arXiv subjects
Publications and source records attributed to Miriam Centurion.
In this work, we study argon abundances in the interstellar medium of high-redshift galaxies (2 3.5 are required to probe the final stages of this process of cosmic reionisation.
We report here a study of nitrogen and $α$-capture element (O, S, and Si) abundances in 18 Damped Ly$α$ Absorbers (DLAs) and sub-DLAs drawn from the ESO-UVES Advanced Data Products (EUADP) database. We report 9 new measurements, 5 upper and 4 lower limits of nitrogen that when compiled with available nitrogen measurements from the literature makes a sample of 108 systems. The extended sample presented here confirms the [N/$α$] bimodal behaviour suggested in previous studies. Three-quarter of the systems show $\langle$[N/$α$]$\rangle=-0.85$ ($\pm$0.20 dex) and one-quarter ratios are clustered at $\langle$[N/$α$]$\rangle= -1.41$ ($\pm$0.14 dex). The high [N/$α$] plateau is consistent with the HII regions of dwarf irregular and blue compact dwarf galaxies although extended to lower metallicities and could be interpreted as the result of a primary nitrogen production by intermediate mass stars. The low [N/$α$] values are the lowest ever observed in any astrophysical site. In spite of this fact, even lower values could be measured with the present instrumentation, but we do not find them below [N/$α$] $\approx$ $-1.7$. This suggests the presence of a floor in [N/$α$] abundances, which along with the lockstep increase of N and Si may indicate a primary nitrogen production from fast rotating, massive stars in relatively young or unevolved systems.
Nitrogen is thought to have both primary and secondary origins depending on whether the seed carbon and oxygen are produced by the star itself (primary) or already present in the interstellar medium (secondary) from which star forms. DLA and sub-DLA systems with typical metallicities of -3.0<Z/Z_sun<-0.5 are excellent tools to study nitrogen production. We made a search for nitrogen in the ESO-UVES advanced data products (EUADP) database. In the EUADP database, we find 10 new measurements and 9 upper limits of nitrogen. We further compiled DLA/sub-DLA data from the literature with estimates available of nitrogen and alpha-elements. This yields a total of 98 systems, i.e. the largest nitrogen abundance sample investigated so far. In agreement with previous studies, we indeed find a bimodal [N/alpha] behaviour: three-quarter systems show a mean value of [N/alpha]=-0.87 with a scatter of 0.21 dex and one-quarter shows ratios clustered at [N/alpha]=-1.43 with a lower dispersion of 0.13 dex. The high [N/alpha] group is consistent with the blue compact dwarves and dwarf irregular galaxies, suggesting primary nitrogen production. The low [N/alpha] group is the lowest ever observed in any astrophysical site and probably provides an evidence of the primary production by fast rotating massive stars in young sites. Moreover, we find a transition between the two [N/alpha] groups around [N/H]=-2.5. The transition is not abrupt and there are a few systems lying in the transition region. Additional observations of DLAs/sub-DLAs below [N/H]<-2.5 would provide more clues.
Precise radial velocity measurements (delta v/c ~ 10^{-7}) of FeII lines in damped Ly-alpha systems from very high quality VLT/UVES spectra of quasars HE0515-4414 and Q1101-264 are used to probe cosmological time dependence of the fine structure constant, alpha. It is found that between two redshifts z1 = 1.15 and z2 = 1.84 the value of Delta alpha/alpha changes at the level of a few ppm: (alpha_z2 - alpha_z1)/alpha_0 = 5.43 +/- 2.52 ppm. Variations of alpha can be considered as one of the most reliable method to constrain the dark energy equation of state and improvements on the accuracy of the wavelength calibration of QSO spectra are of great importance.
We present a study of Ar abundances in 15 damped Ly alpha systems (DLAs) in the redshift interval from 2.3 to 3.4. The sample includes 4 new UVES/VLT measurements of Ar I column densities. The majority of DLAs show significant underabundances of Ar relative to other alpha-capture elements with common nucleosynthetic origin. We show that neither dust depletion nor intervening HII regions inside DLAs offer a viable justification to these underabundances. A natural explanation is found in the framework of photoionization models of HI regions embedded in an ionizing continuum with varying spectral distribution. At z ~ 2.5 the observed Ar deficiencies are large, [Ar/alpha] ~ -0.6/-0.8 dex, suggestive of a hard, QSO-dominated spectrum. At z >~ 3 the deficiencies are instead small, suggestive of a soft, stellar-type spectrum, though more data are needed to generalize this high-z result. We argue that the change of Ar abundances is induced by the evolution of the UV metagalactic continuum, in which case the UV emission internal to DLAs must be small (i.e. DLAs should have modest star formation rates) and the external background must become softer at z > 3. The former requirement is consistent with the modest evolution of DLAs abundances and the lack of Ly_alpha and H_alpha emissions associated with DLAs. The latter requirement is consistent with the observed evolution of SiIV/CIV ratios in the IGM, the claims of high escape fraction of UV photons from Ly-break galaxies at z >~ 3, and the recent finding that the HeII re-ionization seems to occur between z ~ 3.4 and z ~ 3. From the comparison with local interstellar studies, we predict a rise of Ar abundances in the redshift range from z ~ 2.3 to z=0, at the epoch at which the metagalactic field of galaxies overcomes that of quasars.
We analyze extant data of Al++, Al+ and other low ions with the aim of studying the ionization properties of Damped Ly alpha systems (DLAs) from the analysis of the ratio R(Al++/Al+) = N(Al++)/N(Al+). We find good correlations logN(Al+)-logN(Si+) and logN(Al+)-logN(Fe+) that we use to indirectly estimate N(Al+) from N(Si+) and/or N(Fe+) measurements. In this way we determine the ratio R(Al++/Al+) for a sample of 20 DLAs. Contrary to common belief, the ratio can attain relatively high values (up to 0.6), suggesting that gas of intermediate ionization state plays an important role in DLAs. On the other hand, the lack of any trend between abundance ratios, such as Si/H and Si/Fe, and R(Al++/Al+) indicates that abundances are not severely influenced by ionization effects. We find a logR(Al++/Al+) - logN(H I) anticorrelation that we use, in conjunction with idealized photoionization equilibrium calculations, to constrain the ionization properties and to predict ionization corrections in DLAs. We consider two possible origins for the species of low and intermediate ionization state: (1) neutral regions devoid of Al++ and/or (2) partially ionized, (Al++)-bearing regions. The logR(Al++/Al+) - logN(H I) anticorrelation can be naturally explained in terms of a two-region model with a soft, stellar-type ionizing radiation field. We present abundance ionization corrections for 14 elements of astrophysical interest derived with different types of ionizing spectra. For most of these elements the corrections are generally below measurements errors, contrary to the predictions of recent models presented in the literature. We briefly discuss the potential effects of inaccuracies of the Al recombination rates used in the photoionization calculations.
The UV resonance transitions of neutral argon ArI 1066 A, and of singly ionized phosphorus PII 963 A, originated in the damped Ly_alpha system (DLA) at zabs = 3.3901 towards QSO 0000--2620 have been detected by means of the UVES spectrograph at the 8.2m ESO KUEYEN telescope. So far, this is the first measurement of ArI, and the second of PII, ever performed in damped galaxies and in high redshift objects. This DLA is well known for having one of the lowest metal abundances and dust content, and the lowest fractional abundance of molecular hydrogen H_2. The measured Ar abundance is [Ar/H] = - 1.91 (+/- 0.09) which is equal to the abundances of the other alpha-chain elements (O, S and Si). The similarity of the Ar abundance with the other alpha-chain elements implies the absence of significant photoionization by either UV background or stellar sources along the sightline throughout the damped Ly_alpha system. Both log(Ar/O) and log(Ar/S) ratios are found close to those measured in the extragalactic HII regions and in blue compact galaxies where O is more abundant by at least one order of magnitude. This strengthens the universality of the Ar/O and Ar/S ratios and lends support to the existence of a universal IMF. The abundance of the non-refractory element phosphorus [P/H] = - 2.31 (+/- 0.10) confirms the low amount of chemical evolution in the DLA. This is the measurement of P in the most metal-poor material and shows a subsolar [P/Fe] = --0.27 value. The measured ratios [P/Si] = - 0.40 (+/- 0.13) and [P/S] = - 0.33 (+/- 0.13) provide evidence for a mild odd-even effect. Finally, a stringent upper limit to the population of the 3P_1 level in the ground state of OI is derived, which provides a lower limit to the physical dimensions of the zabs = 3.3901 system of L > 7 pc.
Zinc is a good indicator of metallicity in Damped Lyman alpha (DLA) systems because it is almost unaffected by dust depletion. However, the use of zinc as a tracer of metallicity evolution has been hampered by the difficulty of detecting the Zn II resonance lines at high redshift. The measurement of zinc abundance in a DLA system at z_abs > 3 obtained by means of the UVES spectrograph at the VLT prompted us to re-analyse the full sample of zinc abundances present in the literature to search for a metallicity-redshift relation in DLA systems. The study of the metallicities of individual systems shows evidence for an anti-correlation between [Zn/H] and redshift supported by different types of statistical tools. The zinc metallicity decreases by -0.3 +/- 0.1 dex per unit redshift interval in the range 0.5 <~ z_abs <~ 3.5. This rate is in good agreement with that found by Savaglio, Panagia & Stiavelli (2000) in their recent study of DLA abundances corrected for dust depletion. The present result does not require a knowledge of the dust depletion pattern(s) in DLA systems. On the other hand, the analysis of the column-density weighted metallicity of the sample, , does not show a clear evidence for redshift evolution, consistent with previous studies of zinc abundances. We propose that the apparent lack of evolution of is due to the combination of selection bias effects together with the extreme sensitivity of to low-number statistics.
Observations of the QSO 0000-2620 with UVES spectrograph at the 8.2m ESO KUEYEN telescope are used for abundance analysis of the damped Ly-alpha system at z_{abs}=3.3901. Several Oxygen lines are identified in the Ly_alpha forest and a measure for the oxygen abundance is obtained at [O/H]=-1.85 +/- 0.1 by means of the unsaturated OI 925 A and OI 950 A lines. This represents the most accurate O measurement in a damped Ly_alpha galaxy so far. We have also detected ZnII 2026 A and CrII 2056, 2062 A redshifted at about 8900 A and found abundances [Zn/H] = -2.07 +/- 0.1 and [Cr/H]=-1.99 +/- 0.1. Furthermore, previous measurements of Fe, Si, Ni and N have been refined yielding [Fe/H]=-2.04 +/- 0.1, [Si/H]=-1.90 +/- 0.1, [Ni/H]=-2.27 +/- 0.1, and [N/H]=-2.68 +/- 0.1. The abundance of the non-refractory element zinc is the lowest among the damped Ly-alpha systems showing that the associated intervening galaxy is indeed in the early stages of its chemical evolution. The fact that the Zn abundance is identical to that of the refractory elements Fe and Cr suggests that dust grains have not formed yet. In this Damped Ly-alpha system the observed [O,S,Si/Zn,Fe,Cr] ratios, in whatever combination are taken, are close to solar (i.e 0.1-0.2 dex) and do not show the [alpha-element/Fe] enhancement observed in Milky Way stars of comparable metallicity. The observed behavior supports a galaxy evolution model characterized by either episodic or low star formation rate rather than a Milky-Way-type evolutionary model.
Relative elemental abundances, and in particular the alpha/Fe ratio, are an important diagnostic tool of the chemical evolution of damped Ly alpha systems (DLAs). The S/Zn ratio is not affected by differential dust depletion and is an excellent estimator of the alpha/Fe ratio. We report 6 new determinations of sulphur abundance in DLAs at zabs greater than or equal to 2 with already known zinc abundances. The combination with extant data from the literature provides a measure of the S/Zn abundance ratio for a total of 11 high redshift DLA systems. The observed [S/Zn] ratios do not show the characteristic [alpha/Fe] enhancement observed in metal-poor stars of the Milky Way at comparable level of metallicity ([Zn/H] ~ -1). The behaviour of DLAs data is consistent with a general trend of decreasing [S/Zn] ratio with increasing metallicity [Zn/H]. This would be the first evidence of the expected decrease of the alpha/Fe ratio in the course of chemical evolution of DLA systems. However, in contrast to what observed in our Galaxy, the alpha/iron-peak ratio seems to attain solar values when the metallicity is still low ([Zn/H] < -1) and to decrease below solar values at higher metallicities. The behaviour of the alpha/Fe ratio challenges the frequently adopted hypothesis that high redshift DLAs are progenitors of spiral galaxies and favours instead an origin in galaxies characterized by low star formation rates, in agreement with the results from imaging studies of low redshift DLAs, where the candidate DLA galaxies show a variety of morphological types including dwarfs and LSBs and only a minority of spirals.
A high-resolution, high signal-to-noise HIRES-Keck spectrum of APM 08279+5255 reveals a feature in the HI Ly alpha profile of the zabs=3.514 absorber at the expected position of the corresponding DI Lyalpha line. The absorber shows a relatively simple velocity structure, with a major component detected in several metallic lines including CII, CIV, SiII, SiIII and Si IV. Modeling of the hydrogen column density with the minimum number of components yields D/H ~ 1.5 x 10E-5. However, a more complex structure for the hydrogen cloud with somewhat ad hoc components would allow a higher D/H. The system has a very low metallicity with [Si/H] ~ -3.5 and [C/H] ~ -4.0 and is therefore representative of essentially unprocessed material, with a deuterium abundance close to the primordial value. The present analysis favours the low value of D/H =3.39(+/-0.25) x 10E-5 measured towards QSO 1009+2956 and QSO 1937-1009 (Burles & Tytler 1998a, 1998b) as the fiducial value of deuterium at high redshift.
We present high-resolution UES spectra of the quasar PHL 957 obtained for studying the foreground Damped Ly-alpha galaxy at z=2.309. Measurements of absorption lines lead to accurate abundance determinations of Fe, S and N which complement measurements of Zn, Cr and Ni already available for this system. We find [Fe/H]= -2.0 (+/-0.1), [S/H]= -1.54 (+/-0.06) and [N/H]=-2.76 (+/-0.07). The ratio [Fe/Zn]=-0.44 provides evidence that about 74% of iron and about 28% of zinc are locked into dust grains with a dust-to-gas ratio of about 3% of the Galactic one. The total iron content in both gas and dust in the DLA system is [Fe/H]=-1.4. This confirms a rather low metallicity in the galaxy, which is in the early stages of its chemical evolution. The detection of SII allows us to measure the SII/ZnII ratio, which is a unique diagnostic tool for tracing back its chemical history, since it is not affected by the presence of dust. Surprisingly, the resulting relative abundance is [S/Zn]=0.0 (+/-0.1), at variance with the overabundance found in the Galactic halo stars with similar metallicity. We emphasize that the [S/Zn] ratio is solar in all the three DLA absorbers with extant data. Upper limits are also found for Mn, Mg, O and P and, once the dust depletion is accounted for, we obtain [Mg/Fe]_{c} <+0.2, [O/Fe]_{c}< 0.4, [Mn/Fe]_{c}<+0.0 and [P/Fe]_{c}<-0.7. The [alpha/Fe] values do not support a Galactic halo-like abundances implying that the chemical evolution of this young galaxy is not reproducing our own Galaxy's evolution.