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Register a new userSuper-Solar Metallicity in Weak Mg II Absorption Systems at z ~ 1.7
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Through photoionization modeling, constraints on the physical conditions of three z ~ 1.7 single-cloud weak Mg II systems (W_r(2796) < 0.3A) are derived. Constraints are provided by high resolution R = 45,000, high signal-to-noise spectra of the three quasars HE0141-3932, HE0429-4091, and HE2243-6031 which we have obtained from the ESO archive of VLT/UVES. Results are as follows: (1) The single-cloud weak Mg II absorption in the three z ~ 1.7 systems is produced by clouds with ionization parameters of -3.8 < logU < -2.0 and sizes of 1-100 pc. (2) In addition to the low-ionization phase Mg II clouds, all systems need an additional 1-3 high-ionization phase C IV clouds within 100 km/s of the Mg II component. The ionization parameters of the C IV phases range from -1.9 < logU < -1.0, with sizes of tens of parsecs to kiloparsecs. (3) Two of the three single-cloud weak Mg II absorbers have near-solar or super-solar metallicities, if we assume a solar abundance pattern. Although such large metallicities have been found for z < 1 weak Mg II absorbers, these are the first high metallicities derived for such systems at higher redshifts. (4) Two of the three weak Mg II systems also need additional low-metallicity, broad Lya absorption lines, offset in velocity from the metal-line absorption, in order to reproduce the full Lya profile. (5) Metallicity in single-cloud weak Mg II systems are more than an order of magnitude larger than those in Damped Lya systems at z ~ 1.7. In fact, there appears to be a gradual decrease in metallicity with increasing N(HI), from these, the most metal-rich Lya forest clouds, to Lyman limit systems, to sub-DLAs, and finally to the DLAs.
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We present the results of a MgII absorption-line survey using QSO spectra from the SDSS EDR. Over 1,300 doublets with rest equivalent widths greater than 0.3AA and redshifts $0.366 le z le 2.269$ were identified and measured. We find that the $lambda2796$ rest equivalent width ($W_0^{lambda2796}$) distribution is described very well by an exponential function $partial N/partial W_0^{lambda2796} = frac{N^*}{W^*} e^{-frac{W_0}{W^*}}$, with $N^*=1.187pm0.052$ and $W^*=0.702pm0.017$AA. Previously reported power law fits drastically over-predict the number of strong lines. Extrapolating our exponential fit under-predicts the number of $W_0 le 0.3$AA systems, indicating a transition in $dN/dW_0$ near $W_0 simeq 0.3$AA. A combination of two exponentials reproduces the observed distribution well, suggesting that MgII absorbers are the superposition of at least two physically distinct populations of absorbing clouds. We also derive a new redshift parameterization for the number density of $W_0^{lambda2796} ge 0.3$AA lines: $N^*=1.001pm0.132(1+z)^{0.226pm0.170}$ and $W^*=0.443pm0.032(1+z)^{0.634pm 0.097}$AA. We find that the distribution steepens with decreasing redshift, with $W^*$ decreasing from $0.80pm0.04$AA at $z=1.6$ to $0.59pm0.02$AA at $z=0.7$. The incidence of moderately strong MgII $lambda2796$ lines does not show evidence for evolution with redshift. However, lines stronger than $approx 2$AA show a decrease relative to the no-evolution prediction with decreasing redshift for $z lesssim 1$. The evolution is stronger for increasingly stronger lines. Since $W_0$ in saturated absorption lines is an indicator of the velocity spread of the absorbing clouds, we interpret this as an evolution in the kinematic properties of galaxies from moderate to low z.
We present a sample of 34 weak metal line absorbers at $z< 0.3$ complied via the simultaneous detections ($3sigma$) of the SiII$lambda1260$ and CII$lambda1334$ absorption lines, with $W_{r}$(SiII)$<0.2$ AA and $W_{r}$(CII)$<0.3$ AA, in archival HST/COS spectra. Our sample increases the number of known low-$z$ weak absorbers by a factor of $>5$. The column densities of HI and low-ionization metal lines obtained from Voigt profile fitting are used to build simple photoionization models using CLOUDY. The inferred densities and total hydrogen column densities are in the ranges of $-3.3 < log n_{rm H}/{rm cm^{-3}} < -2.4$ and $16.0 < log N_{rm H}/{rm cm^{-2}}<20.3$, respectively. The line of sight thicknesses of the absorbers have a wide range of $sim$1 pc$-$50 kpc with a median value of $sim$500 pc. The high-ionization OVI absorption, detected in 12/18 cases, always stems from a different gas-phase. Most importantly, 85% (50%) of these absorbers show a metallicity of $rm [Si/H] > -1.0$ (0.0). The fraction of systems showing high metallicity (i.e., $rm [Si/H]>-1.0$) in our sample is significantly higher than the HI-selected sample (Wotta et al. 2016) and the galaxy-selected sample (Prochaska et al. 2017) of absorbers probing the circum-galactic medium (CGM) at similar redshift. A search for galaxies has revealed a significant galaxy-overdensity around these weak absorbers compared to random places with a median impact parameter of 166 kpc to the nearest galaxy. Moreover, we find the presence of multiple galaxies in $sim80$% of the cases, suggesting group environments. The observed $dmathcal{N}/dz$ of $0.8pm0.2$ indicates that such metal-enriched, compact, dense structures are ubiquitous in the halos of low-$z$ galaxies that are in groups. We suggest that these are transient structures that are related to outflows and/or stripping of metal-rich gas from galaxies.
We present the rest-frame optical spectrum of a strongly lensed galaxy at redshift z =1.7 behind the cluster Abell 1689. We detect the temperature sensitive auroral line [O III] 4363, which allows the first direct metallicity measurement for galaxies at z > 1. Our high signal-to-noise spectrum indicates that the target is an extremely low metallicity star-forming galaxy.We estimate an intrinsic absolute B band magnitude of M_{B}=-18.3 pm 0.1$, with a stellar mass of 4.4$pm1.2times10^{8}$ M$_{odot}$. This galaxy extends the luminosity-metallicity relation of star-forming galaxies at z > 2 by more than an order of magnitude. Given the double-nuclei like morphology and velocity profile of ha, we tentatively suggest that it could be a merger or a proto-rotating disk galaxy.
We present a study of strong intervening absorption systems in the near-IR spectra of 31 luminous quasars at $z>5.7$. The quasar spectra were obtained with {it Gemini} GNIRS that provide continuous wavelength coverage from $sim$0.9 to $sim$2.5 $mu$m. We detect 32 strong Mg II doublet absorbers with rest-frame equivalent width $W_r$ ($lambda2796$) $>1.0$ AA at $2.2 < z < 6.0$. Each Mg II absorber is confirmed by at least two associated Fe II absorption lines in the rest-frame wavelength range of $sim 1600-2600$ AA. We find that the comoving line density ($dN/dX$) of the strong Fe II-bearing Mg II absorbers decreases towards higher redshift at $z>3$, consistent with previous studies. Compared with strong Mg II absorbers detected in damped Ly$alpha$ systems at 2 $<z<$ 4, our absorbers are potentially less saturated and show much larger rest-frame velocity widths. This suggests that the gas traced by our absorbers are potentially affected by galactic superwinds. We analyze the {it Hubble Space Telescope} near-IR images of the quasars and identify possible associated galaxies for our strong absorbers. There are a maximum of two galaxy candidates found within 5 radius of each absorber. The median F105W-band magnitude of these galaxy candidates is 24.8 mag, which is fainter than the $L^*$ galaxy luminosity at $zsim$ 4. By using our observed $dN/dX$ of strong Mg II absorbers and galaxy candidates median luminosity, we suggest that at high redshift, strong Mg II absorbers tend to have a more disturbed environment but smaller halo size than that at $z <$ 1.
We present Atacama Large Millimeter/submillimeter Array observations of CO lines and dust continuum emission of the source RCSGA 032727--132609, a young $z=1.7$ low-metallicity starburst galaxy. The CO(3-2) and CO(6-5) lines, and continuum at rest-frame $450,mu m$ are detected and show a resolved structure in the image plane. We use the corresponding lensing model to obtain a source plane reconstruction of the detected emissions revealing intrinsic flux density of $S_{450,mu m}=23.5_{-8.1}^{+26.8}$ $mu$Jy and intrinsic CO luminosities $L_{rm CO(3-2)}=2.90_{-0.23}^{+0.21}times10^{8}$ ${rm K,km,s^{-1},pc^{2}}$ and $L_{rm CO(6-5)}=8.0_{-1.3}^{+1.4}times10^{7}$ ${rm K,km,s^{-1},pc^{2}}$. We used the resolved properties in the source plane to obtain molecular gas and star-formation rate surface densities of $Sigma_{rm H2}=16.2_{-3.5}^{+5.8},{rm M}_{odot},{rm pc}^{-2}$ and $Sigma_{rm SFR}=0.54_{-0.27}^{+0.89},{rm M}_{odot},{rm yr}^{-1},{rm kpc}^{-2}$ respectively. The intrinsic properties of RCSGA 032727--132609 show an enhanced star-formation activity compared to local spiral galaxies with similar molecular gas densities, supporting the ongoing merger-starburst phase scenario. RCSGA 032727--132609 also appears to be a low--density starburst galaxy similar to local blue compact dwarf galaxies, which have been suggested as local analogs to high-redshift low-metallicity starburst systems. Finally, the CO excitation level in the galaxy is consistent with having the peak at ${rm J}sim5$, with a higher excitation concentrated in the star-forming clumps.
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