Rational points in the moduli space of genus two

We build a database of genus 2 curves defined over $\mathbb Q$ which contains all curves with minimal absolute height $h \leq 5$, all curves with moduli height $\mathfrak h \leq 20$, and all curves with extra automorphisms in standard form $y^2=f(x^2)$ defined over $\mathbb Q$ with height $h \leq 101$. For each isomorphism class in the database, an equation over its minimal field of definition is provided, the automorphism group of the curve, Clebsch and Igusa invariants. The distribution of rational points in the moduli space $\mathcal M_2$ for which the field of moduli is a field of definition is discussed and some open problems are presented

A state-of-the-art analysis of the dwarf irregular galaxy NGC 6822

We present a detailed photometric study of the dwarf irregular galaxy NGC 6822 aimed at investigating the properties of its stellar populations and, in particular, the presence of stellar radial gradients. Our goal is to analyse the stellar populations in six fields, which cover the whole bar of this dwarf galaxy. We derived the quantitative star formation history (SFH) of the six fields using the IAC method, involving IAC-pop/MinnIAC codes. The solutions we derived show an enhanced star formation rate (SFR) in Fields 1 and 3 during the past 500 Myr. The SFRs of the other fields are almost extinguished at very recent epochs and. We study the radial gradients of the SFR and consider the total mass converted into stars in two time intervals (between 0 and 0.5 Gyr ago and between 0.5 and 13.5 Gyr ago). We find that the scale lengths of the young and intermediate-to-old populations are perfectly compatible, with the exception of the young populations in Fields 1 and 3. The recent SF in these two fields is greater than in the other ones. This might be an indication that in these two fields we are sampling incipient spiral arms. Further evidence and new observations are required to prove this hypothesis. In addition, we derived the age-metallicity relations. As expected, the metallicity increases with time for all of the fields. We do not observe any radial gradient in the metallicity.Comment: 9 pages, 11 figures, Accepted to A&

Spatially resolved LMC star formation history: I. Outside in evolution of the outer LMC disk

We study the evolution of three fields in the outer LMC disk Rgc=3.5-6.2 Kpc. Their star formation history indicates a stellar populations 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% 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 Rgc=5.5 degrees and 7.1 degrees, respectively. This gradient is correlated with the measured HI column density and implies an outside-in quenching of the star formation, possibly related to a variation of the size of the HI disk. This could either result from gas depletion due to star formation or ram-pressure stripping, or from to the compression of the gas disk 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.Comment: 15 pages, 13 figures, 4 tables. MNRAS, in pres

The ACS LCID project VII: the blue stragglers population in the isolated dSph galaxies Cetus and Tucana

We present the first investigation of the Blue Straggler star (BSS) population in two isolated dwarf spheroidal galaxies of the Local Group, Cetus and Tucana. Deep HST/ACS photometry allowed us to identify samples of 940 and 1214 candidates, respectively. The analysis of the star formation histories of the two galaxies suggests that both host a population of BSSs. Specifically, if the BSS candidates are interpreted as young main sequence stars, they do not conform to their galaxy's age-metallicity relationship. The analysis of the luminosity function and the radial distributions support this conclusion, and suggest a non-collisional mechanism for the BSS formation, from the evolution of primordial binaries. This scenario is also supported by the results of new dynamical simulations presented here. Both galaxies coincide with the relationship between the BSS frequency and the absolute visual magnitude Mv found by Momany et al (2007). If this relationship is confirmed by larger sample, then it could be a valuable tool to discriminate between the presence of BSSs and galaxies hosting truly young populations.Comment: Accepted for publication on ApJ. 15 pages, 3 tables, 13 figures. A version with high resolution figure can be downloaded from http://rialto.ll.iac.es/proyecto/LCID/?p=publication

The star formation history of the Sextans dwarf spheroidal galaxy: a true fossil of the pre-reionization era

We present the star formation history (SFH) of the Sextans dwarf spheroidal galaxy based on deep archive $B$,$I$ photometry taken with Suprime-Cam at Subaru telescope focusing our analysis on the inner region of the galaxy, fully located within the core radius. Within the errors of our SFH we have not detected any metallicity gradient along the considered radial distance interval. As a main result of this work we can state that the Sextans dwarf spheroidal stopped forming stars less than $\sim1.3$ Gyr after Big Bang in correspondance to the end of the reionization epoch. We have been able to constrain the duration of the main burst of star formation to $\sim0.6$ Gyr. From the calculation of the mechanical luminosity released from supernovae (SNe) during the brief episode of star formation, there are strong indications that SNe could have played an important role in the fate of Sextans, by removing almost completely the gas component, so preventing a prolonged star formation.Comment: 11 pages, 10 figures. Accepted for publication in MNRA

Diffuse Ionized Gas in the Dwarf Irregular Galaxy DDO 53

The spectral characteristics throughout the dwarf irregular galaxy DDO 53 are studied. The results are very similar to those for other irregular galaxies: high excitation and low values of the [SII]/Halpha ratio. The most likely ionization source is photon leakage from the classical HII regions, without any other source, although the interstellar medium of the galaxy is quite perturbed. Moreover, the physical conditions throughout the galaxy do not change very much because both the photon leakage percentage and the ionization temperature are very similar. In addition, the determined metal content for two HII regions indicates that DDO 53 is a low-metallicity galaxy.Comment: 32 pages, 9 figures, 7 tables. AJ, in pres

Mean temperature evolution on the Spanish mainland 1916-2015

An analysis of the evolution of annual and seasonal mean temperatures on the Spanish mainland (western Mediterranean basin) was carried out, using the new MOnthly TEmperature Dataset of Spain century (MOTEDAS-century) data set. This data set was developed by combining archives from the National Meteorological Agency and newly digitised legacy data from the Annual Books published between 1916 and 1949. Both the annual and seasonal regional mean temperature series experienced increasing trends during the study period. However, the in-crement was neither constant in time (monotonic) nor homogeneous between seasons. Four main periods were identified in the annual mean regional series. The first 3 corresponded to those previously confirmed in global temperature series (rise-pause-rise); a second pause or hiatus was also detected at the end of the period of analysis. The seasonal regional series followed specific patterns: Winter mean temperature only increased in the second rising period, autumn in the first and spring and summer during the 2 rising periods. Also, negative trends were found in extended areas in the first pause in spring, and to a lesser extent in summer. From the middle of the 1980s, the trends in annual and seasonal mean values were not significant up until 2015. Furthermore, spatial variations were found in the significance of the trends, revealing regional differences in the intensity of warming through the seasons. In comparison with other versions of secular mean temperature series developed for the Spanish mainland, MOTEDAS-century seems to better capture the spatial variability