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The Dual Origin of Stellar Halos  [PDF]
Adi Zolotov,Beth Willman,Alyson M. Brooks,Fabio Governato,Chris B. Brook,David W. Hogg,Tom Quinn,Greg Stinson
Physics , 2009, DOI: 10.1088/0004-637X/702/2/1058
Abstract: We investigate the formation of the stellar halos of four simulated disk galaxies using high resolution, cosmological SPH + N-Body simulations. These simulations include a self-consistent treatment of all the major physical processes involved in galaxy formation. The simulated galaxies presented here each have a total mass of ~10^12 M_sun, but span a range of merger histories. These simulations allow us to study the competing importance of in-situ star formation (stars formed in the primary galaxy) and accretion of stars from subhalos in the building of stellar halos in a LambdaCDM universe. All four simulated galaxies are surrounded by a stellar halo, whose inner regions (r < 20 kpc) contain both accreted stars, and an in-situ stellar population. The outer regions of the galaxies' halos were assembled through pure accretion and disruption of satellites. Most of the in-situ halo stars formed at high redshift out of smoothly accreted cold gas in the inner 1 kpc of the galaxies' potential wells, possibly as part of their primordial disks. These stars were displaced from their central locations into the halos through a succession of major mergers. We find that the two galaxies with recently quiescent merger histories have a higher fraction of in-situ stars (~20-50%) in their inner halos than the two galaxies with many recent mergers (~5-10% in-situ fraction). Observational studies concentrating on stellar populations in the inner halo of the Milky Way will be the most affected by the presence of in-situ stars with halo kinematics, as we find that their existence in the inner few tens of kpc is a generic feature of galaxy formation.
Chemical abundances as population tracers  [PDF]
Poul Erik Nissen
Physics , 2011,
Abstract: Elemental abundance ratios as tracers of stellar populations are discussed with emphasis on F, G, and K stars providing a `fossil' record of the chemical evolution of the Galaxy. Most abundance studies have been based on homogeneous 1D model atmospheres and the assumption of local thermodynamic equilibrium (LTE), but recent works have shown that 3D non-LTE corrections can change the derived trends of abundance ratios as a function of stellar metallicity very significantly. However, when comparing stars having similar effective temperatures, surface gravities, and metallicities, 3D non-LTE corrections tend to cancel out. When applying such a differential approach to stars in the Galactic disk, bulge, and halo, abundance ratios like C/O, Na/Fe, alpha/Fe, Cu/Fe, Ba/Y, and Eu/Ba point to the existence of multiple discrete populations in each of these Galactic components.
Hot Gas and Halos in Elliptical Galaxies  [PDF]
William G. Mathews,Fabrizio Brighenti
Physics , 1997,
Abstract: We review recent progress in understanding the evolution of hot interstellar gas in the halos of elliptical galaxies. Non-homologous variations in the physical size of the hot gas account for the large variations of x-ray luminosity among ellipticals of similar L_B. By combining ROSAT and Einstein data we derive the distribution of total mass in NGC 4472. From 0.1r_e to 1r_e the total mass is identical to the expected stellar mass. Therefore stellar mass to light ratios can be determined from x-ray observations! Also the widely used ``mass dropout'' assumption must be incorrect in this important part of the cooling flow. Recent ROSAT observations indicate gas temperatures in excess of the virial stellar temperature, totally unlike standard cooling flow models. However, these new results can be understood if an additional massive component of ``circumgalactic'' gas is assumed to fill the outer galactic halos beyond most of the stars. This old hot gas, first heated during the epoch of galaxy formation, continues to flow into the stellar parts of ellipticals today, combining with gas expelled from evolving stars. This dual origin of hot interstellar gas further complicates recent discussions of abundances in the hot interstellar gas.
The evolution of C and O abundances in stellar populations  [PDF]
Poul E. Nissen,William J. Schuster
Physics , 2013, DOI: 10.1017/S1743921313006212
Abstract: Carbon and oxygen abundances in F and G main-sequence stars ranging in metallicity from [Fe/H] = -1.6 to +0.5 are determined from a non-LTE analysis of CI and OI atomic lines in high-resolution spectra. Both C and O are good tracers of stellar populations; distinct trends of [C/Fe] and [O/Fe] as a function of [Fe/H] are found for high- and low-alpha halo stars and for thick- and thin-disk stars. These trends and that of [C/O] provide new information on the nucleosynthesis sites of carbon and the time-scale for the chemical enrichment of the various Galactic components.
Stellar abundances of beryllium and CUBES  [PDF]
R. Smiljanic
Physics , 2014, DOI: 10.1007/s10509-014-1916-9
Abstract: Stellar abundances of beryllium are useful in different areas of astrophysics, including studies of the Galactic chemical evolution, of stellar evolution, and of the formation of globular clusters. Determining Be abundances in stars is, however, a challenging endeavor. The two Be II resonance lines useful for abundance analyses are in the near UV, a region strongly affected by atmospheric extinction. CUBES is a new spectrograph planned for the VLT that will be more sensitive than current instruments in the near UV spectral region. It will allow the observation of fainter stars, expanding the number of targets where Be abundances can be determined. Here, a brief review of stellar abundances of Be is presented together with a discussion of science cases for CUBES. In particular, preliminary simulations of CUBES spectra are presented, highlighting its possible impact in investigations of Be abundances of extremely metal-poor stars and of stars in globular clusters.
Dual Halos and Formation of Early-Type Galaxies  [PDF]
Hong Soo Park,Myung Gyoon Lee
Physics , 2013, DOI: 10.1088/2041-8205/773/2/L27
Abstract: We present a determination of the two-dimensional shape parameters of the blue and red globular cluster systems (GCSs) in a large number of elliptical galaxies and lenticular galaxies (early-type galaxies, called ETGs). We use a homogeneous data set of the globular clusters in 23 ETGs obtained from the HST/ACS Virgo Cluster Survey. The position angles of both blue and red GCSs show a correlation with those of the stellar light distribution, showing that the major axes of the GCSs are well aligned with those of their host galaxies. However, the shapes of the red GCSs show a tight correlation with the stellar light distribution as well as with the rotation property of their host galaxies, while the shapes of the blue GCSs do much less. These provide clear geometric evidence that the origins of the blue and red globular clusters are distinct and that ETGs may have dual halos: a blue (metal-poor) halo and a red (metal-rich) halo. These two halos show significant differences in metallicity, structure, and kinematics, indicating that they are formed in two distinguishable ways. The red halos might have formed via dissipational processes with rotation, while the blue halos are through accretion.
The outer halos of elliptical galaxies  [PDF]
Ortwin Gerhard
Physics , 2010, DOI: 10.1007/978-1-4419-7317-7_29
Abstract: Recent progress is summarized on the determination of the density distributions of stars and dark matter, stellar kinematics, and stellar population properties, in the extended, low surface brightness halo regions of elliptical galaxies. With integral field absorption spectroscopy and with planetary nebulae as tracers, velocity dispersion and rotation profiles have been followed to ~4 and ~5-8 effective radii, respectively, and in M87 to the outer edge at ~150 kpc. The results are generally consistent with the known dichotomy of elliptical galaxy types, but some galaxies show more complex rotation profiles in their halos and there is a higher incidence of misalignments, indicating triaxiality. Dynamical models have shown a range of slopes for the total mass profiles, and that the inner dark matter densities in ellipticals are higher than in spiral galaxies, indicating earlier assembly redshifts. Analysis of the hot X-ray emitting gas in X-ray bright ellipticals and comparison with dynamical mass determinations indicates that non-thermal components to the pressure may be important in the inner ~10 kpc, and that the properties of these systems are closely related to their group environments. First results on the outer halo stellar population properties do not yet give a clear picture. In the halo of one bright galaxy, lower [alpha/Fe] abundances indicate longer star formation histories pointing towards late accretion of the halo. This is consistent with independent evidence for on-going accretion, and suggests a connection to the observed size evolution of elliptical galaxies with redshift.
Studying stellar halos with future facilities  [PDF]
Laura Greggio,Renato Falomo,Michela Uslenghi
Physics , 2015,
Abstract: Stellar halos around galaxies retain fundamental evidence of the processes which lead to their build up. Sophisticated models of galaxy formation in a cosmological context yield quantitative predictions about various observable characteristics, including the amount of substructure, the slope of radial mass profiles and three dimensional shapes, and the properties of the stellar populations in the halos. The comparison of such models with the observations provides constraints on the general picture of galaxy formation in the hierarchical Universe, as well as on the physical processes taking place in the halos formation. With the current observing facilities, stellar halos can be effectively probed only for a limited number of nearby galaxies. In this paper we illustrate the progress that we expect in this field with the future ground based large aperture telescopes (E-ELT) and with space based facilities as JWST.
Substructure in the stellar halos of the Aquarius simulations  [PDF]
Amina Helmi,A. P. Cooper,S. D. M. White,S. Cole,C. S. Frenk,J. F. Navarro
Physics , 2011, DOI: 10.1088/2041-8205/733/1/L7
Abstract: We characterize substructure in the simulated stellar halos of Cooper et al. (2010) which were formed by the disruption of satellite galaxies within the cosmological N-body simulations of galactic halos of the Aquarius Project. These stellar halos exhibit a wealth of tidal features: broad overdensities and very narrow faint streams akin to those observed around the Milky Way. The substructures are distributed anisotropically on the sky, a characteristic that should become apparent in the next generation of photometric surveys. The normalized RMS of the density of stars on the sky appears to be systematically larger for our halos compared to the value estimated for the Milky Way from main sequence turn-off stars in the Sloan Digital Sky Survey. We show that this is likely to be due in part to contamination by faint QSOs and redder main sequence stars, and might suggest that ~10% of the Milky Way halo stars have formed in-situ.
SAGA: Stellar Abundances for Galactic Archaeology  [PDF]
Takuma Suda
Physics , 2012,
Abstract: A tutorial for the Stellar Abundances for Galactic Archaeology (SAGA) database is presented. This paper describes the outline of the database, reports the current status of the data compilation and known problems, and presents plans for future updates and extensions.
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