18803126
Star Formation in Satellite Galaxies
We present narrowband observations of the Hα emission in a sample of 31 satellites orbiting isolated giant spiral galaxies. The sample studied spans the range -19 mag < MB < -15 mag. Hα emission was detected in all the spiral and irregular objects with fluxes in the range × 10-14 ergs cm-2 s-1. The average and maximum values for the current star formation rates are 0.68 and 3.66 M⊙ yr-1, respectively. Maps of the spatial distribution of ionized gas are presented. The star-forming regions show a rich structure in which frequently discrete complexes are imposed over more diffuse structures. In general, the current star formation rates are smaller than the mean values in the past, obtained from the stellar content; this probably indicates a declining rate with time in the generation of new stars. However, the reserve of gas is enough to continue fueling the current levels of star formation activity for at least another Hubble time. Four of the objects (NGC 2718b, NGC 4541e, and NGC 5965a1 and NGC 5965a2) with higher current star formation rates show clear signs of interaction with close companions of comparable brightness at projected distances of 25, 20, and 2 kpc, respectively. The only two galaxies in our sample that do not show star formation activity are members of these interacting systems, and it is unclear if this is a consequence of intrinsic properties (both are early Hubble types) or if it is related to possible disruption of the external parts due to the interaction. In the case of the pair NGC 2718a-2718b there are indications of gas transport between the galaxies.
119181800
RADIAL DENSITY STATISTICS OF THE GALAXY DISTRIBUTION AND THE LUMINOSITY FUNCTION
This paper discusses a connection between the relativistic number counts of cosmological sources and the observed galaxy luminosity function (LF). Observational differential number densities are defined and obtained from published LF data using such connection. We observe a distortion in the observational quantities that increases with higher redshift values as compared to the theoretical predictions. The use of different cosmological distance measures plays a role in such a distortion
15468380
Spatial and observational homogeneities of the galaxy distribution in standard cosmologies
Context. An important aim of standard relativistic cosmology is the empirical verification of its geometrical concept of homogeneity by considering various definitions of distance and astronomical observations occurring along the past light cone. Aims. We analyze the physical consequences of distinguishing between spatial homogeneity (SH), defined by the Cosmological Principle, and observational homogeneity (OH). We argue that OH is falsifiable by means of astronomical observations, whereas SH can be verified only indirectly. Methods. We simulate observational counts of cosmological sources, such as galaxies, by means of a generalized number-distance expression that can be specialized to produce either the counts of the Einstein-de Sitter (EdS) cosmology, which has SH by construction, or other types of counts, which do, or do not, have OH by construction. Expressions for observational volumes are derived using the various cosmological-distance definitions in the EdS cosmological model. The observational volumes and simulated counts are then used to derive differential densities. We present the behavior of these densities for increasing redshift values. Results. Simulated counts that have OH by construction do not always exhibit SH features. The reverse situation is also true. In addition, simulated counts with no OH features at low redshift begin to show OH characteristics at high redshift. The comoving distance appears to be the only distance definition for which both SH and OH are applicable simultaneously, even though with limitations. Conclusions. We demonstrate that observations indicative of a possible absence of OH do not necessarily falsify the standard Friedmannian cosmology, which implies that this cosmology does not always produce observable homogeneous densities. We conclude that using different cosmological distances in the characterization of the galaxy distribution can produce significant ambiguities in reaching conclusions about the large-scale galaxy distribution in the Universe.
18839519
The Apparent Fractal Conjecture: Scaling Features in Standard Cosmologies
This paper presents an analysis of the smoothness problem in cosmology by focussing on the ambiguities originated in the simplifying hypotheses aimed at observationally verifying if the large-scale distribution of galaxies is homogeneous, and conjecturing that this distribution should follow a fractal pattern, in the sense of having a power-law type average density profile, in perturbed standard cosmologies. This is due to a geometrical effect, appearing when certain types of average densities are calculated along the past light cone. The paper starts by reviewing the argument concerning the possibility that the galaxy distribution follows such a scale invariant pattern, and the premises behind the assumption that the spatial homogeneity of standard cosmology can be observable. Next, it is argued that in order to discuss observable homogeneity one needs to make a clear distinction between local and average relativistic densities, and showing how the different distance definitions strongly affect them, leading the various average densities to display asymptotically opposite behaviours. Then the paper revisits Ribeiro's (1995) results, showing that in a fully relativistic treatment some observational average densities of the flat Friedmann model are not well defined at z ∼ 0.1, implying that at this range average densities behave in a fundamentally different manner as compared to the linearity of the Hubble law, well valid for z < 1. This conclusion brings into question the widespread assumption that relativistic corrections can always be neglected at low z. It is also shown how some key features of fractal cosmologies can be found in the Friedmann models. In view of those findings, it is suggested that the so-called contradiction between the cosmological principle, and the galaxy distribution forming an unlimited fractal structure, may not exist.
118702693
Cosmological Models and the Brightness Profile of Distant Galaxies
The aim of this project is to determine the consistency of an assumed cosmological model by means of a detailed analysis of the brightness profiles of distant galaxies.Starting from the theory developed by Ellis and Perry (1979) connecting the angular diameter distance obtained from a relativistic cosmological model and the detailed photometry of galaxies, we assume the presently most accepted cosmological model with non-zero cosmological constant and attempt to predict the brightness profiles of galaxies of a given redshift. Then this theoretical profile can be compared to observational data already available for distant, that is, high redshift, galaxies. By comparing these two curves we may reach conclusions about the observational feasibility of the underlying cosmological model.
119270953
Cosmological model dependence of the galaxy luminosity function: far-infrared results in the Lemaître-Tolman-Bondi model
This is the first paper of a series aiming at investigating galaxy formation and evolution in the giant-void class of the Lemaitre-Tolman-Bondi (LTB) models that best fits current cosmological observations. Here we investigate the Luminosity Function (LF) methodology, and how its estimates would be affected by a change on the cosmological model assumed in its computation. Are the current observational constraints on the allowed Cosmology enough to yield robust LF results? We use the far-infrared source catalogues built on the observations performed with the Herschel/PACS instrument, and selected as part of the PACS evolutionary probe (PEP) survey. Schechter profiles are obtained in redshift bins up to z approximately 4, assuming comoving volumes in both the standard model, that is, Friedmann-Lemaitre-Robertson-Walker metric with a perfect fluid energy-momentum tensor, and non-homogeneous LTB dust models, parametrized to fit the current combination of results stemming from the observations of supernovae Ia, the cosmic microwave background, and baryonic acoustic oscillations. We find that the luminosity functions computed assuming both the standard model and LTB void models show in general good agreement. However, the faint-end slope in the void models shows a significant departure from the standard model up to redshift 0.4. We demonstrate that this result is not artificially caused by the used LF estimator which turns out to be robust under the differences in matter-energy density profiles of the models. The differences found in the LF slopes at the faint end are due to variation in the luminosities of the sources, which depend on the geometrical part of the model. It follows that either the standard model is over-estimating the number density of faint sources or the void models are under-estimating it.
9827982
The infrared surface brightness fluctuation hubble constant
We measured infrared surface brightness fluctuation (SBF) distances to an isotropically distributed sample of 16 distant galaxies with redshifts reaching 10,000 km s-1 using the near-IR camera and multiobject spectrometer (NICMOS) on the Hubble Space Telescope (HST). The excellent spatial resolution, very low background, and brightness of the IR fluctuations yielded the most distant SBF measurements to date. Twelve nearby galaxies were also observed and used to calibrate the F160W (1.6 μm) SBF distance scale. Of these, three have Cepheid variable star distances measured with HST and eleven have optical I-band SBF distance measurements. A distance modulus of 18.5 mag to the Large Magellanic Cloud was adopted for this calibration. We present the F160W SBF Hubble diagram and find a Hubble constant H0 = 76 ± 1.3 (1 σ statistical) ±6 (systematic) km s-1 Mpc-1. This result is insensitive to the velocity model used to correct for local bulk motions. Restricting the fit to the six most distant galaxies yields the smallest value of H0 = 72 ± 2.3 km s-1 Mpc-1 that is consistent with the data. This 6% decrease in the Hubble constant is consistent with the hypothesis that the Local Group inhabits an underdense region of the universe, but is also consistent with the best-fit value of H0 = 76 km s-1 Mpc-1 at the 1.5 σ level.