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Astrophysics and Astronomy

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University of Massachusetts Amherst

1999

Articles 1 - 20 of 20

Full-Text Articles in Physical Sciences and Mathematics

Tracing Interactions In Hickson Compact Groups Through The Hi, L Verdes-Montenegro, Min S. Yun, B A. Williams, W K. Huchtmeier, A Del Olmo, J Perea Sep 1999

Tracing Interactions In Hickson Compact Groups Through The Hi, L Verdes-Montenegro, Min S. Yun, B A. Williams, W K. Huchtmeier, A Del Olmo, J Perea

Min S. Yun

We present a global study of HI spectral line mapping for 16 Hickson Compact Groups (HCGs) combining new and unpublished VLA data, plus the analysis of the HI content of individual galaxies. Sixty percent of the groups show morphological and kinematical signs of perturbations (from multiple tidal features to concentration of the HI in a single enveloping cloud) and sixty five of the resolved galaxies are found to be HI deficient with respect to a sample of isolated galaxies. In total, 77% of the groups suffer interactions among all its members which provides strong evidence of their reality. We find …

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Luminosity Profiles Of Merger Remnants, J E. Hibbard, Min S. Yun Aug 1999

Luminosity Profiles Of Merger Remnants, J E. Hibbard, Min S. Yun

Min S. Yun

Using published luminosity and molecular gas profiles of the late-stage mergers NGC 3921, NGC 7252, and Arp 220, we examine the expected luminosity profiles of the evolved merger remnants, especially in light of the massive CO complexes that are observed in their nuclei. For NGC 3921 and NGC 7252, we predict that the resulting luminosity profiles will be characterized by an r1/4 law. In view of previous optical work on these systems, it seems likely that they will evolve into normal elliptical galaxies as regards their optical properties. Due to a much higher central molecular gas column density, Arp 220 …

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Detection Of Co (2-1) And Radio Continuum Emission From The Z = 4.4 Qso Bri 1335–0417, C L. Carilli, Karl M. Menten, Min S. Yun Aug 1999

Detection Of Co (2-1) And Radio Continuum Emission From The Z = 4.4 Qso Bri 1335–0417, C L. Carilli, Karl M. Menten, Min S. Yun

Min S. Yun

We have detected redshifted CO (2-1) emission at 43 GHz and radio continuum emission at 1.47 and 4.86 GHz from the z = 4.4 QSO BRI 1335-0417 using the Very Large Array. The CO data imply optically thick emission from warm (>30 K) molecular gas with a total mass, M(H2), of (1.5 ± 0.3) × 1011 Msun, using the standard Galactic gas mass-to-CO luminosity conversion factor. We set an upper limit to the CO source size of 1farcs1 and a lower limit of 0farcs23(Tex/50 K)-1/2, where Tex is the gas excitation temperature. We derive an upper limit to the …

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Perturbations Of Spherical Stellar Systems During Fly-By Encounters, E. Vesperini, Martin D. Weinberg Aug 1999

Perturbations Of Spherical Stellar Systems During Fly-By Encounters, E. Vesperini, Martin D. Weinberg

Astronomy Department Faculty Publication Series

We study the internal response of a galaxy to an unbound encounter and present a survey of orbital parameters covering typical encounters in different galactic environments. Overall, we conclude that relatively weak encounters by low-mass interloping galaxies can cause observable distortions in the primaries. The resulting asymmetries may persist long after the interloper is evident. We focus our attention on the production of structure in dark halos and in cluster ellipticals. Any distortion produced in a dark halo can distort the embedded stellar disk, possibly leading to the formation of lopsided and warped disks. We show that distant encounters with …

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Molecular Gas In The Z = 2.565 Submillimeter Galaxy Smm J14011+0252, D T. Frayer, R J. Ivison, N Z. Scoville, A S. Evans, Min S. Yun, Ian Smail, A J. Barger, A W. Blain, J P. Kneib Mar 1999

Molecular Gas In The Z = 2.565 Submillimeter Galaxy Smm J14011+0252, D T. Frayer, R J. Ivison, N Z. Scoville, A S. Evans, Min S. Yun, Ian Smail, A J. Barger, A W. Blain, J P. Kneib

Min S. Yun

We report the detection of CO (3 r 2) emission from the submillimeter-selected luminous galaxy SMM J1401110252. The optical counterpart of the submillimeter source has been identified as a merger system with spectral characteristics consistent with a starburst at z 5 2.565. The CO emission confirms the optical identification of the submillimeter source and implies a molecular gas mass of M,5#1010 h22 , after correcting 75 for a lensing amplification factor of 2.75. The large molecular gas mass and the radio emission are consistent with the starburst interpretation of the source. These results are similar to those found for SMM …

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The Radio-To-Submillimeter Spectral Index As A Redshift Indicator, C L. Carilli, Min S. Yun Mar 1999

The Radio-To-Submillimeter Spectral Index As A Redshift Indicator, C L. Carilli, Min S. Yun

Min S. Yun

We present models of the 1.4 to 350 GHz spectral index 350 for starburst galaxies as a function of redshift. a1.4 The models include a semianalytic formulation, based on the well-quantified radio–to–far-infrared correlation for low-redshift star-forming galaxies, and an empirical formulation, based on the observed spectrum of the starburst galaxies M82 and Arp 220. We compare the models to the observed values of 350 for starburst galaxies at low a1.4 and high redshift. We find reasonable agreement between the models and the observations and, in particular, that an observed spectral index of 350 indicates that the target source is likely …

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Co Distribution And Kinematics Along The Bar In The Strongly Barred Spiral Ngc 7479, S Laine, J D P Kenny, Min S. Yun, S T. Gottesman Feb 1999

Co Distribution And Kinematics Along The Bar In The Strongly Barred Spiral Ngc 7479, S Laine, J D P Kenny, Min S. Yun, S T. Gottesman

Min S. Yun

We report on the 2farcs5 (400 pc) resolution CO (J=1→0) observations covering the whole length of the bar in the strongly barred late-type spiral galaxy NGC 7479. CO emission is detected only along a dust lane that traverses the whole length of the bar, including the nucleus. The emission is strongest in the nucleus. The distribution of emission is clumpy along the bar outside the nucleus and consists of gas complexes that are unlikely to be gravitationally bound. The CO kinematics within the bar consist of two separate components. A kinematically distinct circumnuclear disk, <500 pc in diameter, is undergoing predominantly circular motion with a maximum rotational velocity of 245 km s-1 at a radius of 1'' (160 pc). The CO-emitting gas in the bar outside the circumnuclear disk has substantial noncircular motions that are consistent with a large radial velocity component, directed inward. The CO emission has a large velocity gradient across the bar dust lane, ranging from 0.5 to 1.9 km s-1 pc-1 after correcting for inclination, and the projected velocity change across the dust lane is as high as 200 km s-1. This sharp velocity gradient is consistent with a shock front at the location of the bar dust lane. A comparison of Hα and CO kinematics across the dust lane shows that, although the Hα emission is often observed both upstream and downstream from the dust lane, the CO emission is observed only where the velocity gradient is large. We also compare the observations with hydrodynamic models and discuss star formation along the bar.

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An Adaptive Algorithm For N-Body Field Expansions, Md Weinberg Jan 1999

An Adaptive Algorithm For N-Body Field Expansions, Md Weinberg

Astronomy Department Faculty Publication Series

An expansion of a density field or particle distribution in basis functions that solve the Poisson equation both provides an easily parallelized N-body force algorithm and simplifies perturbation theories. The expansion converges quickly and provides the highest computational advantage if the lowest order potential-density pair in the basis looks like the unperturbed galaxy or stellar system. Unfortunately, there are only a handful of such bases in the literature that limit this advantage. This paper presents an algorithm for deriving these bases to match a wide variety of galaxy models. The method is based on efficient numerical solution of the Sturm-Liouville …

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Closing In On Omega(M): The Amplitude Of Mass Fluctuations From Galaxy Clusters And The Ly Alpha Forest, Dh Weinberg, Rac Croft, L Hernquist, N Katz, M Pettini Jan 1999

Closing In On Omega(M): The Amplitude Of Mass Fluctuations From Galaxy Clusters And The Ly Alpha Forest, Dh Weinberg, Rac Croft, L Hernquist, N Katz, M Pettini

Astronomy Department Faculty Publication Series

We estimate the present-day value of the matter density parameter ΩM by combining constraints from the galaxy cluster mass function with Croft et al.'s recent measurement of the mass power spectrum, P(k), from Lyα forest data. The key assumption of the method is that cosmic structure formed by gravitational instability from Gaussian primordial fluctuations. For a specified value of ΩM, matching the observed cluster mass function then fixes the value of σ8, the rms amplitude of mass fluctuations in 8 h-1 Mpc spheres, and it thus determines the normalization of P …

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Properties Of Galaxy Clusters: Mass And Correlation Functions, F Governato, A Babul, T Quinn, P Tozzi, Cm Baugh, N Katz, G Lake Jan 1999

Properties Of Galaxy Clusters: Mass And Correlation Functions, F Governato, A Babul, T Quinn, P Tozzi, Cm Baugh, N Katz, G Lake

Astronomy Department Faculty Publication Series

We analyse parallel N-body simulations of three cold dark matter (CDM) universes to study the abundance and clustering of galaxy clusters. The simulation boxes are 500 h−1 Mpc on a side and cover a volume comparable to that of the forthcoming Sloan Digital Sky Survey. The use of a treecode algorithm and 47 million particles allows us at the same time to achieve high mass and force resolution. We are thus able to make robust measurements of cluster properties with good number statistics up to a redshift larger than unity. We extract haloes using two independent, public domain …

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The Clustering Of High-Redshift Galaxies In The Cold Dark Matter Scenario, N Katz, L Hernquist, Dh Weinberg Jan 1999

The Clustering Of High-Redshift Galaxies In The Cold Dark Matter Scenario, N Katz, L Hernquist, Dh Weinberg

Astronomy Department Faculty Publication Series

We investigate the clustering of high-redshift galaxies in five variants of the cold dark matter (CDM) scenario, using hydrodynamic cosmological simulations that resolve the formation of systems with circular velocities vc ≥ 100 km s-1 (Ω = 1) or vc ≥ 70 km s-1 (Ω = 0.4). Although the five models differ in their cosmological parameters and in the shapes and amplitudes of their mass power spectra, they predict remarkably similar galaxy clustering at z = 2, 3, and 4. The galaxy correlation functions show almost no evolution over this redshift range, even though the mass …

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The Power Spectrum Of Mass Fluctuations Measured From The Ly Alpha Forest At Redshift Z=2.5, Rac Croft, Dh Weinberg, M Pettini, L Hernquist, N Katz Jan 1999

The Power Spectrum Of Mass Fluctuations Measured From The Ly Alpha Forest At Redshift Z=2.5, Rac Croft, Dh Weinberg, M Pettini, L Hernquist, N Katz

Astronomy Department Faculty Publication Series

We measure the linear power spectrum of mass-density fluctuations at redshift z = 2.5 from the Lyα forest absorption in a sample of 19 QSO spectra, using the method introduced by Croft et al. The P(k) measurement covers the range 2π/k ~ 450-2350 km s-1 (2-12 comoving h-1 Mpc for Ω = 1), limited on the upper end by uncertainty in fitting the unabsorbed QSO continuum and on the lower end by finite spectral resolution (0.8-2.3 Å FWHM) and by nonlinear dynamical effects. We examine a number of possible sources of systematic error and …

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The Nature Of Lyman Break Galaxies In Cosmological Hydrodynamic Simulations, R Dave, J Gardner, L Hernquist, N Katz, D Weinberg Jan 1999

The Nature Of Lyman Break Galaxies In Cosmological Hydrodynamic Simulations, R Dave, J Gardner, L Hernquist, N Katz, D Weinberg

Astronomy Department Faculty Publication Series

What type of objects are being detected as $z\sim 3$ "Lyman break galaxies"? Are they predominantly the most massive galaxies at that epoch, or are many of them smaller galaxies undergoing a short-lived burst of merger-induced star formation? We attempt to address this question using high-resolution cosmological hydrodynamic simulations including star formation and feedback. Our $\Lambda$CDM simulation, together with Bruzual-Charlot population synthesis models, reproduces the observed number density and luminosity function of Lyman break galaxies when dust is incorporated. The inclusion of dust is crucial for this agreement. In our simulation, these galaxies are predominantly the most massive objects at …

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Submillimeter Imaging Of The Luminous Infrared Galaxy Pair Vv 114, D T. Frayer, R J. Ivison, I Smail, L Armus, Min S. Yun Jan 1999

Submillimeter Imaging Of The Luminous Infrared Galaxy Pair Vv 114, D T. Frayer, R J. Ivison, I Smail, L Armus, Min S. Yun

Min S. Yun

We report on 450 and 850 μm observations of the interacting galaxy pair VV 114 E+W (IC 1623), taken with the SCUBA camera on the James Clerk Maxwell Telescope, and near-infrared observations taken with the UKIRT Fast-Track Imager on the UK Infrared Telescope. The system VV 114 is in an early stage of a gas-rich merger. We detect submillimeter emission extended over 30'' (12 kpc) and find a good correlation between the spatial distribution of the submillimeter and CO emission. Both the CO and submillimeter emission peak near the reddest region of VV 114 E and extend toward VV 114 …

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Counterrotating Nuclear Disks In Arp 220, K Sakamoto, N Z. Scoville, Min S. Yun, M Crosas, R Genzel, L J. Tacconi Jan 1999

Counterrotating Nuclear Disks In Arp 220, K Sakamoto, N Z. Scoville, Min S. Yun, M Crosas, R Genzel, L J. Tacconi

Min S. Yun

The ultraluminous infrared galaxy Arp 220 has been observed at 0farcs5 resolution in CO (2-1) and 1 mm continuum using the newly expanded Owens Valley Millimeter Array. The CO and continuum peaks at the double nuclei and the surrounding molecular gas disk are clearly resolved. We find steep velocity gradients across each nucleus (ΔV~500 km s-1 within r=0farcs3) whose directions are not aligned with each other and with that of the outer gas disk. We conclude that the double nuclei have their own gas disks (r~100 pc), are counterrotating with respect to each other, and are embedded in the outer …

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The Low-Redshift Ly Alpha Forest In Cold Dark Matter Cosmologies, R Dave, L Hernquist, N Katz, Dh Weinberg Jan 1999

The Low-Redshift Ly Alpha Forest In Cold Dark Matter Cosmologies, R Dave, L Hernquist, N Katz, Dh Weinberg

Astronomy Department Faculty Publication Series

We study the physical origin of the low-redshift Lyα forest in hydrodynamic simulations of four cosmological models, all variants of the cold dark matter scenario. Our most important conclusions are insensitive to the cosmological model, but they depend on our assumption that the UV background declines at low redshift in concert with the declining population of quasar sources. We find that the expansion of the universe drives rapid evolution of dN/dz (the number of absorbers per unit redshift above a specified equivalent width threshold) at z1.7, but that at lower redshift the fading of the UV background …

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Evolution Of Clustering And Bias In A Lambda-Cdm Universe, R Dave, L Hernquist, N Katz, Dh Weinberg Jan 1999

Evolution Of Clustering And Bias In A Lambda-Cdm Universe, R Dave, L Hernquist, N Katz, Dh Weinberg

Astronomy Department Faculty Publication Series

We determine the evolution from $z=3\to 0$ of the galaxy and mass correlation functions and bias factor in a $50h^{-1}$Mpc $\Lambda$CDM hydrodynamic simulation with $10h^{-1}$kpc resolution. The mass correlation function grows with time, but the galaxy correlation function shows little evolution and is well described by a power law. At early times, galaxies are biased traces of mass, with bias being higher on smaller scales. By z=0, galaxies trace the mass, and the bias shows little scale dependence.

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Effect Of The Magellanic Clouds On The Milky Way Disk And Vice Versa, Martin D. Weinberg Jan 1999

Effect Of The Magellanic Clouds On The Milky Way Disk And Vice Versa, Martin D. Weinberg

Astronomy Department Faculty Publication Series

We introduce a new galaxy image decomposition tool, GALPHAT (GALaxy PHotometric ATtributes), to provide full posterior probability distributions and reliable confidence intervals for all model parameters. GALPHAT is designed to yield a high speed and accurate likelihood computation, using grid interpolation and Fourier rotation. We benchmark this approach using an ensemble of simulated Sersic model galaxies over a wide range of observational conditions: the signal-to-noise ratio S/N, the ratio of galaxy size to the PSF and the image size, and errors in the assumed PSF; and a range of structural parameters: the half-light radius $r_e$ and the Sersic index $n$. …

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Discovery Of Four Field Methane (T-Type) Dwarfs With The Two Micron All-Sky Survey, Adam J. Burgasser, J. Davy Kirkpatrick, Michael E. Brown, I. Neill Reid, John E. Gizis, Conard C. Dahn, David G. Monet, Charles A. Beichman, James Liebert, Roc M. Cutri, Michael F. Skrutskie Jan 1999

Discovery Of Four Field Methane (T-Type) Dwarfs With The Two Micron All-Sky Survey, Adam J. Burgasser, J. Davy Kirkpatrick, Michael E. Brown, I. Neill Reid, John E. Gizis, Conard C. Dahn, David G. Monet, Charles A. Beichman, James Liebert, Roc M. Cutri, Michael F. Skrutskie

Astronomy Department Faculty Publication Series

We report the discovery of four field methane ("T"-type) brown dwarfs using Two Micron All-Sky Survey (2MASS) data. One additional methane dwarf, previously discovered by the Sloan Digital Sky Survey, was also identified. Near-infrared spectra clearly show the 1.6 and 2.2 m CH4 absorption bands characteristic of objects with Teff 1300 K as well as broadened H2O bands at 1.4 and 1.9 m. Comparing the spectra of these objects with that of Gl 229B, we propose that all new 2MASS T dwarfs are warmer than 950 K, in order from warmest to coolest: 2MASS J1217-03, 2MASS J1225-27, 2MASS J1047+21, and …

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L Dwarfs And The Substellar Mass Function, I. Neill Reid, J. Davy Kirkpatrick, J. Liebert, A. Burrows, J. E. Gizis, A. Burgasser, C. C. Dahn, D. Monet, R. Cutri, C. A. Beichman, M. Skrutskie Jan 1999

L Dwarfs And The Substellar Mass Function, I. Neill Reid, J. Davy Kirkpatrick, J. Liebert, A. Burrows, J. E. Gizis, A. Burgasser, C. C. Dahn, D. Monet, R. Cutri, C. A. Beichman, M. Skrutskie

Astronomy Department Faculty Publication Series

Analysis of initial observations sky surveys has shown that the resulting photometric catalogs, combined with far-red optical data, provide an extremely effective method of finding isolated, very low-temperature objects in the general field. Follow-up observations have already identified more than 25 sources with temperatures cooler than the latest M dwarfs. A comparison with detailed model predictions (Burrows & Sharp 1999) indicates that these L dwarfs have effective temperatures between 2000 ± 100 K and 1500 ± 100 K, while the available trigonometric parallax data place their luminosities at between 10-3.5 and 10. Those properties, together with the detection of lithium …

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