arXiv subjects

S. Taibi

Publications and source records attributed to S. Taibi.

14 recordsLinked to original sources

arXiv2026

The Pristine Dwarf Galaxy Survey -- VII. The metallicity distributions of 12 Milky Way faint satellites

Spectroscopic studies of ultra-faint dwarf galaxies are typically limited to small samples of stars due to the scarcity of sufficiently bright targets. The small number statistics and possible presence of contaminants still hamper solid determinations of their metallicity distribution function. In this work, we characterise the metallicity distributions of 12 Milky Way faint satellites by exploiting deep narrow-band CaHK photometry from the Pristine dwarf galaxy survey. In order to derive accurate stellar photometric metallicities, we combined it with deep broad-band g and r photometry from Mu\~noz et al. (2018) and Pan-STARRS1, covering each system out to 5-8 times their half-light radius Rh, and reaching magnitudes as faint as g~23. Membership probabilities were determined incorporating the available spatial, photometric, astrometric, and spectroscopic information, further refined using the derived photometric metallicities. We identified 3917 probable member stars across the 12 systems, more than doubling the numbers recovered by previous spectroscopic studies. We deliver complete metallicity distributions that yield robust average metallicities and dispersions previously inaccessible for most of the systems examined in this study. We identify 170 candidate extremely metal-poor stars distributed across all systems, and confirm a departure from the linear luminosity-metallicity relation in the ultra-faint regime, with systems scattering around [Fe/H]~-2.3 dex. Given the extensive mass coverage of our sample, we were able to investigate the presence of metallicity gradients, finding clear evidence of radial variations in massive systems, but none in the ultra-faint dwarfs within 2.5xRh. The photometric strategy presented in this paper will continue to serve as an effective complement to future spectroscopic surveys.

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arXiv2025

An LBT view of the co-rotating group of galaxies around NGC 2750: Deep imaging and new satellite candidates

Some galaxies such as the Milky Way and Andromeda display coherently rotating satellite planes, posing tensions with cosmological simulations. NGC 2750 has emerged as an additional candidate system hosting a co-rotating group of galaxies. We aim to assess the presence of a coherent satellite plane around NGC 2750 by identifying new candidate dwarf galaxies and low surface brightness features. We conducted deep, wide-field photometric observations of NGC 2750 using the Large Binocular Telescope in the g- and r-bands. Standard data reduction techniques were applied to enhance the detection of low-surface-brightness features down to about 31 mag/arcsec^2 in r. Our observations led to the discovery of six new candidate dwarf galaxies, including one with properties consistent with an ultra-diffuse galaxy. We also identified tidal features around NGC 2750, indicating past interactions with nearby satellites. The spatial distribution of satellites suggests a moderate flattening, further supported by the newly identified candidates. Follow-up spectroscopic measurements will be critical in confirming or challenging the strong kinematic coherence observed previously. The luminosity function of NGC 2750 reveals an excess of bright satellites compared to similar systems, adding to the growing tension between observed satellite populations and cosmological simulations.

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arXiv2024

From gas to stars: MUSEings on the internal evolution of IC 1613

The kinematics and chemical composition of stellar populations of different ages provide crucial information about the evolution of a galaxy. We aim to provide such information for IC 1613, an isolated, gas-rich, star-forming dwarf galaxy in the Local Group. We present here the results of a new spectroscopic study performed with MUSE, an integral-field spectrograph on the Very Large Telescope. We extracted from the data cubes more than 2000 sources from which we separated stellar objects for further spectroscopic analysis. The quality of the data set allowed us to obtain accurate classifications and line-of-sight velocities for about 800 stars. Our sample includes not only Red Giant Branch (RGB) and Main Sequence (MS) stars, but also a number of probable Be and C stars. We also obtained reliable metallicities for about 300 RGB stars. The kinematic analysis revealed for the first time the presence of stellar rotation with high significance. We found general agreement with the rotation velocity of the neutral gas component. Examining the kinematics of stars as a function of broad age ranges, we find that the velocity dispersion increases as a function of age, with the behaviour being very clear in the outermost pointings, while the rotation-to-velocity dispersion support decreases. On timescales shorter than a Gyr, the stellar kinematics still follow very closely that of the neutral gas, while the two components decouple on longer timescales. The chemical analysis of the RGB stars revealed average properties comparable to other Local Group dwarf galaxies. We also provide a new estimation of the inclination angle using only independent stellar tracers. Our work provides the largest spectroscopic sample of an isolated LG dwarf galaxy. The results obtained seem to support the scenario in which the stars of a dwarf galaxy are born from a less turbulent gas over time.

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arXiv2023

A portrait of the Vast Polar Structure as a young phenomenon: hints from its member satellites

It has been observed that several Milky Way (MW) satellite dwarf galaxies are distributed along a coherent planar distribution known as the Vast Polar Structure (VPOS). Here we investigate whether MW satellites located on the VPOS have different physical and orbital properties from those not associated with it. Using the proper motion measurements of the MW satellites from the \textit{Gaia} mission and literature values for their observational parameters, we first discriminate between systems that may or may not be associated with the VPOS, and then compare their chemical and dynamical properties. Comparing the luminosity distributions of the on-plane and off-plane samples, we find an excess of bright satellites observed on the VPOS. Despite this luminosity gap, we do not observe a significant preference for on-plane and off-plane systems to follow different scaling relations. The on-plane systems also show a striking pattern in their radial velocities and orbital phases: co-orbiting satellites are almost all approaching their pericenters, while both counter-orbiting satellites are leaving their last pericenters. This contrasts with the more random distribution of the off-plane sample. The on-plane systems also tend to have the lowest orbital energies for a given value of angular momentum. These results are robust to the assumed MW potential, even in the case of a potential perturbed by the arrival of a massive LMC. Considering them a significant property of the VPOS, we explore several scenarios, all related to the late accretion of satellite systems, which interpret the VPOS as a young structure. We hypothesise that the VPOS formed as a result of the accretion of a group of dwarf galaxies. More accurate proper motions and dedicated studies in the context of cosmological simulations are needed to confirm this scenario.

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arXiv2022

Metallicity profiles of Ultra Diffuse Galaxies in NIHAO simulations

Supernovae feedback driven expansion has proven to be a viable mechanism to explain the average properties of Ultra Diffuse Galaxies (UDGs) such as the sizes, colors, mass and internal kinematics. Here, we explore the origin of stellar metallicity gradients in feedback driven simulated UDGs from the NIHAO project and compare them with the observed distribution of metallicity gradients of both Local Group dwarfs as well as of the recently observed UDG DF44. Simulated UDGs display a large variety of metallicity profiles, showing flat to negative gradients, similarly to what is observed in LG dwarfs, while DF44 data suggest a flat to positive gradient. The variety of metallicity gradients in simulations is set by the interplay between the radius at which star formation occurs and the subsequent supernovae feedback driven stellar redistribution: rotation supported systems tend to have flat metallicity profiles while dispersion supported galaxies show negative and steep profiles. Our results suggest that UDGs are not peculiar in what regards their metallicity gradients, when compared to regular dwarfs. Desirably, a larger observational sample of UDGs' gradients shall be available in the future, in order to test our predictions.

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arXiv2022

The stellar metallicity gradients of Local Group dwarf galaxies

Through a homogeneous analysis of spectroscopic literature data of red giant stars, we determine the radial metallicity profiles of 30 dwarf galaxies in the Local Group. We explore correlations between the calculated metallicity gradients and stellar mass, star formation history and environment, delivering the largest compilation to date of this type. The dwarf galaxies in our sample mostly show metallicity profiles decreasing with radius, with some exhibiting rather steep profiles. The derived metallicity gradients as a function of the half-light radius, $\nabla_{\rm [Fe/H]} (R/R_e)$, show no statistical differences when compared with the galaxies' morphological type, nor with their distance from the Milky Way or M31. No correlations are found with either stellar mass or star formation timescales. In particular, we do not find the linear relationship between $\nabla_{\rm [Fe/H]} (R/R_e)$ and the galaxies' median age $t_{50}$, as instead shown in the literature for a set of simulated systems. The presence of high angular momentum in some of our galaxies does not seem to have an impact on the gradient values. The strongest gradients in our sample are observed in systems that are likely to have experienced a past merger event. By excluding them, the analysed dwarf galaxies show mild gradients ($\sim -0.1$ dex $R_e^{-1}$) with little scatter between them, regardless of their stellar mass, dynamical state, and star formation history. These results are in good agreement with different sets of simulations presented in the literature and analysed using the same method as for the observed sample. The interplay between the multitude of factors that could drive the formation of metallicity gradients in dwarf galaxies likely combine in complex ways to produce in general comparable values.

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arXiv2021

Variable stars in Local Group Galaxies -- V. The fast and early evolution of the low-mass Eridanus II dSph galaxy

We present a detailed study of the variable star population of Eridanus II (Eri II), an ultra-faint dwarf galaxy that lies close to the Milky Way virial radius. We analyze multi-epoch $g,r,i$ ground-based data from Goodman and the Dark Energy Camera, plus $F475W, F606W, F814W$ space data from the Advanced Camera for Surveys. We report the detection of 67 RR Lyrae (RRL) stars and 2 Anomalous Cepheids, most of them new discoveries. With the RRL stars, we measure the distance modulus of Eri II, $\mu_0=22.84\pm 0.05$ mag (D$_{\odot}=370\pm9$ kpc) and derive a metallicity spread of 0.3 dex (0.2 dex intrinsic). The colour distribution of the horizontal branch (HB) and the period distribution of the RRL stars can be nicely reproduced by a combination of two stellar models of [Fe/H]=($-2.62$, $-2.14$). The overall low metallicity is consistent with the red giant branch bump location, 0.65 mag brighter than the HB. These results are in agreement with previous spectroscopic studies. The more metal-rich RRL and the RRab stars have greater central concentration than the more metal-poor RRL and the RRc stars that are mainly located outside $\sim 1$ r$_{\rm h}$. This is similar to what is found in larger dwarf galaxies such as Sculptor, and in agreement with an outside-in galaxy formation scenario. This is remarkable in such a faint dwarf galaxy with an apparently single and extremely short ($<1$ Gyr) star formation burst. Finally, we have derived new and independent structural parameters for Eri II and its star cluster using our new data that are in very good agreement with previous estimates.

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arXiv2021

Gaia early DR3 systemic motions of Local Group dwarf galaxies and orbital properties with a massive Large Magellanic Cloud

We perform a comprehensive determination of the systemic proper motions of 74 dwarf galaxies and dwarf galaxy candidates in the Local Group based on Gaia early data release 3. The outputs of the analysis for each galaxy, including probabilities of membership, will be made publicly available. The analysis is augmented by a determination of the orbital properties of galaxies within 500 kpc. We adopt the flexible Bayesian methodology presented by McConnachie \& Venn (2020), which takes into account the location of the stars on the sky, on the colour-magnitude diagram and on the proper motion plane. We apply some modifications, in particular to the way the colour-magnitude diagram and spectroscopic information are factored in, e.g. by including stars in several evolution phases. The bulk motions are integrated in three gravitational potentials: two where the Milky Way is treated in isolation and has a mass 0.9 \& 1.6 $\times 10^{12}$M$_{\odot}$ and the time-varying potential by Vasiliev et al. (2021), which includes the infall of a massive Large Magellanic Cloud (LMC). We are able to determine bulk proper motions for 73 systems, and we consider reliable 66 of these measurements. For the first time, systemic motions are presented for galaxies out to a distance of 1.4 Mpc, in the NGC~3109 association. The inclusion of the infall of a massive LMC significantly modifies the orbital trajectories of the objects, with respect to orbit integration in static Milky Way-only potentials, and leads to 6 galaxies being likely associated to the LMC, 3 possibly associated and 1 recently captured object. We discuss the results of the orbit integration in the context of the relation of the galaxies to the system of Milky Way satellites, implications for the too-big-to-fail problem, impact on star formation histories, and tidal disruption.

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arXiv2020

The Tucana dwarf spheroidal galaxy: not such a massive failure after all

Isolated Local Group (LG) dwarf galaxies have evolved most or all of their life unaffected by interactions with the large LG spirals and therefore offer the opportunity to learn about the intrinsic characteristics of this class of objects. Here we explore the internal kinematic and metallicity properties of one of the three isolated LG dwarf spheroidal galaxies, i.e. the Tucana dSph. This is an intriguing system, as it has been found in the literature to have an internal rotation of up to 16 km/s, a much higher velocity dispersion than other dSphs of similar luminosity, and a possible exception to the too-big-too-fail problem. We present results for a new VLT/FORS2 spectroscopic dataset in the CaII triplet region for 50 candidate red giant branch stars in the direction of Tucana, which yielded line-of-sight velocity and metallicity ([Fe/H]) measurements of 39 effective members. This doubles the number of Tucana's stars with such measurements. In addition, we re-reduce and include in our analysis the other two spectroscopic datasets presented in the literature, the VLT/FORS2 sample by Fraternali et al. (2009) and the VLT/FLAMES one by Gregory et al. (2019). We measure a systemic velocity of $180.0\pm1.3$ km/s, consistently across the various datasets analyzed, and find that a dispersion-only model is moderately favored over models accounting also for internal rotation. Our best estimate of the internal velocity dispersion is $6.2_{-1.3}^{+1.6}$ km/s, much smaller than the values reported in the literature and in line with similarly luminous dSphs; this is consistent with Tucana not being an exception to the too-big-to-fail problem, nor living in a dark matter halo much more massive than those of its siblings. As for the metallicity properties, we do not find anything unusual; there are hints of the presence of a [Fe/H] gradient but more data are needed to pin its presence down.

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arXiv2020

The mass of our Galaxy from satellite proper motions in the Gaia era

We use Gaia DR2 systemic proper motions of 45 satellite galaxies to constrain the mass of the Milky Way using the scale free mass estimator of Watkins et al. (2010). We first determine the anisotropy parameter $\beta$, and the tracer satellites' radial density index $\gamma$ to be $\beta$=$-0.67^{+0.45}_{-0.62}$ and $\gamma=2.11\pm0.23$. When we exclude possible former satellites of the Large Magellanic Cloud, the anisotropy changes to $\beta$=$-0.21^{+0.37}_{-0.51}$. We find that the index of the Milky Way's gravitational potential $\alpha$, which is dependent on the mass itself, is the parameter with the largest impact on the mass determination. Via comparison with cosmological simulations of Milky Way-like galaxies, we carried out a detailed analysis of the estimation of the observational uncertainties and their impact on the mass estimator. We found that the mass estimator is biased when applied naively to the satellites of simulated Milky Way halos. Correcting for this bias, we obtain for our Galaxy a mass of $0.58^{+0.15}_{-0.14}\times10^{12}$M$_\odot$ within 64 kpc, as computed from the inner half of our observational sample, and $1.43^{+0.35}_{-0.32}\times10^{12}$M$_\odot$ within 273 kpc, from the full sample; this latter value extrapolates to a virial mass of $M_\mathrm{vir\,\Delta=97}$=$1.51^{+0.45}_{-0.40} \times 10^{12}M_{\odot}$ corresponding to a virial radius of R$_\mathrm{vir}$=$308\pm29$ kpc. This value of the Milky Way mass lies in-between other mass estimates reported in the literature, from various different methods.

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arXiv2019

Kinematic and metallicity properties od the Aquarius dwarf galaxy from FORS2 MXU spectroscopy

Dwarf galaxies found in isolation in the Local Group (LG) are unlikely to have interacted with the large LG spirals, and therefore environmental effects should not be the main drivers of their evolution. We aim to provide insight into the internal mechanisms shaping LG dwarf galaxies by increasing our knowledge of the internal properties of isolated systems. We focus on the evolved stellar component of the Aquarius dwarf, whose kinematic and metallicity properties have only recently started to be explored. We have obtained spectroscopic data in the the near-infrared CaII triplet lines region with FORS2 at the Very Large Telescope for 53 red giant branch (RGB) stars, to derive line-of-sight velocities and [Fe/H] of the individual RGB stars. We have derived a systemic velocity of $-142.2^{+1.8}_{-1.8}$ km s$^{-1}$, in agreement with previous measurements from both the HI gas and stars. The internal kinematics of Aquarius appears to be best modelled by a combination of random motions (l.o.s. velocity dispersion of $10.3^{+1.6}_{-1.3}$ km s$^{-1}$) and linear rotation (with a gradient $-5.0^{+1.6}_{-1.9}$ km s$^{-1}$ arcmin$^{-1}$) along a P.A.=$139_{-27}^{+17}$ deg, consistent with the optical projected major axis. This rotation signal is significantly misaligned or even counter-rotating to that derived from the HI gas. We also find the tentative presence of a mild negative metallicity gradient and indications that the metal-rich stars have a colder velocity dispersion than the metal-poor ones. This work represents a significant improvement with respect to previous measurements of the RGB stars of Aquarius, as it doubles the number of member stars studied in the literature. We speculate that the misaligned rotation between the HI gas and evolved stellar component might have been the result of recent accretion of HI gas or re-accretion after gas-loss due to internal stellar feedback.

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arXiv2019

Star Cluster Catalogs for the LEGUS Dwarf Galaxies

We present the star cluster catalogs for 17 dwarf and irregular galaxies in the $HST$ Treasury Program "Legacy ExtraGalactic UV Survey" (LEGUS). Cluster identification and photometry in this subsample are similar to that of the entire LEGUS sample, but special methods were developed to provide robust catalogs with accurate fluxes due to low cluster statistics. The colors and ages are largely consistent for two widely used aperture corrections, but a significant fraction of the clusters are more compact than the average training cluster. However, the ensemble luminosity, mass, and age distributions are consistent suggesting that the systematics between the two methods are less than the random errors. When compared with the clusters from previous dwarf galaxy samples, we find that the LEGUS catalogs are more complete and provide more accurate total fluxes. Combining all clusters into a composite dwarf galaxy, we find that the luminosity and mass functions can be described by a power law with the canonical index of $-2$ independent of age and global SFR binning. The age distribution declines as a power law, with an index of $\approx-0.80\pm0.15$, independent of cluster mass and global SFR binning. This decline of clusters is dominated by cluster disruption since the combined star formation histories and integrated-light SFRs are both approximately constant over the last few hundred Myr. Finally, we find little evidence for an upper-mass cutoff ($<2\sigma$) in the composite cluster mass function, and can rule out a truncation mass below $\approx10^{4.5}$M$_{\odot}$ but cannot rule out the existence of a truncation at higher masses.

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arXiv2018

Stellar chemo-kinematics of the Cetus dwarf spheroidal galaxy

In order to minimize environmental effects and gain an insight into the internal mechanisms that shape the properties of the early-type dwarf systems, we study one of the few isolated dwarf spheroidal galaxies (dSphs) of the Local Group (LG): Cetus. We obtained VLT/FORS2 spectra ($R\sim2600$) in the region of the nIR CaII triplet lines for 80 candidate red giant branch stars. The analysis yielded line-of-sight velocities and metallicities ([Fe/H]) for 54 bona fide member stars. The kinematic analysis shows that Cetus is a mainly pressure-supported ($\sigma_v = 11.0_{-1.3}^{+1.6}$ km/s), dark-matter-dominated system ($M_{1/2}/L_V = 23.9_{-8.9}^{+9.7} M_\odot/L_\odot$) with no significant signs of internal rotation. We find Cetus to be a metal-poor system with a significant [Fe/H] spread (median [Fe/H] = -1.71 dex, median-absolute-deviation = 0.49 dex), as expected for its stellar mass. We report the presence of a mild metallicity gradient compatible with those found in other dSphs of the same luminosity; we trace the presence of a stellar population gradient also in the spatial distribution of stars in different evolutionary phases in ancillary SuprimeCam photometric data. There are tentative indications of two chemo-kinematically distinct sub-populations, with the more metal-poor stars showing a hotter kinematics than the metal-richer ones. Furthermore, the photometric dataset reveals the presence of a foreground population that most likely belongs to the Sagittarius stream. This study represents a first comprehensive analysis of Cetus chemo-kinematic properties. Our results add Cetus to the growing scatter in stellar-dark matter halo properties in low-mass galactic systems. The presence of a metallicity gradient akin to those found in similar systems inhabiting different environments may hint at metallicity gradients in LG early-type dwarfs being driven by internal mechanisms.

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arXiv2018

A Gaia DR 2 and VLT/FLAMES search for new satellites of the LMC

A wealth of tiny galaxies populates the surroundings of the Milky Way. Some of these objects might have their origin as former satellites of the Large Magellanic Cloud. Examples of the importance of understanding how many systems are genuine satellites of the MW or the LMC are the implications that the number and mass function of satellites have for dark matter theories and the treatment of baryonic physics in simulations of structure formation. Here we aim at deriving the bulk motions and estimates of the internal velocity dispersion and metallicity properties in four recently discovered distant southern dwarf galaxy candidates, Columba I, Reticulum III, Phoenix II and Horologium II. We combine Gaia DR2 astrometric measurements, photometry and new FLAMES spectroscopic data in the region of the CaII triplet lines; such combination is essential for finding potential member stars in these low luminosity systems. We find very likely member stars in all four satellites and are able to determine (or place limits on) the systems bulk motions and average internal properties. The systems are found to be very metal-poor, in agreement with dwarf galaxies and dwarf galaxy candidates of similar luminosity. The only system that we can place firmly in the category of dwarf galaxies is Phx II, given its resolved large velocity dispersion ($9.5_{-4.4}^{+6.8}$ km/s) and intrinsic metallicity spread (0.33 dex). Also for Col I we measure a clear metallicity spread. The orbital pole of Phx II is well constrained and close to that of the LMC, suggesting a prior association. The uncertainty on the orbital poles of the other systems are presently very large, so that an association cannot be excluded, apart from Col I. Using the numbers of potential former satellites of the LMC identified here and in the literature, we obtain for the LMC a dark matter mass of M$_{200}=1.9_{-0.9}^{+1.3}\times10^{11}$ M$_\odot$.

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