Browsing by Author "Ellison, SL"

Now showing 1 - 5 of 5
  • No Thumbnail Available
    Item
    Distinct abundance patterns in multiple damped Lyα galaxies
    (2003) Lopez, S; Ellison, SL
    Following our previous work on metal abundances of a double damped Lyalpha system with a line-of-sight separation similar to2000 km s(-1) (Ellison & Lopez 2001), we present VLT UVES abundances of 3 new systems spanning a total of similar to6000 km s-1 at z similar to 2.5 toward the southern QSO CTQ247. These abundances are supplemented with echelle observations of another "double" damped Lyalpha system in the literature. We propose a definition in terms of velocity shift of the sub-class "multiple damped Lyalpha system", which is motivated by its possible connection with large-scale structure. We find that the abundance ratio [S/Fe] is systematically low in multiple systems compared with single systems, and with a small scatter. The same behavior is found in 2 more single DLA systems taken from the literature that show evidence of belonging to a galaxy group. Although [Si/Fe] ratios are also generally lower in multiple DLAs than in single DLAs, the effect is less striking since the scatter is larger and there are a number of low [Si/Fe] DLAs in the literature. We suggest that this can be explained with a combination of detection bias and, to a lesser extent, the scatter in ionization corrections for different absorbers. We investigate whether consistently low alpha/Fe ratios could be due to dust depletion or ionization corrections and find that the former effect would emphasize the observed trend of low alpha/Fe in multiple systems even further. Ionization may have a minor effect in some cases, but at a level that would not change our conclusions. We thus conclude that the low alpha/Fe ratios in multiple DLAs have a nucleosynthetic origin and suggest that they could be explained by reduced star formation in multiple damped Lyalpha systems, possibly due to environmental effects. There seems to be independent evidence for this scenario from the mild odd-even effect and from the relatively high N/alpha ratios we observe in these multiple systems.
  • No Thumbnail Available
    Item
    Hydrogen concentrations on C-class asteroids derived from remote sensing
    (2003) Rivkin, AS; Davies, JK; Johnson, JR; Ellison, SL; Trilling, DE; Brown, RH; Lebofsky, LA
    We present spectroscopic observations of 16 asteroids from 1.9-3.6 mum collected from the United Kingdom Infrared Telescope (UKIRT) from 1996-2000. Of these 16 asteroids, I I show some evidence of a 3 mum hydrated mineral absorption feature greater than 2sigma at 2.9 mum. Using relations first recognized for carbonaceous chondrite powders by Miyamoto and Zolensky (1994) and Sato et al. (1997), we have determined the hydrogen to silicon ratio for these asteroids and calculated their equivalent water contents, assuming all the hydrogen was in water. The asteroids split into 2 groups, roughly defined as equivalent water contents greater than similar to7% (8 asteroids, all with 3 mum band depths greater than similar to20%) and less than similar to3% for the remaining 8 asteroids. This latter group includes some asteroids for which a weak but statistically significant 3 mum band of non-zero depth exists. The G-class asteroids in the survey have higher water contents, consistent with CM chondrites. This strengthens the connection between CM chondrites and G asteroids that was proposed by Burbine (1998). We find that the 0.7 mum and 3 mum band depths are correlated for the population of target objects.
  • No Thumbnail Available
    Item
    The absorption and emission kinematics in the z=0.7450 MgII absorber toward Q1331+17
    (2003) Ellison, SL; Mallén-Ornelas, G; Sawicki, M
    We present a comparative analysis of the galaxy emission and quasar (QSO) absorption kinematics of a z(abs) similar to 0.7450 Mg II system and its candidate absorbing galaxy (G5) located 3."86 (28.3 h(70)(-1) kpc) from the QSO. We have obtained a spectrum of the galaxy candidate, previously identified as a luminous edge-on disk, and detect the [O II] lambda3727 doublet at a systemic redshift of z(sys) = 0.7450. From slit spectroscopy of this galaxy, we find v(rot) greater than or similar to 210 km s(-1), possibly as large as 350 km s(-1). Plotted on the same velocity scale, the systemic redshift of the galaxy coincides with the center of the absorption system, although the absorption components span more than 100 km s(-1) in either direction. However, once the sense of the rotation is taken into account, there is no absorbing gas at the projected velocity of the disk rotation curve. This implies incompatibility with a simple disk scenario. Moreover, a reanalysis of archival Hubble Space Telescope (HST) data reveals that the galaxy is only 0.3L(*), considerably less luminous than previously reported in the literature. This is incompatible with the established Tully-Fisher relation at this redshift, unless approximately 2 mag of total extinction is invoked. Careful inspection of the archival HST data reveals that G5 may well be composed of two galaxies, although the quality of the data does not permit a detailed investigation of this. This possibility is further supported by the identification of a second faint emission line at lambda(obs) = 5674 Angstrom, whose distinct spatial and velocity profiles indicate that it arises in a different galaxy at a different redshift. Analysis of the absorption lines shows evidence for superbubbles in the interstellar medium of the absorbing galaxy, based on the striking symmetry between components and large Mg I/Mg II and Mg I/Fe II ratios, indicative of large densities. The large velocity separations between line pairings, Deltav similar to 150 km s(-1), indicate that these bubbles may be powered by OB associations comparable to the largest observed at z = 0 and that the gas is probably enriched to at least 1/10 solar metallicity. This is consistent with observations at low redshift that extended Mg II halos are often seen in galaxies that contain disturbed gas. Superbubbles may also explain why the absorber has a relatively large Mg II equivalent width relative to the luminosity of the associated galaxy ( or galaxies).
  • No Thumbnail Available
    Item
    The CIV mass density of the universe at redshift 5
    (2003) Pettini, M; Madau, P; Bolte, M; Prochaska, JX; Ellison, SL; Fan, XH
    In order to search for metals in the Lyalpha forest at redshifts z(abs) > 4, we have obtained spectra of high signal-to-noise ratio and moderately high resolution of three QSOs at z(em) > 5: 4 discovered by the Sloan Digital Sky Survey. These data allow us to probe to metal enrichment of the intergalactic medium at early times with higher sensitivity than previous studies. We find 16 C IV absorption systems with column densities log N (C IV) = 12.50-13.98 over a total redshift path DeltaX = 3.29. In the redshift interval z = 4.5-5.0, where our statistics are most reliable, we deduce a comoving mass density of C3+ ions Omega(CIV) = (4.3 +/- 2.5) x 10(-8) ( 90% confidence limits) for absorption systems with log N (C-IV) greater than or equal to 13.0 ( for an Einstein - de Sitter cosmology with h = 0.65). This value of Omega(CIV) is entirely consistent with those measured at z < 4; we confirm the earlier finding by Songaila that neither the column density distribution of C iv absorbers nor its integral show significant redshift evolution over a period of time that stretches from similar to 1.25 to similar to 4.5 Gyr after the big bang. This somewhat surprising conclusion may be an indication that the intergalactic medium was enriched in metals at z >> 5, perhaps by the sources responsible for its reionization. Alternatively, the C iv systems we see may be associated with outflow from massive star- forming galaxies at later times, while the truly intergalactic metals may reside in regions of the Ly alpha forest of lower density than those probed up to now.
  • No Thumbnail Available
    Item
    The sizes and kinematic structure of absorption systems towards the lensed quasar APM08279+5255
    (2004) Ellison, SL; Ibata, R; Pettini, M; Lewis, GF; Aracil, B; Petitjean, P; Srianand, R
    We have obtained spatially resolved spectra of the z(em)=3.911 triply imaged QSO APM08279+5255 using the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST). We study the line of sight equivalent width (EW) differences and velocity shear of high and low ionization absorbers (including a damped Lyman alpha [DLA] system identified in a spatially unresolved ground based spectrum) in the three lines of sight. The combination of a particularly rich spectrum and three sight-lines allow us to study 27 intervening absorption systems over a redshift range 1.180%) over kpc scales. A minimum radius for strong (EW>0.3Angstrom) Mg II systems of >1.4 h(70)(-1) kpc is inferred from absorption coincidences in all lines of sight. For weak Mg II absorbers (EW<0.3 angstrom), a maximum likelihood analysis indicates a most probable coherence scale of 2.0 h(70)(-1) kpc for a uniform spherical geometry, with 95% confidence limits ranging between 1.5 and 4.4 h(70)(-1) kpc. The weak Mg II absorbers may therefore represent a distinct population of smaller galaxies compared with the strong Mg II systems which we know to be associated with luminous galaxies whose halos extend over tens of kpc. Alternatively, the weak systems may reside in the outer parts of larger galaxies, where their filling factor may be lower. By cross-correlating spectra along different lines of sight, we infer shear velocities of typically less than 20 km s(-1) for both high and low ionization absorbers. Finally, for systems with weak absorption that can be confidently converted to column densities, we find constant N(C IV)/N(Si IV) across the three lines of sight. Similarly, the [Al/Fe] ratios in the z(abs)=2.974 DLA are consistent with solar relative abundances over a transverse distance of similar to0.35 h(70)(-1) kpc.