Redshift Number Density Research Articles

A high signal-to-noise HST spectrum towards J1009+0713: precise absorption measurements in the CGM of two galaxies

ABSTRACT High signal-to-noise spectra towards background quasars are crucial for uncovering weak absorption in the circumgalactic medium (CGM) of intervening galaxies, such as the diagnostic lines of N v that provide insight to the ionization process of warm gas but typically have low equivalent widths. We present a new spectrum from the Hubble Space Telescope with a signal-to-noise ratio of ∼20–35 towards the quasar SDSS J1009+0713 and analyse absorption systems in the CGM of two L⋆ galaxies close to the line of sight. We identify additional absorption in the CGM of these galaxies that was not reported by the previous lower signal-to-noise spectrum, as well as Milky Way absorbers and quasar outflows from J1009+0713. We measure log (NN v/NO vi) ∼ −1.1 for two CGM absorbers, inconsistent with gas in collisional ionization equilibrium and consistent with a radiatively cooling bulk flow of ∼50–150 km s−1, which could be produced by galactic winds. These column density ratios are also consistent with those found for other L⋆ galaxies and for some gas in the Milky Way’s halo. We place upper limits of log (NN v/NO vi) < −1.8 to −1.2 for other O vi absorbers in the same haloes, which suggests that O vi is produced by different processes in different parts of the CGM, even within the same galactic halo. Together with the kinematically different structure of high- and low-ionization lines, these results indicate there are many components to a single galaxy’s gaseous halo. We find the redshift number density of Ly α forest absorbers and broad Ly α absorbers are consistent with expectations at this redshift.

The Associated Absorption Features in Quasar Spectra of the Sloan Digital Sky Survey. I. Mg ii Absorption Doublets

Abstract Using the SDSS spectra of quasars included in the DR7Q or DR12Q catalogs, we search for Mg ii λλ2796, 2803 narrow absorption doublets in the spectra data around Mg ii λ2798 emission lines. We obtain 17,316 Mg ii doublets, within the redshift range of 0.3299 ≤ z abs ≤ 2.5663. We find that a velocity offset of υ r < 6000 km s−1 is a safe boundary to constrain the vast majority of associated Mg ii systems, although we find some doublets at υ r > 6000 km s−1. If associated Mg ii absorbers are defined by υ r < 6000 km s−1, ∼33.3% of the absorbers are supposed to be contaminants of intervening systems. Removing the 33.3% contaminants, ∼4.5% of the quasars present at least one associated Mg ii system with . The fraction of associated Mg ii systems with high-velocity outflows correlates with the average luminosities of their central quasars, indicating a relationship between outflows and the quasar feedback power. The υ r distribution of the outflow Mg ii absorbers is peaked at 1023 km s−1, which is smaller than the corresponding value of the outflow C iv absorbers. The redshift number density evolution of absorbers (dn/dz) limited by υ r > −3000 km s−1 differs from that of absorbers constrained by υ r > 2000 km s−1. Absorbers limited by υ r > 2000 km s−1 and higher values exhibit profiles similar to dn/dz. In addition, the dn/dz is smaller when absorbers are constrained with larger υ r . The distributions of equivalent widths, and the ratio of , are the same for associated and intervening systems, and independent of quasar luminosity.

Collective Properties of Quasar Narrow Associated Absorption Lines

Abstract This paper statistically investigates the properties of C iv and Mg ii narrow absorption lines (NALs) to look for velocity cuts that can constrain quasar-associated NALs. The coverage fraction (f c ) is defined as the ratio between the number of quasars exhibiting at least one detected absorber and the total number of quasars that can be used to detect absorptions with given criteria. We find that, for both C iv and Mg ii absorbers, both the number density of absorbers in given velocity intervals ( ) and the f c show very significant excess at the low-velocity offset from the quasars, relative to the random occurrence that is expected for cosmologically intervening absorbers. These relative excess extensions for Mg ii absorptions are not only evidently related to absorption strength but also to quasar luminosity, while they are mainly constrained within 2000 no matter what quasar luminosity and absorption strength are. In addition, we find that the redshift number density (dn/dz) evolution of Mg ii absorbers with v abs < 2000 evidently differs from that with v abs > 2000 . Turning to C iv absorptions, the relative excess extensions of both and f c are mainly limited within v abs < 4000 , and depend neither on absorption strength nor on quasar luminosity. And also, the absorbers with v abs < 4000 show obviously different redshift number density evolution from those with v abs > 4000 . We suggest velocity cuts of 4000 and 2000 to define quasar C iv and Mg ii associated NALs, respectively.

New Results on DLA Systems: Statistics

AbstractThe damped Lyman-α absorption-line systems (DLAs) that are observed in quasar spectra arise in neutral-gas-rich regions of intervening galaxies. With the highest neutral hydrogen column densities observed (N(HI) ⩾ 2 × 1020 atoms cm−2), they are known to trace the bulk of the neutral gas content of the Universe, and are thus powerful probes of galaxy formation and evolution. However, DLAs are extremely rare, and since the Lyman-α line falls in the UV for redshifts z < 1.65, not many are known at low redshift due to the limited availability of space data. Our HST surveys for DLAs in strong MgII absorbers have been successful at showing that MgII can be used as an unbiased tracer of DLAs. We present new results on their incidence, or redshift number density, dn/dz, and cosmological neutral gas mass density, ΩDLA, at redshifts 0.11 ⩽ z ⩽ 1.65, and incorporate results from higher and lower redshift studies in the literature to derive the evolution of neutral gas in the Universe.

A BLIND GREEN BANK TELESCOPE MILLIMETER-WAVE SURVEY FOR REDSHIFTED MOLECULAR ABSORPTION

We present the methodology for 'blind' millimeter-wave surveys for redshifted molecular absorption in the CO/HCO{sup +} rotational lines. The frequency range 30-50 GHz appears optimal for such surveys, providing sensitivity to absorbers at z ≳ 0.85. It is critical that the survey is 'blind', i.e., based on a radio-selected sample, including sources without known redshifts. We also report results from the first large survey of this kind, using the Q-band receiver on the Green Bank Telescope (GBT) to search for molecular absorption toward 36 sources, 3 without known redshifts, over the frequency range 39.6-49.5 GHz. The GBT survey has a total redshift path of Δz ≈ 24, mostly at 0.81 < z < 1.91, and a sensitivity sufficient to detect equivalent H{sub 2} column densities ≳ 3 × 10{sup 21} cm{sup –2} in absorption at 5σ significance (using CO-to-H{sub 2} and HCO{sup +}-to-H{sub 2} conversion factors of the Milky Way). The survey yielded no confirmed detections of molecular absorption, yielding the 2σ upper limit n(z = 1.2) < 0.15 on the redshift number density of molecular gas at column densities N(H{sub 2}) ≳ 3 × 10{sup 21} cm{sup –2}.

The redshift number density evolution of Mg II absorption systems

We make use of the recent large sample of 17 042 Mg II absorption systems from Quider et al. to analyze the evolution of the redshift number density. Regardless of the strength of the absorption line, we find that the evolution of the redshift number density can be clearly distinguished into three different phases. In the intermediate redshift epoch (0.6 ≲ z ≲ 1.6), the evolution of the redshift number density is consistent with the non-evolution curve, however, the non-evolution curve over-predicts the values of the redshift number density in the early (z ≲ 0.6) and late (z ≳ 1.6) epochs. Based on the invariant cross-section of the absorber, the lack of evolution in the redshift number density compared to the non-evolution curve implies the galaxy number density does not evolve during the middle epoch. The flat evolution of the redshift number density tends to correspond to a shallow evolution in the galaxy merger rate during the late epoch, and the steep decrease of the redshift number density might be ascribed to the small mass of halos during the early epoch.

A SURVEY OF METAL LINES AT HIGH REDSHIFT. II. SDSS ABSORPTION LINE STUDIES—O VI LINE DENSITY, SPACE DENSITY, AND GAS METALLICITY ATzabs∼ 3.0

We have analysed a large data set of OVI absorber candidates found in the spectra of 3702 SDSS quasars, focusing on a subsample of 387 AGN sightlines with an average S/N>5.0, allowing for detection of absorbers above rest-frame equivalents widths W_r>0.19 A for the OVI 1032 A component. Accounting for random interlopers mimicking an OVI doublet, we derive for the first time a secure lower limit for the redshift number density $\Delta N / \Delta z$ for redshifts z_abs>2.8. With extensive Monte Carlo simulations we quantify the losses of absorbers due to blending with the ubiquitous Lyman forest lines, and estimate the success rate of retrieving each individual candidate as a function of its redshift, the emission redshift of the quasar, the strength of the absorber and the measured S/N of the spectrum by modelling typical Ly forest spectra. These correction factors allow us to derive the 'incompleteness and S/N corrected' redshift number densities of OVI absorbers :$\Delta N _{OVI, c} / \Delta z_{c} (2.8 < z < 3.2) = 4.6+-0.3, at 3.2 < z < 3.6 = 6.7+-0.8,and at 3.6 < z < 4.0 = 8.4+-2.9. We can place a secure lower limit for the contribution of OVI to the closure mass density at the redshifts probed here: $\Omega _{OVI} (2.8 < z < 3.2) >1.9x10^{-8} h^-1. We show that the strong lines we probe account for over 65\% of the mass in the OVI absorbers; the weak absorbers, while dominant in line number density, do not contribute significantly to the mass density. Making a conservative assumption about the ionisation fraction, and adopting the Anders (1989) solar abundance values, we derive the mean metallicty of the gas probed in our search : $\zeta (2.8 < z < 3.2) > 3.6 x 10^-4 h, in good agreement with other studies. These results demonstrate that large spectroscopic datasets such as SDSS can play an important role in QSO absorption line studies, in spite of the relatively low resolution.

A CROSS-CORRELATION ANALYSIS OF Mg II ABSORPTION LINE SYSTEMS AND LUMINOUS RED GALAXIES FROM THE SDSS DR5

We analyze the cross-correlation of 2,705 unambiguously intervening Mg II (2796,2803A) quasar absorption line systems with 1,495,604 luminous red galaxies (LRGs) from the Fifth Data Release of the Sloan Digital Sky Survey within the redshift range 0.36<=z<=0.8. We confirm with high precision a previously reported weak anti-correlation of equivalent width and dark matter halo mass, measuring the average masses to be log M_h(M_[solar]h^-1)=11.29 [+0.36,-0.62] and log M_h(M_[solar]h^-1)=12.70 [+0.53,-1.16] for systems with W[2796A]>=1.4A and 0.8A<=W[2796A]<1.4A, respectively. Additionally, we investigate the significance of a number of potential sources of bias inherent in absorber-LRG cross-correlation measurements, including absorber velocity distributions and the weak lensing of background quasars, which we determine is capable of producing a 20-30% bias in angular cross-correlation measurements on scales less than 2'. We measure the Mg II - LRG cross-correlation for 719 absorption systems with v<60,000 km s^-1 in the quasar rest frame and find that these associated absorbers typically reside in dark matter haloes that are ~10-100 times more massive than those hosting unambiguously intervening Mg II absorbers. Furthermore, we find evidence for evolution of the redshift number density, dN/dz, with 2-sigma significance for the strongest (W>2.0A) absorbers in the DR5 sample. This width-dependent dN/dz evolution does not significantly affect the recovered equivalent width-halo mass anti-correlation and adds to existing evidence that the strongest Mg II absorption systems are correlated with an evolving population of field galaxies at z<0.8, while the non-evolving dN/dz of the weakest absorbers more closely resembles that of the LRG population.

Where Is the Molecular Hydrogen in Damped Lyα Absorbers?

We show in this paper why molecular millimeter absorption line searches in DLAs have been unsuccessful. We use CO emission-line maps of local galaxies to derive the H2 column density distribution function f(N) at z = 0. We show that it forms a natural extension to f(N): the H2 distribution exceeds f(N) at NH ≈ 1022cm-2 and exhibits a power-law dropoff with slope ~-2.5. Approximately 97% of the H2 mass density ρ is in systems above N = 1021 cm-2. We derive a value ρ = 1.1 × 107 h70 M☉ Mpc-3, which is ≈25% the mass density of atomic hydrogen. Yet the redshift number density of H2 above this N limit is only ≈ 3 × 10-4, a factor 150 lower than that for H I in DLAs at z = 0. Furthermore, we show that the median impact parameter between a N > 1021 cm-2 absorber and the center of the galaxy hosting the H2 gas is only 2.5 kpc. On the basis of arguments related to the Schmidt law, we argue that H2 gas above this column density limit is associated with a large fraction of the integral star formation rate density. Even allowing for an increased molecular mass density at higher redshifts, the derived cross sections indicate that it is very unlikely to identify the bulk of the molecular gas in present quasar absorption lines samples. We discuss the prospects for identifying this molecular mass in future surveys.

High‐Redshift Superwinds as the Source of the Strongest MgiiAbsorbers: A Feasibility Analysis

We present High Resolution Echelle Spectrometer/Keck profiles of four extremely strong (Wr > 1.8 Å) Mg II absorbers at 1 < z < 2. The profiles display a common kinematic structure, having a sharp drop in optical depth near the center of the profile and strong, black-bottomed absorption on either side. This "symmetric-inverted" structure, with a velocity spread of several hundred kilometers per second, is suggestive of superwinds arising in actively star-forming galaxies. Low-ionization absorption of similar strength has been observed in local star-forming galaxies. The Mg II absorbers with Wr > 1.8 Å evolve away from z = 2 to the present. We propose that a substantial fraction of these very strong absorbers are due to superwinds and that their evolution is related to the redshift evolution of star-forming galaxies. Based on the observed redshift number density of Wr > 1.8 Å Mg II absorbers at 1 < z < 2, we explore whether it is realistic that superwinds from starbursting galaxies could give rise to these absorbers. Finally, we do an analysis of the superwind connection to damped Lyα absorbers (DLAs). DLAs and superwinds evolve differently and usually have different kinematic structure, indicating that superwinds probably do not give rise to the majority of DLAs.

MgiiAbsorber Number Density atz ≃ 0.05: Implications for ΩDLAEvolution

An unbiased sample of 147 quasar/active galactic nuclei spectra, obtained with the Faint Object Spectrograph on board the Hubble Space Telescope, has been searched for intervening Mg II ??2796, 2803 absorbers over the redshift range 0 < z ? 0.15. The total redshift path searched is 18.8, with the survey being 80% complete to a 5 ? rest-frame equivalent width Wr(?2796) of 0.6 ? and 20% complete to Wr(?2796) = 0.3 ?. Main results of this work are the following: (1) Four systems were found, with a mean redshift of z = 0.06, yielding a redshift number density dN/dz = 0.22 for absorbers with Wr(?2796) ? 0.6 ?. This is consistent with the value expected if these systems do not evolve from higher redshifts (z = 2.2). (2) No systems with Wr(?2796) < 0.6 ? were found. Based on no evolution expectations and accounting for the survey completeness, it is a 2 ? result to have a null detection of smaller Wr(?2796) systems. If this implies a turnover in the low Wr(?2796) region of the equivalent width distribution at z ~ 0, then there is at least a 25% reduction in the average galaxy gas cross section from z ? 0.2 galaxies. (3) These systems have strong Fe II absorption and, based on the results of Rao & Turnshek, are good candidates for damped Ly? absorbers (DLAs). This translates to a redshift number density of dN/dz = 0.08 for DLAs at z ~ 0. In tandem with the data analyzed by Rao & Turnshek, these results indicate that the redshift number density of DLAs does not evolve from z 4 to z 0. If the H I mass function does not evolve from z 0.5 to z 0, then the cosmological H I mass density ?DLA is also deduced so as to not evolve from z 4 to z 0. These z 0 results for Mg II absorption-selected DLAs are at odds with those based on 21 cm emission from H I galaxies by a factor of 5-6.

Clustering Properties of Low‐Redshift QSO Absorption Systems Toward the Galactic Poles

We examine the clustering properties of low-redshift Lya and heavy-element QSO absorption line systems seen in the spectra of 13 QSOs at the Galactic poles. This is the densest sample of D1i separat- ed QSOs observed spectroscopically with the Hubble Space Telescope to date. At the median redshift of the Lya sample (z B 0.7), the QSO lines of sight are separated on transverse scales from about 15 to 200 h~1 Mpc H 100 h km s~1 Mpc~1), allowing the three-dimensional clustering of the (q 0 0.5, absorbers to be examined on those scales. The Galactic poles are also regions where relatively deep and wide-—eld galaxy redshift surveys have taken place, so the distributions of galaxies and Lya systems can be compared within the same volume of space. There are 545 total absorption lines detected in the com- plete sample from 13 QSOs. We identify 307 Lya systems, of which 18 contain heavy-element lines. We con—rm the relatively slow redshift number density evolution for Lya systems at There are also z ( 1. —ve likely C IV doublets in our sample, for which the Lya line is not accessible. The main results are summarized as follows: 1. Most, but not all, of the very low-z (z 0.2) Lya systems appear to be associated with large-scale structures traced by luminous galaxies, based upon the spectrum of the low-redshift QSO PG 1309)355. 2. On scales of 2.5 to 5 h~1 Mpc, the line-of-sight correlation function shows a positive and signi—cant clustering amplitude, for systems with rest equivalent width, W , exceeding 0.24 m los 0.95 ^ 0.28, Ae . Most of this signal can be accounted for by a small number of Lya ii clumps, ˇˇ which typically contain about —ve systems spanning D20 h~1 Mpc. There is qualitative evidence that the strength of the corre- lation function rises with the limiting Lya rest equivalent width. 3. Over half of the heavy-element systems are either in groups or have a nearby ii clump ˇˇ of Lya lines. However, there is only weak evidence for a signi—cant heavy-element/Lya cross-correlation function. 4. There is no clear evidence that the Lya systems are distributed in redshift ii peaks ˇˇ and ii voids ˇˇ as is characteristic for the galaxies. In particular, there is no signi—cant periodicity detected on the scale of 128 h~1 Mpc, which had been detected for the galaxies located in the same direction of the sky. There is marginal evidence for a 41.5 h~1 Mpc ii periodicity ˇˇ in the distribution, which is seen for all equivalent width-limited samples. 5. The three-dimensional Lya radial correlation function shows no signi—cant departures from a random sample at any scale or using any limiting equivalent width. This indicates that if there are sheet- like structures spanning separations on the order of 30 h~1 Mpc or more, they are not clearly traced by Lya systems. 6. The fraction of Lya lines with W ( 0.24 which may be clustered in the same way as the heavy- Ae , element systems detected mostly at higher redshifts, is about 39% ^ 6%. This value is likely to depend on the limiting equivalent width and redshift of the sample. These results are consistent with a picture of the Lya forest at low redshift in which some fraction of the Lya lines arise in close proximity to galaxies and structures traced by galaxies, but in which a substantial fraction of the systems are also more uniformly distributed and are largely unrelated to the distribution of luminous galaxies. The results also appear to be in broad agreement with numerical simulations of the