In the last 30 days

• Open/close Cloud Fragmentation and Proplyd-like Features in H II Regions Imaged by the Hubble Space Telescope

  Orsola De Marco et al. 2006 The Astronomical Journal 131 2580 Tag this article

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  We have analyzed Hubble Space Telescope ACS and WFPC2 new and archival images of eight H II regions to look for new protoplanetary disks (proplyds) similar to those found in the Orion Nebula. We find a wealth of features similar in size (although many are larger) to the bright cusps around the Orion Nebula proplyds. None of them, however, contains a definitive central star. From this, we deduce that the new cusps may not be proplyds but instead fragments of molecular cloud material. Out of all the features found in the eight H II regions examined, only one, an apparent edge-on silhouette in M17, may have a central star. This feature might join the small number of bona fide proplyds found outside the Orion Nebula, in M8, M20, and possibly M16. In line with the results found recently by Smith et al., the paucity of proplyds outside the Orion Nebula can be explained by their transient nature, as well as by the specific environmental conditions under which they can be observed. Several fragments are seen as dark silhouettes against a bright background. We have reanalyzed those found in IC 2944 by Reipurth et al. and found new, similar ones in M16. None of these fragments contains a central star, and we exclude the possibility that they are disks. Reipurth et al. concluded that the IC 2944 silhouettes are not star forming. We argue here that their assumption of a constant optical depth for these fragments is not physical and that it is more likely that these fragments are star forming, a condition that is supported, although not proved, by their shapes and distributions. The process of cloud fragmentation and photoevaporation produces a large number of small fragments, while the size hierarchy expected in a photoevaporative environment would not favor small fragments. The size distributions observed will constrain any future theories of cloud fragmentation. One bright microjet candidate is found in M17, protruding from a large, limb-brightened fragment. A second, larger, jetlike feature, similar in shape and size to a Herbig-Haro jet, is found in Pismis 24. No central star appears to be associated with either of these jet candidates.

• Open/close Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant

  Adam G. Riess et al. 1998 The Astronomical Journal 116 1009 Tag this article

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  We present spectral and photometric observations of 10 Type Ia supernovae (SNe Ia) in the redshift range 0.16 ≤ z ≤ 0.62. The luminosity distances of these objects are determined by methods that employ relations between SN Ia luminosity and light curve shape. Combined with previous data from our High- z Supernova Search Team and recent results by Riess et al., this expanded set of 16 high-redshift supernovae and a set of 34 nearby supernovae are used to place constraints on the following cosmological parameters: the Hubble constant ( H ₀), the mass density (Ω _M ), the cosmological constant (i.e., the vacuum energy density, Ω _Λ), the deceleration parameter ( q ₀), and the dynamical age of the universe ( t ₀). The distances of the high-redshift SNe Ia are, on average, 10%–15% farther than expected in a low mass density (Ω _M = 0.2) universe without a cosmological constant. Different light curve fitting methods, SN Ia subsamples, and prior constraints unanimously favor eternally expanding models with positive cosmological constant (i.e., Ω _Λ > 0) and a current acceleration of the expansion (i.e., q ₀ < 0). With no prior constraint on mass density other than Ω _M ≥ 0, the spectroscopically confirmed SNe Ia are statistically consistent with q ₀ < 0 at the 2.8 σ and 3.9 σ confidence levels, and with Ω _Λ > 0 at the 3.0 σ and 4.0 σ confidence levels, for two different fitting methods, respectively. Fixing a "minimal" mass density, Ω _M = 0.2, results in the weakest detection, Ω _Λ > 0 at the 3.0 σ confidence level from one of the two methods. For a flat universe prior (Ω _M + Ω _Λ = 1), the spectroscopically confirmed SNe Ia require Ω _Λ > 0 at 7 σ and 9 σ formal statistical significance for the two different fitting methods. A universe closed by ordinary matter (i.e., Ω _M = 1) is formally ruled out at the 7 σ to 8 σ confidence level for the two different fitting methods. We estimate the dynamical age of the universe to be 14.2 ± 1.7 Gyr including systematic uncertainties in the current Cepheid distance scale. We estimate the likely effect of several sources of systematic error, including progenitor and metallicity evolution, extinction, sample selection bias, local perturbations in the expansion rate, gravitational lensing, and sample contamination. Presently, none of these effects appear to reconcile the data with Ω _Λ = 0 and q ₀ ≥ 0.

• Open/close The Wide-field Infrared Survey Explorer (WISE): Mission Description and Initial On-orbit Performance

  Edward L. Wright et al. 2010 The Astronomical Journal 140 1868 Tag this article

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  The all sky surveys done by the Palomar Observatory Schmidt, the European Southern Observatory Schmidt, and the United Kingdom Schmidt, the InfraRed Astronomical Satellite, and the Two Micron All Sky Survey have proven to be extremely useful tools for astronomy with value that lasts for decades. The Wide-field Infrared Survey Explorer (WISE) is mapping the whole sky following its launch on 2009 December 14. WISE began surveying the sky on 2010 January 14 and completed its first full coverage of the sky on July 17. The survey will continue to cover the sky a second time until the cryogen is exhausted (anticipated in 2010 November). WISE is achieving 5σ point source sensitivities better than 0.08, 0.11, 1, and 6 mJy in unconfused regions on the ecliptic in bands centered at wavelengths of 3.4, 4.6, 12, and 22 μm. Sensitivity improves toward the ecliptic poles due to denser coverage and lower zodiacal background. The angular resolution is 6 1, 6 4, 6 5, and 12 0 at 3.4, 4.6, 12, and 22 μm, and the astrometric precision for high signal-to-noise sources is better than 0 15.

• Open/close The ugriz Standard-Star System

  J. Allyn Smith et al. 2002 The Astronomical Journal 123 2121 Tag this article

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  We present the 158 standard stars that define the u' g' r' i' z' photometric system. These stars form the basis for the photometric calibration of the Sloan Digital Sky Survey. The defining instrument system and filters, the observing process, the reduction techniques, and the software used to create the stellar network are all described. We briefly discuss the history of the star selection process, the derivation of a set of transformation equations for the UBVR _C I _C system, and plans for future work.

• Open/close Composite Quasar Spectra from the Sloan Digital Sky Survey

  Daniel E. Vanden Berk et al. 2001 The Astronomical Journal 122 549 Tag this article

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  We have created a variety of composite quasar spectra using a homogeneous data set of over 2200 spectra from the Sloan Digital Sky Survey (SDSS). The quasar sample spans a redshift range of 0.044 ≤ z ≤ 4.789 and an absolute r' magnitude range of -18.0 to -26.5. The input spectra cover an observed wavelength range of 3800–9200 Å at a resolution of 1800. The median composite covers a rest-wavelength range from 800 to 8555 Å and reaches a peak signal-to-noise ratio of over 300 per 1 Å resolution element in the rest frame. We have identified over 80 emission-line features in the spectrum. Emission-line shifts relative to nominal laboratory wavelengths are seen for many of the ionic species. Peak shifts of the broad permitted and semiforbidden lines are strongly correlated with ionization energy, as previously suggested, but we find that the narrow forbidden lines are also shifted by amounts that are strongly correlated with ionization energy. The magnitude of the forbidden line shifts is 100 km s ^-1, compared with shifts of up to 550 km s ^-1 for some of the permitted and semiforbidden lines. At wavelengths longer than the Lyα emission, the continuum of the geometric mean composite is well fitted by two power laws, with a break at ≈5000 Å. The frequency power-law index, α _ν, is -0.44 from ≈1300 to 5000 Å and -2.45 redward of ≈5000 Å. The abrupt change in slope can be accounted for partly by host-galaxy contamination at low redshift. Stellar absorption lines, including higher order Balmer lines, seen in the composites suggest that young or intermediate-age stars make a significant contribution to the light of the host galaxies. Most of the spectrum is populated by blended emission lines, especially in the range 1500–3500 Å, which can make the estimation of quasar continua highly uncertain unless large ranges in wavelength are observed. An electronic table of the median quasar template is available.

• Open/close UBVRI Photometric Standard Stars Around the Celestial Equator: Updates and Additions

  Arlo U. Landolt 2009 The Astronomical Journal 137 4186 Tag this article

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  New broadband UBVRI photoelectric observations on the Johnson-Kron-Cousins photometric system have been made of 202 stars around the sky, and centered at the celestial equator. These stars constitute both an update of and additions to a previously published list of equatorial photometric standard stars. The list is capable of providing, for both celestial hemispheres, an internally consistent homogeneous broadband standard photometric system around the sky. When these new measurements are included with those previously published by Landolt (1992), the entire list of standard stars in this paper encompasses the magnitude range 8.90 < V < 16.30, and the color index range –0.35 < ( B – V) < +2.30.

• Open/close High-Resolution Rotation Curves and Galaxy Mass Models from THINGS

  W. J. G. de Blok et al. 2008 The Astronomical Journal 136 2648 Tag this article

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  We present rotation curves of 19 galaxies from The H I Nearby Galaxy Survey (THINGS). The high spatial and velocity resolution of THINGS make these the highest quality H I rotation curves available to date for a large sample of nearby galaxies, spanning a wide range of H I masses and luminosities. The high quality of the data allows us to derive the geometric and dynamical parameters using H I data alone. We do not find any declining rotation curves unambiguously associated with a cut-off in the mass distribution out to the last measured point. The rotation curves are combined with 3.6 μm data from the Spitzer Infrared Nearby Galaxies Survey to construct mass models. Our best-fit dynamical disk masses, derived from the rotation curves, are in good agreement with photometric disk masses derived from the 3.6 μm images in combination with stellar population synthesis arguments and two different assumptions for the stellar initial mass function (IMF). We test the cold dark matter (CDM) motivated cusp model, and the observationally motivated central density core model and find that (independent of IMF) for massive, disk-dominated galaxies, all halo models fit apparently equally well; for low-mass galaxies, however, a core-dominated halo is clearly preferred over a cusp-like halo. The empirically derived densities of the dark matter halos of the late-type galaxies in our sample are half of what is predicted by CDM simulations, again independent of the assumed IMF.

• Open/close A Catalog of Neighboring Galaxies

  Igor D. Karachentsev et al. 2004 The Astronomical Journal 127 2031 Tag this article

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  We present an all-sky catalog of 451 nearby galaxies, each having an individual distance estimate D 10 Mpc or a radial velocity V _LG < 550 km s ^-1. The catalog contains data on basic optical and H I properties of the galaxies, in particular, their diameters, absolute magnitudes, morphological types, circumnuclear region types, optical and H I surface brightnesses, rotational velocities, and indicative mass-to-luminosity and H I mass-to-luminosity ratios, as well as a so-called tidal index, which quantifies the galaxy environment. We expect the catalog completeness to be roughly 70%–80% within 8 Mpc. About 85% of the Local Volume population are dwarf (dIr, dIm, and dSph) galaxies with M _B > -17.0, which contribute about 4% to the local luminosity density, and roughly 10%–15% to the local H I mass density. The H I mass-to-luminosity and the H I mass-to-total (indicative) mass ratios increase systematically from giant galaxies toward dwarfs, reaching maximum values about 5 in solar units for the most tiny objects. For the Local Volume disklike galaxies, their H I masses and angular momentum follow Zasov's linear relation, expected for rotating gaseous disks being near the threshold of gravitational instability, favorable for active star formation. We found that the mean local luminosity density exceeds 1.7–2.0 times the global density, in spite of the presence of the Tully void and the absence of rich clusters in the Local Volume. The mean local H I density is 1.4 times its "global" value derived from the H I Parkes Sky Survey. However, the mean local baryon density Ω _b (< 8 Mpc) = 2.3% consists of only a half of the global baryon density, Ω _b = (4.7 ± 0.6)% (Spergel et al., published in 2003). The mean-square pairwise difference of radial velocities is about 100 km s ^-1 for spatial separations within 1 Mpc, increasing to ~300 km s ^-1 on a scale of ~3 Mpc. also We calculated the integral area of the sky occupied by the neighboring galaxies. Assuming the H I size of spiral and irregular galaxies to be 2.5 times their standard optical diameter and ignoring any evolution effect, we obtain the expected number of the line-of-sight intersections with the H I galaxy images to be dn/ dz ~ 0.4, which does not contradict the observed number of absorptions in QSO spectra.

• Open/close K-Corrections and Filter Transformations in the Ultraviolet, Optical, and Near-Infrared

  Michael R. Blanton and Sam Roweis 2007 The Astronomical Journal 133 734 Tag this article

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  Template fits to observed galaxy fluxes allow calculation of K-corrections and conversions among observations of galaxies at various wavelengths. We present a method for creating model-based template sets given a set of heterogeneous photometric and spectroscopic galaxy data. Our technique, nonnegative matrix factorization, is akin to principal component analysis (PCA), except that it is constrained to produce nonnegative templates, it can use a basis set of models (rather than the delta-function basis of PCA), and it naturally handles uncertainties, missing data, and heterogeneous data (including broadband fluxes at various redshifts). The particular implementation we present here is suitable for ultraviolet, optical, and near-infrared observations in the redshift range 0 < z < 1.5. Since we base our templates on stellar population synthesis models, the results are interpretable in terms of approximate stellar masses and star formation histories. We present templates fitted with this method to data from Galaxy Evolution Explorer, Sloan Digital Sky Survey spectroscopy and photometry, the Two Micron All Sky Survey, the Deep Extragalactic Evolutionary Probe, and the Great Observatories Origins Deep Survey. In addition, we present software for using such data to estimate K-corrections.

• Open/close WFPC2 Observations of Star Clusters in the Magellanic Clouds. II. The Oldest Star Clusters in the Small Magellanic Cloud

  Kenneth J. Mighell et al. 1998 The Astronomical Journal 116 2395 Tag this article

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  We present our analysis of archival Hubble Space T elescope Wide Field Planetary Camera 2 (WFPC2) observations in F450W (~ B) and F555W (~ V) of the intermediate-age populous star clusters NGC 121, NGC 339, NGC 361, NGC 416, and Kron 3 in the Small Magellanic Cloud. We use published photometry of two other SMC populous star clusters, Lindsay 1 and Lindsay 113, to investigate the age sequence of these seven star clusters in order to improve our understanding of the formation chronology of the SMC. We analyzed the V versus B- V and M _V versus ( B- V) ₀ color-magnitude diagrams of these populous Small Magellanic Cloud star clusters using a variety of techniques and determined their ages, metallicities, and reddenings. These new data enable us to improve the age-metallicity relation of star clusters in the Small Magellanic Cloud. In particular, we find that a closed-box continuous star formation model does not reproduce the age-metallicity relation adequately. However, a theoretical model punctuated by bursts of star formation is in better agreement with the observational data presented herein.