Browsing by Subject "dark-matter universe"

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The First Galaxies: Assembly Under Radiative Feedback From The First Stars
(2013-04) Pawlik, Andreas H.; Milosavljevic, Milos; Bromm, Volker; Pawlik, Andreas H.; Milosavljevic, Milos; Bromm, Volker
We investigate how radiative feedback from the first stars affects the assembly of the first dwarf galaxies. To this end, we perform cosmological zoomed smoothed particle hydrodynamics simulations of a dwarf galaxy assembling inside a halo reaching a virial mass similar to 10(9) M-circle dot at z = 10. The simulations follow the non-equilibrium chemistry and cooling of primordial gas and the subsequent conversion of the cool dense gas into massive metal-free stars. To quantify the radiative feedback, we compare a simulation in which stars emit both molecular hydrogen dissociating and hydrogen/helium ionizing radiation with a simulation in which stars emit only molecular hydrogen dissociating radiation, and further with a simulation in which stars remain dark. Photodissociation and photoionization exert a strong negative feedback on the assembly of the galaxy inside the main minihalo progenitor. Gas condensation is strongly impeded, and star formation is strongly suppressed in comparison with the simulation in which stars remain dark. The feedback on the gas from either dissociating or ionizing radiation implies a suppression of the central dark matter densities in the minihalo progenitor by factors of up to a few, which is a significant deviation from the singular isothermal density profile characterizing the dark matter distribution inside the virial radius in the absence of radiative feedback. The evolution of gas densities, star formation rates, and the distribution of dark matter becomes insensitive to the inclusion of dissociating radiation in the late stages of the minihalo assembly, and it becomes insensitive to the inclusion of ionizing radiation once the minihalo turns into an atomically cooling galaxy. The formation of a rotationally supported extended disk inside the dwarf galaxy is a robust outcome of our simulations not affected by the inclusion of radiation. Low-mass galaxies in the neighborhood of the dwarf galaxy show a large scatter in the baryon fraction which is driven by radiative feedback from sources both internal and external to these galaxies. Our estimates of the observability of the first galaxies show that dwarf galaxies such as simulated here will be among the faintest galaxies the upcoming James Webb Space Telescope will detect. Our conclusions regarding the structure and observability of the first galaxies are subject to our neglect of feedback from supernovae and chemical enrichment as well as to statistical uncertainties implied by the limited number of galaxies in our simulations.
Insight Into The Formation Of The Milky Way Through Cold Halo Substructure. I. The Echos Of Milky Way Formation
(2009-10) Schlaufman, Kevin C.; Rockosi, Constance M.; Prieto, Carlos Allende; Beers, Timothy C.; Bizyaev, Dmitry; Brewington, Howard; Lee, Young Sun; Malanushenko, Viktor; Malanushenko, Elena; Oravetz, Dan; Pan, Kaike; Simmons, Audrey; Snedden, Stephanie; Yanny, Brian; Prieto, Carlos Allende
We identify 10-seven for the first time-elements of cold halo substructure (ECHOS) in the volume within 17.5 kpc of the Sun in the inner halo of the Milky Way. Our result is based on the observed spatial and radial velocity distribution of metal-poor main-sequence turnoff (MPMSTO) stars in 137 Sloan Extension for Galactic Understanding and Exploration lines of sight. We point out that the observed radial velocity distribution is consistent with a smooth stellar component of the Milky Way's inner halo overall, but disagrees significantly at the radial velocities that correspond to our detections. We show that all of our detections are statistically significant and that we expect no false positives. These ECHOS represent the observable stellar debris of ancient merger events in the stellar accretion history of the Milky Way, and we use our detections and completeness estimates to infer a formal upper limit of 0.34(-0.02)(+0.02) on the fraction of the MPMSTO population in the inner halo that belong to ECHOS. Our detections and completeness calculations also suggest that there is a significant population of low fractional overdensity ECHOS in the inner halo, and we predict that 1/3 of the inner halo (by volume) harbors ECHOS with MPMSTO star number densities n approximate to 15 kpc(-3). In addition, we estimate that there are of order 10(3) ECHOS in the entire inner halo. ECHOS are likely older than known surface brightness substructure, so our detections provide us with a direct measure of the accretion history of the Milky Way in a region and time interval that has yet to be fully explored. In concert with previous studies, our result suggests that the level of merger activity has been roughly constant over the past few Gyr and that there has been no accretion of single stellar systems more massive than a few percent of a Milky Way mass in that interval.
The Kinematics Of Thick Disks In Nine External Galaxies
(2008-08) Yoachim, Peter; Dalcanton, Julianne J.; Yoachim, Peter
We present kinematic measurements of thin- and thick-disk components in a sample of nine edge-on galaxies. We extract stellar and ionized gas rotation curves at and above the galaxies' midplanes using the Ca II triplet absorption features and H alpha emission lines measured with the GMOS spectrographs on Gemini-North and Gemini-South. For the higher mass galaxies in the sample, we fail to detect differences between the thin- and thick-disk kinematics. In the lower mass galaxies, there is a wide range of thick-disk behavior, including thick disks with substantial lag and one counterrotating thick disk. We compare our rotation curves with expectations from thick-disk formation models and conclude that the wide variety of thick-disk kinematics favors a formation scenario in which thick-disk stars are accreted or formed during merger events as opposed to models that form thick disks through gradual thin- disk heating.
Lick Indices In The Thin And Thick Disks Of Edge-On Disk Galaxies
(2008-08) Yoachim, Peter; Dalcanton, Julianne J.; Yoachim, Peter
We have measured Lick index equivalent widths to derive luminosity weighted stellar ages and metallicities for thin-and thick-disk-dominated regions of nine edge-on disk galaxies with the ARC 3.5 m telescope at Apache Point Observatory. In all cases, the thick disks are confirmed to be old stellar populations, with typical ages between 4 and 10 Gyr. The thin disks are uniformly younger than the thick disks, and show strong radial age gradients, with the outer regions of the disks being younger than 1 Gyr. We do not detect any significant metallicity differences or alpha-element enhancement in the thick-disk stars compared to the thin disk, due to the insensitivity of the Lick indices to these differences at low metallicity. We compare these results to thick disks measured in other systems and to predictions from thick-disk formation models.
On The Survival And Abundance Of Disk-Dominated Galaxies
(2009-05) Koda, Jin; Milosavljevic, Milos; Shapiro, Paul R.; Koda, Jin; Milosavljevic, Milos; Shapiro, Paul R.
We study the formation of disk-dominated galaxies in a Lambda cold dark matter (Lambda CDM) universe. Their existence is considered to be a challenge for the Lambda CDM cosmology, because galaxy mergers isotropize stellar disks and trigger angular momentum transport in gas disks, thus fostering the formation of central stellar spheroids. Here, we postulate that the formation of stellar spheroids from gas-rich disks is controlled by two parameters that characterize galaxy mergers, the mass ratio of merging dark matter halos, and the virial velocity of the larger merging halo. We utilize merger histories generated from realizations of the cosmological density field to calculate the fraction of dark matter halos that have avoided spheroid formation, and compare the derived statistics with the spheroid occupation fractions in surveys of nearby galaxies. We find, for example, that the survival rate of disk-dominated galaxies in Lambda CDM is just high enough to explain the observed fractional representation of disk-dominated galaxies in the universe if the only mergers which lead to central spheroid formation are those with mass ratios M(2)/M(1) > 0.3 and virial velocities V(vir), 1 > 55 km s(-1). We discuss the physical origin of this criterion.
Oxygen Abundances In Nearby FGK Stars And The Galactic Chemical Evolution Of The Local Disk And Halo
(2013-02) Ramirez, Ivan; Prieto, Carlos Allende; Lambert, David L.; Ramirez, Ivan; Prieto, Carlos Allende; Lambert, David L.
Atmospheric parameters and oxygen abundances of 825 nearby FGK stars are derived using high-quality spectra and a non-local thermodynamic equilibrium analysis of the 777 nm O I triplet lines. We assign a kinematic probability for the stars to be thin-disk (P-1), thick-disk (P-2), and halo (P-3) members. We confirm previous findings of enhanced [O/Fe] in thick-disk (P-2 > 0.5) relative to thin-disk (P-1 > 0.5) stars with [Fe/H] less than or similar to -0.2, as well as a "knee" that connects the mean [O/Fe]-[Fe/H] trend of thick-disk stars with that of thin-disk members at [Fe/H] greater than or similar to -0.2. Nevertheless, we find that the kinematic membership criterion fails at separating perfectly the stars in the [O/Fe]-[Fe/H] plane, even when a very restrictive kinematic separation is employed. Stars with "intermediate" kinematics (P-1 < 0.7, P-2 < 0.7) do not all populate the region of the [O/Fe]-[Fe/H] plane intermediate between the mean thin-disk and thick-disk trends, but their distribution is not necessarily bimodal. Halo stars (P-3 > 0.5) show a large star-to-star scatter in [O/Fe]-[Fe/H], but most of it is due to stars with Galactocentric rotational velocity V < -200 km s(-1); halo stars with V > -200 km s(-1) follow an [O/Fe]-[Fe/H] relation with almost no star-to-star scatter. Early mergers with satellite galaxies explain most of our observations, but the significant fraction of disk stars with "ambiguous" kinematics and abundances suggests that scattering by molecular clouds and radial migration have both played an important role in determining the kinematic and chemical properties of solar neighborhood stars.