arXiv subjects

Rodrigo Ibata

Publications and source records attributed to Rodrigo Ibata.

At least 91 records · Page 5Linked to original sources

arXiv2019

The Streams of the Gaping Abyss: A population of entangled stellar streams surrounding the Inner Galaxy

We present the discovery of a large population of stellar streams that surround the inner Galaxy, found in the Gaia DR2 catalog using the new STREAMFINDER algorithm. Here we focus on the properties of eight new high-significance structures found at Heliocentric distances between 1 and 10 kpc and at Galactic latitudes $|b|>20°$, named Slidr, Sylgr, Ylgr, Fimbulthul, Svöl, Fjörm, Gjöll and Leiptr. Spectroscopic measurements of seven of the streams confirm the detections, which are based on Gaia astrometry and photometry alone, and show that these streams are predominantly metal-poor. The sample possesses diverse orbital properties, although most of the streams appear to be debris of inner-halo globular clusters. Many more candidate streams are visible in our maps, but require follow-up spectroscopy to confirm their nature. We also explain in detail the workings of the algorithm, and gauge the incidence of false detections by running the algorithm on a smooth model of the Gaia catalog.

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arXiv2019

The outer halo globular cluster system of M31 - III. Relationship to the stellar halo

We utilise the final catalogue from the Pan-Andromeda Archaeological Survey to investigate the links between the globular cluster system and field halo in M31 at projected radii $R_p=25-150$ kpc. In this region the cluster radial density profile exhibits a power-law decline with index $Γ=-2.37\pm0.17$, matching that for the stellar halo component with [Fe/H] $<-1.1$. Spatial density maps reveal a striking correspondence between the most luminous substructures in the metal-poor field halo and the positions of many globular clusters. By comparing the density of metal-poor halo stars local to each cluster with the azimuthal distribution at commensurate radius, we reject the possibility of no correlation between clusters and field overdensities with high confidence. We use our stellar density measurements and previous kinematic data to demonstrate that $\approx35-60\%$ of clusters exhibit properties consistent with having been accreted into the outskirts of M31 at late times with their parent dwarfs. Conversely, at least $\sim40\%$ of remote clusters show no evidence for a link with halo substructure. The radial density profile for this subgroup is featureless and closely mirrors that observed for the apparently smooth component of the metal-poor stellar halo. We speculate that these clusters are associated with the smooth halo; if so, their properties appear consistent with a scenario where the smooth halo was built up at early times via the destruction of primitive satellites. In this picture the entire M31 globular cluster system outside $R_p=25$ kpc comprises objects accumulated from external galaxies over a Hubble time of growth.

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arXiv2018

A-type stars in the Canada-France Imaging Survey II. Tracing the height of the disc at large distances with Blue Stragglers

We present the kinematics of Blue Straggler (BS) stars identified in the Canada-France-Imaging-Survey (CFIS), covering 4000 deg$^2$ on the sky in the $u$-band. The BS sample, characterised through CFIS and Pan-STARRS photometry, has been kinematically decomposed into putative halo and disc populations after cross-matching with Gaia astrometry and SDSS/SEGUE/LAMOST spectroscopy. This decomposition clearly reveals the strong flaring of the outer Milky Way disc. In particular, we show that we can detect this flaring up to a vertical height of $|Z| \simeq 8$ kpc at a Galactocentric distance of $R\sim $27 kpc. While some small level of flaring is expected for extended discs built up by radial migration, we demonstrate that this observed very strong flaring of the Milky Way disc is more likely consistent with it being dynamically heated by the repeated passage of the Sagittarius dwarf spheroidal galaxy through the midplane.

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arXiv2018

The large-scale structure of the halo of the Andromeda galaxy II. Hierarchical structure in the Pan-Andromeda Archaeological Survey

The Pan-Andromeda Archaeological Survey is a survey of $>400$ square degrees centered on the Andromeda (M31) and Triangulum (M33) galaxies that has provided the most extensive panorama of a $L_\star$ galaxy group to large projected galactocentric radii. Here, we collate and summarise the current status of our knowledge of the substructures in the stellar halo of M31, and discuss connections between these features. We estimate that the 13 most distinctive substructures were produced by at least 5 different accretion events, all in the last 3 or 4 Gyrs. We suggest that a few of the substructures furthest from M31 may be shells from a single accretion event. We calculate the luminosities of some prominent substructures for which previous estimates were not available, and we estimate the stellar mass budget of the outer halo of M31. We revisit the problem of quantifying the properties of a highly structured dataset; specifically, we use the OPTICS clustering algorithm to quantify the hierarchical structure of M31's stellar halo, and identify three new faint structures. M31's halo, in projection, appears to be dominated by two `mega-structures', that can be considered as the two most significant branches of a merger tree produced by breaking M31's stellar halo into smaller and smaller structures based on the stellar spatial clustering. We conclude that OPTICS is a powerful algorithm that could be used in any astronomical application involving the hierarchical clustering of points. The publication of this article coincides with the public release of all PAndAS data products.

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arXiv2018

STREAMFINDER I: A New Algorithm for detecting Stellar Streams

We have designed a powerful new algorithm to detect stellar streams in an automated and systematic way. The algorithm, which we call the STREAMFINDER, is well suited for finding dynamically cold and thin stream structures that may lie along any simple or complex orbits in Galactic stellar surveys containing any combination of positional and kinematic information. In the present contribution we introduce the algorithm, lay out the ideas behind it, explain the methodology adopted to detect streams and detail its workings by running it on a suite of simulations of mock Galactic survey data of similar quality to that expected from the ESA/Gaia mission. We show that our algorithm is able to detect even ultra-faint stream features lying well below previous detection limits. Tests show that our algorithm will be able to detect distant halo stream structures $>10^{\circ}$ long containing as few as $\sim 15$ members ($Σ_{\rm G} \sim 33.6\, {\rm mag \, arcsec^{-2}}$) in the Gaia dataset.

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arXiv2017

On the derivation of radial velocities of SB2 components: a "CCF vs TODCOR" comparison

The radial velocity (RV) of a single star is easily obtained from cross-correlation of the spectrum with a template, but the treatment of double-lined spectroscopic binaries (SB2s) is more difficult. Two different approaches were applied to a set of SB2s: the fit of the cross-correlation function with two normal distributions, and the cross-correlation with two templates, derived with the TODCOR code. It appears that the minimum masses obtained through the two methods are sometimes rather different, although their estimated uncertainties are roughly equal. Moreover, both methods induce a shift in the zero point of the secondary RVs, but it is less pronounced for TODCOR. All-in-all the comparison between the two methods is in favour of TODCOR.

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arXiv2017

The programme "accurate masses for SB2 components"

Accurate stellar masses are requested in order to improve our understanding of stellar interiors, but they are still rather rare. Fortunately, the forthcoming Gaia Mission will provide astrometric measurements permitting the derivation of the orbital inclinations of nearby binaries which are also observed as double-lined spectroscopic binaries (SB2s) with ground-based telescopes. A programme of radial velocity (RV) measurements was initiated in 2010 with the Sophie spectrograph of the Haute-Provence observatory in order to derive accurate SB2 orbits for a large set of stars. Therefore, combined SB2+astrometric orbits will be derived thanks to Gaia, and masses with errors around 1~\% are expected for both components. The programme includes 70 SB2s, and the accurate SB2 orbits of 24 of them were already derived. In addition, two complementary programmes devoted to southern stars or to late-type dwarf stars were also initiated with the HERMES and the CARMENES spectrographs, respectively. Interferometric measurements were obained with the VLTI/PIONIER for 7 SB2s, and were taken from other sources for 4 others. Currently, combined "visual binary" (VB) +SB2 solutions were derived for 7 binaries, leading to the masses of the components and to the parallaxes. The parallaxes from the Hipparcos 2 catalogue were corrected for orbital motion and compared to our solution, confirming the high quality of Hipparcos 2.

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arXiv2017

Stellar streams as gravitational experiments II. Asymmetric tails of globular cluster streams

Kinematically cold tidal streams of globular clusters (GC) are excellent tracers of the Galactic gravitational potential at moderate Galactocentric distances, and can also be used as probes of the law of gravity on Galactic scales. Here, we compare for the first time the generation of such streams in Newtonian and Milgromian gravity (MOND). We first compute analytical results to investigate the expected shape of the GC gravitational potential in both frameworks, and we then run N-body simulations with the Phantom of Ramses code. We find that the GCs tend to become lopsided in MOND. This is a consequence of the external field effect which breaks the strong equivalence principle. When the GC is filling its tidal radius the lopsidedness generates a strongly asymmetric tidal stream. In Newtonian dynamics, such markedly asymmetric streams can in general only be the consequence of interactions with dark matter subhalos, giant molecular clouds, or interaction with the Galactic bar. In these Newtonian cases, the asymmetry is the consequence of a very large gap in the stream, whilst in MOND it is a true asymmetry. This should thus allow us in the future to distinguish these different scenarios by making deep observations of the environment of the asymmetric stellar stream of Palomar 5. Moreover, our simulations indicate that the high internal velocity dispersion of Palomar 5 for its small stellar mass would be natural in MOND.

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arXiv2017

The Canada-France Imaging Survey: First results from the u-band component

The Canada-France Imaging Survey (CFIS) will map the northern high Galactic latitude sky in the $u$-band ("CFIS-u", 10,000$\, {\rm deg^2}$) and in the $r$-band ("CFIS-r", 5,000$\, {\rm deg^2}$), enabling a host of stand-alone science investigations, and providing some of the ground-based data necessary for photometric redshift determination for the Euclid mission. In this first contribution we present the $u$-band component of the survey, describe the observational strategy, and discuss some first highlight results, based on approximately one third of the final area. We show that the Galactic anticenter structure is distributed continuously along the line of sight, out to beyond 20 kpc, and possesses a metallicity distribution that is essentially identical to that of the outer disk sampled by APOGEE. This suggests that it is probably a buckled disk of old metal-rich stars, rather than a stream or a flare. We also discuss the future potential for CFIS-u in discovering star-forming dwarf galaxies around the Local Group, the characterization of the white dwarf and blue straggler population of the Milky Way, as well as its sensitivity to low-surface brightness structures in external galaxies.

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arXiv2017

Chemical Mapping of the Milky Way With The Canada-France Imaging Survey: A Non-parametric Metallicity-Distance Decomposition of the Galaxy

We present the chemical distribution of the Milky Way, based on 2,900$\, {\rm deg^2}$ of $u$-band photometry taken as part of the Canada-France Imaging Survey. When complete, this survey will cover 10,000$\, {\rm deg^2}$ of the Northern sky. By combing the CFHT $u$-band photometry together with SDSS and Pan-STARRS $g,r,$ and $i$, we demonstrate that we are able to measure reliably the metallicities of individual stars to $\sim 0.2$ dex, and hence additionally obtain good photometric distance estimates. This survey thus permits the measurement of metallicities and distances of the dominant main-sequence population out to approximately 30 kpc, and provides much higher number of stars at large extraplanar distances than have been available from previous surveys. We develop a non-parametric distance-metallicity decomposition algorithm and apply it to the sky at $30°< |b| < 70°$ and to the North Galactic Cap. We find that the metallicity-distance distribution is well-represented by three populations whose metallicity distributions do not vary significantly with vertical height above the disk. As traced in main-sequence stars, the stellar halo component shows a vertical density profile that is close to exponential, with a scale height of around 3 kpc. This may indicate that the inner halo was formed partly from disk stars ejected in an ancient minor merger.

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arXiv2017

Theia: Faint objects in motion or the new astrometry frontier

In the context of the ESA M5 (medium mission) call we proposed a new satellite mission, Theia, based on relative astrometry and extreme precision to study the motion of very faint objects in the Universe. Theia is primarily designed to study the local dark matter properties, the existence of Earth-like exoplanets in our nearest star systems and the physics of compact objects. Furthermore, about 15 $\%$ of the mission time was dedicated to an open observatory for the wider community to propose complementary science cases. With its unique metrology system and "point and stare" strategy, Theia's precision would have reached the sub micro-arcsecond level. This is about 1000 times better than ESA/Gaia's accuracy for the brightest objects and represents a factor 10-30 improvement for the faintest stars (depending on the exact observational program). In the version submitted to ESA, we proposed an optical (350-1000nm) on-axis TMA telescope. Due to ESA Technology readiness level, the camera's focal plane would have been made of CCD detectors but we anticipated an upgrade with CMOS detectors. Photometric measurements would have been performed during slew time and stabilisation phases needed for reaching the required astrometric precision.

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arXiv2017

Stellar streams as gravitational experiments I. The case of Sagittarius

Tidal streams of disrupting dwarf galaxies orbiting around their host galaxy offer a unique way to constrain the shape of galactic gravitational potentials. Such streams can be used as leaning tower gravitational experiments on galactic scales. The most well motivated modification of gravity proposed as an alternative to dark matter on galactic scales is Milgromian dynamics (MOND), and we present here the first ever N-body simulations of the dynamical evolution of the disrupting Sagittarius dwarf galaxy in this framework. Using a realistic baryonic mass model for the Milky Way, we attempt to reproduce the present-day spatial and kinematic structure of the Sagittarius dwarf and its immense tidal stream that wraps around the Milky Way. With very little freedom on the original structure of the progenitor, constrained by the total luminosity of the Sagittarius structure and by the observed stellar mass-size relation for isolated dwarf galaxies, we find reasonable agreement between our simulations and observations of this system. The observed stellar velocities in the leading arm can be reproduced if we include a massive hot gas corona around the Milky Way that is flattened in the direction of the principal plane of its satellites. This is the first time that tidal dissolution in MOND has been tested rigorously at these mass and acceleration scales.

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arXiv2016

The lopsided distribution of satellite galaxies

The distribution of smaller satellite galaxies around large central galaxies has attracted attention because peculiar spatial and kinematic configurations have been detected in some systems. A particularly striking example of such behavior is seen in the satellite system of the Andromeda galaxy, where around 80\% are on the nearside of that galaxy, facing the Milky Way. Motivated by this departure from anisotropy, we examined the spatial distribution of satellites around pairs of galaxies in the SDSS. By stacking tens of thousands of satellites around galaxy pairs we found that satellites tend to bulge towards the other central galaxy, preferably occupying the space between the pair, rather than being spherically or axis-symmetrically distributed around each host. The bulging is a function of the opening angle examined and is fairly strong -- there are up to $\sim$10\% more satellites in the space between the pair, than expected from uniform. Consequently, it is a statistically very strong signal, being inconsistent with a uniform distribution at the 5$σ$ level. The possibility that the observed signal is the result of the overlap of two haloes with extended satellite distributions, is ruled out by testing this hypothesis by performing the same tests on isolated galaxies (and their satellites) artificially placed at similar separations. These findings highlight the unrelaxed and interacting nature of galaxies in pairs.

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arXiv2016

The Detailed Science Case for the Maunakea Spectroscopic Explorer: the Composition and Dynamics of the Faint Universe

MSE is an 11.25m aperture observatory with a 1.5 square degree field of view that will be fully dedicated to multi-object spectroscopy. More than 3200 fibres will feed spectrographs operating at low (R ~ 2000 - 3500) and moderate (R ~ 6000) spectral resolution, and approximately 1000 fibers will feed spectrographs operating at high (R ~ 40000) resolution. MSE is designed to enable transformational science in areas as diverse as tomographic mapping of the interstellar and intergalactic media; the in-situ chemical tagging of thick disk and halo stars; connecting galaxies to their large scale structure; measuring the mass functions of cold dark matter sub-halos in galaxy and cluster-scale hosts; reverberation mapping of supermassive black holes in quasars; next generation cosmological surveys using redshift space distortions and peculiar velocities. MSE is an essential follow-up facility to current and next generations of multi-wavelength imaging surveys, including LSST, Gaia, Euclid, WFIRST, PLATO, and the SKA, and is designed to complement and go beyond the science goals of other planned and current spectroscopic capabilities like VISTA/4MOST, WHT/WEAVE, AAT/HERMES and Subaru/PFS. It is an ideal feeder facility for E-ELT, TMT and GMT, and provides the missing link between wide field imaging and small field precision astronomy. MSE is optimized for high throughput, high signal-to-noise observations of the faintest sources in the Universe with high quality calibration and stability being ensured through the dedicated operational mode of the observatory. (abridged)

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arXiv2016

Exploring the reality of density substructures in the Palomar 5 stellar stream

We present an analysis of the presence of substructures in the stellar stream of the Palomar 5 globular cluster, as derived from Sloan Digital Sky Survey data. Using a matched filter technique, we recover the positions and sizes of overdensities reported in previous studies. To explore the reality of these structures, we also create an artificial model of the stream, in which we construct a realistic background on top of which we add a perfectly smooth stream structure, taking into account the effects of photometric completeness and interstellar extinction. We find that the smooth artificial stream then shows similarly-pronounced substructures as the real structure. Interestingly, our best-fit N-body simulation does display real projected density variations linked to stellar epicyclic motions, but these become less significant when taking into account the SDSS star-count constraints. The substructures found when applying our matched filter technique to the N-body particles converted into observable stars are thus mostly unrelated to these epicyclic motions. This analysis suggests that the majority of the previously-detected substructures along the tidal tail of Palomar 5 are artefacts of observational inhomogeneities.

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arXiv2015

A Detection of Gas Associated with the M 31 Stellar Stream

Detailed studies of stellar populations in the halos of the Milky Way and the Andromeda (M 31) galaxies have shown increasing numbers of tidal streams and dwarf galaxies, attesting to a complicated and on-going process of hierarchical structure formation. The most prominent feature in the halo of M 31 is the Giant Stellar Stream, a structure ~4.5 degrees in extent along the sky, which is close to, but not coincident with the galaxy's minor axis. The stars that make up this stream are kinematically and chemically distinct from the other stars in the halo. Here, we present HST/COS high-resolution ultraviolet absorption spectra of three Active Galactic Nuclei sight lines which probe the M 31 halo, including one that samples gas in the main southwestern portion of the Giant Stream. We see two clear absorption components in many metal species at velocities typical of the M 31 halo and a third, blue-shifted component which arises in the stream. Photoionization modeling of the column density ratios in the different components shows gas in an ionization state typical of that seen in other galaxy halo environments and suggests solar to slightly super-solar metallicity, consistent with previous findings from stellar spectroscopy.

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arXiv2015

Comment on "Evidence for dark matter in the inner Milky Way"

This is a brief rebuttal to arXiv:1502.03821, which claims to provide the first observational proof of dark matter interior to the solar circle. We point out that this result is not new, and can be traced back at least a quarter century.

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arXiv2015

The intrinsic ellipticity of dwarf spheroidal galaxies: constraints from the Andromeda system

We present a study of the intrinsic deprojected ellipticity distribution of the satellite dwarf galaxies of the Andromeda galaxy, assuming that their visible components have a prolate shape, which is a natural outcome of simulations. Different possibilities for the orientation of the major axis of the prolate dwarf galaxies are tested, pointing either as close as possible to the radial direction towards the centre of Andromeda, or tangential to the radial direction, or with a random angle in the plane that contains the major axis and the observer. We find that the mean intrinsic axis ratio is ~ 1/2, with small differences depending on the assumed orientation of the population. Our deprojections also suggest that a significant fraction of the satellites, ~ 10%, are tidally disrupted remnants. We find that there is no evidence of any obvious difference in the morphology and major axis orientation between satellites that belong to the vast thin plane of co-rotating galaxies around Andromeda and those that do not belong to this structure.

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