arXiv subjects

Emma V. Ryan-Weber

Publications and source records attributed to Emma V. Ryan-Weber.

25 records · Page 2Linked to original sources

arXiv2009

A downturn in intergalactic CIV as redshift 6 is approached

We present the results of the largest survey to date for intergalactic metals at redshifts z > 5, using near-IR spectra of nine QSOs with emission redshifts z(em) > 5.7. We find, for the first time, a change in the comoving mass density of C IV ions as we look back to redshifts z > 5. At a mean = 5.76, we deduce Omega(C IV)=(4.4+/-2.6)x10^(-9) which implies a drop by a factor of about 3.5 compared to the value at z < 4.7, after accounting for the differing sensitivities of different surveys. The observed number of C IV doublets is also lower by a similar factor, compared to expectations for a non-evolving column density distribution of absorbers. These results point to a rapid build-up of intergalactic C IV over a period of only 300 Myr; such a build-up could reflect the accumulation of metals associated with the rising levels of star formation activity from z = 9 indicated by galaxy counts, and/or an increasing degree of ionisation of the intergalactic medium (IGM), following the overlap of ionisation fronts from star-forming regions. If the value of Omega(C IV) we derive is typical of the IGM at large, it would imply a metallicity Z(IGM) = 10^(-4) Z(Sun). The early-type stars responsible for synthesising these metals would have emitted only about one Lyman continuum photon per baryon prior to z = 5.8; such a background is insufficient to keep the IGM ionised and we speculate on possible factors which could make up the required shortfall.

astro-ph.CO↗

arXiv2007

The Local Group dwarf Leo T: HI on the brink of star formation

We present Giant Meterwave Radio Telescope (GMRT) and Westerbork ynthesis Radio Telescope (WSRT) observations of the recently discovered Local Group dwarf galaxy, Leo T. The peak HI column density is measured to be 7x10^20 cm^-2, and the total HI mass is 2.8Xx10^5 Msun, based on a distance of 420 kpc. Leo T has both cold (~ 500 K) and warm (~ 6000 K) HI at its core, with a global velocity dispersion of 6.9 km/s, from which we derive a dynamical mass within the HI radius of 3.3x10^6 Msun, and a mass-to-light ratio of greater than 50. We calculate the Jeans mass from the radial profiles of the HI column density and velocity dispersion, and predict that the gas should be globally stable against star formation. This finding is inconsistent with the half light radius of Leo T, which extends to 170 pc, and indicates that local conditions must determine where star formation takes place. Leo T is not only the lowest luminosity galaxy with on-going star formation discovered to date, it is also the most dark matter dominated, gas-rich dwarf in the Local Group.

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arXiv2007

The host galaxies of strong CaII QSO absorption systems at z<0.5

We present new imaging and spectroscopic observations of the fields of five QSOs with very strong intervening CaII absorption systems at redshifts z<0.5 selected from the Sloan Digital Sky Survey. Recent studies of these very rare absorbers indicate that they may be related to damped Lyman alpha systems (DLAs). In all five cases we identify a galaxy at the redshift of the CaII system with impact parameters up to ~24 kpc. In four out of five cases the galaxies are luminous (L ~L*), metal-rich (Z ~Zsun), massive (velocity dispersion, sigma ~100 km/s) spirals. Their star formation rates, deduced from Halpha emission, are high, in the range SFR = 0.3 - 30 Msun/yr. In our analysis, we paid particular attention to correcting the observed emission line fluxes for stellar absorption and dust extinction. We show that these effects are important for a correct SFR estimate; their neglect in previous low-z studies of DLA-selected galaxies has probably led to an underestimate of the star formation activity in at least some DLA hosts. We discuss possible links between CaII-selected galaxies and DLAs and outline future observations which will help clarify the relationship between these different classes of QSO absorbers.

astro-ph↗

arXiv2006

Intergalactic CIV absorption at redshifts 5.4 to 6

We report the discovery of a strong CIV 1548,1550 absorption system at z_abs = 5.7238 in the near-infrared spectrum (J-band) of the z_em = 6.28 QSO SDSS J1030+0524. These observations, obtained with the Infrared Spectrometer And Array Camera (ISAAC) on the European Southern Observatory Very Large Telescope (ESO VLT), demonstrate that, with modern instrumentation, QSO absorption line spectroscopy can be successfully extended to near-infrared wavelengths to probe the intergalactic medium near the end of the reionization epoch. Although the statistics of this pilot study are limited, the mass density of triply ionized carbon implied by our data is comparable to the values of Omega_CIV reported at lower redshifts. Neither the column density distribution of CIV absorbers nor its integral show significant redshift evolution over a period of time which stretches from 1 to 4.5 Gyr after the big bang, suggesting that a large fraction of intergalactic metals may already have been in place at redshifts above 6. Alternatively, the strong CIV system we have detected may be associated with outflowing, highly-ionized, gas from a foreground massive galaxy; deep imaging and spectroscopy of galaxies near the QSO sightline should be able to distinguish between these two possibilities.

astro-ph↗

arXiv2006

Cross correlation of Lyman-alpha absorbers with gas-rich galaxies

The HI Parkes All Sky Survey (HIPASS) galaxy catalogue is cross-correlated with known low redshift, low column density (N_HI <10^15 cm^-2) Lyman-alpha absorbers from the literature. The redshift-space correlation is found to be similar in strength to HIPASS galaxy self-clustering (correlation length s_0,ag=6+/-4 and s_0,gg=3.1+/-0.5 h^-1 Mpc respectively). In real-space the cross-correlation is stronger than the galaxy auto-correlation (correlation length r_0,ag=7.2+/-1.4 and r_0,gg=3.5+/-0.7 h^-1 Mpc respectively) on scales from 1-10 h^-1 Mpc, ruling out the mini-halo model for the confinement Lyman-alpha absorbers at the 99 percent confidence level. Provided that the cause of the strong cross-correlation is purely gravitational, the ratio of correlation lengths suggest that absorbers are embedded in dark matter haloes with masses log(M/Msun)=14.2 h^-1, similar to those of galaxy groups. The flattening of the cross-correlation at separations less than ~600 h^-1 kpc could correspond to the thickness of filaments in which absorbers are embedded. This work provides indirect statistical evidence for the notion that galaxy groups and large-scale filaments, particularly those that comprise gas-rich galaxies, are the dominant environments of low column density Lyman-alpha absorbers at z=0.

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arXiv2005

Evolution of damped Lyman alpha kinematics and the effect of spatial resolution on 21-cm measurements

We have investigated the effect of spatial resolution on determining pencil-beam like velocity widths and column densities in galaxies. Three 21-cm datasets are used, the HIPASS galaxy catalogue, a subset of HIPASS galaxies with ATCA maps and a high-resolution image of the LMC. Velocity widths measured from 21-cm emission in local galaxies are compared with those measured in intermediate redshift Damped Lyman alpha (DLA) absorbers. We conclude that spatial resolution has a severe effect on measuring pencil-beam like velocity widths in galaxies. Spatial smoothing by a factor of 240 is shown to increase the median velocity width by a factor of two. Thus any difference between velocity widths measured from global profiles or low spatial resolution 21-cm maps at z=0 and DLAs at z>1 cannot unambiguously be attributed to galaxy evolution. The effect on column density measurements is less severe and the values of dN/dz from local low-resolution 21-cm measurements are expected to be overestimated by only ~10 per cent.

astro-ph↗

arXiv2005

Correlation of Low z Lyman-alpha Absorbers with HI-selected Galaxies

In this work, observational evidence for the connection between low column density Lyman-alpha absorbers and large-scale structure traced by gas-rich galaxies is investigated. The HI Parkes All Sky Survey (HIPASS) galaxy catalogue is cross-correlated with known low redshift, low column density Lyman-alpha absorbers from the literature. The absorber-galaxy cross-correlation function shows that on scales from 1 to 10 h^-1 Mpc, absorbers are imbedded in halos with masses similar to that of galaxy groups. This statistical evidence suggests that galaxy groups could be the dominant environment of low column density Lyman-alpha absorbers at z=0.

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