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Status of the large-scale dark-matter axion search

Description: If axions constitute the dark matter of our galactic halo they can be detected by their conversion into monochromatic microwave photons in a high-Q microwave cavity permeated by a strong magnetic field. A large-scale experiment is under construction at LLNL to search for halo axions in the mass range 1.3 - 13 {mu}eV, where axions may constitute closure density of the universe. The search builds upon two pilot efforts at BNL and the University of Florida in the late 1980`s, and represents a large improvement in power sensitivity ({approximately}50) both due to the increase in magnetic volume (B{sup 2}V = 14 T{sup 2}m{sup 3}), and anticipated total noise temperature (T{sub n} {approximately}3K). This search will also mark the first use of multiple power-combined cavities to extend the mass range accessible by this technique. Data will be analyzed in two parallel streams. In the first, the resolution of the power spectrum will be sufficient to resolve the expected width of the overall axion line, {approximately}{bigcirc} (1kHz). In the second, the resolution will be {bigcirc}(O.01-1 Hz) to look for extremely narrow substructure reflecting the primordial phase-space of the axions during infall. This experiment will be the first to have the required sensitivity to detect axions, for plausible axion models.
Date: September 1, 1994
Creator: Van Bibber, K.; Hagmann, C.; Stoeffl, W.; Daw, E.; Rosenberg, L.; Sikivie, P. et al.
Partner: UNT Libraries Government Documents Department

Precision galactic structure

Description: Optical and IR surveys in progress or in the planning stages will lead to substantial improvements in our picture of the Milky Way as a consequence of their providing large volumes of data with much improved photometric and positional measurements compared with existing datasets.
Date: January 4, 2001
Creator: Kent, S.M.
Partner: UNT Libraries Government Documents Department

''Where is everybody. '' An account of Fermi's question

Description: Fermi's famous question, now central to debates about the prevalence of extraterrestrial civilizations, arose during a luncheon conversation with Emil Konopinski, Edward Teller, and Herbert York in the summer of 1950. Fermi's companions on that day have provided accounts of the incident.
Date: March 1, 1985
Creator: Jones, E.M.
Partner: UNT Libraries Government Documents Department

Long gamma-ray bursts and core-collapse supernovae have differentenvironments

Description: When massive stars exhaust their fuel they collapse andoften produce the extraordinarily bright explosions known ascore-collapse supernovae. On occasion, this stellar collapse also powersan even more brilliant relativistic explosion known as a long-durationgamma-ray burst. One would then expect that long gamma-ray bursts andcore-collapse supernovae should be found in similar galacticenvironments. Here we show that this expectation is wrong. We find thatthe long gamma-ray bursts are far more concentrated on the very brightestregions of their host galaxies than are the core-collapse supernovae.Furthermore, the host galaxies of the long gamma-ray bursts aresignificantly fainter and more irregular than the hosts of thecore-collapse supernovae. Together theseresults suggest thatlong-duration gamma-ray bursts are associated with the most massive starsand may be restricted to galaxies of limited chemical evolution. Ourresults directly imply that long gamma-ray bursts are relatively rare ingalaxies such as our own MilkyWay.
Date: May 1, 2006
Creator: Fruchter, A.S.; Levan, A.J.; Strolger, L.; Vreeswijk, P.M.; Thorsett, S.E.; Bersier, D. et al.
Partner: UNT Libraries Government Documents Department

High Rate for Type IC Supernovae

Description: Using an automated telescope we have detected 20 supernovae in carefully documented observations of nearby galaxies. The supernova rates for late spiral (Sbc, Sc, Scd, and Sd) galaxies, normalized to a blue luminosity of 10{sup 10} L{sub Bsun}, are 0.4 h{sup 2}, 1.6 h{sup 2}, and 1.1 h{sup 2} per 100 years for SNe type la, Ic, and II. The rate for type Ic supernovae is significantly higher than found in previous surveys. The rates are not corrected for detection inefficiencies, and do not take into account the indications that the Ic supernovae are fainter on the average than the previous estimates; therefore the true rates are probably higher. The rates are not strongly dependent on the galaxy inclination, in contradiction to previous compilations. If the Milky Way is a late spiral, then the rate of Galactic supernovae is greater than 1 per 30 {+-} 7 years, assuming h = 0.75. This high rate has encouraging consequences for future neutrino and gravitational wave observatories.
Date: September 1, 1991
Creator: Muller, R.A.; Marvin-Newberg, H.J.; Pennypacker, Carl R.; Perlmutter, S.; Sasseen, T.P. & Smith, C.K.
Partner: UNT Libraries Government Documents Department

Numerical Simulations of the Metallicity Distribution in Dwarf Spheroidal Galaxies

Description: Recent observations show that the number of stars with very low metallicities in the dwarf spheroidal satellites of the Milky Way is low, despite the low average metallicities of stars in these systems. We undertake numerical simulations of star formation and metal enrichment of dwarf galaxies in order to verify whether this result can be reproduced with ''standard'' assumptions. The answer is likely to be negative, unless some selection bias against very low metallicity stars is present in the observations.
Date: December 12, 2006
Creator: Ripamonti, Emanuele; Tolstoy, E.; Helmi, A.; Battaglia, G.; /Kapteyn Astron. Inst., Groningen; Abel, T. et al.
Partner: UNT Libraries Government Documents Department

Discovery of a New, Polar-Orbiting Debris Stream in the Milky Way Stellar Halo

Description: We show that there is a low metallicity tidal stream that runs along l = 143{sup o} in the South Galactic Cap, about 34 kpc from the Sun, discovered from SEGUE stellar velocities. Since the most concentrated detections are in the Cetus constellation, and the orbital path is nearly polar, we name it the Cetus Polar Stream (CPS). Although it is spatially coincident with the Sgr dwarf trailing tidal tail at b = -70{sup o}, the metallicities ([Fe/H] = -2.1), ratio of blue straggler to blue horizontal branch stars, and velocities of the CPS stars differ from Sgr. Some CPS stars may contaminate previous samples of Sgr dwarf tidal debris. The unusual globular cluster NGC 5824 is located along an orbit fit to the CPS, with the correct radial velocity.
Date: June 1, 2009
Creator: Newberg, Heidi Jo; Poly., /Rensselaer; Yanny, Brian; /Fermilab; Willett, Benjamin A. & Poly., /Rensselaer
Partner: UNT Libraries Government Documents Department

SPECTRAL IDENTIFICATION OF AN ANCIENT SUPERNOVA USING LIGHT ECHOES IN THE LMC

Description: We report the successful identification of the type of the supernova responsible for the supernova remnant SNR 0509-675 in the Large Magellanic Cloud (LMC) using Gemini spectra of surrounding light echoes. The ability to classify outbursts associated with centuries-old remnants provides a new window into several aspects of supernova research and is likely to be successful in providing new constraints on additional LMC supernovae as well as their historical counterparts in the Milky Way Galaxy (MWG). The combined spectrum of echo light from SNR 0509-675 shows broad emission and absorption lines consistent with a supernova (SN) spectrum. We create a spectral library consisting of 26 SNe Ia and 6 SN Ib/c that are time-integrated, dust-scattered by LMC dust, and reddened by the LMC and MWG. We fit these SN templates to the observed light echo spectrum using {chi}{sup 2} minimization as well as correlation techniques, and we find that overluminous 91T-like SNe Ia with {Delta}m{sub 15} < 0.9 match the observed spectrum best.
Date: February 7, 2008
Creator: Rest, A.; Matheson, T.; Blondin, S.; Bergmann, M.; Welch, D. L.; Suntzeff, N. B. et al.
Partner: UNT Libraries Government Documents Department

A Survey of Open Clusters in the u'g'r'i'z' Filter System. 3. Results for the Cluster NGC 188

Description: The authors continue the series of papers describing the results of a photometric survey of open star clusters, primarily in the southern hemisphere, taken in the u'g'r'i'z' filter system. The entire observed sample covered more than 100 clusters, but here they present data only on NGC 188, which is one of the oldest open clusters known in the Milky Way. They fit the Padova theoretical isochrones to the data. Assuming a solar metallicity for NGC 188, they find a distance of 1700 {+-} 100 pc, an age of 7.5 {+-} 0.7 Gyr, and a reddening E(B-V) of 0.025 {+-} 0.005. This yields a distance modulus of 11.23 {+-} 0.14.
Date: November 1, 2006
Creator: Fornal, Bartosz; Tucker, Douglas L.; Smith, J.Allyn; Allam, Sahar S.; Rider, Cristin J.; Sung, Hwankyung et al.
Partner: UNT Libraries Government Documents Department

Connecting Reionization to the Local Universe

Description: We present results of combined N-body and three-dimensional reionization calculations to determine the relationship between reionization history and local environment in a volume 1 Gpc h{sup -1} across and a resolution of about 1 Mpc. We resolve the formation of about 2 x 10{sup 6} halos of mass greater than {approx} 10{sup 12} M{sub {circle_dot}} at z = 0, allowing us to determine the relationship between halo mass and reionization epoch for galaxies and clusters. For our fiducial reionization model, in which reionization begins at z {approx} 15 and ends by z {approx} 6, we find a strong bias for cluster-size halos to be in the regions which reionized first, at redshifts 10 < z < 15. Consequently, material in clusters was reionized within relatively small regions, on the order of a few Mpc, implying that all clusters in our calculation were reionized by their own progenitors. Milky Way mass halos were on average reionized later and by larger regions, with a distribution most similar to the global one, indicating that low mass halos are nearly uncorrelated with reionization when only their mass is taken as a prior. On average, we find that most halos with mass less than 10{sup 13} M{sub {circle_dot}} were reionized internally, while almost all halos with masses greater than 10{sup 14} M{sub {circle_dot}} were reionized by their own progenitors. We briefly discuss the implications of this work in light of the 'missing satellites' problem and how this new approach may be extended further.
Date: August 3, 2009
Creator: Alvarez, Marcelo A.; Busha, Michael; Abel, Tom; Wechsler, Risa H. & /KIPAC, Menlo Park
Partner: UNT Libraries Government Documents Department

New resources to explore the old galaxy: Mining the SDSS

Description: The Sloan Digital Sky Survey (SDSS) is collecting photometry and intermediate resolution spectra for {approx} 10{sup 5} stars in the thick-disk and stellar halo of the Milky Way. This massive dataset can be used to infer the properties of the stars that make up these structures, and considerably deepen our vision of the old components of the Galaxy. We devise tools for automatic analysis of the SDSS photometric and spectroscopic data based on plane-parallel line-blanketed LTE model atmospheres and fast optimization algorithms. A preliminary study of about 5000 stars in the Early Data Release gives a hint of the vast amount of information that the SDSS stellar sample contains.
Date: July 1, 2003
Creator: al., C. Allende Prieto et
Partner: UNT Libraries Government Documents Department

MACHO RR lyrae in the inner halo and bulge

Description: The RR Lyrse in the bulge have been proposed to be the oldest populations in the Milky Way, tracers of how the galaxy formed. We study here the distribution of ?{approximately}1600 bulge RR Lyrae stars found by the MACHO Project. The RR Lyrae with 0.4 ? R ? 3 kpc show a density law that is well fit by the extension of the metal-poor stellar halo present in the outer regions of the Milky Way.
Date: October 1, 1998
Creator: Drake, A.; Alcock, C.; Allsman, R. A.; Alves, D. R.; Axelrod, T.S.; Becker, A. et al.
Partner: UNT Libraries Government Documents Department

Cosmological gamma-ray bursts

Description: This is the final report of a three-year, Laboratory-Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). Gamma-ray bursts are brief events that dominate the emission from all other gamma-ray objects in the sky, flicker for tens of seconds, and then turn off. Their nature remains uncertain despite years of efforts to understand them. One hypothesis is that the bursts arise within our galaxy albeit in an extended halo of neutron stars. Another hypothesis uses the isotropic distribution of gamma-ray bursts to argue that they come from nearly the edge of the universe. If gamma-ray bursts originate from cosmological distances, then the expansion of the universe should cause the dimmer (and presumably further) bursts to last longer. The authors have developed methods for measuring this time stretching, related the time stretching to the distance to the bursts, determined how the detailed physics causes temporal variations, and found the amount of total energy and peak luminosity that the events must be producing.
Date: April 1, 1996
Creator: Fenimore, E.; Epstein, R.; Ho, C. & Intzand, J.
Partner: UNT Libraries Government Documents Department

Interstellar colonization and the zoo hypothesis

Description: Michael Hart and others have pointed out that current estimates of the number of technological civilizations arisen in the Galaxy since its formation is in fundamental conflict with the expectation that such a civilization could colonize and utilize the entire Galaxy in 10 to 20 million years. This dilemma can be called Hart's paradox. Resolution of the paradox requires that one or more of the following are true: we are the Galaxy's first technical civilization; interstellar travel is immensely impractical or simply impossible; technological civilizations are very short-lived; or we inhabit a wildnerness preserve. The latter is the zoo hypothesis. (GHT)
Date: January 1, 1978
Creator: Jones, E.M.
Partner: UNT Libraries Government Documents Department

How Common are the Magellanic Clouds

Description: We introduce a probabilistic approach to the problem of counting dwarf satellites around host galaxies in databases with limited redshift information. This technique is used to investigate the occurrence of satellites with luminosities similar to the Magellanic Clouds around hosts with properties similar to the Milky Way in the object catalog of the Sloan Digital Sky Survey. Our analysis uses data from SDSS Data Release 7, selecting candidate Milky-Way-like hosts from the spectroscopic catalog and candidate analogs of the Magellanic Clouds from the photometric catalog. Our principal result is the probability for a Milky-Way-like galaxy to host N{sub sat} close satellites with luminosities similar to the Magellanic Clouds. We find that 81 percent of galaxies like the Milky Way have no such satellites within a radius of 150 kpc, 11 percent have one, and only 3.5 percent of hosts have two. The probabilities are robust to changes in host and satellite selection criteria, background-estimation technique, and survey depth. These results demonstrate that the Milky Way has significantly more satellites than a typical galaxy of its luminosity; this fact is useful for understanding the larger cosmological context of our home galaxy.
Date: May 20, 2011
Creator: Liu, Lulu; Gerke, Brian F.; Wechsler, Risa H.; Behroozi, Peter S.; Busha, Michael T. & /KIPAC, Menlo Park /SLAC
Partner: UNT Libraries Government Documents Department

Fossils of reionization in the local group

Description: We use a combination of high-resolution gas dynamics simulations of high-redshift dwarf galaxies and dissipationless simulations of a Milky Way sized halo to estimate the expected abundance and spatial distribution of the dwarf satellite galaxies that formed most of their stars around z {approx} 8 and evolved only little since then. Such galaxies can be considered as fossils of the reionization era, and studying their properties could provide a direct window into the early, pre-reionization stages of galaxy formation. We show that 5-15% of the objects existing at z {approx} 8 do indeed survive until the present in the MW like environment without significant evolution. This implies that it is plausible that the fossil dwarf galaxies do exist in the Local Group. Because such galaxies form their stellar systems early during the period of active merging and accretion, they should have spheroidal morphology regardless of their current distance from the host galaxy. We show that both the expected luminosity function and spatial distribution of dark matter halos which are likely to host fossil galaxies agree reasonably well with the observed distributions of the luminous (L{sub V} > 10{sup 6} Lsun) Local Group fossil candidates near the host galaxy (d<200 kpc). However, the predicted abundance is substantially larger (by a factor of 2-3) for fainter galaxies (L{sub V} < 10{sup 6} Lsun) at larger distances (d>300 kpc). We discuss several possible explanations for this discrepancy.
Date: January 1, 2006
Creator: Gnedin, Nickolay Y.; /Fermilab /KICP, Chicago /Chicago U., EFI; Kravtsov, Andrey V. & /KICP, Chicago /Chicago U., EFI /Chicago U., Astron. Astrophys. Ctr.
Partner: UNT Libraries Government Documents Department

A New View of the Dwarf Spheroidal Satellites of the Milky Way From VLT/FLAMES: Where are the Very Metal Poor Stars?

Description: As part of the Dwarf galaxies Abundances and Radial-velocities Team (DART) Programme, we have measured the metallicities of a large sample of stars in four nearby dwarf spheroidal galaxies (dSph): Sculptor, Sextans, Fornax and Carina. The low mean metal abundances and the presence of very old stellar populations in these galaxies have supported the view that they are fossils from the early Universe. However, contrary to naive expectations, we find a significant lack of stars with metallicities below [Fe/H] {approx} -3 dex in all four systems. This suggests that the gas that made up the stars in these systems had been uniformly enriched prior to their formation. Furthermore, the metal-poor tail of the dSph metallicity distribution is significantly different from that of the Galactic halo. These findings show that the progenitors of nearby dSph appear to have been fundamentally different from the building blocks of the Milky Way, even at the earliest epochs.
Date: November 20, 2006
Creator: Helmi, Amina; Irwin, M.J.; Tolstoy, E.; Battaglia, G.; Hill, V.; Jablonka, P. et al.
Partner: UNT Libraries Government Documents Department

M dwarfs in the Local Milky Way: The Field Low-Mass Stellar Luminosity and Mass Functions

Description: Modern sky surveys, such as the Sloan Digital Sky Survey (SDSS) and the Two-Micron All Sky Survey, have revolutionized how Astronomy is done. With millions of photometric and spectroscopic observations, global observational properties can be studied with unprecedented statistical significance. Low-mass stars dominate the local Milky Way, with tens of millions observed by SDSS within a few kpc. Thus, they make ideal tracers of the Galactic potential, and the thin and thick disks. In this thesis dissertation, I present my efforts to characterize the local low-mass stellar population, using a collection of observations from the Sloan Digital Sky Survey (SDSS). First, low-mass stellar template spectra were constructed from the co-addition of thousands of SDSS spectroscopic observations. These template spectra were used to quantify the observable changes introduced by chromospheric activity and metallicity. Furthermore, the average ugriz colors were measured as a function of spectral type. Next, the local kinematic structure of the Milky Way was quantified, using a special set of SDSS spectroscopic observations. Combining proper motions and radial velocities (measured using the spectral templates), along with distances, the full UVW space motions of over 7000 low-mass stars along one line of sight were computed. These stars were also separated kinematically to investigate other observational differences between the thin and thick disks. Finally, this dissertation details a project designed to measure the luminosity and mass functions of low-mass stars. Using a new technique optimized for large surveys, the field luminosity function (LF) and local stellar density profile are measured simultaneously. The sample size used to estimate the LF is nearly three orders of magnitude larger than any previous study, offering a definitive measurement of this quantity. The observed LF is transformed into a mass function (MF) and compared to previous studies.
Date: June 1, 2006
Creator: Bochanski, John J., Jr. & /Washington U., Seattle, Astron. Dept.
Partner: UNT Libraries Government Documents Department

Merger Histories of Galaxy Halos and Implications for Disk Survival

Description: The authors study the merger histories of galaxy dark matter halos using a high resolution {Lambda}CDM N-body simulation. The merger trees follow {approx} 17,000 halos with masses M{sub 0} = (10{sup 11} - 10{sup 13})h{sup -1}M{sub {circle_dot}} at z = 0 and track accretion events involving objects as small as m {approx_equal} 10{sup 10} h{sup -1}M{sub {circle_dot}}. They find that mass assembly is remarkably self-similar in m/M{sub 0}, and dominated by mergers that are {approx}10% of the final halo mass. While very large mergers, m {approx}> 0.4 M{sub 0}, are quite rare, sizeable accretion events, m {approx} 0.1 M{sub 0}, are common. Over the last {approx} 10 Gyr, an overwhelming majority ({approx} 95%) of Milky Way-sized halos with M{sub 0} = 10{sup 12} h{sup -1}M{sub {circle_dot}} have accreted at least one object with greater total mass than the Milky Way disk (m > 5 x 10{sup 10} h{sup -1}M{sub {circle_dot}}), and approximately 70% have accreted an object with more than twice that mass (m > 10{sup 11} h{sup -1}M{sub {circle_dot}}). The results raise serious concerns about the survival of thin-disk dominated galaxies within the current paradigm for galaxy formation in a {Lambda}CDM universe. in order to achieve a {approx} 70% disk-dominated fraction in Milky Way-sized {Lambda}CDM halos, mergers involving m {approx_equal} 2 x 10{sup 11} h{sup -1}M{sub {circle_dot}} objects must not destroy disks. Considering that most thick disks and bulges contain old stellar populations, the situation is even more restrictive: these mergers must not heat disks or drive gas into their centers to create young bulges.
Date: May 16, 2008
Creator: Stewart, Kyle R.; Bullock, James S.; Wechsler, Risa H.; Maller, Ariyeh H. & Zentner, Andrew R.
Partner: UNT Libraries Government Documents Department

SDSS spectroscopic survey of stars

Description: In addition to optical photometry of unprecedented quality, the Sloan Digital Sky Survey (SDSS) is also producing a massive spectroscopic database. They discuss determination of stellar parameters, such as effective temperature, gravity and metallicity from SDSS spectra, describe correlations between kinematics and metallicity, and study their variation as a function of the position in the Galaxy. They show that stellar parameter estimates by Beers et al. show a good correlation with the position of a star in the g-r vs. u-g color-color diagram, thereby demonstrating their robustness as well as a potential for photometric parameter estimation methods. Using Beers et al. parameters, they find that the metallicity distribution of the Milky Way stars at a few kpc from the galactic plane is bimodal with a local minimum at [Z/Z{sub {circle_dot}}] {approx} -1.3. The median metallicity for the low-metallicity [Z/Z{sub {circle_dot}}] < =1.3 subsample is nearly independent of Galactic cylindrical coordinates R and z, while it decreases with z for the high-metallicity [Z/Z{sub {circle_dot}}] > -1.3 sample. they also find that the low-metallicity sample has {approx} 2.5 times larger velocity dispersion and that it does not rotate (at the {approx} 10 km/s level), while the rotational velocity of the high-metallicity sample decreases smoothly with the height above the galactic plane.
Date: January 1, 2007
Creator: Ivezic, Zeljko; Schlegel, D.; Uomoto, A.; Bond, N.; Beers, T.; Allende Prieto, C. et al.
Partner: UNT Libraries Government Documents Department

The milky way tomography with sdss

Description: Using the photometric parallax method, we estimate the distances to {approx}48 million stars detected by the Sloan Digital Sky Survey (SDSS), and map their three-dimensional number density distribution in the Galaxy. The currently available data sample the distance range from 100 pc to 15 kpc and cover 6,500 deg{sup 2} of sky, mostly at high galactic latitudes (|b| > 25). These stellar number density maps allow an investigation of the Galactic structure without any a priori assumptions about its components. The data show strong evidence for a Galaxy consisting of an oblate halo, disk components, and a number of localized overdensities. The number density distribution of stars in the Solar neighborhood (D < 1.5kpc) favors a model having a ''thin'' and a ''thick'' exponential disk, with scale heights and lengths of H{sub 1} {approx} 280 pc and L{sub 1} {approx} 2400pc, and H{sub 2} {approx} 1200pc and L{sub 2} {approx} 3500pc, respectively, and local thick-to-thin disk normalization {rho}{sub thick} (R{sub {circle_dot}})/{rho}{sub thin}(R{sub {circle_dot}}) = 4%. Fits applied to the entire dataset are significantly more uncertain due to the presence of clumps and overdensities. The halo power law index is very poorly constrained, but we find an oblate halo with c/a {approx} 0.5 to be strongly preferred. While roughly consistent with this simple model, the measured density distribution shows a number of statistically significant deviations from the model predictions. In addition to known features, such as the Monoceros stream, a remarkable density enhancement covering over a thousand square degrees of sky is detected towards the constellation of Virgo, at distances of {approx} 5-15 kpc. Compared to counts in a region symmetric with respect to the l = 0 line and with the same Galactic latitude, it is responsible for a factor of 2 number density excess, and may be a nearby ...
Date: October 1, 2005
Creator: Juric, Mario; Ivezic, Zeljko; Brooks, Alyson; Lupton, Robert H.; Schlegel, David; Finkbeiner, Douglas et al.
Partner: UNT Libraries Government Documents Department

The Impact of Inhomogeneous Reionization on the Satellite Galaxy Population of the Milky Way

Description: We use the publicly available subhalo catalogs from the via Lactea simulation along with a Gpc-scale N-body simulation to understand the impact of inhomogeneous reionization on the satellite galaxy population of the Milky Way. The large-volume simulation is combined with a model for reionization that allows us to predict the distribution of reionization times for Milky Way mass halos. Motivated by this distribution, we identify candidate satellite galaxies in the simulation by requiring that any subhalo must grow above a specified mass threshold before it is reionized; after this time the photoionizing background will suppress both the formation of stars and the accretion of gas. We show that varying the reionization time over the range expected for Milky Way mass halos can change the number of satellite galaxies by roughly two orders of magnitude. This conclusion is in contradiction with a number of studies in the literature, and we conclude that this is a result of inconsistent application of the results of Gnedin (2000); subtle changes in the assumptions about how reionization affects star formation in small galaxies can lead to large changes in the effect of changing the reionization time on the number of satellites. We compare our satellite galaxies to observations using both abundance matching and stellar population synthesis methods to assign luminosities to our subhalos and account for observational completeness effects. Additionally, if we assume that the mass threshold is set by the virial temperature T{sub vir} = 8 x 10{sup 3} K we find that our model accurately matches the vmax distribution, radial distribution, and luminosity function of observed Milky Way satellites for a reionization time z{sub reion} = 9.6{sub -2.1}{sup 1.0}, assuming that the via Lactea subhalo distribution is representative of the Milky Way. This results in the presence of 119{sub -50}{sup +202} satellite galaxies.
Date: August 3, 2009
Creator: Busha, Michael T.; Alvarez, Marcelo A.; Wechsler, Risa H.; Abel, Tom; Strigari, Louis E. & /KIPAC, Menlo Park
Partner: UNT Libraries Government Documents Department

The Orbit of the Orphan Stream

Description: We use recent SEGUE spectroscopy and SDSS and SEGUE imaging data to measure the sky position, distance, and radial velocities of stars in the tidal debris stream that is commonly referred to as the 'Orphan Stream.' We fit orbital parameters to the data, and find a prograde orbit with an apogalacticon, perigalacticon, and eccentricity of 90 kpc, 16.4 kpc and e = 0.7, respectively. Neither the dwarf galaxy UMa II nor the Complex A gas cloud have velocities consistent with a kinematic association with the Orphan Stream. It is possible that Segue-1 is associated with the Orphan Stream, but no other known Galactic clusters or dwarf galaxies in the Milky Way lie along its orbit. The detected portion of the stream ranges from 19 to 47 kpc from the Sun and is an indicator of the mass interior to these distances. There is a marked increase in the density of Orphan Stream stars near (l, b) = (253{sup o}; 49{sup o}), which could indicate the presence of the progenitor at the edge of the SDSS data. If this is the progenitor, then the detected portion of the Orphan Stream is a leading tidal tail. We find blue horizontal branch (BHB) stars and F turnoff stars associated with the Orphan Stream. The turnoff color is (g-r){sub 0} = 0.22. The BHB stars have a low metallicity of [Fe/H]{sub WBG} = -2.1. The orbit is best fit to a halo potential with a halo plus disk mass of about 2.6 x 10{sup 11} M{sub {circle_dot}}, integrated to 60 kpc from the Galactic center. Our fits are done to orbits rather than full N-body simulations; we show that if N-body simulations are used, the inferred mass of the galaxy would be slightly smaller. Our best fit is found with a logarithmic halo speed ...
Date: January 1, 2010
Creator: Newberg, Heidi Jo; Willett, Benjamin A.; Yanny, Brian & Xu, Yan
Partner: UNT Libraries Government Documents Department