Sloan Fellows

About: Sloan Fellows is a based out in . It is known for research contribution in the topics: Galaxy & Star formation. The organization has 55 authors who have published 253 publications receiving 35008 citations. The organization is also known as: Sloan Fellows.

Chandra Imaging of the X-Ray Nebula Powered by Pulsar B1509?58

Abstract: We present observations with the Chandra X-Ray Observatory of the pulsar wind nebula (PWN) powered by the energetic young pulsar B1509-58. These data confirm the complicated morphology of the system indicated by previous observations, and in addition reveal several new components to the nebula. The overall PWN shows a clear symmetry axis oriented at a position angle 150" +- 5" (north through east), which we argue corresponds to the pulsar spin axis. We show that a previously identified radio feature matches well with the overall extent of the X-ray PWN, and propose the former as the long-sought radio nebula powered by the pulsar. We further identify a bright collimated feature, at least 4' long, lying along the nebula's main symmetry axis; we interpret this feature as a physical outflow from the pulsar, and infer a velocity for this jet of greater than 0.2c. The lack of any observed counterjet implies that the pulsar spin axis is inclined at -30" to the line of sight, contrary to previous estimates made from lower resolution data. We also identify a variety of compact features close to the pulsar. A pair of semicircular X-ray arcs lie 17" and 30" to the north of the pulsar; the latter arc shows a highly polarized radio counterpart. We show that these features can be interpreted as ion compression wisps in a particle-dominated equatorial flow, and use their properties to infer a ratio of electromagnetic to particle energy in pairs at the wind shock 0.005, similar to that seen in the Crab Nebula. We further identify several compact knots seen very close to the pulsar; we use these to infer cr < 0.003 at a separation from the pulsar of 0.1 pc.

Maximal Disks and the Tully-Fisher Relation

Abstract: We show that for luminous, nonbarred, high surface brightness (HSB) spirals the Tully-Fisher relation (TFR) residuals can be used to estimate the relative mass contributions of the stellar disk and the dark halo at the peak of the disk rotation, near 22 exponential scale lengths For maximal disks, a large fraction (085?01) of the total rotational support, V22, at such radii should arise from their stellar mass Therefore, the disk size or surface-brightness should be a significant additional parameter in the TFR At a given absolute luminosity, Mr, more compact disks (as measured by the disk scale length Rexp) should have higher rotation speeds, V22 Using a well-defined sample of late-type spirals, deviations, ?log V22, and ?log Rexp, from the mean relations, V22(Mr) and Rexp(Mr), are not significantly correlated The case of ?log V22/?log Rexp=-05 expected for a maximal disk is ruled out for the majority of these HSB galaxies We model adiabatic infall of varying amounts of luminous matter into dark matter halos to explore the range of possible values for ?log V22/?log Rexp From this, we find that the TFR residuals require a mean value of Vdisk~06Vtotal, fairly insensitive to the details of the initial dark matter halo and to the presence of a bulge This translates to Mhalo~06Mtotal within 22Rexp or roughly twice more dark matter in the inner parts of late-type spirals than previously accounted for by maximum disk fits We show that any stellar population differences between disks of different scale lengths lead to lower values of Vdisk/Vtotal Our result is independent of the shape of the luminosity profile and relies only on the assumption of adiabatic contraction and that the dark matter halo rotation rises in the central parts of the galaxy Submaximal disks establish a natural continuity between HSB and low surface brightness galaxies, which appear to be completely dark matter dominated even in their inner regions

REST-FRAME OPTICAL SPECTRA OF THREE STRONGLY LENSED GALAXIES AT z ~ 2

Abstract: We present Keck II NIRSPEC rest-frame optical spectra for three recently discovered lensed galaxies: the Cosmic Horseshoe (z = 2.38), the Clone (z = 2.00), and SDSS J090122.37+181432.3 (z = 2.26). The boost in signal-to-noise ratio (S/N) from gravitational lensing provides an unusually detailed view of the physical conditions in these objects. A full complement of high S/N rest-frame optical emission lines is measured, spanning from rest frame 3600 to 6800 A, including robust detections of fainter lines such as Hγ, [S II]λ6717,6732, and in one instance [Ne III]λ3869. SDSS J090122.37+181432.3 shows evidence for active galactic nucleus activity, and therefore we focus our analysis on star-forming regions in the Cosmic Horseshoe and the Clone. For these two objects, we estimate a wide range of physical properties. Current lensing models for the Cosmic Horseshoe and the Clone allow us to correct the measured Hα luminosity and calculated star formation rate. Metallicities have been estimated with a variety of indicators, which span a range of values of 12+ log(O/H) = 8.3-8.8, between ~0.4 and ~1.5 of the solar oxygen abundance. Dynamical masses were computed from the Hα velocity dispersions and measured half-light radii of the reconstructed sources. A comparison of the Balmer lines enabled measurement of dust reddening coefficients. Variations in the line ratios between the different lensed images are also observed, indicating that the spectra are probing different regions of the lensed galaxies. In all respects, the lensed objects appear fairly typical of ultraviolet-selected star-forming galaxies at z ~ 2. The Clone occupies a position on the emission-line diagnostic diagram of [O III]/Hβ versus [N II]/Hα that is offset from the locations of z ~ 0 galaxies. Our new NIRSPEC measurements may provide quantitative insights into why high-redshift objects display such properties. From the [S II] line ratio, high electron densities (~1000 cm–3) are inferred compared to local galaxies, and [O III]/[O II] line ratios indicate higher ionization parameters compared to the local population. Building on previous similar results at z ~ 2, these measurements provide further evidence (at high S/N) that star-forming regions are significantly different in high-redshift galaxies, compared to their local counterparts.

A Test of the Collisional Dark Matter Hypothesis from Cluster Lensing

Abstract: Spergel & Steinhardt proposed the possibility that dark matter particles are self-interacting as a solution to two discrepancies between the predictions of cold dark matter models and observations: First, the observed dark matter distribution in some dwarf galaxies has large, constant-density cores, as opposed to the predicted central cusps; and second, small satellites of normal galaxies are much less abundant than predicted. The dark matter self-interaction would produce isothermal cores in halos and expel the dark matter particles from dwarfs orbiting in large halos. Another consequence of the model is that halos should become spherical once most particles have interacted. Several observations show that the mass distribution in relaxed clusters of galaxies is elliptical. Here, I discuss in particular gravitational lensing in the cluster MS 2137-23, where the ellipticity of the dark matter distribution can be measured to a small radius, r ~ 70 kpc, suggesting that most dark matter particles in clusters outside this radius do not collide during the characteristic age of clusters. If true, this implies that any dark matter self-interaction with a cross section independent of velocity is too weak to have affected the observed density profiles in the dark-matter dominated dwarf galaxies, or to have facilitated the destruction of dwarf satellites in galactic halos. If sx is the cross section and mx the mass of the dark matter particle, then sx/mx < 10-25.5 cm2 GeV-1.

A Measurement of the Temperature-Density Relation in the Intergalactic Medium Using a New Lyα Absorption-Line Fitting Method*

Abstract: The evolution of the temperature in the intergalactic medium is related to the reionization of hydrogen and helium and has important consequences for our understanding of the Lyα forest and of galaxy formation. We measure the temperature-density relation of intergalactic gas from Lyα forest observations of eight quasar spectra, using a new line fitting technique to obtain a lower cutoff on the distribution of line widths from which the temperature is derived. Using a numerical simulation, we examine the details of this kind of measurement at different densities, finding that the temperature may be difficult to measure for gas with Δg 1 (Δg is the density of the gas in units of the mean density) because the velocities due to expansion always dominate the widths of the corresponding weak lines, and that the temperature measurement is increasingly ambiguous for gas with Δg 5 because the dispersion in temperature at fixed density is high. From our observed spectra, the temperature is most precisely determined at densities slightly above the mean: T* = (20,200 ± 2700, 20,200 ± 1300, 22,600 ± 1900) K (statistical error bars) for gas densities Δ* = (1.42 ± 0.08, 1.37 ± 0.11, 1.66 ± 0.11) at redshift = (3.9, 3.0, 2.4). Systematic errors in T* should be less than 2000 K. The power-law index of the temperature-density relation, defined by T = T*(Δg/Δ*)γ-1, is γ - 1 = (0.43 ± 0.45, 0.29 ± 0.30, 0.52 ± 0.14) for the same three redshifts. The temperature at fixed overdensity Δ = 1.4 is T1.4 = (20,100 ± 2800, 20,300 ± 1400, 20,700 ± 1900) K. This unchanging temperature is higher than expected for photoionized gas in ionization equilibrium with a cosmic background. If the heat from the He II reionization is responsible for the high measured temperature, then the temperature should not be constant but should have a maximum at the end of the reionization epoch. We update the lower limit to the baryon density implied by the observed mean flux decrement.