Absence Of Galaxies Research Articles

Discovery of intergalactic bridges connecting two faint z ∼ 3 quasars

We used the Multi-Unit Spectroscopic Explore (MUSE) on the Very Large Telescope (VLT) to conduct a survey of z ∼ 3 physical quasar pairs at close separation (<30″) with a fast observation strategy (45 min on source). Our aim is twofold: (i) to explore the Lyα glow around the faint-end of the quasar population; and (ii) to take advantage of the combined illumination of a quasar pair to unveil large-scale intergalactic structures (if any) extending between the two quasars. In this work we report the results for the quasar pair SDSS J113502.03−022110.9 – SDSS J113502.50−022120.1 (z = 3.020, 3.008; i = 21.84, 22.15), separated by 11.6″ (or 89 projected kpc). MUSE reveals filamentary Lyα structures extending between the two quasars with an average surface brightness of SBLyα = 1.8 × 10−18 erg s−1 cm−2 arcsec−2. Photoionization models of the constraints in the Lyα, He IIλ1640, and C IVλ1548 line emissions show that the emitting structures are intergalactic bridges with an extent between ∼89 kpc, the quasars’ projected distance, and up to ∼600 kpc. Our models rule out the possibility that the structure extends for ∼2.9 Mpc, that is, the separation inferred from the uncertain systemic redshift difference of the quasars if the difference was only due to the Hubble flow. At the current spatial resolution and surface brightness limit, the average projected width of an individual bridge is ∼35 kpc. We also detect one strong absorption in H I, N V, and C IV along the background sight-line at higher z, which we interpret to be due to at least two components of cool (T ∼ 104 K), metal enriched (Z > 0.3 Z⊙), and relatively ionized circumgalactic or intergalactic gas surrounding the quasar pair. Two additional H I absorbers are detected along both quasar sight-lines at ∼−900 and −2800 km s−1 from the system; the latter has associated C IV absorption only along the foreground quasar sight-line. The absence of galaxies in the MUSE field of view at the redshifts of these two absorbers suggests that they trace large-scale structures or expanding shells in front of the quasar pair. Combining longer exposures and higher spectral resolution when targeting similar quasar pairs has the potential to firmly constrain the physical properties of gas in large-scale intergalactic structures.

QUENCHING STAR FORMATION AT INTERMEDIATE REDSHIFTS: DOWNSIZING OF THE MASS FLUX DENSITY IN THE GREEN VALLEY

The bimodality in galaxy properties has been observed at low and high redshift, with a clear distinction between star-forming galaxies in the blue cloud and passively evolving objects in the red sequence; the absence of galaxies with intermediate properties indicates that the quenching of star formation and subsequent transition between populations must happen rapidly. In this paper, we present a study of over 100 transiting galaxies in the so-called "green valley" at intermediate redshifts (z ~ 0.8). By using very deep spectroscopy with the DEIMOS instrument at the Keck telescope we are able to infer the star formation histories of these objects and measure the stellar mass flux density transiting from the blue cloud to the red sequence when the universe was half its current age. Our results indicate that the process happened more rapidly and for more massive galaxies in the past, suggesting a top-down scenario in which the massive end of the red sequence is forming first. This represent another aspect of downsizing, with the mass flux density moving towards smaller galaxies in recent times.

Quenching star formation at intermediate redshifts: downsizing of the mass flux density in the green valley

AbstractThe bimodality in galaxy properties has been observed at low and high redshift, with a clear distinction between star-forming galaxies in the blue cloud and passively evolving objects in the red sequence; the absence of galaxies with intermediate properties indicates that the quenching of star formation and subsequent transition between populations must happen rapidly. In this work, we present a study of over 100 transiting galaxies in the so-called “green valley” at intermediate redshifts (z ~ 0.8). By using very deep spectroscopy with the DEIMOS instrument at the Keck telescope, we are able to infer the star formation histories of these objects and measure the stellar mass flux density transiting from the blue cloud to the red sequence when the Universe was half its current age. Our results indicate that the process happened more rapidly, affecting more massive galaxies in the past, suggesting a top-down scenario whereby the massive end of the red sequence assembles first. This represents another aspect of downsizing, with the mass flux density moving towards smaller galaxies in recent times.

X-ray and optical observations of M55 and NGC 6366: evidence for primordial binaries

We present Chandra X-ray Observatory ACIS-S3 X-ray imaging observations and VLT/FORS2 and Hubble Space Telescope optical observations of two low-density Galactic globular clusters; NGC 6366 and M 55. We detect 16 X-ray sources with 0.5–6.0 keV luminosities above LX = 4 × 10 30 erg s −1 within the half-mass radius of M 55, of which 8 or 9 are expected to be background sources, and 5 within the half-mass radius of NGC 6366, of which 4 are expected to be background sources. Optical counterparts are identified for several X-ray sources in both clusters and from these we conclude that 3 of the X-ray sources in M 55 and 2 or 3 of the X-ray sources in NGC 6366 are probably related to the cluster. Combining these results with those for other clusters, we find the best fit for a predicted number of X-ray sources in a globular cluster μc = 1.2 Γ+ 1.1 Mh ,w hereΓ is the collision number and Mh is (half of) the cluster mass, both normalized to the values for the globular cluster M4. Some sources tentatively classified as magnetically active binaries are more luminous in X-rays than the upper limit of LX � 0.001 Lbol of such binaries in the solar neighbourhood. Comparison with XMM and ROSAT observations lead us to conclude that the brightest X-ray source in M 55, a dwarf nova, becomes fainter in X-rays during the optical outburst, in accordance with other dwarf novae. The brightest X-ray source in NGC 6366 is a point source surrounded by a slightly offset extended source. The absence of galaxies and Hα emission in our optical observations argues against a cluster of galaxies and against a planetary nebula, and we suggest that the source may be an old nova.

Small‐Scale Clustering in the Isotropic Arrival Distribution of Ultra–High‐Energy Cosmic Rays and Implications for Their Source Candidates

We present numerical simulations on the propagation of ultra-high-frequency protons with energies of 1019.5-1022 eV in extragalactic magnetic fields over 1 Gpc. We use the Optical Redshift Survey (ORS) galaxy sample, which allows us to accurately quantify the contribution of nearby sources to the energy spectrum and the arrival distribution, as a source model. The sample is corrected, taking the selection effect and absence of galaxies in the zone of avoidance (|b| < 20°) into account. We calculate three observable quantities—cosmic-ray spectrum, harmonic amplitude, and two-point correlation function—from our data of numerical simulations. With these quantities, we compare the results of our numerical calculations with the observation. We find that the arrival distribution of ultra-high-energy cosmic rays (UHECRs) becomes most isotropic as sources are restricted to to luminous galaxies (Mlim = -20.5). However, it is not isotropic enough to be consistent with the Akeno Giant Air Shower Array (AGASA) observation, even for Mlim = -20.5. In order to obtain a sufficiently isotropic arrival distribution, we randomly select sources more luminous than -20.5 mag from the ORS sample, which contribute to the observed cosmic-ray flux, and investigate the dependence of the results on their number. We show that three observable quantities, including the Greisen-Zatsepin-Kuz'min (GZK) cutoff of the energy spectrum, can be reproduced in the case that the number fraction ~10-1.7 of the ORS galaxies more luminous than -20.5 mag is selected as UHECR sources. In terms of the source number density, this constraint corresponds to ~10-6 Mpc-3. However, since the mean number of sources within the GZK sphere is only ~0.5 in this case, the eight AGASA events above 1020.0 eV, which do not together constitute clustered events, cannot be reproduced. On the other hand, if the cosmic-ray flux measured by the High Resolution Fly's Eye EHE Cosmic-Ray Detector (HiRes), which is consistent with the GZK cutoff, is correct and observational features about the arrival distribution of UHECRs are same as the AGASA, our source model can explain both the arrival distribution and the flux at the same time. Thus, we conclude that a large fraction of the eight AGASA events above 1020 eV might originate in the top-down scenarios or that the cosmic-ray flux measured by the HiRes experiment might be better. We also discuss the origin of UHECRs below 1020.0 eV through comparisons between the number density of astrophysical source candidates and our result (~10-6 Mpc-3).

ChandraObservation of A2256: A Cluster at the Early Stage of Merging

We present Chandra observations of the rich cluster of galaxies A2256. In addition to the known cool subcluster, a new structure (we call it the in this paper, based on its morphology) was resolved 2' east of the peak of the main cluster. It is shown as a localized feature embedded in the main cluster. Its position is roughly at the center of a low-brightness radio relic. Spectral analysis shows that the shoulder has a high iron abundance, ~1 (after the decomposition). The gas mass within it is around 2 × 1011 M☉. We suggest that this structure is either another merging component or an internal structure of the main cluster. The previously known subcluster has a low temperature (~4.5 keV) and high iron abundance (~0.6) in the central 150 kpc. The main cluster has a temperature of 7-8 keV and an iron abundance of 0.2-0.3 around the center. The Chandra image shows a relatively sharp brightness gradient at the south of the subcluster peak running south-southeast. A temperature jump was found across the edge, with lower temperature inside the subcluster. This phenomenon is qualitatively similar to the cold fronts found in A2142 and A3667. While a simple interpretation is not possible because of the projection, the edge indicates relative motion and contact of the two gas clouds. The Chandra temperature map shows only moderate temperature variations across the cluster, but not as strong as those expected in a major merger. If the shoulder is ignored, the temperature map resembles simulations at the early stage of merging while the subcluster approached the main cluster from somewhere west. The observed temperature map and the edgelike feature near the south of the subcluster imply that the ongoing merger is still at the early stage. The X-ray redshifts of several regions were measured. The results are consistent with a single value and all agree with the optical value. At least three member galaxies, including a radio head-tail galaxy, were found to have corresponding X-ray emission, with X-ray luminosity from several times 1040 to 1041 ergs s-1. We find that the observed characteristics (temperature and iron abundance gradient) of the subcluster are similar to those of some poor clusters. The absence of galaxies around the peak of the subcluster is proposed to be the result of different falling velocities of the galaxies and the core gas.

N-body experiments with gas in a cosmological model

view Abstract Citations (24) References (49) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS N-Body Experiments with Gas in a Cosmological Model Carlberg, R. G. Abstract The formation of galaxies and their clustering is studied with an N-body experiment altered to include gas along with collisionless dark matter. The gas is isothermal, the simplest equation of state that dissipates energy under compression. The gas resists only shallow potential wells, that is, it has a "minimal" physical biasing for galaxy formation. The background cosmology has a local {OMEGA} > 1 and the particles are perturbed with a spectrum of waves having P(k) proportional to k^-1^. About one-fourth of the particles form into galaxies, originating in "1 σ" peaks, where σ is the amplitude of density fluctuations on the mass scale of galaxies, although there is a large dispersion of overdensities. The expected correlation between initial overdensity and the clustering environment of the galaxy clearly emerges, the highest peaks tending to become galaxies in the core of the cluster. Galaxies completely avoid the initially underdense regions. The autocorrelation function of the galaxies is generally a factor of 2-3 times stronger than that of the dark matter. The most massive galaxies form from 2 and more σ peaks, making their autocorrelation 2-4 times higher than that of all galaxies. In this {OMEGA} > 1 model all the galaxies eventually come together to form a stable group of 21 galaxies, corresponding closely to a poor cD cluster. The galaxies orbit in a dark matter background having 1000 times the mass of a typical galaxy, where the residual dynamical friction is defeated by continuing infall. The virial mass provides a reliable indication of the mass within the orbits of the galaxies. Global mass estimators are vexed by the absence of galaxies in low-density regions, underestimating the mass present by a factor of 2-10. The main conclusion is that this relatively conservative gas model provides a physical justification for significant "biasing" in the sites of galaxy formation. Publication: The Astrophysical Journal Pub Date: January 1988 DOI: 10.1086/165926 Bibcode: 1988ApJ...324..664C Keywords: Astronomical Models; Cosmology; Dark Matter; Galactic Clusters; Many Body Problem; Autocorrelation; Galactic Evolution; Galactic Structure; Star Formation; Astrophysics; COSMOLOGY; GALAXIES: CLUSTERING; GALAXIES: FORMATION full text sources ADS |