arXiv subjects

Ruben Sanchez-Janssen

Publications and source records attributed to Ruben Sanchez-Janssen.

At least 19 recordsLinked to original sources

arXiv2026

The Next Generation Virgo Cluster Survey (NGVS). II. A Catalog of Galaxies in the Virgo Cluster

The Next Generation Virgo Cluster Survey (NGVS) is a deep, high resolution imaging campaign that used the 1 deg$^2$ MegaCam instrument on the Canada-France-Hawaii Telescope to carry out a comprehensive optical survey of the Virgo cluster, from its core to its virial radius. The NGVS covers a contiguous area of 104 deg$^2$ (8.63 Mpc$^2$ at the 16.5 Mpc distance of Virgo) in the $u^*$-,$g$-,$i$-, and $z$-band, with additional limited coverage in $r$. In this paper, we present the final catalog of Virgo galaxies across the entire NGVS area. The catalog includes 3680 galaxies considered to be $bona~fide$ members of the cluster, spanning a factor of 2.5 million in luminosity, from $g = 8.42$ mag to $g = 24.41$ mag ($M_g = -22.67$ mag to $M_g = -6.68$ mag). With 2100 previously uncataloged galaxies, the NGVS catalog augments the number of known Virgo members by a factor 2.3. The catalog is complete down to $g = 18.6$ mag ($M_g=-12.5$ mag, corresponding to a stellar mass $M_* \sim 1.6\times10^7~M_{\odot}$ for an old stellar population) and 50% complete at $g = 22.0$ mag ($M_g=-9.1$ mag, $M_* \sim 6.2\times10^5~M_{\odot}$), three magnitudes deeper than the venerable Virgo Cluster Catalog (VCC), which for over 40 years has served as the reference standard for Virgo. Photometric and structural parameters are derived for all NGVS galaxies and presented in a series of tables, alongside nuclear and morphological classification, as well as stellar masses and, when available, radial velocities.

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arXiv2026

The primordial nature of the C-19 stellar stream

Stellar streams, remnants of compact star systems stretched out by the tidal forces of the Milky Way, offer a unique way to study stellar populations that formed billions of years ago. A particularly unique stream is C-19, the most metal-poor stellar stream known at less than a thousandth of the Sun's metallicity. The nature of C-19 is not yet clear, with properties that resemble both star clusters and ultra faint dwarf galaxies, yet in either case its extremely low metallicity indicates very early star formation, <1 Gyr after the Big Bang. Here, we present the first detailed study on the nature of C-19 based on the chemical abundances of 14 member stars from high-resolution spectroscopy. These reveal that C-19 formed stars in an early, rapid, and prolific star formation event, with mild inhomogeneous mixing of elements produced in massive stars. There is otherwise no evidence for subsequent star formation, multiple stellar populations, nor chemical evolution. Although C-19 is currently disrupted in the Milky Way halo, it offers a rare and complementary window into the details of star formation and chemical evolution in the early universe, ideal for comparisons with current studies of primordial star formation in the high-redshift universe.

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arXiv2026

JWST/MIRI-MRS view of the metal-poor galaxy CGCG 007-025: the spatial location of PAHs and very highly ionized gas

Polycyclic Aromatic Hydrocarbons (PAHs) are key diagnostics of the physical conditions in the interstellar medium and are widely used to trace star formation in the mid-infrared (mid-IR). The relative strengths of mid-IR PAH emission features (e.g., 6.2, 7.7, 11.3 um) are sensitive to both the size and ionization state of the molecules and can be strongly influenced by the local radiation field. However, at low metallicities ( Z < 0.2 Zsun), detecting PAHs remains notoriously difficult, likely reflecting a combination of suppressed formation and enhanced destruction mechanisms. We present new JWST/MIRI MRS observations of the metal-poor (Z = 0.1 Zsun) dwarf galaxy CGCG 007-025. We confirm the tentative PAH detection previously reported from Spitzer data and, for the first time, identify a compact (approx. 50 pc) PAH-emitting region nearly co-spatial with the newly detected [NeV](I.P. = 97 eV) emission and the galaxy's most metal-poor, strongly star-forming region. The 11.3 and 12.7 um PAH features are detected, while no emission is found from the other typically brighter features, suggesting a PAH population dominated by large, neutral molecules resilient to hard ionizing fields. When compared with models, mid-IR line ratios involving [NeIII], [OIV], and [NeV] can only be reproduced by a combination of star formation and AGN ionization, with the latter contributing 4--8%. The [OIV] and [NeV] luminosities exceed what massive stars or shocks can produce, highlighting a puzzling scenario in line with recent JWST observations of similar galaxies. This work provides a crucial reference for studying the physical conditions of the dust and star formation in low-metallicity starburst regions, environments typical of the early universe.

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arXiv2025

The WEAVE-TwiLight-Survey: Expanding WEAVE's Reach to Bright and Low-Surface-Density Targets with a Novel Observing Mode

Current-day multi-object spectroscopic surveys are often limited in their ability to observe bright stars due to their low surface densities, resulting in increased observational overheads and reduced efficiency. Addressing this, we have developed a novel observing mode for WEAVE (William Herschel Telescope Enhanced Area Velocity Explorer) that enables efficient observations of low-surface-density target fields without incurring additional overheads from calibration exposures. As a pilot for the new mode, we introduce the WEAVE-TwiLight-Survey (WTLS), focusing on bright exoplanet-host stars and their immediate surroundings on the sky. High observational efficiency is achieved by superimposing multiple low-target-density fields and allocating the optical fibres in this configuration. We use a heuristic method to define fields relative to a central guide star, which serves as a reference for their superposition. Suitable guide fibres for each merged configuration are selected using a custom algorithm. Test observations have been carried out, demonstrating the feasibility of the new observing mode. We show that merged field configurations can be observed with WEAVE using the proposed method. The approach minimizes calibration times and opens twilight hours to WEAVE's operational schedule. WTLS is built upon the new observing mode and sourced from the ESA PLATO long-duration-phase fields. This survey will result in a homogeneous catalogue of approximately 6300 bright stars, including 62 known planet hosts, laying the groundwork for future elemental abundance studies tracing chemical patterns of planetary formation. This new observing mode (WEAVE-Tumble-Less) expands WEAVE's capabilities to rarely used on-sky time and low-density field configurations without sacrificing efficiency.

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arXiv2025

Galaxies OBserved as Low-luminosity Identified Nebulae (GOBLIN): a catalog of 43,000 high-probability dwarf galaxy candidates in the UNIONS survey

The detection of low surface brightness galaxies beyond the Local Group poses significant observational challenges, yet these faint systems are fundamental to our understanding of dark matter, hierarchical galaxy formation, and cosmic structure. Their abundance and distribution provide crucial tests for cosmological models, particularly regarding the small-scale predictions of $Λ$CDM. We present a systematic detection framework for dwarf galaxy candidates in Ultraviolet Near Infrared Optical Northern Survey (UNIONS) data covering 4,861 deg$^{2}$. Our pipeline preprocesses UNIONS gri-band data through binning, artifact removal, and stellar masking, then employs MTObjects (MTO) for low surface brightness detection. After parameter cuts and cross-matching, we obtain $\sim$360 candidates per deg$^{2}$, totaling $\sim$1.5 million candidates forming our GOBLIN (Galaxies OBserved as Low-luminosity Identified Nebulae) catalog. We fine-tuned the deep learning model Zoobot, pre-trained on Galaxy Zoo labels, for classification. Training data came from visual inspection of literature candidates with probability labels from expert assessments, capturing consensus and uncertainty. Applied to all MTO objects, our method identifies 42,965 dwarf candidates with probability $>$ 0.8, including 23,072 with probability $>$ 0.9. High-probability candidates correlate spatially with massive galaxies (log$(M_{*}/M_{\odot}) \geq$ 10) within 120 Mpc. While some of these objects may have been previously identified in other surveys, we present this extensive catalog of candidates, including their positions, structural parameter estimates, and classification probabilities, as a resource for the community to enable studies of galaxy formation, evolution, and the distribution of dwarf galaxies in different environments.

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arXiv2024

Dwarf Galaxies in the MATLAS Survey: Hubble Space Telescope Observations of the Globular Cluster Systems of 74 Ultra Diffuse Galaxies

Ultra diffuse galaxies, characterized by their low surface brightness and large physical size, constitute a subclass of dwarf galaxies that challenge our current understanding of galaxy formation and evolution. In this paper, we probe the properties of 74 UDGs, identified in the MATLAS survey, based on a comprehensive study of their globular cluster (GC) populations. We obtained high resolution HST imaging of these galaxies using the ACS F606W and F814W filters, allowing us to select GCs based on color and concentration index. After background subtraction and completeness correction, we calculate an overall total of 387 GCs. The number of GCs per galaxy ranges from 0 to 38, with the majority (64%) having low counts (0-2 GCs). On average, the more massive UDGs host a larger number of GCs. We find that our UDGs have specific frequencies (S_N) ranging from 0 to 91, with a small population (9%) with S_N > 30. The median S_N of our sample is similar to the one for the Perseus cluster UDGs, despite the fact that our UDGs are found in lower density environments. The S_N measurements for individual galaxies can extend beyond those found in Perseus, but remain below the values found for UDGs in the Virgo and Coma cluster. Based on a trending analysis of the S_N values with the host galaxy properties, we find trends with host galaxy size, roundness, color, and local density. For the UDGs with sufficiently high statistics, we study 2D density maps of the GC distributions, which show a variety of appearances: symmetric, asymmetric, off-center, and elongated. The UDGs with disturbed density maps also show disturbed stellar light morphologies. We further quantify the distribution by modeling it with a Sersic profile, finding R_{e,GC}/R_{e,gal} ~ 1.0, which indicates that the GCs follow the stellar light of the host galaxy.

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arXiv2024

WST -- Widefield Spectroscopic Telescope: Motivation, science drivers and top-level requirements for a new dedicated facility

In this paper, we describe the wide-field spectroscopic survey telescope (WST) project. WST is a 12-metre wide-field spectroscopic survey telescope with simultaneous operation of a large field-of-view (3 sq. degree), high-multiplex (20,000) multi-object spectrograph (MOS), with both a low and high-resolution modes, and a giant 3x3 arcmin2 integral field spectrograph (IFS). In scientific capability, these specifications place WST far ahead of existing and planned facilities. In only 5 years of operation, the MOS would target 250 million galaxies and 25 million stars at low spectral resolution, plus 2 million stars at high resolution. Without need for pre-imaged targets, the IFS would deliver 4 billion spectra offering many serendipitous discoveries. Given the current investment in deep imaging surveys and noting the diagnostic power of spectroscopy, WST will fill a crucial gap in astronomical capability and work in synergy with future ground and space-based facilities. We show how it can address outstanding scientific questions in the areas of cosmology; galaxy assembly, evolution, and enrichment, including our own Milky Way; the origin of stars and planets; and time domain and multi-messenger astrophysics. WST's uniquely rich dataset may yield unforeseen discoveries in many of these areas. The telescope and instruments are designed as an integrated system and will mostly use existing technology, with the aim to minimise the carbon footprint and environmental impact. We will propose WST as the next European Southern Observatory (ESO) project after completion of the 39-metre ELT.

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arXiv2021

HI observations of the MATLAS dwarf and ultra-diffuse galaxies

The presence of HI gas in galaxies is inextricably linked to their morphology and evolution. This paper aims to understand the HI content of the already identified 2210 dwarfs located in the low-to-moderate density environments of the MATLAS deep imaging survey. We combine the HI observations from the ATLAS$^{3D}$ survey, with the extragalactic HI sources from the ALFALFA survey, to extract the HI line width, velocity and mass of the MATLAS dwarfs. From the 1773 dwarfs in our sample with available HI observations, 8% (145) have an HI line detection. The majority of the dwarfs show irregular morphology, while 29% (42) are ellipticals, the largest sample of HI-bearing dwarf ellipticals (dEs) to date. Of the HI dwarf sample, 2% (3) are ultra-diffuse galaxies (UDGs), 12% have a transition-type morphology, 5% are tidal dwarf candidates, and 10% appear to be disrupted objects. In our optically selected sample, 9.5% of the dEs, 7% of the UDGs and 10% of the classical dwarfs are HI-bearing. The HI-bearing dwarfs have on average bluer colors than the dwarfs without detected HI. We find relations between the stellar and HI masses, gas fraction, color and absolute magnitude consistent with previous studies of dwarfs probing similar masses and environments. For 79% of the dwarfs identified as satellites of massive early-type galaxies, we find that the HI mass increases with the projected distance to the host. Using the HI line width, we estimate dynamical masses and find that 5% (7) of the dwarfs are dark matter deficient.

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arXiv2021

Ultra diffuse galaxies in the MATLAS low-to-moderate density fields

Recent advances in deep dedicated imaging surveys over the past decade have uncovered a surprisingly large number of extremely faint low surface brightness galaxies with large physical sizes called ultra diffuse galaxies (UDGs) in clusters and, more recently, in lower density environments. As part of the MATLAS survey, a deep imaging large program at the Canada-France-Hawaii Telescope (CFHT), our team has identified 2210 dwarf galaxies, 59 (~3%) of which qualify as UDGs. Averaging over the survey area, we find ~0.4 UDG per square degree. They are found in a range of low to moderate density environments, although 61% of the sample fall within the virial radii of groups. Based on a detailed analysis of their photometric and structural properties, we find that the MATLAS UDGs do not show significant differences from the traditional dwarfs, except from the predefined size and surface brightness cut. Their median color is as red as the one measured in galaxy clusters, albeit with a narrower color range. The majority of the UDGs are visually classified as dwarf ellipticals with log stellar masses of ~6.5-8.7. The fraction of nucleated UDGs (~34%) is roughly the same as the nucleated fraction of the traditional dwarfs. Only five (~8%) UDGs show signs of tidal disruption and only two are tidal dwarf galaxy candidates. A study of globular cluster (GC) candidates selected in the CFHT images finds no evidence of a higher GC specific frequency S_N for UDGs than for classical dwarfs, contrary to what is found in most clusters. The UDG halo-to-stellar mass ratio distribution, as estimated from the GC counts, peaks at roughly the same value as for the traditional dwarfs, but spans the smaller range of ~10-2000. We interpret these results to mean that the large majority of the field-to-group UDGs do not have a different formation scenario than traditional dwarfs.

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arXiv2021

Structure and morphology of the MATLAS dwarf galaxies and their central nuclei

We present a photometric study of the dwarf galaxy population in the low to moderate density environments of the MATLAS (Mass Assembly of early-Type gaLAxies with their fine Structures) deep imaging survey. The sample consists of 2210 dwarfs, including 508 nucleated. We define a nucleus as a compact source that is close to the galaxy photocentre (within 0.5 $R_e$) which is also the brightest such source within the galaxy's effective radius. The morphological analysis is performed using a 2D surface brightness profile modelling on the g-band images of both the galaxies and nuclei. Our study reveals that, for similar luminosities, the MATLAS dwarfs show ranges in the distribution of structural properties comparable to cluster (Virgo and Fornax) dwarfs and a range of sizes comparable to the Local Group and Local Volume dwarfs. Colour measurements using the r- and i-band images indicate that the dwarfs in low and moderate density environments are as red as cluster dwarfs on average. The observed similarities between dwarf ellipticals in vastly different environments imply that dEs are not uniquely the product of morphological transformation due to ram-pressure stripping and galaxy harassment in high density environments. We measure that the dwarf nuclei are located predominantly in massive, bright and round dwarfs and observe fewer nuclei in dwarfs with a faint centre and a small size. The colour of the galaxy nucleus shows no clear relation to the colour of the dwarf, in agreement with the migration and wet migration nucleus formation scenarios. The catalogues of the MATLAS dwarfs photometric and structural properties are provided.

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arXiv2020

The Strikingly Metal-Rich Halo of the Sombrero Galaxy

The nature of the Sombrero galaxy (M 104 = NGC 4594) has remained elusive despite many observational studies at a variety of wavelengths. Here we present Hubble Space Telescope imaging of two fields at $\sim$16 and 33 kpc along the minor axis to examine stellar metallicity gradients in the extended spheroid. We use this imaging, extending more than 2 mag below the tip of the red giant branch (TRGB), in combination with artificial star tests to forward model observed color-magnitude diagrams (CMDs), measuring metallicity distribution functions (MDFs) at different radii along the minor axis. An important and unexpected result is that the halo of the Sombrero is strikingly metal-rich: even the outer field, located at $\sim$17 effective radii of the bulge, has a median metallicity [Z/H]$\sim$-0.15 and the fraction of stars with [Z/H]<-1.0 is negligible. This is unprecedented among massive galaxy halos studied to date, even among giant ellipticals. We find significant radial metallicity gradients, characterized by an increase in the fraction of metal-poor stars with radius and a gradient in median metallicity of $\sim$-0.01 dex/kpc. The density profile is well fit by power laws with slopes that exhibit a dependence on metallicity, with flatter slopes for more metal-poor stars. We discuss our results in the context of recent stellar MDF studies of other nearby galaxies and potential formation scenarios for the Sombrero galaxy.

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arXiv2019

The Pristine survey X: a large population of low-metallicity stars permeates the Galactic disk

The orbits of the least chemically enriched stars open a window on the formation of our Galaxy when it was still in its infancy. The common picture is that these low-metallicity stars are distributed as an isotropic, pressure-supported component since these stars were either accreted from the early building blocks of the assembling Milky Way, or were later brought by the accretion of faint dwarf galaxies. Combining the metallicities and radial velocities from the Pristine and LAMOST surveys and Gaia DR2 parallaxes and proper motions for an unprecedented large and unbiased sample of very metal-poor stars at $[Fe/H]\leq-2.5$ we show that this picture is incomplete. This sample shows strong statistical evidence (at the $5.0σ$ level) of asymmetry in their kinematics, favouring prograde motion. Moreover, we find that $31\%$ of the stars that currently reside in the disk do not venture outside of the disk plane throughout their orbit. The discovery of this population implies that a significant fraction of stars with iron abundances $[Fe/H]\leq-2.5$ formed within or concurrently with the Milky Way disk and that the history of the disk was quiet enough to allow them to retain their disk-like orbital properties.

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arXiv2019

The Pristine survey -- VII. A cleaner view of the Galactic outer halo using blue horizontal branch stars

We use the Pristine survey CaHK narrow-band photometry, combined with the SDSS ugr photometry, to provide a cleaner sample of blue horizontal branch stars in the Galactic halo out to large distances. We demonstrate a completeness of 91% and a purity of 93% with respect to available spectroscopic classifications. We subsequently use our new clean sample of these standard candles to investigate the substructure in the Galactic halo over the Pristine footprint. Among other features, this allows for a careful tracing of multiple parts of the Sagittarius stream, providing a measurement independent from other tracers used and reaching larger distances. Moreover, we demonstrate with this clean and complete sample that the halo follows a density profile with a negative power-law slope of 3.5 - 4.0. As the relatively shallow SDSS u-band is the limiting factor in this technique, we foresee large potential for combining Pristine survey photometry with the much deeper u-band photometry from the Canada-France-Imaging Survey.

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arXiv2019

The Next Generation Fornax Survey (NGFS): VI. The Alignment of Dwarf Galaxies in The Fornax Cluster

Using the photometric data from the Next Generation Fornax Survey, we find a significant radial alignment signal among the Fornax dwarf galaxies. For the first time, we report that the radial alignment signal of nucleated dwarfs is stronger than that of non-nucleated ones at 2.4$σ$ confidence level, and the dwarfs located in the outer region ($R>R_{\rm{vir}}/3$; $R_{\rm{vir}}$ is the Fornax virial radius) show slightly stronger radial alignment signal than those in the inner region ($R<R_{\rm{vir}}/3$) at $1.5σ$ level. We also find that the significance of radial alignment signal is independent of the luminosities or sizes of the dwarfs.

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arXiv2019

The ultra-diffuse galaxy NGC 1052-DF2 with MUSE: I. Kinematics of the stellar body

The so-called ultra-diffuse galaxy NGC~1052-DF2 was announced to be a galaxy lacking dark matter based on a spectroscopic study of its constituent globular clusters. Here we present the first spectroscopic analysis of the stellar body of this galaxy using the MUSE integral-field spectrograph at the (ESO) Very Large Telescope. The MUSE datacube simultaneously provides DF2's stellar velocity field and systemic velocities for seven globular clusters (GCs). We further discovered three planetary nebulae (PNe) that are likely part of this galaxy. While five of the clusters had velocities measured in the literature, we were able to confirm the membership of two more candidates through precise radial velocity measurements, which increases the measured specific frequency of GCs in DF2. The mean velocity of the diffuse stellar body, 1792.9$^{-1.8}_{+1.4}$~\kms, is consistent with the mean globular cluster velocity. We detect a weak but significant velocity gradient within the stellar body, with a kinematic axis close to the photometric major-axis, making it a prolate-like rotator. We estimate a velocity dispersion from the clusters and PNe of $σ_{\mathrm{int}}=10.6^{+3.9}_{-2.3}$~\kms. The velocity dispersion $σ_{\rm{DF2}\star}$(\re) for the stellar body within one effective radius is $10.8^{-4.0}_{+3.2}$~\kms. Considering various sources of systemic uncertainties this central value varies between 5 and 13~\kms, and we conservatively report a 95\% confidence upper limit to the dispersion within one \re\ of 21~\kms. We provide updated mass estimates based on these dispersions corresponding to the different distances to NGC~1052-DF2 that have been reported in the recent literature.

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arXiv2019

A close look at the dwarf AGN of NGC 4395: optical and near-IR integral field spectroscopy

Intermediate mass black holes (10$^3$-10$^5$ M$_\odot$) in the center of dwarf galaxies are believed to be analogous to growing Active Galactic Nuclei (AGN) in the early Universe. Their characterization can provide insight about the early galaxies. We present optical and near-infrared integral field spectroscopy of the inner $\sim$50 pc of the dwarf galaxy NGC4395, known to harbor an AGN. NGC 4395 is an ideal candidate to investigate the nature of dwarf AGN, as it is nearby ($d\approx4.4$ Mpc) enough to allow a close look at its nucleus. The optical data were obtained with the Gemini GMOS-IFU covering the 4500 A to 7300 A spectral range at a spatial resolution of 10 pc. The J and K-band spectra were obtained with the Gemini NIFS at spatial resolutions of $\sim$5 pc. The gas kinematics show a compact, rotation disk component with a projected velocity amplitude of 25 km s$^{-1}$. We estimate a mass of $7.7\times10^5$ M$_\odot$ inside a radius of 10 pc. From the H$α$ broad line component, we estimate the AGN bolometric luminosity as $L_{ bol}=(9.9\pm1.4)\times10^{40}$ erg s$^{-1}$ and a mass $M_{ BH}=(2.5^{+1.0}_{-0.8})\times10^5$ M$_\odot$ for the central black hole. The mean surface mass densities for the ionized and molecular gas are in the ranges (1-2) M$_{\odot} $pc$^{-2}$ and (1-4)$\times10^{-3}$ M${_\odot}$ pc$^{-2}$ and the average ratio between ionized and hot molecular gas masses is $\sim$500. The emission-line flux distributions reveal an elongated structure at 24 pc west of the nucleus, which is blueshifted relative to the systemic velocity of the galaxy by $\approx$30 km s$^{-1}$. We speculate that this structure is originated by the accretion of a gas-rich small satellite or by a low metallicity cosmic cloud.

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arXiv2019

The Pristine Dwarf-Galaxy survey - II. In-depth observational study of the faint Milky Way satellite Sagittarius II

We present an extensive study of the Sagittarius II (Sgr II) stellar system using MegaCam g and i photometry, narrow-band, metallicity-sensitive Calcium H&K doublet photometry, augmented with Keck II/DEIMOS multi-object spectroscopy. We are able to derive and refine the Sgr II structural and stellar properties: the colour-magnitude diagram implies Sgr II is old (12.0 +- 0.5 Gyr) and metal-poor. The CaHK photometry confirms the metal-poor nature of the satellite ([Fe/H]CaHK = -2.32 +- 0.04 dex) and suggests that Sgr II hosts more than one single stellar population (σ CaHK = 0.11+0.05-0.03 dex). From the deep spectroscopic data, the velocity dispersion of the system is found to be σvr = 2.7+1.3-1.0 km s-1 after excluding two potential binary stars. Using the Ca infrared triplet measured from our highest signal-to-noise spectra, we are able to confirm the metallicity and dispersion inferred from the Pristine photometric metallicities: ([Fe/H] = -2.23 +- 0.05 dex, σspectro = 0.10+0.06-0.04 dex). Sgr II's metallicity and absolute magnitude (MV = -5.7 +- 0.1 mag) place the system on the luminosity-metallicity relation of the Milky Way dwarf galaxies despite its small size. The low, but resolved metallicity and velocity dispersions paint the picture of a slightly dark matter-dominated satellite. Furthermore, using the Gaia Data Release 2, we constrain the orbit of the satellite and find an apocenter of 118.4+28.4-23.7 kpc and a pericenter of 54.8+3.3-6.1 kpc. The orbit of Sgr II is consistent with the trailing arm of the Sgr stream and indicates that it is possibly a satellite of the Sgr dSph that was tidally stripped from the dwarf's influence.

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arXiv2018

The Pristine Survey IV: Approaching the Galactic metallicity floor with the discovery of an ultra metal-poor star

The early Universe presented a star formation environment that was almost devoid of heavy elements. The lowest metallicity stars thus provide a unique window into the earliest Galactic stages, but are exceedingly rare and difficult to find. Here we present the discovery of an ultra-metal-poor star, Pristine_221.8781+9.7844, using narrow-band Ca H&K photometry from the Pristine survey. Follow-up medium and high-resolution spectroscopy confirms the ultra-metal-poor nature of Pristine_221.8781+9.7844 ([Fe/H] = -4.66 +/- 0.13 in 1D LTE) with an enhancement of 0.3-0.4 dex in alpha-elements relative to Fe, and an unusually low carbon abundance. We derive an upper limit of A(C) = 5.6, well below typical A(C) values for such ultra metal-poor stars. This makes Pristine_221.8781+9.7844 one of the most metal-poor stars; in fact, it is very similar to the most metal-poor star known (SDSS J102915+172927). The existence of a class of ultra metal-poor stars with low(er) carbon abundances suggest that there must have been several formation channels in the early Universe through which long-lived, low-mass stars were formed.

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