Star-forming Galactic Contrails at z=3.2 as a Source of Metal Enrichment and Ionizing Radiation

A spectroscopically detected Lyman alpha emitting halo at redshift 3.216 in the GOODS-N field is found to reside at the convergence of several Lyman alpha filaments. HST images show that some of the filaments are inhabited by galaxies. Several of the galaxies in the field have pronounced head-tail structures, which are partly aligned with each other. The blue colors of most tails suggest the presence of young stars, with the emission from at least one of the galaxies apparently dominated by high equivalent width Lyman alpha. Faint, more diffuse, and similarly elongated, apparently stellar features, can be seen over an area with a linear extent of at least 90 kpc. The region within several arcseconds of the brightest galaxy exhibits spatially extended emission by HeII, NV and various lower ionization metal lines. The gas-dynamical features present are strongly reminiscent of ram-pressure stripped galaxies, including evidence for recent star formation in the stripped contrails. Spatial gradients in the appearance of several galaxies may represent a stream of galaxies passing from a colder to a hotter intergalactic medium. The stripping of gas from the in-falling galaxies, in conjunction with the occurrence of star formation and stellar feedback in the galactic contrails suggests a mechanism for the metal enrichment of the high redshift intergalactic medium that does not depend on long-range galactic winds, at the same time opening a path for the escape of ionizing radiation from galaxies.Comment: 12 pages, 9 figures, submitted to MNRA

Substructure around M31 : Evolution and Effects

We investigate the evolution of a population of 100 dark matter satellites orbiting in the gravitational potential of a realistic model of M31. We find that after 10 Gyr, seven subhalos are completely disrupted by the tidal field of the host galaxy. The remaining satellites suffer heavy mass loss and overall, 75% of the mass initially in the subhalo system is tidally stripped. Not surprisingly, satellites with pericentric radius less than 30 kpc suffer the greatest stripping and leave a complex structure of tails and streams of debris around the host galaxy. Assuming that the most bound particles in each subhalo are kinematic tracers of stars, we find that the halo stellar population resulting from the tidal debris follows an r^{-3.5} density profile at large radii. We construct B-band photometric maps of stars coming from disrupted satellites and find conspicuous features similar both in morphology and brightness to the observed Giant Stream around Andromeda. An assumed star formation efficiency of 5-10% in the simulated satellite galaxies results in good agreement with the number of M31 satellites, the V-band surface brightness distribution, and the brightness of the Giant Stream. During the first 5 Gyr, the bombardment of the satellites heats and thickens the disk by a small amount. At about 5 Gyr, satellite interations induce the formation of a strong bar which, in turn, leads to a significant increase in the velocity dispersion of the disk.Comment: 45 pages, 18 figures. To be submitted to the Astrophysical Journal, version 2.0 : scale height value corrected, references added, and some figures have been modifie

A z = 3.045 Lyα emitting halo hosting a QSO and a possible candidate for AGN-triggered star formation

In this third paper in a series on the nature of extended, asymmetric Lyα emitters at z ∼ 3 we report the discovery, in an ultra-deep, blind, spectroscopic long-slit survey, of a Lyα emitting halo around a QSO at redshift 3.045. The QSO is a previously known, obscured active galactic nucleus (AGN). The Lyα emitting halo appears extended along the direction of the slit and exhibits two faint patches separated by 17 proper kpc in projection from the QSO. Comparison of the two-dimensional spectrum with archival Hubble Space Telescope ACS images shows that these patches coincide spatially with emission from a peculiar, dumbbell-shaped, faint galaxy. The assumptions that the Lyα emission patches are originating in the galaxy and that the galaxy is physically related to the QSO are at variance with photometric estimates of the galaxy redshift. We show, however, that a population of very young stars at the redshift of the QSO may fit the existing rest-frame broad-band UV photometry of the galaxy. If this scenario is correct, then the symmetry of the galaxy in continuum and Lyα emission, the extension of the QSO’s Lyα emission in its direction, and the likely presence of a young stellar population in close proximity to a (short-lived) AGN suggest that this may be an example of AGN feedback triggering external star formation in high-redshift galaxies

Extended and filamentary Lyα emission from the formation of a protogalactic halo at z = 2.63

We report the observation of a further asymmetric, extended Lyα emitting halo at z = 2.63, from our ultra-deep, long-slit spectroscopic survey of faint high-redshift emitters, undertaken with Magellan LDSS3 in the GOODS-S field. The Lyα emission, detected over more than 30 kpc, is spatially coincident with a statistically significant concentration of galaxies visible in deep broad-band imaging. While these faint galaxies without spectroscopic redshifts cannot all with certainty be associated with one another or with the Lyα emission, there are a number of compelling reasons why they very probably form a Milky Way halo-mass group at the Lyα redshift. Filamentary structure, possibly consisting of Lyα emission with very high equivalent width, blue stellar continua and evidence for disturbed stellar populations, suggest that the properties of the emitting region reflect ongoing galaxy assembly, with recent galaxy mergers and star formation occurring in the group. The Lyα emission may be powered by cooling radiation or spatially extended star formation in the halo, but is unlikely to be fluorescence driven by either an active galactic nucleus or one of the galaxies. A comparison with the Lyα surface brightness profiles of more typical, bright Lyα emitters or Lyman break galaxies from similarly deep two-dimensional spectra shows them to be conspicuously different from the extended, asymmetric object studied here. This is consistent with the picture that typical Lyα emitters represent Lyα resonantly scattering from single, kinematically quiescent, compact sources of ionizing radiation, whereas extended emission of the kind seen in the current halo reflect a more active, kinematically disturbed stage in the galaxy formation process. Hence, unusual Lyα emission as observed here may provide unique insights into what is probably a key mode of galaxy formation at high redshifts. Our observations provide further, circumstantial evidence that galaxy mergers may promote the production and/or escape of ionizing radiation, and that the haloes of interacting galaxies may be significant sources for ionizing photons during the epoch of reionization

Characterizing Circumgalactic Gas around Massive Ellipticals at z ~ 0.4 I. Initial Results

We present a new Hubble Space Telescope (HST) Cosmic Origins Spectrograph (COS) absorption-line survey to study halo gas around 16 luminous red galaxies (LRGs) at z=0.21-0.55. The LRGs are selected uniformly with stellar mass Mstar>1e11 Msun and no prior knowledge of the presence/absence of any absorption features. Based on observations of the full Lyman series, we obtain accurate measurements of neutral hydrogen column density N(HI) and find that high-N(HI) gas is common in these massive quiescent halos with a median of <log N(HI)> = 16.6 at projected distances d<~160 kpc. We measure a mean covering fraction of optically-thick gas with log N(HI)>~17.2 of LLS=0.44^{+0.12}_{-0.11} at d<~160 kpc and LLS=0.71^{+0.11}_{-0.20} at d<~100 kpc. The line-of-sight velocity separations between the HI absorbing gas and LRGs are characterized by a mean and dispersion of =29 km/s and \sigma_v_{gas-gal}=171 km/s. Combining COS FUV and ground-based echelle spectra provides an expanded spectral coverage for multiple ionic transitions, from low-ionization MgII and SiII, to intermediate ionization SiIII and CIII, and to high-ionization OVI absorption lines. We find that intermediate ions probed by CIII and SiIII are the most prominent UV metal lines in LRG halos with a mean covering fraction of _{0.1}=0.75^{+0.08}_{-0.13} for W(977)>=0.1 Ang at d<160 kpc, comparable to what is seen for CIII in L* and sub-L* star-forming and red galaxies but exceeding MgII or OVI in quiescent halos. The COS-LRG survey shows that massive quiescent halos contain widespread chemically-enriched cool gas and that little distinction between LRG and star-forming halos is found in their HI and CIII content.Comment: 19 pages, 6 figures, accepted by MNRA

Filamentary Infall of Cold Gas and Escape of Lyman Alpha and Hydrogen Ionizing Radiation from an Interacting High-Redshift Galaxy

We present observations of a peculiar Lyman alpha-emitting galaxy at redshift 3.344, discovered in a deep, blind spectroscopic survey for faint Lyman alpha emitters with the Magellan II telescope in the Hubble Ultra Deep Field (HUDF). The galaxy exhibits complex Lyman alpha emission, including an extended, asymmetric component that is partially suppressed by damped Lyman alpha absorption, and two spatially elongated, narrow emission features. Archival HST ACS imaging shows evidence for tidal disruption of the stellar component. This V=27 galaxy appears to give us unprecedented insights into two fundamental stages in the formation of structure at high redshift: the inflow of gas into ordinary galaxies, and the escape of ionizing radiation into the intergalactic medium. Neutral hydrogen, falling in partly in form of a narrow filament, appears to emit fluorescent Lyman alpha photons induced by the stellar ionizing flux escaping from the disturbed galaxy. The in-falling material may represent primary cold accretion or an interaction-triggered inflow. The rate of ionizing photons required by the observed Lyman alpha emission is consistent with the rate of photons produced by the observed stellar population, with roughly 50 percent of ionizing photons escaping from the immediate galaxy and encountering the in-falling gas. The observational properties of the galaxy lend support to a picture where galaxy interactions facilitate the escape of both Lyman alpha and ionizing radiation. We argue that galaxies like the present object may be common at high redshift. This galaxy may therefore be a late example of an interacting population of dwarf galaxies contributing significantly to the reionization of the universe.Comment: submitted to MNRA

Extended and Filamentary Lyman Alpha Emission from the Formation of a Protogalactic Halo at z=2.63

We report the observation of a further asymmetric, extended Lyman alpha emitting halo at z=2.63, from our ultra-deep, long-slit spectroscopic survey of faint high redshift emitters, undertaken with Magellan LDSS3 in the GOODS-S field. The Lya emission, detected over more than 30 kpc, is spatially coincident with a concentration of galaxies visible in deep broad-band imaging. While these faint galaxies without spectroscopic redshifts cannot with certainty be associated with one another or with the Lya emission, there are a number of compelling reasons why they very probably form a Milky Way halo-mass group at the Lya redshift. A filamentary structure, possibly consisting of Lya emission at very high equivalent width, and evidence for disturbed stellar populations, suggest that the properties of the emitting region reflect ongoing galaxy assembly, with recent galaxy mergers and star formation occurring in the group. Hence, the Lya provides unique insights into what is probably a key mode of galaxy formation at high redshifts. The Lya emission may be powered by cooling radiation or spatially extended star formation in the halo, but is unlikely to be fluorescence driven by either an AGN or one of the galaxies. The spatial profile of the emission is conspicuously different from that of typical Lya emitters or Lyman break galaxies, which is consistent with the picture that extended emission of this kind represents a different stage in the galaxy formation process. Faint, extended Lya emitters such as these may be lower-mass analogues of the brighter Lya blobs. Our observations provide further, circumstantial evidence that galaxy mergers may promote the production and / or escape of ionizing radiation, and that the halos of interacting galaxies may be significant sources for ionizing photons during the epoch of reionization.Comment: 18 pages, submitted to MNRA