Background Quasi-stellar Objects Research Articles

CloudFlex: A Flexible Parametric Model for the Small-scale Structure of the Circumgalactic Medium

We present CloudFlex, an open-source tool for predicting absorption-line signatures of cool gas in galaxy halos with small-scale structure. Motivated by analyses of ∼104 K material in hydrodynamical simulations of turbulent, multiphase media, we model cool gas structures as complexes of cloudlets sampled from a power-law distribution of mass ∝mcl−α with velocities drawn from a turbulent velocity field. The user may specify α, the lower limit of the cloudlet mass distribution ( mcl,min ), and several other parameters that set the mass, size, and velocity distribution of the complex. This permits investigation of the relation between these parameters and absorption-line observables. As a proof-of-concept, we calculate the Mg ii λ2796 absorption induced by the cloudlets in background quasi-stellar object (QSO) spectra. We demonstrate that, at fixed metallicity, the covering fraction of sight lines with equivalent widths W 2796 < 0.3 Å increases significantly with decreasing mcl,min , cloudlet number density (n cl), and complex size. We then use this framework to predict the halo-scale W 2796 distribution around ∼L * galaxies. We show that the observed incidences of W 2796 > 0.3 Å sight lines with impact parameters 10 kpc < R ⊥ < 50 kpc in projected QSO–galaxy studies are consistent with our model over much of parameter space. However, they are underpredicted by models with mcl,min≥100M⊙ and n cl ≥ 0.03 cm−3, in keeping with a picture in which the inner cool circumgalactic medium (CGM) is dominated by numerous low-mass cloudlets (m cl ≲ 100M ⊙) with a volume filling factor ≲1%. When used to model absorption-line data sets built from multi-sight line and/or spatially extended background probes, CloudFlex enables detailed constraints on the size and velocity distributions of structures comprising the photoionized CGM.

Compact [C II] emitters around a C IV absorption complex at redshift 5.7.

The physical conditions of the circumgalactic medium are investigated by means of intervening absorption-line systems in the spectrum of background quasi-stellar objects (QSOs) out to the epoch of cosmic reionization1-4. A correlation between the ionization state of the absorbing gas and the nature of the nearby galaxies has been suggested by the sources detected in either Lyα or [C II] 158 μm near to, respectively, highly ionized and neutral absorbers5,6. This is also probably linked to the global changes in the incidence of absorption systems of different types and the process of cosmic reionization7-12. Here we report the detection of two [C II]-emitting galaxies at redshift z ≈ 5.7 that are associated with a complex, high-ionization C IV absorption system. These objects are part of an overdensity of galaxies and have compact sizes (<2.4 kpc) and narrow linewidths (full width at half maximum (FWHM) ≈ 62-64 km s-1). Hydrodynamic simulations predict that similar narrow [C II] emission may arise from the heating of small (≲3 kpc) clumps of cold neutral medium or a compact photodissociation region13,14. The lack of counterparts in the rest-frame ultraviolet (UV) indicates severe obscuration of the sources that are exciting the [C II] emission. These results may suggest a connection between the properties of the [C II] emission, the rare overdensity of galaxies and the unusual high ionization state of the gas in this region.

A [C ii] 158 μm emitter associated with an O i absorber at the end of the reionization epoch

The physical and chemical properties of the circumgalactic medium (CGM) at $z\gtrsim6$ have been studied successfully through the absorption in the spectra of background Quasi-Stellar Objects (QSOs). One of the most crucial questions is to investigate the nature and location of the source galaxies that give rise to these early metal absorbers. Theoretical models suggest that momentum-driven outflows from typical star-forming galaxies can eject metals into the CGM and the intergalactic medium (IGM) at z=5-6. Deep, dedicated surveys have searched for Ly$\alpha$ emission associated with strong CIV absorbers at $z\approx 6$, but only a few Ly$\alpha$ emitter candidates have been detected. Interpreting these detections is moreover ambiguous because Ly$\alpha$ is a resonant line, raising the need for complementary techniques for detecting absorbers' host galaxies. Here, using Atacama Large Millimeter Array (ALMA), we report a [C II] 158$\mu$m emitter associated with a strong low-ionization absorber, OI, at $z=5.978$. The projected impact parameter between OI and [C II] emitter is 20.0 kpc. The measured [C II] luminosity is $7.0\times 10^7$ solar luminosities. Further analysis indicates that strong OI absorbers may reside in the circumgalactic medium of massive halos one to two orders of magnitude more massive than expected values.

A complete census of circumgalactic Mg ii at redshift z ≲ 0.5

ABSTRACT This paper presents a survey of Mg ii absorbing gas in the vicinity of 380 random galaxies, using 156 background quasi-stellar objects (QSOs) as absorption-line probes. The sample comprises 211 isolated (73 quiescent and 138 star-forming galaxies) and 43 non-isolated galaxies with sensitive constraints for both Mg ii absorption and H α emission. The projected distances span a range from d = 9 to 497 kpc, redshifts of the galaxies range from z = 0.10 to 0.48, and rest-frame absolute B-band magnitudes range from MB = −16.7 to −22.8. Our analysis shows that the rest-frame equivalent width of Mg ii, Wr(2796), depends on halo radius (Rh), B-band luminosity(LB), and stellar mass (Mstar) of the host galaxies, and declines steeply with increasing d for isolated, star-forming galaxies. At the same time, Wr(2796) exhibits no clear trend for either isolated, quiescent galaxies or non-isolated galaxies. In addition, the covering fraction of Mg ii absorbing gas 〈κ〉 is high with 〈κ〉 ≳ 60 per cent at &amp;lt;40 kpc for isolated galaxies and declines rapidly to 〈κ〉 ≈ 0 at d ≳ 100 kpc. Within the gaseous radius, the incidence of Mg ii gas depends sensitively on both Mstar and the specific star formation rate inferred from H α. Different from what is known for massive quiescent haloes, the observed velocity dispersion of Mg ii absorbing gas around star-forming galaxies is consistent with expectations from virial motion, which constrains individual clump mass to $m_{\rm cl} \gtrsim 10^5 \, \rm M_\odot$ and cool gas accretion rate of $\sim 0.7\!-\!2 \, \mathrm{ M}_\odot \, \rm yr^{-1}$. Finally, we find no strong azimuthal dependence of Mg ii absorption for either star-forming or quiescent galaxies. Our results demonstrate that multiple parameters affect the properties of gaseous haloes around galaxies and highlight the need of a homogeneous, absorption-blind sample for establishing a holistic description of chemically enriched gas in the circumgalactic space.

The influence of metagalactic ultraviolet background fluctuations on the high-redshift Lyα forest

ABSTRACT Under the assumption that galaxies and quasi-stellar objects (QSOs) dominate the metagalactic ultraviolet (UV) background, it is shown that at high redshifts, fluctuations in the UV background are dominated by QSO shot noise and have an autocorrelation length of a few to several comoving Mpcs, depending on the bright end of the QSO luminosity function. The correlations create long-range spatial coherence in the neutral hydrogen fraction. Using a semi-analytic model, it is demonstrated that the coherence may account for the broad distribution in effective optical depths measured in the Lyα forest spectra of background QSOs, for line-of-sight segments of comoving length $50\, h^{-1}$ Mpc at redshifts 5 &amp;lt; z &amp;lt; 6. Capturing the fluctuations in a numerical simulation requires a comoving box size of ∼1 Gpc, although a box half this size may be adequate if sufficient random realizations of the QSO population are performed.

Discovery of a Damped Lyα System in a Low-z Galaxy Group: Possible Evidence for Gas Inflow and Nuclear Star Formation

Abstract We present a low-redshift (z = 0.029) damped Lyα (DLA) system in the spectrum of a background quasi-stellar object (QSO). The DLA is associated with an interacting galaxy pair within a galaxy group. We detected weak Lyα emission centered at the absorption trough of the DLA. The emission was likely tracing the neutral H i reservoir around the galaxies in the interacting pair, which scattered the Lyα generated by star formation within those galaxies. We also found that the interacting pair is enveloped by a large H i cloud with M ( H I ) = 2 × 10 10 M ⊙ . We discovered blueshifted 21 cm H i emission, corresponding to M(H i) = 2 × 10 9 M ⊙ , associated with J151225.15+012950.4—one of the galaxies in the interacting pair. The blueshifted H i was tracing gas flowing into the galaxy from behind and toward us. Gas at similar blueshifted velocities was seen in the QSO sight line, thus suggesting the presence of a filamentary structure of the order of 100 kpc feeding the galaxy. We estimated a mass inflow rate of 2 M ⊙ yr − 1 into the galaxy, which matches the star formation rate estimated from Hα emission. It is likely that the inflow of enormous amounts of gas has triggered star formation in this galaxy. The sudden acquisition of cold gas may lead to a starburst in this galaxy like those commonly seen in simulations.

ALMA Observations of Molecular Absorption in the Gravitational Lens PMN 0134−0931 at z = 0.7645

Abstract We report the detection of molecular absorption lines at z = 0.7645 toward the radio-loud quasi-stellar object (QSO) PMN 0134−0931. The CO J = 2–1 and HCO+ J = 2–1 lines are seen in absorption along two different lines of sight to lensed images of the background QSO. The lines of sight are separated by ∼0.″7, corresponding to 5 kpc in the lens plane. PMN 0134−0931 represents one out of only five known molecular absorption line systems at cosmologically significant distances. Moreover, it is also one of three such systems where the absorption occurs in a galaxy acting as a gravitational lens. The absorption lines through the two lines of sight are shifted by 215 ± 8 km s−1, possibly representing rotational motion in one of the lensing galaxies. The absorption profiles are wide, ∼200 km s−1, suggesting that the absorption occurs in a highly inclined disk galaxy with a flat rotation curve and a cloud–cloud velocity dispersion ∼30 km s−1. Gravitational lens models require two equal mass galaxies to account for the observed configuration of lensed images. The presence of two galaxies in close proximity means that they might be interacting and potentially merging and the kinematics of the molecular gas may not reflect ordered rotational motion. Compared with other high-redshift molecular absorption systems, the column densities of both CO and HCO+ are normal for diffuse molecular gas toward one of the lensed images, but significantly higher toward the other. Also, the abundance ratio is 2 − 3 times higher than in typical diffuse molecular gas. It is plausible that the second line of sight probes denser molecular gas than what is normally the case for absorption.

Probing Large Galaxy Halos at Z ∼ 0 with Automated Lyα-absorption Matching

Abstract We present initial results from an ongoing large-scale study of the circumgalactic medium in the nearby Universe ( 10,000 ), using archival Cosmic Origins Spectrograph spectra of background quasi-stellar objects. This initial sample contains 33 sightlines chosen for their proximity to large galaxies ( kpc) and high signal-to-noise ratios (S/N 10), yielding 48 Lyα absorption lines that we have paired with 33 unique galaxies, with 29 cases where multiple absorbers within a single sightline are paired with the same galaxy. We introduce a likelihood parameter to facilitate the matching of galaxies to absorption lines in a reproducible manner. We find the usual anti-correlation between Lyα equivalent width (EW) and impact parameter (ρ) when we normalize by galaxy virial radius (R vir). Galaxies associated with a Lyα absorber are found to be more highly inclined than galaxies in the survey volume at a confidence level (equivalent to for a normal distribution). In contrast with suggestions in other recent papers of a correlation with azimuth angle for Mg ii absorption, we find no such correlation for Lyα.

CONNECTION BETWEEN THE CIRCUMGALACTIC MEDIUM AND THE INTERSTELLAR MEDIUM OF GALAXIES: RESULTS FROM THE COS-GASS SURVEY

We present a study exploring the nature and properties of the Circum-Galactic Medium (CGM) and its connection to the atomic gas content in the interstellar medium (ISM) of galaxies as traced by the HI 21cm line. Our sample includes 45 low-z (0.026-0.049) galaxies from the GALEX Arecibo SDSS Survey. Their CGM was probed via absorption in the spectra of background Quasi-Stellar Objects at impact parameters of 63 to 231kpc. The spectra were obtained with the Cosmic Origins Spectrograph aboard the Hubble Space Telescope. We detected neutral hydrogen (Ly$\alpha$ absorption-lines) in the CGM of 92% of the galaxies. We find the radial profile of the CGM as traced by the Ly$\alpha$ equivalent width can be fit as an exponential with a scale length of roughly the virial radius of the dark matter halo. We found no correlation between the orientation of sightline relative to the galaxy major axis and the Ly$\alpha$ equivalent width. The velocity spread of the circumgalactic gas is consistent with that seen in the atomic gas in the interstellar medium. We find a strong correlation (99.8% confidence) between the gas fraction (M(HI)/M*) and the impact-parameter-corrected Ly$\alpha$ equivalent width. This is stronger than the analogous correlation between corrected Ly$\alpha$ equivalent width and SFR/M* (97.5% confidence). These results imply a physical connection between the HI disk and the CGM, which is on scales an order-of-magnitude larger. This is consistent with the picture in which the HI disk is nourished by accretion of gas from the CGM.

Metal-line absorption around z ≈ 2.4 star-forming galaxies in the Keck Baryonic Structure Survey★

We study metal absorption around 854 z ≈ 2.4 star-forming galaxies taken from the Keck Baryonic Structure Survey. The galaxies examined in this work lie in the fields of 15 hyperluminous background quasi-stellar objects, with galaxy impact parameters ranging from 35 proper kpc (pkpc) to 2 proper Mpc (pMpc). Using the pixel optical depth technique, we present the first galaxy-centred 2D maps of the median absorption by O vi, N v, C iv, C iii, and Si iv, as well as updated results for H i. At small galactocentric radii we detect a strong enhancement of the absorption relative to randomly located regions that extend out to at least 180 pkpc in the transverse direction, and ±240 km s−1 along the line of sight (LOS, ∼1 pMpc in the case of pure Hubble flow) for all ions except N v. For C iv (and H i) we detect a significant enhancement of the absorption signal out to 2 pMpc in the transverse direction, corresponding to the maximum impact parameter in our sample. After normalizing the median absorption profiles to account for variations in line strengths and detection limits, in the transverse direction we find no evidence for a sharp drop-off in metals distinct from that of H i. We argue instead that non-detection of some metal-line species in the extended circumgalactic medium is consistent with differences in the detection sensitivity. Along the LOS, the normalized profiles reveal that the enhancement in the absorption is more extended for O vi, C iv, and Si iv than for H i. We also present measurements of the scatter in the pixel optical depths, covering fractions, and equivalent widths as a function of projected galaxy distance. Limiting the sample to the 340 galaxies with redshifts measured from nebular emission lines does not decrease the extent of the enhancement along the LOS compared to that in the transverse direction. This rules out redshift errors as the source of the observed redshift-space anisotropy and thus implies that we have detected the signature of gas peculiar velocities from infall, outflows, or virial motions for H i, O vi, C iv, C iii, and Si iv.

The VLT LBG Redshift Survey – III. The clustering and dynamics of Lyman-break galaxies at z ∼ 3★

We present a catalogue of 2135 galaxy redshifts from the VLT LBG Redshift Survey (VLRS), a spectroscopic survey of z ~ 3 galaxies in wide fields centred on background quasi-stellar objects. We have used deep optical imaging to select galaxies via the Lyman-break technique. Spectroscopy of the Lyman-break galaxies (LBGs) was then made using the Visible Multi-Object Spectrograph (VIMOS), giving a mean redshift of z=2.79. We analyse the clustering properties of the VLRS sample and also of the VLRS sample combined with the smaller area Keck-based survey of Steidel et al. From the semiprojected correlation function, wp({\sigma}) we find that the results are well fit with a single power-law model, with clustering scale lengths of r0=3.46+-0.41 and 3.83+-0.24 Mpc/h, respectively. We note that the corresponding combined {\xi}(r) slope is flatter than for local galaxies at {\gamma} = 1.5-1.6 rather than {\gamma}=1.8. This flat slope is confirmed by the z-space correlation function, {\xi}(s), and in the range 10<s<100 Mpc/h the VLRS shows ~2.5{\sigma} excess over the {\Lambda} cold dark matter. This excess may be consistent with recent evidence for non-Gaussianity in clustering results at z~1. We then analyse the LBG z-space distortions using the 2D correlation function, {\xi}({\sigma}, {\pi}), finding for the combined sample a large-scale infall parameter of $\beta$ = 0.38+-0.19 and a velocity dispersion of 420km/s. Based on our measured {\beta}, we are able to determine the gravitational growth rate, finding a value of f(z = 3)=0.99+-0.50 (or f{\sigma}8 = 0.26+-0.13), which is the highest redshift measurement of the growth rate via galaxy clustering and is consistent with {\Lambda}CDM. Finally, we constrain the mean halo mass for the LBG population, finding that the VLRS and combined sample suggest mean halo masses of log(MDM/Msun) = 11.57+-0.15 and 11.73+-0.07, respectively.

The transverse proximity effect in thez∼ 2 Lyman α forest suggests quasi-stellar object episodic lifetimes of ∼1 Myr

We look for signs of the H i transverse proximity effect in the spectra of 130 quasi-stellar object (QSO) pairs, most with transverse separations in the plane of the sky of 0.1–3 Mpc at z∼ 2.2. We expected to see a decrease in Lyα forest H i absorption in the spectrum of background QSOs near the position of foreground QSOs. Instead, we see no change in the absorption in front of the foreground QSOs, and we see evidence for a 50 per cent increase in the absorption out to 6 Mpc behind the foreground QSOs. Further, we see no change in the H i absorption along the line-of-sight to the foreground QSOs, the normal line-of-sight proximity effect. We may account for the lack of change in the H i absorption if the effect of extra ultraviolet photons is cancelled by higher gas density around QSOs. If so, the increase in absorption behind the QSOs then suggests that the higher gas density there is not cancelled by the UV radiation from the QSOs. We can explain our observations if QSOs have had their current UV luminosities for less than approximately a million years, a time-scale that has been suggested for accretion disc instabilities and gas depletion.

Measurements of Ca ii absorption, metals and dust in a sample ofz≃ 1 DLAs and subDLAs★

We present observations of Ca ii, Zn ii and Cr ii absorption lines in 16 damped Lyman alpha (DLA) systems and six subDLAs at redshifts 0.6 < zabs < 1.3, obtained for the dual purposes of (i) clarifying the relationship between DLAs and absorption systems selected from their strong Ca ii lines, and (ii) increasing the still limited sample of Zn and Cr abundance determinations in this redshift range. We find only partial overlap between current samples of intermediate redshift DLAs (which are drawn from magnitude-limited surveys) and strong Ca ii absorbers: approximately 25 per cent of known DLAs at these redshifts have an associated Ca iiλ3935 line with a rest-frame equivalent width greater than 0.35 A, the threshold of the Sloan Digital Sky Survey sample assembled by Wild and her collaborators. The lack of the strongest Ca ii systems (with equivalent widths greater than 0.5 A) is consistent with these authors' conclusion that such absorbers are often missed in current DLA surveys because they redden and dim the light of the background quasi-stellar objects. We rule out the suggestion that strong Ca ii absorption is associated exclusively with the highest column density DLAs. Furthermore, we find no correlation between the strength of the Ca ii lines and either the metallicity or degree of depletion of refractory elements, although the strongest Ca ii absorber in our sample is also the most metal-rich DLA yet discovered, with [Zn/H]≃ solar. We conclude that a complex mix of parameters must determine the strengths of the Ca ii lines, including the density of particles and ultraviolet photons in the interstellar media of the galaxies hosting the DLAs. We find tentative evidence (given the small size of our sample) that strong Ca ii systems may preferentially sample regions of high gas density, perhaps akin to the DLAs exhibiting molecular hydrogen absorption at redshifts z > 2. If this connection is confirmed, strong Ca ii absorbers would trace possibly metal rich, H2 bearing columns of cool, dense gas at distances up to tens of kpc from normal galaxies.

The Proper Motion of the Large Magellanic Cloud: A Reanalysis

We have determined the proper motion (PM) of the Large Magellanic Cloud (LMC) relative to four background quasi-stellar objects, combining data from two previous studies made by our group and new observations carried out in three epochs not included in the original investigations. The new observations provided a significant increase in the time base and the number of frames, relative to what was available in our previous studies. We have derived a total LMC PM of μ = (+2.0 ± 0.1) mas yr-1, with a position angle of θ = 624 ± 31. Our new values agree well with most results obtained by other authors, and we believe we have clarified the large discrepancy between previous results from our group. Using published values of the radial velocity for the center of the LMC, in combination with the transverse velocity vector derived from our measured PM, we have calculated the absolute space velocity of the LMC. This value, along with some assumptions regarding the mass distribution of the Galaxy, has in turn been used to calculate the mass of the Milky Way. Our measured PM also indicates that the LMC is not a member of a proposed stream of galaxies with similar orbits around our Galaxy.

Proper Motion of the Large Magellanic Cloud Using QSO[CSC]s[/CSC] as an Inertial Reference System: The Q0459−6427 Field

The proper motion of the Large Magellanic Cloud relative to one background quasi-stellar object is determined using 44 CCD frames obtained from 1989.0 to 2000.0 at the Cassegrain focus of the Cerro Tololo Inter-American Observatory 1.5 m telescope. This is the continuation of a previous study made for three other LMC quasar fields observed with the same telescope and equipment and processed in a similar way. The results obtained here are compared with those by other authors who used different approaches and reference systems for the proper-motion determinations and with those in the previous study. The results of our proper-motion determination are in agreement with those of the former studies, but they show discrepancies when compared with the results of the latter. The possible causes of this inconsistency are discussed. Also, from the newly determined proper motion, the LMC spatial velocity vector is calculated, which in turn is used to determine the lower limit of the mass of the Galaxy contained within 50 kpc from its center.

A Physical Upper Limit on the H [CSC]i[/CSC] Column Density of Gas Clouds

An intriguing fact about cosmic gas clouds is that they all appear to have neutral (atomic) hydrogen column densities smaller than 1022 cm-2. Observations of damped Lyα (DLA) absorption systems further indicate that the maximum NH i decreases with increasing metallicity. It is generally assumed that this trend is due to a dust-induced selection bias: DLA systems with high N and high metallicity contain so much dust that the background quasi-stellar object becomes too dim to be included in optically selected surveys. Here it is argued that this explanation may not be viable. Instead, it is proposed that conversion to molecular hydrogen determines the maximum H I column density. Molecular hydrogen forms on the surface of dust grains and is destroyed by photodissociation. Therefore, the molecular fraction correlates with both the dust content and, because of self-shielding, the total hydrogen column density, and anticorrelates with the intensity of the incident UV radiation. It is shown that the first relation can account for the observed anticorrelation between the maximum N and metallicity.

Proper Motion of the Large Magellanic Cloud using [CLC]QSOs[/CLC] as an Inertial Reference System

The proper motion of the Large Magellanic Cloud (LMC) relative to three background quasi-stellar objects has been determined using 125 CCD frames taken from 1989.0 to 1997.2 at the Cassegrain focus of the CTIO 1.5 m telescope. The observation and reduction methods are fully described. The results are compared with those obtained by other authors who use different methods and reference systems for the proper motions. This comparison results in a good agreement for μα cos δ and a rather large discrepancy for μδ. Our LMC proper motion seems to indicate that the LMC is not leading the Magellanic Stream. Finally, from the newly determined proper motion, the LMC spatial velocity vector is calculated, which in turn is used to determine the lower limit of the mass of the Galaxy contained within 50 kpc from its center. The possible consequences of our results on the dynamical status of the LMC are discussed.

THE LYMAN ALPHA FOREST IN THE SPECTRA OF QUASISTELLAR OBJECTS

▪ Abstract Observations of redshifted Lyman α (Lyα) forest absorption in the spectra of quasistellar objects (QSOs) provide a highly sensitive probe of the distribution of gaseous matter in the universe. Over the past two decades, optical spectroscopy with large ground-based telescopes, and more recently ultraviolet (UV) spectroscopy from space, have yielded a wealth of information on what appears to be a gaseous, photoionized intergalactic medium (IGM), partly enriched by the products of stellar nucleosynthesis, residing in coherent structures over many hundreds of kiloparsecs. Recent progress with cosmological hydro-simulations based on hierarchical structure formation models has led to important insights into the physical structures giving rise to the forest. If these ideas are correct, a truly inter- and protogalactic medium [at high redshift (z ∼ 3), the main repository of baryons] collapses under the influence of dark matter gravity into flattened or filamentary structures, which are seen in absorption against background QSOs. With decreasing redshift, galaxies forming in the denser regions may contribute an increasing part of the Lyα absorption cross section. Comparisons between large data samples from the new generation of telescopes and artificial Lyα forest spectra from cosmological simulations promise to become a useful cosmological tool.