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Register a new userResolving the Galaxies within a Giant Lya Nebula: Witnessing the Formation of a Galaxy Group?
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Moire K. M. Prescott
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Detailed analysis of the substructure of Lya nebulae can put important constraints on the physical mechanisms at work and the properties of galaxies forming within them. Using high resolution HST imaging of a Lya nebula at z~2.656, we have taken a census of the compact galaxies in the vicinity, used optical/near-infrared colors to select system members, and put constraints on the morphology of the spatially-extended emission. The system is characterized by (a) a population of compact, low luminosity (~0.1 L*) sources --- 17 primarily young, small (Re~1-2 kpc), disky galaxies including an obscured AGN --- that are all substantially offset (>20 kpc) from the line-emitting nebula; (b) the lack of a central galaxy at or near the peak of the Lya emission; and (c) several nearly coincident, spatially extended emission components --- Lya, HeII, and UV continuum --- that are extremely smooth. These morphological findings are difficult to reconcile with theoretical models that invoke outflows, cold flows, or resonant scattering, suggesting that while all of these physical phenomena may be occurring, they are not sufficient to explain the powering and large extent of Lya nebulae. In addition, although the compact galaxies within the system are irrelevant as power sources, the region is significantly overdense relative to the field galaxy population (by at least a factor of 4). These observations provide the first estimate of the luminosity function of galaxies within an individual Lya nebula system, and suggest that large Lya nebulae may be the seeds of galaxy groups or low-mass clusters.
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Recent theoretical work has suggested that Lyman-alpha nebulae could be substantially polarized in the Lyman-alpha emission line, depending on the geometry, kinematics, and powering mechanism at work. Polarization observations can therefore provide a useful constraint on the source of ionization in these systems. In this Letter, we present the first Lyman-alpha polarization measurements for a giant Lyman-alpha nebula at z~2.656. We do not detect any significant linear polarization of the Lyman-alpha emission: P_{Lyman-alpha}=2.6+/-2.8% (corrected for statistical bias) within a single large aperture. The current data also do not show evidence for the radial polarization gradient predicted by some theoretical models. These results rule out singly scattered Lyman-alpha (e.g., from the nearby AGN) and may be inconsistent with some models of backscattering in a spherical outflow. However, the effects of seeing, diminished signal-to-noise ratio, and angle averaging within radial bins make it difficult to put strong constraints on the radial polarization profile. The current constraints may be consistent with higher density outflow models, spherically symmetric infall models, photoionization by star formation within the nebula or the nearby AGN, resonant scattering, or non-spherically symmetric cold accretion (i.e., along filaments). Higher signal-to-noise ratio data probing to higher spatial resolution will allow us to harness the full diagnostic power of polarization observations in distinguishing between theoretical models of giant Lyman-alpha nebulae.
This paper reports the discovery of spatially-extended line-emitting nebula, reaching to ~100 physical kpc (pkpc) from a damped Ly{alpha} absorber (DLA) at z_DLA=0.313 along the sightline toward QSO PKS1127-145 (z_QSO=1.188). This DLA was known to be associated with a galaxy group of dynamical mass M_group ~3e12 M_sun, but its physical origin remained ambiguous. New wide-field integral field observations revealed a giant nebula detected in [OII], H{beta}, [OIII], H{alpha}, and [NII] emission, with the line-emitting gas following closely the motions of group galaxies. One of the denser streams passes directly in front of the QSO with kinematics consistent with the absorption profiles recorded in the QSO echelle spectra. The emission morphology, kinematics, and line ratios of the nebula suggest that shocks and turbulent mixing layers, produced as a result of stripped gaseous streams moving at supersonic speed across the ambient hot medium, contribute significantly to the ionization of the gas. While the DLA may not be associated with any specific detected member of the group, both the kinematic and dust properties are consistent with the DLA originating in streams of gas stripped from sub-L* group members at <~25 pkpc from the QSO sightline. This study demonstrates that gas stripping in low-mass galaxy groups is effective in releasing metal-enriched gas from star-forming regions, producing absorption systems in QSO spectra, and that combining absorption and emission-line observations provides an exciting new opportunity for studying gas and galaxy co-evolution.
We study the properties of a sample of 3967 LINER galaxies selected from SDSS-DR7, respect to their proximity to galaxy groups. The host galaxies of LINER have been analysed and compared with a well defined control sample of 3841 non-LINER galaxies matched in redshift, luminosity, colour, morphology, age and stellar mass content. We find no difference between LINER and control galaxies in terms of colour and age of stellar population as function of the virial mass and distance to the geometric centre of the group. However, we find that LINER are more likely to populate low density environments in spite of their morphology, which is typical of high density regions such as rich galaxy clusters. For rich (poor) galaxy groups, the occurrence of LINER is $sim$2 times lower (higher) than the occurrence of matched, non-LINER galaxies. Moreover, LINER hosts do not seem to follow the expected morphology-density relation in groups of high virial mass. The high frequency of LINERS in low density regions could be due to the combination of a sufficiently ample gas reservoir to power the low ionization emission and/or enhanced galaxy interaction rates benefiting the gas flow toward their central regions.
Using deep Chandra and optical spectroscopic observations, we investigate an intriguing, young massive group, RXJ1648.7+6109, at z=0.376, and we combine these observations with previous measurements to fit the scaling relations of intermediate-redshift groups and poor clusters. RXJ1648 appears to be in an early stage of formation; while it follows X-ray scaling relations, its X-ray emission is highly elongated and it lacks a central, dominant BCG. Instead, RXJ1648 contains a central string of seven bright galaxies, which have a smaller velocity dispersion, are on average brighter, and have less star formation (lower EW([OII]) and EW(H_delta)) than other group galaxies. The 4-5 brightest galaxies in this string should sink to the center and merge through dynamical friction by z=0, forming a BCG consistent with a system of RXJ1648s mass even if 5-50% of the light is lost to an intracluster light component (ICL). The L_X-T_X relation for intermediate-redshift groups/poor clusters is very similar to the low-redshift cluster relation and consistent with the low-redshift group relation. In contrast, the L_X-sigma_v and sigma_v-T_X relations reveal that intermediate-redshift groups/poor clusters have significantly lower velocity dispersions for their X-ray properties compared to low-redshift systems, however the intermediate-redshift relations are currently limited to a small range in luminosity.
The confinement of most satellite galaxies in the Local Group to thin planes presents a challenge to the theory of hierarchical galaxy clustering. The PAndAS collaboration has identified a particularly thin configuration with kinematic coherence among companions of M31 and there have been long standing claims that the dwarf companions to the Milky Way lie in a plane roughly orthogonal to the disk of our galaxy. This discussion investigates the possible origins of four Local Group planes: the plane similar, but not identical to that identified by PAndAS, an adjacent slightly tilted plane, and two planes near the Milky Way: one with nearer galaxies and the other with more distant ones. Plausible orbits are found by using a combination of Numerical Action methods and a backward in time integration procedure. For M31, M33, IC10, and LeoI, solutions are found that are consistent with measurements of their proper motions. For galaxies in planes, there must be commonalities in their proper motions, and this constraint greatly limits the number of physically plausible solutions. Key to the formation of the planar structures has been the evacuation of the Local Void and consequent build-up of the Local Sheet, a wall of this void. Most of the M31 companion galaxies were born in early-forming filamentary or sheet-like substrata that chased M31 out of the void. M31 is a moving target because of its attraction toward the Milky Way, and the result has been alignments stretched toward our galaxy. In the case of the configuration around the Milky Way, it appears that our galaxy was in a three-way competition for companions with M31 and Centaurus A. Only those within a modest band fell our way. The Milky Ways attraction toward the Virgo Cluster resulted in alignments along the Milky Way-Virgo Cluster line.
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