather a large sample of the most metal-poor stars in the Galaxy, to further characterise the faintest Milky Way satellites, and to map the (metal-poor) substructure in the Galactic halo. The current Pristine footprint comprises over 1,000 deg2 in the Galactic halo ranging from b~30 to 78 and covers many known stellar substructures. We demonstrate that, for SDSS stellar objects, we can calibrate the photometry at the 0.02-magnitude level. The comparison with existing spectroscopic metallicities from SDSS/SEGUE and LAMOST shows that, when combined with SDSS broad-band g and i photometry, we can use the CaHK photometry to infer photometric metallicities with an accuracy of ~0.2 dex from [Fe/H]=-0.5 down to the extremely metal-poor regime ([Fe/H]<-3.0). After the removal of various contaminants, we can efficiently select metal-poor stars and build a very complete sample with high purity. The success rate of uncovering [Fe/H]SEGUE<-3.0 stars among [Fe/H]Pristine<-3.0 selected stars is 24% and 85% of the remaining candidates are still very metal poor ([Fe/H]<-2.0). We further demonstrate that Pristine is well suited to identify the very rare and pristine Galactic stars with [Fe/H]<-4.0, which can teach us valuable lessons about the early Universe.

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arXiv2016

On the Stability of Satellite Planes I: Effects of Mass, Velocity, Halo Shape and Alignment

The recently discovered vast thin plane of dwarf satellites orbiting the Andromeda Galaxy (M31) adds to the mystery of the small scale distribution of the Local Group's galaxy population. Such well defined planar structures are apparently rare occurrences in cold dark matter cosmological simulations, and we lack a coherent explanation of their formation and existence. In this paper, we explore the long-term survivability of thin planes of dwarfs in galactic halos, focusing, in particular, on systems mimicking the observed Andromeda distribution. The key results show that, in general, planes of dwarf galaxies are fragile, sensitive to the shape of the dark matter halo and other perturbing effects. In fact, long lived planes of satellites only exist in polar orbits in spherical dark matter halos, presenting a challenge to the observed Andromeda plane which is significantly tilted with respect to the optical disk. Our conclusion is that, in standard cosmological models, planes of satellites are generally short lived, and hence we must be located at a relatively special time in the evolution of the Andromeda Plane, lucky enough to see its coherent pattern.

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arXiv2016

The PAndAS view of the Andromeda satellite system - II. Detailed properties of 23 M31 dwarf spheroidal galaxies

We present a comprehensive analysis of the structural properties and luminosities of the 23 dwarf spheroidal galaxies that fall within the footprint of the Pan-Andromeda Archaeological Survey (PAndAS). These dwarf galaxies represent the large majority of Andromeda's known satellite dwarf galaxies and cover a wide range in luminosity ($-11.6<M_V<-5.8$ or $10^{4.2}< L <10^{6.5} L_\odot$) and surface brightness ($25.1<μ_0<29.3$ mag/arcsec$^2$). We confirm most previous measurements, but find And XIX to be significantly larger than before ($r_h=3065^{+1065}_{-935}$ pc, $M_V=-10.1^{+0.8}_{-0.4}$) and cannot derive parameters for And XXVII as it is likely not a bound stellar system. We also significantly revise downward the luminosities of And~XV and And~XVI, which are now $M_V\sim-7.5$ or $L\sim10^5 L_\odot$. Finally, we provide the first detailed analysis of Cas II/And XXX, a fairly faint system ($M_V=-8.0^{+0.4}_{-0.3}$) of typical size ($r_h=270\pm50$ pc), located in close proximity to the two bright elliptical dwarf galaxies NGC 147 & 185. Combined with the set of homogeneous distances published in an earlier contribution, our analysis dutifully tracks all relevant sources of uncertainty in the determination of the properties of the dwarf galaxies from the PAndAS photometric catalogue. We further publish the posterior probability distribution functions of all the parameters we fit for in the form of MCMC chains available online; these inputs should be used in any analysis that aims to remain truthful to the data and properly account for covariance between parameters.

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arXiv2016

The dynamics of Andromeda's dwarf galaxies and stellar streams

As part of the Z-PAndAS Keck II DEIMOS survey of resolved stars in our neighboring galaxy, Andromeda (M31), we have built up a unique data set of measured velocities and chemistries for thousands of stars in the Andromeda stellar halo, particularly probing its rich and complex substructure. In this contribution, we will discuss the structural, dynamical and chemical properties of Andromeda's dwarf spheroidal galaxies, and how there is no observational evidence for a difference in the evolutionary histories of those found on and off M31's vast plane of satellites. We will also discuss a possible extension to the most significant merger event in M31 - the Giant Southern Stream - and how we can use this feature to refine our understanding of M31's mass profile, and its complex evolution.

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arXiv2016

Major Substructure in the M31 Outer Halo: Distances and Metallicities along the Giant Stellar Stream

We present a renewed look at M31's Giant Stellar Stream along with the nearby structures Stream C and Stream D, exploiting a new algorithm capable of fitting to the red giant branch (RGB) of a structure in both colour and magnitude space. Using this algorithm, we are able to generate probability distributions in distance, metallicity and RGB width for a series of subfields spanning these structures. Specifically, we confirm a distance gradient of approximately 20 kpc per degree along a 6 degree extension of the Giant Stellar Stream, with the farthest subfields from M31 lying ~ 120 kpc more distant than the inner-most subfields. Further, we find a metallicity that steadily increases from -0.7^{+0.1}_{-0.1} dex to -0.2^{+0.2}_{-0.1} dex along the inner half of the stream before steadily dropping to a value of -1.0^{+0.2}_{-0.2} dex at the farthest reaches of our coverage. The RGB width is found to increase rapidly from 0.4^{+0.1}_{-0.1} dex to 1.1^{+0.2}_{-0.1} dex in the inner portion of the stream before plateauing and decreasing marginally in the outer subfields of the stream. In addition, we estimate Stream C to lie at a distance between 794 and 862 kpc and Stream D between 758 kpc and 868 kpc. We estimate the median metallicity of Stream C to lie in the range -0.7 to -1.6 dex and a metallicity of -1.1^{+0.3}_{-0.2} dex for Stream D. RGB widths for the two structures are estimated to lie in the range 0.4 to 1.2 dex and 0.3 to 0.7 dex respectively. In total, measurements are obtained for 19 subfields along the Giant Stellar Stream, 4 along Stream C, 5 along Stream D and 3 general M31 spheroid fields for comparison. We thus provide a higher resolution coverage of the structures in these parameters than has previously been available in the literature.

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