arXiv subjects

S. L. Ellison

Publications and source records attributed to S. L. Ellison.

33 records · Page 2Linked to original sources

arXiv2011

Rapid-Response Mode VLT/UVES spectroscopy of GRB060418: Conclusive evidence for UV pumping from the time evolution of FeII and NiII excited- and metastable-level populations

We present high-resolution spectroscopic observations of GRB060418, obtained with VLT/UVES. These observations were triggered using the VLT Rapid Response Mode (RRM), which allows for automated observations of transient phenomena, without any human intervention. This resulted in the first UVES exposure of GRB060418 to be started only 10 minutes after the initial Swift satellite trigger. A sequence of spectra covering 330-670 nm were acquired at 11, 16, 25, 41 and 71 minutes (mid-exposure) after the trigger, with a resolving power of 7 km/s, and a signal-to-noise ratio of 10-15. This time-series clearly shows evidence for time variability of allowed transitions involving FeII fine-structure levels, and metastable levels of both FeII and NiII, at the host-galaxy redshift z=1.490. This is the first report of absorption lines arising from metastable levels of FeII and NiII along any GRB sightline. We model the observed evolution of the level populations with three different excitation mechanisms: collisions, excitation by infra-red photons, and fluorescence following excitation by ultraviolet photons. Our data allow us to reject the collisional and IR excitation scenarios with high confidence. The UV pumping model, in which the GRB afterglow UV photons excite a cloud of atoms with a column density N, distance d, and Doppler broadening parameter b, provides an excellent fit, with best-fit values: log N(FeII)=14.75+0.06-0.04, log N(NiII)=13.84+/-0.02, d=1.7+/-0.2 kpc (but see Appendix A), and b=25+/-3 km/s. The success of our UV pumping modeling implies that no significant amount of FeII or NiII is present at distances smaller than ~1.7 kpc (but see erratum in Appendix A), most likely because it is ionized by the GRB X-ray/UV flash. Because neutral hydrogen is more easily ionized than FeII and NiII, this minimum distance also applies to any HI present. [abridged]

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arXiv2010

Cosmological concordance or chemical coincidence? Deuterated molecular hydrogen abundances at high redshift

We report two detections of deuterated molecular hydrogen (HD) in QSO absorption-line systems at $z > 2$. Toward J2123-0500, we find $N$(HD) $= 13.84 \pm 0.2$ for a sub-DLA with metallicity $\simeq 0.5Z_{\odot}$ and $N$(H$_2$) = $17.64 \pm 0.15$ at $z = 2.0594$. Toward FJ0812+32, we find $N$(HD) $= 15.38 \pm 0.3$ for a solar-metallicity DLA with $N$(H$_2$) = $19.88 \pm 0.2$ at $z = 2.6265$. These systems have ratios of HD to H$_2$ above that observed in dense clouds within the Milky Way disk and apparently consistent with a simple conversion from the cosmological ratio of D/H. These ratios are not readily explained by any available model of HD chemistry and there are no obvious trends with metallicity or molecular content. Taken together, these two systems and the two published $z > 2$ HD-bearing DLAs indicate that HD is either less effectively dissociated or more efficiently produced in high-redshift interstellar gas, even at low molecular fraction and/or solar metallicity. It is puzzling that such diverse systems should show such consistent HD/H$_2$ ratios. Without clear knowledge of all the aspects of HD chemistry that may help determine the ratio HD/H$_2$, we conclude that these systems are potentially more revealing of gas chemistry than of D/H itself and that it is premature to use such systems to constrain D/H at high-redshift.

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arXiv2010

Probing fundamental constant evolution with neutral atomic gas lines

We have detected narrow HI 21cm and CI absorption at $z \sim 1.4 - 1.6$ towards Q0458$-$020 and Q2337$-$011, and use these lines to test for possible changes in the fine structure constant $α$, the proton-electron mass ratio $μ$, and the proton gyromagnetic ratio $g_p$. A comparison between the HI 21cm and CI line redshifts yields $ΔX/X = [+6.8 \pm 1.0] \times 10^{-6}$ over $0 < \le 1.46$, where $X = g_p α^2/μ$, and the errors are purely statistical, from the gaussian fits. The simple line profiles and the high sensitivity of the spectra imply that statistical errors in this comparison are an order of magnitude lower than in previous studies. Further, the CI lines arise in cold neutral gas that also gives rise to HI 21cm absorption, and both background quasars are core-dominated, reducing the likelihood of systematic errors due to local velocity offsets between the hyperfine and resonance lines. The dominant source of systematic error lies in the absolute wavelength calibration of the optical spectra, which appears uncertain to $\sim 2$ km/s, yielding a maximum error in $ΔX/X$ of $\sim 6.7 \times 10^{-6}$. Including this, we obtain $ΔX/X = [+6.8 \pm 1.0 (statistical) \pm 6.7 (max. systematic)] \times 10^{-6}$ over $0 < \le 1.46$. Using literature constraints on $Δμ/μ$, this is inconsistent with claims of a smaller value of $α$ from the many-multiplet method, unless fractional changes in $g_p$ are larger than those in $α$ and $μ$.

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arXiv2010

New limit on a varying proton-to-electron mass ratio from high-resolution optical quasar spectra

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arXiv2010

Keck telescope constraint on cosmological variation of the proton-to-electron mass ratio

Molecular transitions recently discovered at redshift z_abs=2.059 toward the bright background quasar J2123-0050 are analysed to limit cosmological variation in the proton-to-electron mass ratio, mu=m_p/m_e. Observed with the Keck telescope, the optical echelle spectrum has the highest resolving power and largest number (86) of H_2 transitions in such analyses so far. Also, (seven) HD transitions are used for the first time to constrain mu-variation. These factors, and an analysis employing the fewest possible free parameters, strongly constrain mu's relative deviation from the current laboratory value: dmu/mu =(+5.6+/-5.5_stat+/-2.9_sys)x10^{-6}, indicating an insignificantly larger mu in the absorber. This is the first Keck result to complement recent null constraints from three systems at z_abs>2.5 observed with the Very Large Telescope. The main possible systematic errors stem from wavelength calibration uncertainties. In particular, distortions in the wavelength solution on echelle order scales are estimated to contribute approximately half the total systematic error component, but our estimate is model dependent and may therefore under or overestimate the real effect, if present. To assist future mu-variation analyses of this kind, and other astrophysical studies of H_2 in general, we provide a compilation of the most precise laboratory wavelengths and calculated parameters important for absorption-line work with H_2 transitions redwards of the hydrogen Lyman limit.

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arXiv2009

Physical conditions in high-redshift GRB-DLA absorbers observed with VLT/UVES: Implications for molecular hydrogen searches

We aim to understand the nature of the absorbing neutral gas in the galaxies hosting high-redshift long-duration GRBs and to determine their physical conditions. We report the detection of a significant number of previously unidentified allowed transition lines of Fe+, involving the fine structure of the ground term and that of other excited levels, from the zabs=3.969, log N(H0)=22.10 DLA system located in the host galaxy of GRB 050730. The time-dependent evolution of the observed Fe+ energy-level populations is modelled by assuming the excitation mechanism is fluorescence following excitation by ultraviolet photons. This UV pumping model successfully reproduces the observations, yielding a burst/cloud distance (defined to the near-side of the cloud) of d=440\pm 30 pc and a linear cloud size of l=520{+240}{-190} pc. We discuss these results in the context of no detections of H2 and CI lines in a sample of seven z>1.8 GRB host galaxies observed with VLT/UVES. We show that the lack of H2 can be explained by the low metallicities, [X/H]<-1, low depletion factors and, at most, moderate particle densities of the systems. This points to a picture where GRB-DLAs typically exhibiting very high H0 column densities are diffuse metal-poor atomic clouds with high kinetic temperatures, Tkin>~1000 K, and large physical extents, l>~100 pc. The properties of GRB-DLAs observed at high spectral resolution towards bright GRB afterglows differ markedly from the high metal and dust contents of GRB-DLAs observed at lower resolution. This difference likely results from the effect of a bias, against systems of high metallicity and/or close to the GRB, due to dust obscuration in the magnitude-limited GRB afterglow samples observed with high-resolution spectrographs.

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arXiv2009

A search for HI 21cm absorption in strong MgII absorbers in the redshift desert

We report results from a deep search for redshifted HI 21cm absorption in 55 strong MgII$λ$2796 absorbers (having $W (MgII) > 0.5 Å$) at intermediate redshifts, $0.58 < z_{\rm abs} < 1.70$, with the Green Bank Telescope (GBT) and the Giant Metrewave Radio Telescope (GMRT). Nine detections of HI 21cm absorption were obtained, all at $1.17 < z_{\rm abs} < 1.68$, including three systems reported earlier by Gupta et al. (2007). Absorption was not detected at $> 3σ$ significance in 32 other MgII absorbers, with 26 of these providing strong upper limits to the HI 21cm optical depth, $τ_{3σ} < 0.013$ per $\sim 10$ km/s. For the latter 26 systems, the spin temperature $T_s$ of the absorber must be $> [800 \times f]$ K (where $f$ is the covering factor), if the HI column density is $\ge 2 \times 10^{20}$ cm$^{-2}$, i.e. if the absorber is a damped Lyman-$α$ system (DLA). Data on the remaining 13 systems of the sample were affected by radio frequency interference and were hence not useful. Excluding "associated" systems (within 3000 km/s of the quasar redshift), the detection rate of HI 21cm absorption in strong MgII absorbers is $x_{\rm 21,MgII} ({\bar z} = 1.1) = 25^{+11}_{-8}$%, at a $3σ$ optical depth sensitivity of $\sim 0.013$ per 10 km/s. Comparing the detection rates of HI 21cm and damped Lyman-$α$ absorption in strong MgII absorber samples yields a detection rate of HI 21cm absorption in DLAs of $x_{\rm 21,DLA} ({\bar z} = 1.1) = (73 \pm 27)$%, consistent with the detection rate of HI 21cm absorption in low-$z$ DLAs. Since HI 21cm absorption arises in cold neutral gas, this indicates that most gas-rich galaxies contain significant fractions of cold HI by $z \sim 1$. (abridged)

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arXiv2007

Searching for the Precursors of Life in External Galaxies

Are the organic molecules crucial for life on Earth abundant in early-epoch galaxies? To address this, we searched for organic molecules in extragalactic sources via their absorption features, known as diffuse interstellar bands (DIBs). There is strong evidence that DIBs are associated with polycyclic aromatic hydrocarbons (PAHs) and carbon chains. Galaxies with a preponderance of DIBs may be the most likely places in which to expect life. We use the method of quasar absorption lines to probe intervening early-epoch galaxies for the DIBs. We present the equivalent width measurements of DIBs in one neutral hydrogen (HI) abundant galaxy and limits for five DIB bands in six other HI-rich galaxies (damped Lyman-alpha systems--DLAs). Our results reveal that HI-rich galaxies are dust poor and have significantly lower reddening than known DIB-rich Milky Way environments. We find that DIBs in HI-rich galaxies do not show the same correlation with hydrogen abundance as observed in the Milky Way; the extragalactic DIBs are underabundant by as much as 10 times. The lower limit gas-to-dust ratios of four of the HI-rich early epoch galaxies are much higher than the gas-to-dust ratios found in the Milky Way. Our results suggest that the organic molecules responsible for the DIBs are underabundant in HI-rich early epoch galaxies relative to the Milky Way.

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arXiv2007

The nature of the dwarf starforming galaxy associated with GRB 060218 / SN2006aj

We present high resolution VLT UVES and low resolution FORS optical spectroscopy of supernova 2006aj and its host galaxy, associated with the nearby (z = 0.03342) gamma-ray burst GRB 060218. This host galaxy is a unique case, as it is one of the few nearby GRB host galaxies known, and it is only the second time high resolution spectra have been taken of a nearby GRB host galaxy (after GRB 980425). The resolution, wavelength range and S/N of the UVES spectrum combined with low resolution FORS spectra allow a detailed analysis of the circumburst and host galaxy environments. We analyse the emission and absorption lines in the spectrum, combining the high resolution UVES spectrum with low resolution FORS spectra and find the metallicity and chemical abundances in the host. We probe the geometry of the host by studying the emission line profiles. Our spectral analysis shows that the star forming region in the host is metal poor with 12 + log(O/H) = 7.54 (+0.17, -0.10) (~0.07 Z_sun), placing it among the most metal deficient subset of emission-line galaxies. It is also the lowest metallicity found so far for a GRB host from an emission-line analysis. Given the stellar mass of the galaxy of ~10^7 M_sun and the SFR (H alpha) = 0.065 +/- 0.005 M_sun/yr, the high specific star formation rate indicates an age for the galaxy of less than ~200 Myr. The brightest emission lines are clearly asymmetric and are well fit by two Gaussian components separated by ~22$ km/s. We detect two discrete Na I and Ca II absorption components at the same redshifts as the emission components. We tentatively interpret the two components as arising from two different starforming regions in the host, but high resolution imaging is necessary to confirm this.

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arXiv2006

HI 21cm Absorption at $z \sim 2.347$ towards PKS B0438-436

We report the detection of redshifted HI~21cm absorption in the $z \sim 2.347$ damped Lyman-$α$ absorber (DLA) towards PKS B0438-436, with the Green Bank Telescope. This is the second-highest redshift at which 21cm absorption has been detected in a DLA. The absorption extends over $\sim 60$ km/s and has two distinct components, at $z = 2.347477 (12)$ and $z = 2.347869 (20)$. A similar velocity structure is seen in optical metal lines, although the peak absorption here is offset by $\sim 11$ km/s from the peak in the 21cm line. We obtain a high spin temperature $T_s \sim (886 \pm 248) \times (f/0.58)$ K, using a covering factor estimated from 2.3 GHz VLBI data. However, the current data cannot rule out a low spin temperature. The non-detection of CO 3-2 absorption places the upper limit $N_{CO} < 3.8 \times 10^{15} \times (T_x/10)$ cm$^{-2}$ on the CO column density.

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arXiv2005

Gas and dust properties in the afterglow spectra of GRB 050730

We present early WHT ISIS optical spectroscopy of the afterglow of gamma-ray burst GRB 050730. The spectrum shows a DLA system with the highest measured hydrogen column to date: N(HI) = 22.1 +/- 0.1 at the third-highest GRB redshift z = 3.968. Our analysis of the Swift XRT X-ray observations of the early afterglow show X-ray flares accompanied by decreasing X-ray absorption. From both the optical and the X-ray spectra we constrain the dust and gas properties of the host galaxy. We find the host to be a low metallicity galaxy, with low dust content. Much of the X-ray absorbing gas is situated close to the GRB, whilst the HI absorption causing the DLA is most likely located further out.

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arXiv2004

The host of GRB 030323 at z=3.372: a very high column density DLA system with a low metallicity

We present photometry and spectroscopy of the afterglow of GRB 030323. VLT spectra of the afterglow show damped Lya (DLA) absorption and low- and high-ionization lines at a redshift z=3.3718+-0.0005. The inferred neutral hydrogen column density, log N(HI)=21.90+-0.07, is larger than any (GRB- or QSO-) DLA HI column density inferred directly from Lya in absorption. From the afterglow photometry, we derive a conservative upper limit to the host-galaxy extinction: A(V)<0.5 mag. The iron abundance is [Fe/H]=-1.47+-0.11, while the metallicity of the gas as measured from sulphur is [S/H]=-1.26+-0.20. We derive an upper limit on the H2 molecular fraction of 2N(H2)/(2N(H2)+N(HI))<~10^-6. In the Lya trough, a Lya emission line is detected, which corresponds to a star-formation rate (not corrected for dust extinction) of roughly 1 Msun per year. All these results are consistent with the host galaxy of GRB 030323 consisting of a low metallicity gas with a low dust content. We detect fine-structure lines of silicon, SiII*, which have never been clearly detected in QSO-DLAs; this suggests that these lines are produced in the vicinity of the GRB explosion site. Under the assumption that these fine-structure levels are populated by particle collisions, we estimate the HI volume density to be n_HI=100-10000 cm^-3. HST/ACS imaging 4 months after the burst shows an extended AB(F606W)=28.0+-0.3 mag object at a distance of 0.14" (1kpc) from the early afterglow location, which presumably is the host galaxy of GRB 030323.

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arXiv2003

Clues to the origin of metals in the Ly-alpha forest

We analyze the z=2 Ly-alpha and CIV forest along two adjacent lines-of-sight at high S/N and spectral resolution to assess the question of the origin of metals in the intergalactic medium. We find that the gas containing detectable amounts of metals (as seen in absorption by CIV) is less quiescent on scales of a few kpc than the gas without metals. Although it is difficult to discriminate unequivocally between possible sources of enrichment and the extent to which each contributes to line-of-sight differences, those suggest that the metal-enriched systems are bound to galaxies (i.e., they do not arise in the IGM nor in galactic winds).

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arXiv2001

The host galaxy of GRB010222: The strongest damped Lyman-alpha system known

Analysis of the absorption lines in the afterglow spectrum of the gamma-ray burst GRB010222 indicates that its host galaxy (at a redshift of z=1.476) is the strongest damped Lyman-alpha (DLA) system known, having a very low metallicity and modest dust content. This conclusion is based on the detection of the red wing of Lyman-alpha plus a comparison of the equivalent widths of ultraviolet Mg I, Mg II, and Fe II lines with those in other DLAs. The column density of H I, deduced from a fit to the wing of Lyman-alpha, is (5 +/- 2) 10^22 cm^-2. The ratio of the column densities of Zn and Cr lines suggests that the dust content in our line of sight through the galaxy is low. This could be due to either dust destruction by the ultraviolet emission of the afterglow or to an initial dust composition different to that of the diffuse interstellar material, or a combination of both.

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arXiv1996

The Parkes Half-Jansky Flat-Spectrum Sample

We present a new sample of Parkes half-Jansky flat-spectrum radio sources having made a particular effort to find any previously unidentified sources. The sample contains 323 sources selected according to a flux limit of 0.5 Jy at 2.7 GHz, a spectral index measured between 2.7 and 5.0 GHz of alpha > -0.5 (where S(v) is proportional to v to the power of alpha), Galactic latitude abs(b) > 20 deg and -45 deg < Declination (B1950) < +10 deg. The sample was selected from a region 3.90 steradians in area. We have obtained accurate radio positions for all the unresolved sources in this sample and combined these with accurate optical positions from digitised photographic sky survey data to check all the optical identifications. We report new identifications based on \R- and \Kn-band imaging and new spectroscopic measurements of many of the sources. We present a catalogue of the 323 sources of which 321 now have identified optical counterparts and 277 have measured spectral redshifts.

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