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2001

Series

Astronomy Department Faculty Publication Series

Articles 1 - 26 of 26

Full-Text Articles in Physical Sciences and Mathematics

Investigating The Long-Term Evolution Of Galaxies: Noise, Cuspy Halos And Bars, Martin D. Weinberg Oct 2001

Investigating The Long-Term Evolution Of Galaxies: Noise, Cuspy Halos And Bars, Martin D. Weinberg

Astronomy Department Faculty Publication Series

I review the arguments for the importance of halo structure in driving galaxy evolution and coupling a galaxy to its environment. We begin with a general discussion of the key dynamics and examples of structure dominated by modes. We find that simulations with large numbers of particles (N ≥ 106) are required to resolve the dynamics. Finally, I will describe some new results which demonstrates that a disk bar can produce cores in a cuspy CDM dark-matter profile within a gigayear. An inner Lindblad-like resonance couples the rotating bar to halo orbits at all radii through the cusp, rapidly flattening …

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Structure Of The Large Magellanic Cloud From 2mass, Md Weinberg, S Nikolaev Jan 2001

Structure Of The Large Magellanic Cloud From 2mass, Md Weinberg, S Nikolaev

Astronomy Department Faculty Publication Series

We derive structural parameters and evidence for extended tidal debris from star count and preliminary standard candle analyses of the Large Magellanic Cloud based on Two-Micron All-Sky Survey (2MASS) data. The full-sky coverage and low extinction in Ks present an ideal sample for structural analysis of the LMC. The star count surface densities and deprojected inclination for both young and older populations are generally consistent with previous work. We fit the projected density with an exponential disk or spherical power-law model with an optional two-dimensional elliptical bar. The derived disk scale length is R = 1.42 ± 0.01 kpc for …

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Noise-Driven Evolution In Stellar Systems - Ii. A Universal Halo Profile, Md Weinberg Jan 2001

Noise-Driven Evolution In Stellar Systems - Ii. A Universal Halo Profile, Md Weinberg

Astronomy Department Faculty Publication Series

Disc instabilities such as arm and bar formation, minor mergers and tidal encounters drive a galaxy from equilibrium. Using the theory that describes the evolution of a galaxy halo as a result of stochastic fluctuations developed in the companion paper to this one, we show that this sort of noise evolves a halo toward a standard profile, independent of its initial profile and concentration. This process can substantially redistribute the mass in dark-matter haloes in the 10 Gyr since formation. Three different noise processes are studied: (i) a bombardment by blobs of mass that are small compared to the halo …

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Periodic Photometric Variability In The Becklin-Neugebauer Object, Lynne A. Hillenbrand, John M. Carpenter, Michael F. Skrutskie Jan 2001

Periodic Photometric Variability In The Becklin-Neugebauer Object, Lynne A. Hillenbrand, John M. Carpenter, Michael F. Skrutskie

Astronomy Department Faculty Publication Series

The Becklin-Neugebauer (BN) object in the Orion Nebula Cluster (ONC) is a well-studied optically invisible, infrared-bright young stellar object, thought to be an intermediate-mass protostar. We report here that BN exhibited nearly-sinusoidal periodic variability at the near-infrared H- and Ks-bands during a one month observing campaign in 2000 March/April. The period was 8.28 days and the peak-to-peak amplitude ~0.2 mag. Plausible mechanisms for producing the observed variability characteristics are explored.

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Three Wide Separation L-Dwarf Companions From The Two Micron All Sky Survey: Gl 337c, Gl 618.1b, And Hd 89744b, J. C. Wilson, Davy Kirkpatrick, G. E. Gizis, M. F. Skrutskie, D. G. Monet, J. R. Houck Jan 2001

Three Wide Separation L-Dwarf Companions From The Two Micron All Sky Survey: Gl 337c, Gl 618.1b, And Hd 89744b, J. C. Wilson, Davy Kirkpatrick, G. E. Gizis, M. F. Skrutskie, D. G. Monet, J. R. Houck

Astronomy Department Faculty Publication Series

We present two confirmed wide separation L-dwarf common proper motion companions to nearby stars and one candidate identified from the Two Micron All Sky Survey. Spectral types from optical spectroscopy are L0 V, L2.5 V, and L8 V. Near-infrared low resolution spectra of the companions are provided as well as a grid of known objects spanning M6 V -- T dwarfs to support spectral type assignment for these and future L-dwarfs in the z'JHK bands. Using published measurements, we estimate ages of the companions from physical properties of the primaries. These crude ages allow us to estimate companion masses using …

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Near-Infrared Photometric Variability Of Stars Toward The Orion A Molecular Cloud, John M. Carpenter, Lynne A. Hillenbrand, M. F. Skrutskie Jan 2001

Near-Infrared Photometric Variability Of Stars Toward The Orion A Molecular Cloud, John M. Carpenter, Lynne A. Hillenbrand, M. F. Skrutskie

Astronomy Department Faculty Publication Series

We present an analysis of J, H, and Ks time-series photometry obtained with the southern 2MASS telescope over a 084 × 6° region centered near the Trapezium region of the Orion Nebula cluster. These data are used to establish the near-infrared variability properties of pre–main-sequence stars in Orion on timescales of ∼1–36 days, ∼2 months, and ∼2 years. A total of 1235 near-infrared variable stars are identified, ∼93% of which are likely associated with the Orion A molecular cloud. The variable stars exhibit a diversity of photometric behavior with time, including cyclic fluctuations with periods up to 15 days, aperiodic …

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Cormass: A Compact And Efficient Near‐Infrared Spectrograph For Studying Low‐Mass Objects, J. C. Wilson, M. F. Skrutskie, M. R. Colonno, A. T. Enos, J. D. Smith, C. P. Henderson, J. E. Gizis, D. G. Monet, J. R. Houck Jan 2001

Cormass: A Compact And Efficient Near‐Infrared Spectrograph For Studying Low‐Mass Objects, J. C. Wilson, M. F. Skrutskie, M. R. Colonno, A. T. Enos, J. D. Smith, C. P. Henderson, J. E. Gizis, D. G. Monet, J. R. Houck

Astronomy Department Faculty Publication Series

This is a pre-published version which is collected from arXiv. The published version is at http://www.journals.uchicago.edu/doi/abs/10.1086/318619.

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Cooling Radiation And The Ly Alpha Luminosity Of Forming Galaxies, Ma Fardal, N Katz, Jp Gardner, L Hernquist, Dh Weinberg, R Dave Jan 2001

Cooling Radiation And The Ly Alpha Luminosity Of Forming Galaxies, Ma Fardal, N Katz, Jp Gardner, L Hernquist, Dh Weinberg, R Dave

Astronomy Department Faculty Publication Series

We examine the cooling radiation from forming galaxies in hydrodynamic simulations of the LCDM model (cold dark matter with a cosmological constant), focusing on the Lyα line luminosities of high-redshift systems. Primordial composition gas condenses within dark matter potential wells, forming objects with masses and sizes comparable to the luminous regions of observed galaxies. As expected, the energy radiated in this process is comparable to the gravitational binding energy of the baryons, and the total cooling luminosity of the galaxy population peaks at z 2. However, in contrast to the classical picture of gas cooling from the ~106 K …

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Metal Enrichment Of The Intergalactic Medium In Cosmological Simulations, A Aguirre, L Hernquist, J Schaye, N Katz, Dh Weinberg, J Gardner Jan 2001

Metal Enrichment Of The Intergalactic Medium In Cosmological Simulations, A Aguirre, L Hernquist, J Schaye, N Katz, Dh Weinberg, J Gardner

Astronomy Department Faculty Publication Series

Observations have established that the diffuse intergalactic medium (IGM) at z ~ 3 is enriched to ~10-2.5 solar metallicity and that the hot gas in large clusters of galaxies (ICM) is enriched to - Z at z = 0. Metals in the IGM may have been removed from galaxies (in which they presumably form) during dynamical encounters between galaxies, by ram-pressure stripping, by supernova-driven winds, or as radiation-pressure-driven dust efflux. This study develops a method of investigating the chemical enrichment of the IGM and of galaxies, using already completed cosmological simulations. To these simulations we add dust and (gaseous) …

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Hydrodynamic Simulation Of The Cosmological X-Ray Background, Rac Croft, T Di Matteo, L Hernquist, N Katz, Ma Fardal, Dh Weinberg Jan 2001

Hydrodynamic Simulation Of The Cosmological X-Ray Background, Rac Croft, T Di Matteo, L Hernquist, N Katz, Ma Fardal, Dh Weinberg

Astronomy Department Faculty Publication Series

We use a hydrodynamic simulation of an inflationary cold dark matter model with a cosmological constant to predict properties of the extragalactic X-ray background (XRB). We focus on emission from the intergalactic medium (IGM), with particular attention to diffuse emission from warm-hot gas that lies in relatively smooth filamentary structures between galaxies and galaxy clusters. We also include X-rays from point sources associated with galaxies in the simulation, and we make maps of the angular distribution of the emission. Although much of the X-ray luminous gas has a filamentary structure, the filaments are not evident in the simulated maps because …

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Group Scaling Relations From A Cosmological Hydrodynamic Simulation: No Pre-Heating Required?, R Dave, N Katz, L Hernquist, D Weinberg Jan 2001

Group Scaling Relations From A Cosmological Hydrodynamic Simulation: No Pre-Heating Required?, R Dave, N Katz, L Hernquist, D Weinberg

Astronomy Department Faculty Publication Series

We investigate the X-ray vs. optical scaling relations of poor groups to small clusters (sigma~100-700 km/s) identified in a cosmological hydrodynamic simulation of a Lambda-CDM universe, with cooling and star formation but no pre-heating. We find that the scaling relations between X-ray luminosity, X-ray temperature, and velocity dispersion show significant departures from the relations predicted by simple hydrostatic equilibrium models or simulations without cooling, having steeper L_X-sigma and L_X-T_X slopes and a "break" at \~200 km/s (~0.3 keV). These departures arise because the hot (X-ray emitting) gas fraction varies substantially with halo mass in this regime. Our predictions roughly agree …

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Rosat X-Ray Observations Of The Spiral Galaxy M81, S Immler, Qd Wang Jan 2001

Rosat X-Ray Observations Of The Spiral Galaxy M81, S Immler, Qd Wang

Astronomy Department Faculty Publication Series

We present results from the analysis of deep ROSAT HRI and PSPC observations of the spiral galaxy M81. The inferred total (0.5-2 keV band) luminosity of M81 is ~3 × 1040 ergs s-1, excluding the contribution from identified interlopers found within the D25 ellipse. The nucleus of the galaxy alone accounts for about 65% of this luminosity. The rest is due to 26 other X-ray sources (contributing ~10%) and to apparently diffuse emission, which is seen across much of the galactic disk and is particularly bright in the bulge region around the nucleus. Spectral analysis further …

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Ngc 5775: Anatomy Of A Disk-Halo Interface, S-W Lee, Ja Irwin, R-J Dettmar, Ct Cunningham, G Golla, Qd Wang Jan 2001

Ngc 5775: Anatomy Of A Disk-Halo Interface, S-W Lee, Ja Irwin, R-J Dettmar, Ct Cunningham, G Golla, Qd Wang

Astronomy Department Faculty Publication Series

We present the first high-resolution study of the disk-halo interface in an edge-on galaxy (NGC 5775) in which every component of the interstellar medium is represented and resolved (though not all to the same resolution). New single-dish CO J=2-1 and CO J=1-0 data, ROSAT X-ray data, and HIRES IRAS data are presented along with HI data which emphasizes the high latitude features. In conjunction with previously published radio continuum (6 and 20 cm) and H data, we find spatial correlations between various ISM components in that all components of the ISM are present in the disk-halo features (except …

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Detection Of An X-Ray Pulsar Wind Nebula And Tail In Snr N157b, Qd Wang, Ev Gtthelf, Y-H Chu, Jr Dickel Jan 2001

Detection Of An X-Ray Pulsar Wind Nebula And Tail In Snr N157b, Qd Wang, Ev Gtthelf, Y-H Chu, Jr Dickel

Astronomy Department Faculty Publication Series

We report Chandra X-ray observations of the supernova remnant N157B in the Large Magellanic Cloud, which are presented together with an archival Hubble Space Telescope optical image and a radio continuum map for comparison. This remnant contains the recently discovered 16 ms X-ray pulsar PSR J0537-6910, the most rapidly rotating young pulsar known. Using phase-resolved Chandra imaging, we pinpoint the location of the pulsar at 5h37m47.s36, -69°10'204 (J2000), with an uncertainty of 1''. PSR J0537-6910 is not detected in any other wavelength band. The X-ray observations resolve three distinct features: the pulsar itself, a surrounding compact …

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Discovery Of Molecular Gas In The Outflow And Tidal Arms Around M82, Cl Taylor, F Walter, Min Yun Jan 2001

Discovery Of Molecular Gas In The Outflow And Tidal Arms Around M82, Cl Taylor, F Walter, Min Yun

Astronomy Department Faculty Publication Series

We present the first fully sampled map of 12CO (1-0) emission from M82 covering the entire galaxy. Our map contains an ~12 × 15 kpc2 area. We find that extraplanar CO emission, previously reported at short distances above the galactic plane, extends to heights of up to 6 kpc above the disk. Some of this emission is associated with tidal arms seen in H I, implying either that M82 contained substantial amounts of molecular gas in the outer disk or that molecular gas formed after the tidal features. CO emission along the direction of the outflow extends to …

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X-Ray Detection Of Presupernova Evolution For The Sn 1993j Progenitor, S Immler, B Aschenbach, Qd Wang Jan 2001

X-Ray Detection Of Presupernova Evolution For The Sn 1993j Progenitor, S Immler, B Aschenbach, Qd Wang

Astronomy Department Faculty Publication Series

We report on the first detection of presupernova evolution in the X-ray regime. The results are based on ROSAT observations of SN 1993J ranging from 6 days to 5 years after the outburst. The X-ray observations are used to probe the SN shell interaction with the ambient circumstellar matter (CSM). After exploring various scenarios that might explain the observed X-ray light curve with a t-0.27 rate of decline, we present a coherent picture in terms of the interaction of the SN shock front with the CSM deposited by the progenitor's stellar wind. During the observed period, the SN shell …

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Noise-Driven Evolution In Stellar Systems - I. Theory, Md Weinberg Jan 2001

Noise-Driven Evolution In Stellar Systems - I. Theory, Md Weinberg

Astronomy Department Faculty Publication Series

We present a theory for describing the evolution of a galaxy caused by stochastic events such as weak mergers, transient spiral structure, orbiting blobs, etc. This noise excites large-scale patterns that drive the evolution of the galactic density profile. In a dark matter halo, the repeated stochastic perturbations preferentially ring the lowest-order modes with only a very weak dependence on the details of their source. The subsequent redistribution of halo mass is determined only by the mechanics of these modes. The halo profile then evolves toward a universal asymptotic form for a wide variety of noise sources. Such a convergence …

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The Equilibrium State Of Molecular Regions In The Outer Galaxy, Mh Heyer, Jm Carpenter, Ronald L. Snell Jan 2001

The Equilibrium State Of Molecular Regions In The Outer Galaxy, Mh Heyer, Jm Carpenter, Ronald L. Snell

Astronomy Department Faculty Publication Series

A summary of global properties and an evaluation of the equilibrium state of molecular regions in the outer Galaxy are presented from the decomposition of the FCRAO Outer Galaxy Survey and targeted 12CO and 13CO observations of four giant molecular cloud complexes. The ensemble of identified objects includes both small, isolated clouds and clumps within larger cloud complexes. The 12CO luminosity function and size distribution of a subsample of objects with well-defined distances are determined such that ΔN/ΔLCO = L and ΔN/Δre = r. 12CO velocity dispersions …

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Constraints On Cosmological Parameters From The Ly Alpha Forest Power Spectrum And Cobe Dmr, J Phillips, Dh Weinberg, Rac Croft, L Hernquist, N Katz, M Pettini Jan 2001

Constraints On Cosmological Parameters From The Ly Alpha Forest Power Spectrum And Cobe Dmr, J Phillips, Dh Weinberg, Rac Croft, L Hernquist, N Katz, M Pettini

Astronomy Department Faculty Publication Series

We combine COBE DMR measurements of cosmic microwave background (CMB) anisotropy with a recent measurement of the mass power spectrum at redshift z = 2.5 from Lyα forest data to derive constraints on cosmological parameters and test the inflationary cold dark matter (CDM) scenario of structure formation. By treating the inflationary spectral index n as a free parameter, we are able to find successful fits to the COBE and Lyα forest constraints in Ωm = 1 models with and without massive neutrinos and in low-Ωm models with and without a cosmological constant. Within each class of model, the …

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Simulations Of Damped Ly Alpha And Lyman Limit Absorbers In Different Cosmologies: Implications For Structure Formation At High Redshift, Jp Gardner, N Katz, L Hernquist, Dh Weinberg Jan 2001

Simulations Of Damped Ly Alpha And Lyman Limit Absorbers In Different Cosmologies: Implications For Structure Formation At High Redshift, Jp Gardner, N Katz, L Hernquist, Dh Weinberg

Astronomy Department Faculty Publication Series

We use hydrodynamic cosmological simulations to study damped Lyα (DLA) and Lyman limit (LL) absorption at redshifts z = 2-4 in five variants of the cold dark matter scenario: COBE-normalized (CCDM), cluster-normalized (SCDM), and tilted (n = 0.8) Ωm = 1 models, as well as open (OCDM) and flat (LCDM) Ωm = 0.4 models. Our standard simulations resolve the formation of dense concentrations of neutral gas in halos with circular velocity vc ≥ vc,res 140 km s-1 for Ωm = 1 and 90 km s-1 for Ωm = 0.4, …

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Enrichment Of The Intergalactic Medium By Radiation Pressure-Driven Dust Efflux, A Aguirre, L Hernquist, N Katz, J Gardner, D Weinberg Jan 2001

Enrichment Of The Intergalactic Medium By Radiation Pressure-Driven Dust Efflux, A Aguirre, L Hernquist, N Katz, J Gardner, D Weinberg

Astronomy Department Faculty Publication Series

The presence of metals in hot cluster gas and in Lyα absorbers, as well as the mass-metallicity relation of observed galaxies, suggest that galaxies lose a significant fraction of their metals to the intergalactic medium (IGM). Theoretical studies of this process have concentrated on metal removal by dynamical processes or supernova-driven winds. Here we investigate the enrichment of the IGM by the expulsion of dust grains from galaxies by radiation pressure. We use already completed cosmological simulations to which we add dust, assuming that most dust can reach the equilibrium point between radiation pressure and gravitational forces. We find that …

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Baryons In The Warm-Hot Intergalactic Medium, R Dave, R Cen, Jp Ostriker, Gl Bryan, L Hernquist, N Katz, Dh Weinberg, Ml Norman, B O'Shea Jan 2001

Baryons In The Warm-Hot Intergalactic Medium, R Dave, R Cen, Jp Ostriker, Gl Bryan, L Hernquist, N Katz, Dh Weinberg, Ml Norman, B O'Shea

Astronomy Department Faculty Publication Series

Approximately 30%-40% of all baryons in the present-day universe reside in a warm-hot intergalactic medium (WHIM), with temperatures in the range 105 < T < 107 K. This is a generic prediction from six hydrodynamic simulations of currently favored structure formation models having a wide variety of numerical methods, input physics, volumes, and spatial resolutions. Most of these warm-hot baryons reside in diffuse large-scale structures with a median overdensity around 10-30, not in virialized objects such as galaxy groups or galactic halos. The evolution of the WHIM is primarily driven by shock heating from gravitational perturbations breaking on mildly nonlinear, nonequilibrium …

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The Milky Way As A Key To Structural Evolution In Galaxies, Md Weinberg Jan 2001

The Milky Way As A Key To Structural Evolution In Galaxies, Md Weinberg

Astronomy Department Faculty Publication Series

Much of our effort in understanding the long-term evolution and morphology of theMilkyWay and other galaxies has focused on the equilibrium of its luminous disk. However, the interplay between all components, seen and unseen, is a major cause of observed features and drives evolution. I will review the key underlying dynamics, and in a number of examples, show how this leads to lopsidedness and offset nuclei, may trigger bars and cause warps. Indeed, the Milky Way like most spiral galaxies show exhibit many of these features. In addition, the mechanisms suggest that observed morphology depends on the properties of the …

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Where Is The Neutral Atomic Gas In Hickson Groups?, L Verdes-Montenegro, Min Yun, Ba Williams, Wk Huchtmeier, A Del Olmo, J Perea Jan 2001

Where Is The Neutral Atomic Gas In Hickson Groups?, L Verdes-Montenegro, Min Yun, Ba Williams, Wk Huchtmeier, A Del Olmo, J Perea

Astronomy Department Faculty Publication Series

We have analyzed the total HI contents of 72 Hickson compact groups of galaxies (HCGs) and the detailed spatial distributions and kinematics of HI within a subset of 16 groups using the high angular resolution observations obtained with the VLA in order to investigate a possible evolutionary scenario for these densest systems in the present day galaxy hierarchy. For the more homogeneous subsample of 48 groups, we found a mean HI deficiency of , which corresponds to 40% of the expected HI for the optical luminosities and morphological types of the member galaxies. The individual galaxies show larger degrees of …

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Sub-Mm Galaxies In Cosmological Simulations, Ma Fardal, N Katz, Dh Weinberg, R Dave, L Hernquist Jan 2001

Sub-Mm Galaxies In Cosmological Simulations, Ma Fardal, N Katz, Dh Weinberg, R Dave, L Hernquist

Astronomy Department Faculty Publication Series

We study the predicted sub-mm emission from massive galaxies in a Lambda-CDM universe, using hydrodynamic cosmological simulations. Assuming that most of the emission from newly formed stars is absorbed and reradiated in the rest-frame far-IR, we calculate the number of galaxies that would be detected in sub-mm surveys conducted with SCUBA. The predicted number counts are strongly dependent on the assumed dust temperature and emissivity law. With plausible choices for SED parameters (e.g., T=35 K, beta=1.0), the simulation predictions reproduce the observed number counts above ~ 1 mJy. The sources have a broad redshift distribution with median z ~ 2, …

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Chandra Detection Of A Hot Gaseous Corona Around The Edge-On Galaxy Ngc 4631, Qd Wang, S Immler, R Walterbos, Jt Lauroesch, D Breitschwerdt Jan 2001

Chandra Detection Of A Hot Gaseous Corona Around The Edge-On Galaxy Ngc 4631, Qd Wang, S Immler, R Walterbos, Jt Lauroesch, D Breitschwerdt

Astronomy Department Faculty Publication Series

We present a Chandra X-ray observation that shows, unambiguously for the first time, the presence of a giant diffuse X-ray-emitting corona around the edge-on disk galaxy NGC 4631. This corona, with a characteristic thermal temperature of × 106 K, extends as far as 8 kpc away from the galactic plane. The X-ray morphology resembles the radio halo of the galaxy, indicating a close connection between outflows of hot gas, cosmic rays, and the magnetic field from the galactic disk. Enhanced diffuse X-ray emission is apparently enclosed by numerous Hα-emitting loops blistered out from the central disk of the galaxy, …

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