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Register a new userColour-magnitude diagram in simulations of galaxy formation
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State-of-the-art cosmological hydrodynamical simulations have star particles with typical mass between $sim 10^8$ and $sim 10^3$ M$_{odot}$ according to resolution, and treat them as simple stellar populations. On the other hand, observations in nearby galaxies resolve individual stars and provide us with single star properties. An accurate and fair comparison between predictions from simulations and observations is a crucial task. We introduce a novel approach to consistently populate star particles with stars. We developed a technique to generate a theoretical catalogue of mock stars whose characteristics are derived from the properties of parent star particles from a cosmological simulation. Also, a library of stellar evolutionary tracks and synthetic spectra is used to mimic the photometric properties of mock stars. The aim of this tool is to produce a database of synthetic stars from the properties of parent star particles in simulations: such a database represents the observable stellar content of simulated galaxies and allows a comparison as accurate as possible with observations of resolved stellar populations. With this innovative approach we are able to provide a colour-magnitude diagram from a cosmological hydrodynamical simulation. This method is flexible and can be tailored to fit output of different codes used for cosmological simulations. Also, it is of paramount importance with ongoing survey data releases (e.g. GAIA and surveys of resolved stellar populations), and will be useful to predict properties of stars with peculiar chemical features and to compare predictions from hydrodynamical models with data of different tracers of stellar populations.
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We present a deep Blanco/DECam colour-magnitude diagram (CMD) for the large but very diffuse Milky Way satellite dwarf galaxy Crater II. The CMD shows only old stars with a clearly bifurcated subgiant branch (SGB) that feeds a narrow red giant branch. The horizontal branch (HB) shows many RR Lyrae and red HB stars. Comparing the CMD with [Fe/H] = -2.0 and [$alpha$/Fe] = +0.3 alpha-enhanced BaSTI isochrones indicates a mean age of 12.5 Gyr for the main event and a mean age of 10.5 Gyr for the brighter SGB. With such multiple star formation events Crater II shows similarity to more massive dwarfs that have intermediate age populations, however for Crater II there was early quenching of the star formation and no intermediate age or younger stars are present. The spatial distribution of Crater II stars overall is elliptical in the plane of the sky, the detailed distribution shows a lack of strong central concentration, and some inhomogeneities. The 10.5 Gyr subgiant and upper main sequence stars show a slightly higher central concentration when compared to the 12.5 Gyr population. Matching to Gaia DR2 we find the proper motion of Crater II: $mu_{alpha}cos delta$=-0.14 $pm$ 0.07 , $mu_{delta}$=-0.10 $pm$ 0.04 mas yr$^{-1}$, approximately perpendicular to the semi-major axis of Crater II. Our results provide constraints on the star formation and chemical enrichment history of Crater II, but cannot definitively determine whether or not substantial mass has been lost over its lifetime.
In this work, we study the evolution with redshift of the colour-magnitude relation (CMR) of early-type galaxies. This evolution is analyzed through cosmological numerical simulations from z = 2 to z = 0. The preliminary results shown here represent the starting point of a study aimed at identifying the processes that originated the observed CMR of early-type galaxies at z = 0.
The ESA Gaia mission provides a unique time-domain survey for more than 1.6 billion sources with G ~ 21 mag. We showcase stellar variability across the Galactic colour-absolute magnitude diagram (CaMD), focusing on pulsating, eruptive, and cataclysmic variables, as well as on stars exhibiting variability due to rotation and eclipses. We illustrate the locations of variable star classes, variable object fractions, and typical variability amplitudes throughout the CaMD and illustrate how variability-related changes in colour and brightness induce `motions using 22 months worth of calibrated photometric, spectro-photometric, and astrometric Gaia data of stars with significant parallax. To ensure a large variety of variable star classes to populate the CaMD, we crossmatch Gaia sources with known variable stars. We also used the statistics and variability detection modules of the Gaia variability pipeline. Corrections for interstellar extinction are not implemented in this article. Gaia enables the first investigation of Galactic variable star populations across the CaMD on a similar, if not larger, scale than previously done in the Magellanic Clouds. Despite observed colours not being reddening corrected, we clearly see distinct regions where variable stars occur and determine variable star fractions to within Gaias current detection thresholds. Finally, we show the most complete description of variability-induced motion within the CaMD to date. Gaia enables novel insights into variability phenomena for an unprecedented number of stars, which will benefit the understanding of stellar astrophysics. The CaMD of Galactic variable stars provides crucial information on physical origins of variability in a way previously accessible only for Galactic star clusters or external galaxies.
XMMU J1229+0151 is a rich galaxy cluster with redshift z=0.975, that was serendipitously detected in X-rays within the scope of the XMM-Newton Distant Cluster Project. HST/ACS observations in the i775 and z850 passbands, as well as VLT/FORS2 spectroscopy were further obtained, in addition to follow-up Near-Infrared (NIR) imaging in J- and Ks-bands with NTT/SOFI. We investigate the photometric, structural and spectral properties of the early-type galaxies in the high-redshift cluster XMMU J1229+0151. Source detection and aperture photometry are performed in the optical and NIR imaging. Galaxy morphology is inspected visually and by means of Sersic profile fitting to the 21 spectroscopically confirmed cluster members in the ACS field of view. The i775-z850 colour-magnitude relation (CMR) is derived with a method based on galaxy magnitudes obtained by fitting the surface brightness of the galaxies with Sersic models. The i775-z850 CMR of the spectroscopic members shows a very tight red-sequence with a zero point of 0.86+-0.04 mag and intrinsic scatter equal to 0.039 mag. The CMR obtained with the galaxy models has similar parameters. Stellar masses and formation ages of the cluster galaxies are derived by fitting the observed spectral energy distributions (SED) with models based on Bruzual & Charlot 2003. We obtain a star formation weighted age of 4.3 Gyr for a median mass of 7.4e10 Msun. Instead of an unambiguous brightest cluster galaxy (BCG), we find three bright galaxies with a similar z850 magnitude, which are, in addition, the most massive cluster members, with ~ 2e11 Msun. Our results strengthen the current evidence for a lack of significant evolution of the scatter and slope of the red-sequence out to z~1.
Over the last decades, cosmological simulations of galaxy formation have been instrumental for advancing our understanding of structure and galaxy formation in the Universe. These simulations follow the non-linear evolution of galaxies modeling a variety of physical processes over an enormous range of scales. A better understanding of the physics relevant for shaping galaxies, improved numerical methods, and increased computing power have led to simulations that can reproduce a large number of observed galaxy properties. Modern simulations model dark matter, dark energy, and ordinary matter in an expanding space-time starting from well-defined initial conditions. The modeling of ordinary matter is most challenging due to the large array of physical processes affecting this matter component. Cosmological simulations have also proven useful to study alternative cosmological models and their impact on the galaxy population. This review presents a concise overview of the methodology of cosmological simulations of galaxy formation and their different applications.
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