ALBERT

Description: Open clusters (OCs) are crucial for studying the formation and evolution of the Galactic disc. However, the lack of a large number of OCs analysed homogeneously hampers the investigations about chemical patterns and the existence of Galactocentric radial and vertical gradients, or an age–metallicity relation. To overcome this, we have designed the Open Cluster Chemical Abundances from Spanish Observatories (OCCASO) survey. We aim to provide homogeneous radial velocities, physical parameters and individual chemical abundances of six or more red clump stars for a sample of 25 old and intermediate-age OCs visible from the Northern hemisphere. To do so, we use high-resolution spectroscopic facilities ( R ≥ 62 000) available at Spanish observatories. We present the motivation, design and current status of the survey, together with the first data release of radial velocities for 77 stars in 12 OCs, which represents about 50 per cent of the survey. We include clusters never studied with high-resolution spectroscopy before (NGC 1907, NGC 6991, NGC 7762), and clusters in common with other large spectroscopic surveys like the Gaia -ESO Survey (NGC 6705) and Apache Point Observatory Galactic Evolution Experiment (NGC 2682 and NGC 6819). We perform internal comparisons between instruments to evaluate and correct internal systematics of the results, and compare our radial velocities with previous determinations in the literature, when available. Finally, radial velocities for each cluster are used to perform a preliminary kinematic study in relation with the Galactic disc.

Description: We report the discovery of an optical Einstein ring in the Sculptor constellation, IAC J010127-334319, in the vicinity of the Sculptor dwarf spheroidal galaxy. It is an almost complete ring (~300°) with a diameter of ~4.5 arcsec. The discovery was made serendipitously from inspecting Dark Energy Camera (DECam) archive imaging data. Confirmation of the object nature has been obtained by deriving spectroscopic redshifts for both components, lens and source, from observations at the 10.4 m Gran Telescopio CANARIAS (GTC) with the spectrograph OSIRIS. The lens, a massive early-type galaxy, has a redshift of z  = 0.581, while the source is a starburst galaxy with redshift of z  = 1.165. The total enclosed mass that produces the lensing effect has been estimated to be M tot  = (1.86 ± 0.23) x 10 12 M .

Description: While wide-field surveys of M31 have revealed much substructure at large radii, understanding the nature and origin of this material is not straightforward from morphology alone. Using deep Hubble Space Telescope /Advanced Camera for Surveys data, we have derived further constraints in the form of quantitative star formation histories (SFHs) for 14 inner halo fields which sample diverse substructures. In agreement with our previous analysis of colour–magnitude diagram morphologies, we find the resultant behaviours can be broadly separated into two categories. The SFHs of ‘disc-like’ fields indicate that most of their mass has formed since z  ~ 1, with one quarter of the mass formed in the last 5 Gyr. We find ‘stream-like’ fields to be on average 1.5 Gyr older, with 10 per cent of their stellar mass formed within the last 5 Gyr. These fields are also characterized by an age–metallicity relation showing rapid chemical enrichment to solar metallicity by z  = 1, suggestive of an early-type progenitor. We confirm a significant burst of star formation 2 Gyr ago, discovered in our previous work, in all the fields studied here. The presence of these young stars in our most remote fields suggests that they have not formed in situ but have been kicked-out from the thin disc through disc heating in the recent past.

Description: For the first time, we obtain chemical evolution models (CEMs) for Tucana and Cetus, two isolated dwarf spheroidal galaxies of the Local Group. The CEMs have been built from the star formation histories (SFHs) and the metallicity histories, both obtained independently by the Local Cosmology from Isolated Dwarfs (LCID) project from deep colour–magnitude diagrams. Based on our models, we find that the chemical histories were complex and can be divided into different epochs and scenarios. In particular, during 75 per cent of the SFH, the galaxies behaved as closed boxes and, during the remaining 25 per cent, either received a lot of primordial gas by accretion or they lost metals through metal-rich winds. In order to discriminate between these two scenarios, abundances ratios in old stars are needed. At t  ~ 4.5 Gyr, the galaxies lost most of their gas due to a short-strong, well-mixed wind. We obtain very similar CEMs for both galaxies, although Cetus is twice as massive as Tucana. We conclude that the star formation in both galaxies began with only 1.5 per cent of the baryonic mass fraction predicted by cold dark matter.

Description: We present a comprehensive and detailed study of the stellar populations of the Fornax dwarf spheroidal galaxy. We analyse their spatial distributions along the main body of the galaxy, obtaining their surface density maps, together with their radial density profiles. Results are based on the largest and most complete catalogue of stars in Fornax, with more than 3.5 x 10 5 stars covering the main body of the galaxy up to V ~ 24. We find a differentiated structure in Fornax depending on the stellar ages. Old stars (10 Gyr) follow an elliptical distribution well fitted by King profiles with relatively large core radius ( r c  = 760 ± 60 pc). On another hand, young populations (3 Gyr) concentrate in the central region of the galaxy ( r c  = 210 ± 10 pc), and are better fitted by Sérsic profiles with 0.8 〈 n 〈 1.2, indicating some discy shape. These stars show strong asymmetries and substructures not aligned with the main optical axes of Fornax. This together with the observed differences between metallicity and age distribution maps strongly suggests accretion of material with different angular momentum. These results lead us to propose a scenario in which Fornax has suffered a major merger at z ~ 1.

Description: We study the evolution of three fields in the outer Large Magellanic Cloud (LMC) disc ( R gc  = 3.5–6.2 kpc). Their star formation history indicates a stellar population gradient such that younger stellar populations are more centrally concentrated. We identify two main star-forming epochs, separated by a period of lower activity between ~=7 and ~=4 Gyr ago. Their relative importance varies from a similar amount of stars formed in the two epochs in the innermost field to only 40 per cent of the stars formed in the more recent epoch in the outermost field. The young star-forming epoch continues to the present time in the innermost field, but lasted only till ~=0.8 and 1.3 Gyr ago at R gc  = 5 $_{.}^{\circ}$ 5 and 7 $_{.}^{\circ}$ 1, respectively. This gradient is correlated with the measured H i column density and implies an outside-in quenching of the star formation, possibly related to a variation of the size of the H i disc. This could either result from gas depletion due to star formation or ram-pressure stripping, or from to the compression of the gas disc as ram pressure from the Milky Way halo acted on the LMC interstellar medium. The latter two situations may have occurred when the LMC first approached the Milky Way.

Description: We present the star formation history (SFH) and the age–metallicity relation (AMR) in three fields of the Fornax dwarf spheroidal galaxy. They sample a region spanning from the centre of the galaxy to beyond one core radius, which allows studying galactocentric gradients. In all the cases, we found stars as old as 12 Gyr, together with intermediate-age and young stellar populations. The last star formation events, as young as 1 Gyr old, are mainly located in the central region, which may indicate that the gas reservoir in the outer parts of the galaxy would have been exhausted earlier than in the centre or removed by tidal interactions. The AMR is smoothly increasing in the three analysed regions and similar to each other, indicating that no significant metallicity gradient is apparent within and around the core radius of Fornax. No significant traces of global UV-reionization or local SNe feedback are appreciated in the early SFH of Fornax. Our study is based on VLT photometry as deep as I ~ 24.5 and the iac-star/iac-pop/minniac suite of codes for the determination of the SFH in resolved stellar populations.

Description: We present a novel analysis of the internal kinematics of the Fornax dwarf spheroidal galaxy. Our results are based on the largest sample of spectroscopic data for Fornax stars presently available (〉2500 stars), for which we have chemical and kinematic information. We introduce new software, beacon , designed to detect chemo-kinematic patterns among stars of different stellar populations using their metallicity and velocity along the line of sight. Applying beacon to Fornax, we have detected non-negligible rotation signals around main optical axes of the galaxy, characteristic for a triaxial system partially supported by rotation. The dominant rotation pattern is relatively strong (~12 km s –1 ), but the galaxy also shows additional weaker albeit complex rotation patterns. Using the information available from the star formation history of Fornax, we have also derived the average age of the different chemo-kinematic components found by beacon , which has allowed us to obtain its kinematic history. Our results point to a possible major merger suffered by Fornax at redshift z ~ 1, in agreement with the previous works.

Description: We produce a comprehensive field star age–metallicity relationship (AMR) from the earliest epoch until ~1 Gyr ago for three fields in the Fornax dwarf spheroidal galaxy by using VI photometric data obtained with FORS1 at the VLT. We find that the innermost one does not contain dominant very old stars (age 〉 12 Gyr), whereas the relatively outer field does not account for representative star field populations younger than ~3 Gyr. When focusing on the most prominent stellar populations, we find that the derived AMRs are engraved by the evidence of an outside-in star formation process. The studied fields show bimodal metallicity distributions peaked at [Fe/H] = (–0.95 ± 0.15) dex and (–1.15 or –1.25 ± 0.05) dex, respectively, but only during the first half of the entire galaxy lifetime. Furthermore, the more metal-rich population appears to be more numerous in the outer fields, while in the innermost Fornax field the contribution of both metallicity populations seems to be similar. We also find that the metallicity spread ~6 Gyr ago is remarkable large, while the intrinsic metallicity dispersion at ~1–2 Gyr results smaller than that for the relatively older generations of stars. We interpret these outcomes as a result of a possible merger of two galaxies that would have triggered a star formation bursting process that peaked between ~6 and 9 Gyr ago, depending on the position of the field in the galaxy.

Description: We present the results of a new search for variable stars in the Local Group dwarf galaxy Leo A, based on deep photometry from the Advanced Camera for Surveys onboard the Hubble Space Telescope . We detected 166 bona fide variables in our field, of which about 60 per cent are new discoveries and 33 candidate variables. Of the confirmed variables, we found 156 Cepheids, but only 10 RR Lyrae stars despite nearly 100 per cent completeness at the magnitude of the horizontal branch. The RR Lyrae stars include seven fundamental and three first-overtone pulsators, with mean periods of 0.636 and 0.366 d, respectively. From their position on the period–luminosity (PL) diagram and light-curve morphology, we classify 91, 58 and 4 Cepheids as fundamental, first-overtone and second-overtone mode Classical Cepheids (CC), respectively, and two as Population II Cepheids. However, due to the low metallicity of Leo A, about 90 per cent of the detected Cepheids have periods shorter than 1.5 d. Comparison with theoretical models indicate that some of the fainter stars classified as CC could be Anomalous Cepheids. We estimate the distance to Leo A using the tip of the red giant branch (TRGB) and various methods based on the photometric and pulsational properties of the Cepheids and RR Lyrae stars. The distances obtained with the TRGB and RR Lyrae stars agree well with each other while that from the Cepheid PL relations is somewhat larger, which may indicate a mild metallicity effect on the luminosity of the short-period Cepheids. Due to its very low metallicity, Leo A thus serves as a valuable calibrator of the metallicity dependences of the variable star luminosities.