arXiv subjects

Julio Navarro

Publications and source records attributed to Julio Navarro.

At least 37 records · Page 2Linked to original sources

arXiv2021

Pericentric passage-driven star formation in satellite galaxies and their hosts: CLUES from Local Group simulations

Local Group satellite galaxies show a wide diversity of star formation histories (SFHs) whose origin is yet to be fully understood. Using hydrodynamical simulations from the Constrained Local UniversE project, we study the SFHs of satellites of Milky Way-like galaxies in a cosmological context: while in the majority of the cases the accretion onto their host galaxy causes the satellites to lose their gas, with a subsequent suppression in SF, in about 25$\%$ of our sample we observe a clear enhancement of SF after infall. Peaks in SF clearly correlate with the satellite pericentric passage around its host and, in one case, with a satellite-satellite interaction. We identify two key ingredients that result in enhanced SF after infall: galaxies must enter the host's virial radius with a reservoir of cold gas $M_{\rm gas,inf}/M_{\rm vir,inf}\gtrsim 10^{-2}$ and with a minimum pericentric distance $\gtrsim$10 kpc (mean distance $\sim$50 kpc for the full sample), in order to form new stars due to compression of cold gas at pericentric passage. On the other hand, satellites that infall with little gas or whose pericentric distance is too small, have their gas ram-pressure stripped and subsequent SF quenched. The pericentric passage of satellites likewise correlates with SF peaks in their hosts, suggesting that this mechanism induces bursts of SF in satellites and central galaxies alike, in agreement with recent studies of our Galaxy's SFH. Our findings can explain the recently reported multiple stellar populations observed in dwarf galaxies such as Carina and Fornax, and should be taken into account in semi-analytic models of galaxy formation and satellite quenching.

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arXiv2019

Satellites of Satellites: The Case for Carina and Fornax

We use the Auriga cosmological simulations of Milky Way (MW)-mass galaxies and their surroundings to study the satellite populations of dwarf galaxies in $Λ$CDM. As expected from prior work, the number of satellites above a fixed stellar mass is a strong function of the mass of the primary dwarf. For galaxies as luminous as the Large Magellanic Cloud (LMC), and for halos as massive as expected for the LMC (determined by its rotation speed), the simulations predict about 3 satellites with stellar masses exceeding $M_*>10^5\, M_\odot$. If the LMC is on its first pericentric passage, then these satellites should be near the LMC and should have orbital angular momenta roughly coincident with that of the LMC. We use 3D positions and velocities from the 2nd data release of the Gaia mission to revisit which of the "classical" MW dwarf spheroidals could plausibly be LMC satellites. The new proper motions of the Fornax and Carina dwarf spheroidals place them on orbits closely aligned with the orbital plane of the Magellanic Clouds, hinting at a potential Magellanic association. Together with the Small Magellanic Cloud (SMC), this result raises to $3$ the number of LMC satellites with $M_*>10^5\, M_\odot$, as expected from simulations. This also fills the 12-mag luminosity gap between the SMC and the ultra-faints Hyi1, Car2, Hor1, and Car3, the few ultra-faint satellites confirmed to have orbits consistent with a Magellanic origin.

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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

Single-lined Spectroscopic Binary Star Candidates from a Combination of the RAVE and Gaia DR2 surveys

The combination of the final version of the RAVE spectroscopic survey data release 6 with radial velocities and astrometry from Gaia DR2 allows us to identify and create a catalog of single lined binary star candidates (SB1), their inferred orbital parameters, and to inspect possible double lined binary stars (SB2). A probability function for the detection of radial velocity (RV) variations is used for identifying SB1 candidates. The estimation of orbital parameters for main sequence dwarfs is performed by matching the measured RVs with theoretical velocity curves sampling the orbital parameter space. The method is verified by studying a mock sample from the SB9 catalogue. Studying the boxiness and asymmetry of the spectral lines allows us to identify possible SB2 candidates, while matching their spectra to a synthetic library indicates probable properties of their components. From the RAVE catalog we select 37,664 stars with multiple RV measurements and identify 3838 stars as SB1 candidates. Joining RAVE and Gaia DR2 yields 450,646 stars with RVs measured by both surveys and 27,716 of them turn out to be SB1 candidates, which is an increase by an order of magnitude over previous studies. For main sequence dwarf candidates we calculate their most probable orbital parameters: orbital periods are not longer than a few years and primary components have masses similar to the Solar mass. All our results are available via Vizier/CDS.

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arXiv2019

Subhalo destruction in the Apostle and Auriga simulations

N-body simulations make unambiguous predictions for the abundance of substructures within dark matter halos. However, the inclusion of baryons in the simulations changes the picture because processes associated with the presence of a large galaxy in the halo can destroy subhalos and substantially alter the mass function and velocity distribution of subhalos. We compare the effect of galaxy formation on subhalo populations in two state-of-the-art sets of hydrodynamical CDM simulations of Milky Way mass halos, APOSTLE and AURIGA. We introduce a new method for tracking the orbits of subhalos between simulation snapshots that gives accurate results down to a few kiloparsecs from the centre of the halo. Relative to a dark matter-only simulation, the abundance of subhalos in APOSTLE is reduced by 50% near the centre and by 10% within r200. In AURIGA the corresponding numbers are 80% and 40%. The velocity distributions of subhalos are also affected by the presence of the galaxy, much more so in AURIGA than in APOSTLE . The differences on subhalo properties in the two simulations can be traced back to the mass of the central galaxies, which in AURIGA are typically twice as massive as those in APOSTLE . We show that some of the results from previous studies are inaccurate due to systematic errors in the modelling of subhalo orbits near the centre of halos.

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arXiv2019

The R-Process Alliance: Discovery of a low-alpha, r-Process-Enhanced Metal-Poor Star in the Galactic Halo

A new moderately r-process-enhanced metal-poor star, RAVE J093730.5-062655, has been identified in the Milky Way halo as part of an ongoing survey by the R-Process Alliance. The temperature and surface gravity indicate that J0937-0626 is likely a horizontal branch star. At [Fe/H] = -1.86, J0937-0626 is found to have subsolar [X/Fe] ratios for nearly every light, alpha, and Fe-peak element. The low [alpha/Fe] ratios can be explained by an ~0.6 dex excess of Fe; J0937-0626 is therefore similar to the subclass of "iron-enhanced" metal-poor stars. A comparison with Milky Way field stars at [Fe/H] = -2.5 suggests that J0937-0626 was enriched in material from an event, possibly a Type Ia supernova, that created a significant amount of Cr, Mn, Fe, and Ni and smaller amounts of Ca, Sc, Ti, and Zn. The r-process enhancement of J0937-0626 is likely due to a separate event, which suggests that its birth environment was highly enriched in r-process elements. The kinematics of J0937-0626, based on Gaia DR2 data, indicate a retrograde orbit in the Milky Way halo; J0937-0626 was therefore likely accreted from a dwarf galaxy that had significant r-process enrichment.

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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

The R-Process Alliance: First Release from the Northern Search for r-Process Enhanced Metal-Poor Stars in the Galactic Halo

This paper presents the detailed abundances and r-process classifications of 126 newly identified metal-poor stars as part of an ongoing collaboration, the R-Process Alliance. The stars were identified as metal-poor candidates from the RAdial Velocity Experiment (RAVE) and were followed-up at high spectral resolution (R~31,500) with the 3.5~m telescope at Apache Point Observatory. The atmospheric parameters were determined spectroscopically from Fe I lines, taking into account <3D> non-LTE corrections and using differential abundances with respect to a set of standards. Of the 126 new stars, 124 have [Fe/H]<-1.5, 105 have [Fe/H]<-2.0, and 4 have [Fe/H]<-3.0. Nine new carbon-enhanced metal-poor stars have been discovered, 3 of which are enhanced in r-process elements. Abundances of neutron-capture elements reveal 60 new r-I stars (with +0.3<=[Eu/Fe]<=+1.0 and [Ba/Eu]<0) and 4 new r-II stars (with [Eu/Fe]>+1.0). Nineteen stars are found to exhibit a `limited-r' signature ([Sr/Ba]>+0.5, [Ba/Eu]<0). For the r-II stars, the second- and third-peak main r-process patterns are consistent with the r-process signature in other metal-poor stars and the Sun. The abundances of the light, alpha, and Fe-peak elements match those of typical Milky Way halo stars, except for one r-I star which has high Na and low Mg, characteristic of globular cluster stars. Parallaxes and proper motions from the second Gaia data release yield UVW space velocities for these stars which are consistent with membership in the Milky Way halo. Intriguingly, all r-II and the majority of r-I stars have retrograde orbits, which may indicate an accretion origin.

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arXiv2018

A-type stars in the Canada-France Imaging Survey I. The stellar halo of the Milky Way traced to large radius by blue horizontal branch stars

We present the stellar density profile of the outer halo of the Galaxy traced over a range of Galactocentric radii from $15< R_{GC} < 220$ kpc by blue horizontal branch (BHB) stars. These stars are identified photometrically using deep $u-$band imaging from the new Canada-France-Imaging-Survey (CFIS) that reaches 24.5 mag. This is combined with $griz$ bands from Pan-STARRS 1 and covers a total of $\sim4000$ deg$^2$ of the northern sky. We present a new method to select BHB stars that has low contamination from blue stragglers and high completeness. We use this sample to measure and parameterize the three dimensional density profile of the outer stellar halo. We fit the profile using (i) a simple power-law with a constant flattening (ii) a flattening that varies as a function of Galactocentric radius (iii) a broken power law profile. We find that outer stellar halo traced by the BHB is well modelled by a broken power law with a constant flattening of $q=0.86 \pm 0.02$, with an inner slope of $γ=4.24 \pm 0.08$. This is much steeper than the preferred outer profile that has a slope of $β=3.21\pm 0.07$ after a break radius of $r_b=41.4^{+2.5}_{-2.4}$ kpc. The outer profile of the stellar halo trace by BHB stars is shallower than that recently measured using RR Lyrae, a surprising result given the broad similarity of the ages of these stellar populations.

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arXiv2018

Correlations between age, kinematics, and chemistry as seen by the RAVE survey

We explore the connections between stellar age, chemistry, and kinematics across a Galactocentric distance of $7.5 < R\,(\mathrm{kpc}) < 9.0$, using a sample of $\sim 12\,000$ intermediate-mass (FGK) turnoff stars observed with the RAdial Velocity Experiment (RAVE) survey. The kinematics of this sample are determined using radial velocity measurements from RAVE, and parallax and proper motion measurements from the Tycho-Gaia Astrometric Solution (TGAS). In addition, ages for RAVE stars are determined using a Bayesian method, taking TGAS parallaxes as a prior. We divide our sample into young ($0 < τ< 3$ Gyr) and old ($8 < τ< 13$ Gyr) populations, and then consider different metallicity bins for each of these age groups. We find significant differences in kinematic trends of young and old, metal-poor and metal-rich, stellar populations. In particular, we find a strong metallicity dependence in the mean Galactocentric radial velocity as a function of radius ($\partial {V_{\rm R}}/\partial R$) for young stars, with metal-rich stars having a much steeper gradient than metal-poor stars. For $\partial {V_ϕ}/\partial R$, young, metal-rich stars significantly lag the LSR with a slightly positive gradient, while metal-poor stars show a negative gradient above the LSR. We interpret these findings as correlations between metallicity and the relative contributions of the non-axisymmetries in the Galactic gravitational potential (the spiral arms and the bar) to perturb stellar orbits.

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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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arXiv2016

The ACS LCID Project XI. On the early time resolution of LG dwarf galaxy SFHs: Comparing the effects of reionization in models with observations

The analysis of the early star formation history (SFH) of nearby galaxies, obtained from their resolved stellar populations is relevant as a test for cosmological models. However, the early time resolution of observationally derived SFHs is limited by several factors. Thus, direct comparison of observationally derived SFHs with those derived from theoretical models of galaxy formation is potentially biased. Here we investigate and quantify this effect. For this purpose, we analyze the duration of the early star formation activity in a sample of four Local Group dwarf galaxies and test whether they are consistent with being true fossils of the pre-reionization era; i.e., if the quenching of their star formation occurred before cosmic reionization by UV photons was completed. Two classical dSph (Cetus and Tucana) and two dTrans (LGS-3 and Phoenix) isolated galaxies with total stellar masses between $1.3\times 10^6$ to $7.2\times 10^6$ M$_\odot$ have been studied. Accounting for time resolution effects, the SFHs peak as much as 1.25 Gyr earlier than the optimal solutions. Thus, this effect is important for a proper comparison of model and observed SFHs. It is also shown that none of the analyzed galaxies can be considered a true-fossil of the pre-reionization era, although it is possible that the {\it outer regions} of Cetus and Tucana are consistent with quenching by reionization.

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arXiv2014

The effect of radial migration on galactic disks

We study the radial migration of stars driven by recurring multi-arm spiral features in an exponential disk embedded in a dark matter halo. The spiral perturbations redistribute angular momentum within the disk and lead to substantial radial displacements of individual stars, in a manner that largely preserves the circularity of their orbits and that results, after 5 Gyr (~40 full rotations at the disk scalelength), in little radial heating and no appreciable changes to the vertical or radial structure of the disk. Our results clarify a number of issues related to the spatial distribution and kinematics of migrators. In particular, we find that migrators are a heavily biased subset of stars with preferentially low vertical velocity dispersions. This "provenance bias" for migrators is not surprising in hindsight, for stars with small vertical excursions spend more time near the disk plane and thus respond more readily to non-axisymmetric perturbations. We also find that the vertical velocity dispersion of outward migrators always decreases, whereas the opposite holds for inward migrators. To first order, newly arrived migrators simply replace stars that have migrated off to other radii, thus inheriting the vertical bias of the latter. Extreme migrators might therefore be recognized, if present, by the unexpectedly small amplitude of their vertical excursions. Our results show that migration, understood as changes in angular momentum that preserve circularity, can affect strongly the thin disk, but cast doubts on models that envision the Galactic thick disk as a relic of radial migration.

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arXiv2014

Pseudo Three-dimensional Maps of the Diffuse Interstellar Band at 862 nm

The diffuse interstellar bands (DIBs) are absorption lines observed in visual and near infrared spectra of stars. Understanding their origin in the interstellar medium (ISM) is one of the oldest problems in astronomical spectroscopy, as DIBs ahave been known since 1922. In a completely new approach to understanding DIBs, we combined information from nearly 500,000 stellar spectra obtained by the massive spectroscopic survey RAVE (Radial Velocity Experiment) to produce the first pseudo three-dimensional map of the strength of the DIB at 8620~Ångstroms covering the nearest 3 kiloparsecs from the Sun, and show that it follows our independently constructed spatial distribution of extinction by interstellar dust along the Galactic plane. Despite having a similar distribution in the Galactic plane, the DIB 8620 carrier has a significantly larger vertical scale height than the dust. Even if one DIB may not represent the general DIB population, our observations outline the future direction of DIB research.

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arXiv2014

New distances to RAVE stars

Probability density functions are determined from new stellar parameters for the distance moduli of stars for which the RAdial Velocity Experiment (RAVE) has obtained spectra with S/N>=10. Single-Gaussian fits to the pdf in distance modulus suffice for roughly half the stars, with most of the other half having satisfactory two-Gaussian representations. As expected, early-type stars rarely require more than one Gaussian. The expectation value of distance is larger than the distance implied by the expectation of distance modulus; the latter is itself larger than the distance implied by the expectation value of the parallax. Our parallaxes of Hipparcos stars agree well with the values measured by Hipparcos, so the expectation of parallax is the most reliable distance indicator. The latter are improved by taking extinction into account. The effective temperature absolute-magnitude diagram of our stars is significantly improved when these pdfs are used to make the diagram. We use the method of kinematic corrections devised by Schoenrich, Binney & Asplund to check for systematic errors for general stars and confirm that the most reliable distance indicator is the expectation of parallax. For cool dwarfs and low-gravity giants tends to be larger than the true distance by up to 30 percent. The most satisfactory distances are for dwarfs hotter than 5500 K. We compare our distances to stars in 13 open clusters with cluster distances from the literature and find excellent agreement for the dwarfs and indications that we are over-estimating distances to giants, especially in young clusters.

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arXiv2014

Galactic kinematics and dynamics from RAVE stars

We analyse the kinematics of ~400000 RAVE stars. We split the sample into hot and cold dwarfs, red-clump and non-clump giants. The kinematics of the clump giants are consistent with being identical with those of non-clump giants. We fit Gaussian velocity ellipsoids to the meridional-plane components of velocity of each star class and give formulae from which the shape and orientation of the velocity ellipsoid can be determined at any location. The data are consistent with the giants and the cool dwarfs sharing the same velocity ellipsoids; sigma_z rises from 21 kms in the plane to sim 55 kms at |z|=2 kpc, while sigma_r rises from 37 kms to 82 kms. At (R,z) the longest axis of one of these velocity ellipsoids is inclined to the Galactic plane by an angle ~0.8 arctan(z/R). We use a novel formula to obtain precise fits to the highly non-Gaussian distributions of v_phi components. We compare the observed velocity distributions with the predictions of a dynamical model fitted to the velocities of stars that lie within ~150 pc of the Sun and star counts towards the Galactic pole. The model accurately reproduces the non-Gaussian nature of the v_r and v_z distributions and provides excellent fits to the data for v_z at all locations. The model v_phi distributions for the cool dwarfs fit the data extremely well, while those for the hot dwarfs have displacements to low v_phi that grow with |z| from very small values near the plane. At |z|>0.5 kpc, the theoretical v_phi distributions for giants show a deficit of stars with large v_phi and the model v_r distributions are too narrow. Systematically over-estimating distances by 20 per cent introduces asymmetry into the model v_r and v_z distributions near the plane and but significantly improves the fits to the data at |z|>0.5 kpc. The quality of the fits lends credence to the assumed, disc-dominated, gravitational potential.

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arXiv2013

The large-scale structure of the halo of the Andromeda Galaxy Part I: global stellar density, morphology and metallicity properties

We present an analysis of the large-scale structure of the halo of the Andromeda galaxy, based on the Pan-Andromeda Archeological Survey (PAndAS), currently the most complete map of resolved stellar populations in any galactic halo. Despite copious substructure, the global halo populations follow closely power law profiles that become steeper with increasing metallicity. We divide the sample into stream-like populations and a smooth halo component. Fitting a three-dimensional halo model reveals that the most metal-poor populations ([Fe/H]<-1.7) are distributed approximately spherically (slightly prolate with ellipticity c/a=1.09+/-0.03), with only a relatively small fraction (42%) residing in discernible stream-like structures. The sphericity of the ancient smooth component strongly hints that the dark matter halo is also approximately spherical. More metal-rich populations contain higher fractions of stars in streams (86% for [Fe/H]>-0.6). The space density of the smooth metal-poor component has a global power-law slope of -3.08+/-0.07, and a non-parametric fit shows that the slope remains nearly constant from 30kpc to 300kpc. The total stellar mass in the halo at distances beyond 2 degrees is 1.1x10^10 Solar masses, while that of the smooth component is 3x10^9 Solar masses. Extrapolating into the inner galaxy, the total stellar mass of the smooth halo is plausibly 8x10^9 Solar masses. We detect a substantial metallicity gradient, which declines from [Fe/H]=-0.7 at R=30kpc to [Fe/H]=-1.5 at R=150kpc for the full sample, with the smooth halo being 0.2dex more metal poor than the full sample at each radius. While qualitatively in-line with expectations from cosmological simulations, these observations are of great importance as they provide a prototype template that such simulations must now be able to reproduce in quantitative detail.

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