Astrophysics

We present comprehensive characterization of the Galactic open cluster M 36. Some two hundred member candidates, with an estimated contamination rate of ??8%, have been identified on the basis of proper motion and parallax measured by the Gaia DR2. The cluster has a proper motion grouping around ( μ α cosδ=??0.15 ± 0.01 mas yr ?? , and μ δ =??3.35 ± 0.02 mas yr ?? ), distinctly separated from the field population. Most member candidates have parallax values 0.7 ??0.9 mas, with a median value of 0.82 ± 0.07 mas (distance ??1.20 ± 0.13 kpc). The angular diameter of 27 ??± $0\farcm4$ determined from the radial density profile then corresponds to a linear extent of 9.42 ± 0.14 pc. With an estimated age of ??15 Myr, M 36 is free of nebulosity. To the south-west of the cluster, we discover a highly obscured ( A V up to ??23 mag), compact ( ??$1\farcm9 \times 1\farcm2$) dense cloud, within which three young stellar objects in their infancy (ages ??0.2 Myr) are identified. The molecular gas, 3.6 pc in extent, contains a total mass of (2 ??3) ? 10 2 M ??, and has a uniform velocity continuity across the cloud, with a velocity range of ??20 to ??22 km s ?? , consistent with the radial velocities of known star members. In addition, the cloud has a derived kinematic distance marginally in agreement with that of the star cluster. If physical association between M 36 and the young stellar population can be unambiguously established, this manifests a convincing example of prolonged star formation activity spanning up to tens of Myrs in molecular clouds.

The colour bimodality of galaxies provides an empirical basis for theories of galaxy evolution. However, the balance of processes that begets this bimodality has not yet been constrained. A more detailed view of the galaxy population is needed, which we achieve in this paper by using unsupervised machine learning to combine multi-dimensional data at two different epochs. We aim to understand the cosmic evolution of galaxy subpopulations by uncovering substructures within the colour bimodality. We choose a clustering algorithm that models clusters using only the most discriminative data available, and apply it to two galaxy samples: one from the second edition of the GALEX-SDSS-WISE Legacy Catalogue (GSWLC-2; z??.06 ), and the other from the VIMOS Public Extragalactic Redshift Survey (VIPERS; z??.65 ). We cluster within a nine-dimensional feature space defined purely by rest-frame ultraviolet-through-near-infrared colours. Both samples are similarly partitioned into seven clusters, breaking down into four of mostly star-forming galaxies (including the vast majority of green valley galaxies) and three of mostly passive galaxies. The separation between these two families of clusters suggests differences in the evolution of their galaxies, and that these differences are strongly expressed in their colours alone. The samples are closely related, with star-forming/green-valley clusters at both epochs forming morphological sequences, capturing the gradual internally-driven growth of galaxy bulges. At high stellar masses, this growth is linked with quenching. However, it is only in our low-redshift sample that additional, environmental processes appear to be involved in the evolution of low-mass passive galaxies.

The properties of synthetic CO emission from 3D simulations of forming molecular clouds are studied within the SILCC-Zoom project. Since the time scales of cloud evolution and molecule formation are comparable, the simulations include a live chemical network. Two sets of simulations with an increasing spatial resolution (d x=3.9 pc to d x=0.06 pc) are used to investigate the convergence of the synthetic CO emission, which is computed by post-processing the simulation data with the \textsc{radmc-3d} radiative transfer code. To determine the excitation conditions, it is necessary to include atomic hydrogen and helium alongside H 2 and the resulting CO emission is increased by ~20-30\%. Combining the brightness temperature of 12 CO and 13 CO , we compare different methods to estimate the excitation temperature, the optical depth of the CO line and hence, the CO column density. An intensity-weighted average excitation temperature results in the most accurate estimate of the total CO mass. When the pixel-based excitation temperature is used to calculate the CO mass, it is over-/underestimated at low/high CO column densities where the assumption that 12 CO is optically thick while 13 CO is optically thin is not valid. Further, in order to obtain a converged total CO luminosity and hence < X CO > factor, the 3D simulation must have d x??.1 pc. The < X CO > evolves over time and differs for the two clouds; yet pronounced differences with numerical resolution are found. Since high column density regions with a visual extinction larger than 3~mag are not resolved for d x?? pc, in this case the H 2 mass and CO luminosity both differ significantly from the higher resolution results and the local X CO is subject to strong noise. Our calculations suggest that synthetic CO emission maps are only converged for simulations with d x??.1 pc.

Cosmological simulations of structure formation are invaluable to study the evolution of the Universe and the development of galaxies in it successfully reproducing many observations in the context of the cosmological paradigm ? CDM. However, there are remarkable discrepancies with observations that are a matter of debate. One of the most recently reported is the diversity of shapes in the rotation curves of dwarf galaxies in the local Universe which is in contrast to the apparent homogeneity of rotation curves in cosmological hydrodynamic simulations. Previous studies on similar problems have shown that sometimes can be alleviated by accounting for the impact of observational effects in the comparison. For this reason, in this work we present a set of controlled experiments to measure the impact that some systematic effects, associated with modeling the observation process in a realistic way, have on the diversity of synthetic rotation curves. Our results demonstrate that factors such as spectral power, spatial resolution and inclination angle, can naturally induce noticeable fluctuations on the shape of the rotation curves, reproducing up to 47% of the diversity reported in the observations. This is remarkable, especially considering that we limited the sample to highly-symmetric disks simulated in isolation. This shows that a more realistic modeling of synthetic rotation curves may alleviate the reported tension between simulations and observations, without posing a challenge to the standard cosmological model of cold dark matter.

We search for unresolved X-ray emission from lensed sources in the FOV of 11 CLASH clusters with Chandra data. We consider the solid angle in the lens plane corresponding to a magnification μ>1.5 , that amounts to a total of ~100 arcmin 2 . Our main goal is to assess the efficiency of massive clusters as cosmic telescopes to explore the faint end of X-ray extragalactic source population. We search for X-ray emission from strongly lensed sources identified in the optical, and perform an untargeted detection of lensed X-ray sources. We detect X-ray emission only in 9 out of 849 lensed/background optical sources. The stacked emission of the sources without detection does not reveal any signal in any band. Based on the untargeted detection, we find 66 additional X-ray sources that are consistent with being lensed sources. After accounting for completeness and sky coverage, we measure for the first time the soft- and hard-band number counts of lensed X-ray sources. The results are consistent with current modelization of the AGN population distribution. The distribution of de-lensed fluxes of the sources identified in moderately deep CLASH fields reaches a flux limit of ~ 10 ??6 and ~ 10 ??5 erg/s/cm 2 in the soft and hard bands, respectively. We conclude that, in order to match the depth of the CDFS exploiting massive clusters as cosmic telescopes, the required number of cluster fields is about two orders of magnitude larger than that offered by the 20 years Chandra archive. A significant step forward will be made when future X-ray facilities, with ~1' angular resolution and large effective area, will allow the serendipitous discovery of rare, strongly lensed high- z X-ray sources, enabling the study of faint AGN activity in early Universe and the measurement of gravitational time delays in the X-ray variability of multiply imaged AGN.

We apply BayeSN, our new hierarchical Bayesian model for the SEDs of Type Ia supernovae (SNe Ia), to analyse the griz light curves of 157 nearby SNe Ia ( 0.015<z<0.08 ) from the public Foundation DR1 dataset. We train a new version of BayeSN, continuous from 0.35--0.95 μ m, which we use to model the properties of SNe Ia in the rest-frame z -band, study the properties of dust in their host galaxies, and construct a Hubble diagram of SN Ia distances determined from full griz light curves. Our griz Hubble diagram has a low total RMS of 0.13 mag using BayeSN, compared to 0.16 mag using SALT2. Additionally, we test the consistency of the dust law R V between low- and high-mass host galaxies by using our model to fit the full time- and wavelength-dependent SEDs of SNe Ia up to moderate reddening (peak apparent B?�V??.3 ). Splitting the population at the median host mass, we find R V =2.84±0.31 in low-mass hosts, and R V =2.58±0.23 in high-mass hosts, both consistent with the global value of R V =2.61±0.21 that we estimate for the full sample. For all choices of mass split we consider, R V is consistent across the step within ??.2? . Modelling population distributions of dust laws in low- and high-mass hosts, we find that both subsamples are highly consistent with the full sample's population mean μ( R V )=2.70±0.25 with a 95% upper bound on the population ?( R V )<0.61 . The R V population means are consistent within ??.2? . We find that simultaneous fitting of host-mass-dependent dust properties within our hierarchical model does not account for the conventional mass step.

Supersonic turbulence in the interstellar medium (ISM) is closely linked to the formation of stars, and hence many theories connect the stellar initial mass function (IMF) with the turbulent properties of molecular clouds. Here we test three turbulence-based IMF models (by Padoan & Nordlund 2002, Hennebelle & Chabrier 2008, and Hopkins 2012), which predict the relation between the high-mass slope ( ? ) of the IMF, dN/dlogM??M ? and the exponent n of the velocity power spectrum of turbulence, E v (k)??k ?�n , where n?? corresponds to typical ISM turbulence. Using hydrodynamic simulations, we drive turbulence with an unusual index of n?? , measure ? , and compare the results with n?? . We find that reducing n from 2 to 1 primarily changes the high-mass region of the IMF (beyond the median mass), where we measure high-mass slopes within the 95 per cent confidence interval of ??.5<?<?? for n?? and ??.7<?<??.4 for n?? , respectively. Thus, we find that n=1 results in a significantly flatter high-mass slope of the IMF, with more massive stars formed than for n?? . We compare these simulations with the predictions of the three IMF theories. We find that while the Padoan & Nordlund theory matches our simulations with fair accuracy, the other theories either fail to reproduce the main qualitative outcome of the simulations or require some modifications. We conclude that turbulence plays a key role in shaping the IMF, with a shallower turbulence power spectrum producing a shallower high-mass IMF, and hence more massive stars.

In the star formation process, the vital impact of environmental factors such as feedback from massive stars and stellar density on the form of the initial mass function (IMF) at low-mass end is yet to be understood. Hence a systematic, highly sensitive observational analysis of a sample of regions under diverse environmental conditions is essential. We analyse the IMF of eight young clusters ( < 5 Myr), namely IC1848-West, IC1848-East, NGC 1893, NGC 2244, NGC 2362, NGC 6611, Stock 8 and Cygnus OB2, which are located at the Galactocentric distance ( R g ) range ??6-12 kpc along with nearby cluster IC348 using deep near-IR photometry and Gaia DR2. These clusters are embedded in massive stellar environments of radiation strength log( L FUV / L ??) ??2.6 to 6.8, log( L EUV ) ??42.2 to 50.85 photons/s, with stellar density in the range of ??170 - 1220 stars/pc 2 . After structural analysis and field decontamination we obtain an unbiased, uniformly sensitive sample of pre-main-sequence members of the clusters down to brown-dwarf regime. The lognormal fit to the IMF of nine clusters gives the mean characteristic mass ( m c ) and ? of 0.32 ± 0.02 M ??and 0.47 ± 0.02, respectively. We compare the IMF with that of low- and high-mass clusters across the Milky Way. We also check for any systematic variation with respect to the radiation field strength, stellar density as well with R g . We conclude that there is no strong evidence for environmental effect in the underlying form of the IMF of these clusters.

The ALMA Spectroscopic Survey in the Hubble Ultra Deep Field (ASPECS) Band 6 scan (212-272 GHz) covers potential [CII] emission in galaxies at 6?�z?? throughout a 2.9 arcmin 2 area. By selecting on known Lyman- α emitters (LAEs) and photometric dropout galaxies in the field, we perform targeted searches down to a 5 ? [CII] luminosity depth L [CII] ??.0? 10 8 L ??, corresponding roughly to star formation rates (SFRs) of 10 - 20 M ??yr ?? when applying a locally calibrated conversion for star-forming galaxies, yielding zero detections. While the majority of galaxies in this sample are characterized by lower SFRs, the resulting upper limits on [CII] luminosity in these sources are consistent with the current literature sample of targeted ALMA observations of z=6 - 7 LAEs and Lyman-break galaxies (LBGs), as well as the locally calibrated relations between L [CII] and SFR -- with the exception of a single [CII]-deficient, UV luminous LBG. We also perform a blind search for [CII]-bright galaxies that may have been missed by optical selections, resulting in an upper limit on the cumulative number density of [CII] sources with L [CII] >2.0? 10 8 L ??( 5? ) to be less than 1.8? 10 ?? Mpc ?? (90% confidence level). At this luminosity depth and volume coverage, we present an observed evolution of the [CII] luminosity function from z=6 - 8 to z?? by comparing the ASPECS measurement to literature results at lower redshift.

Sulfur is a volatile chemical element that plays an important role in tracing the chemical evolution of galaxies. However, its nucleosynthesis origin and abundance variations are still unclear. The goal of the present article is to accurately and precisely study the S-content of large number of stars located in the solar neighbourhood. We use the parametrisation of thousands of HR stellar spectra provided by the AMBRE Project, and combine it with the automated abundance determination GAUGUIN to derive LTE sulfur abundances for 1855 slow-rotating FGK-type stars. This is the largest and most precise catalogue of S-abundances published to date. It covers a metallicity domain as high as ~2.5dex starting at [M/H]~-2.0dex. We find that the [S/M] abundances ratio is compatible with a plateau-like distribution in the metal-poor regime, and then starts to decrease continuously at [M/H]~-1.0dex. This decrease continues towards negative values for supersolar metallicity stars as recently reported for Mg and as predicted by Galactic chemical evolution models. Moreover, sulfur-rich stars having [M/H] in the range [-1.0,-0.5] have very different kinematical and orbital properties with respect to more metal-rich and sulfur-poor ones. Two disc components, associated with the thin and thick discs, are thus seen independently in kinematics and sulfur abundances. The sulfur radial gradients in the Galactic discs have also been estimated. Finally, the enrichment in sulfur with respect to iron is nicely correlated with stellar ages: older metal-poor stars have higher [S/M] ratios than younger metal-rich ones. This work has confirmed that sulfur is an alfa-element that could be considered to explore the Galactic populations properties.