1,321 research outputs found
Galaxies in group and field environments: a comparison of optical-NIR luminosities and colors
We compare properties of galaxies in loose groups with those in field
environment by analyzing the Nearby Optical Galaxy (NOG) catalog of galaxy
systems. We consider as group galaxies, objects belonging to systems with at
least five members identified by means of the "friends of friends method", and,
as field galaxies, all galaxies with no companions. We analyze both a
magnitude--limited sample of 959 and 2035 galaxies (groups vs. field galaxies,
respectively, B<14 mag, and 2000<cz<6000 km/s) and a volume-limited sample (M_B
<-19.01 mag, 2000<cz<4000 km/s 369 group and 548 field galaxies). For all these
galaxies, blue corrected magnitudes and morphological types are available. The
cross-correlation of NOG with the 2MASS second release allow us to assign K
magnitudes and obtain B-K colors for about half of the galaxies in our samples.
We analyze luminosity and color segregation-effects in relation with the
morphological segregation. For both B and K bands, we find that group galaxies
are, on average, more luminous than field galaxies and this effect is not
entirely a consequence of the morphological segregation. After taking into
account the morphological segregation, the luminosity difference between group
and field galaxies is about 10%. When considering only very early-type galaxies
(T<-2) the difference is larger than 30%. We also find that group galaxies are
redder than field galaxies, Delta(B-K) about 0.4 mag. However, after taking
into account the morphological segregation, we find a smaller B-K difference,
poorly significant (only at the c.l. of about 80%).Comment: 11 pages, 10 eps figures, A&A in pres
Nearby Optical Galaxies: Selection of the Sample and Identification of Groups
In this paper we describe the Nearby Optical Galaxy (NOG) sample, which is a complete, distance-limited (6000 km/s) and magnitude-limited (B14) sample of 7000 optical galaxies. The sample covers 2/3 (8.27 sr) of the sky () and appears to have a good completeness in redshift (98%). We select the sample on the basis of homogenized corrected total blue magnitudes in order to minimize systematic effects in galaxy sampling. We identify the groups in this sample by means of both the hierarchical and the percolation {\it friends of friends} methods. The resulting catalogs of loose groups appear to be similar and are among the largest catalogs of groups presently available. Most of the NOG galaxies (60%) are found to be members of galaxy pairs (580 pairs for a total of 15% of objects) or groups with at least three members (500 groups for a total of 45% of objects). About 40% of galaxies are left ungrouped (field galaxies). We illustrate the main features of the NOG galaxy distribution. Compared to previous optical and IRAS galaxy samples, the NOG provides a denser sampling of the galaxy distribution in the nearby universe. Given its large sky coverage, the identification of groups, and its high-density sampling, the NOG is suited for the analysis of the galaxy density field of the nearby universe, especially on small scales
Geometrical tests of cosmological models. II. Calibration of rotational widths and disc scaling relations
This series of papers is dedicated to a new technique to select galaxies that
can act as standard rods and standard candles in order to perform geometrical
tests on large samples of high redshift galaxies to constrain different
cosmological parameters. The goals of this paper are (1) to compare different
rotation indicators in order to understand the relation between rotation
velocities extracted from observations of the Halpha line and the [OII]3727
line, and (2) determine the scaling relations between physical size, surface
brightness and magnitude of galaxies and their rotation velocity using the
SFI++, a large catalog of nearby galaxies observed at I-band. A good
correlation is observed between the rotation curve-derived velocities of the
Halpha and [OII] observations, as well as between those calculated from
velocity histograms, justifying the direct comparison of velocities measured
from Halpha rotation curves in nearby galaxies and from [OII] line widths at
higher redshifts. To provide calibration for the geometrical tests, we give
expressions for the different scaling relations between properties of galaxies
(size, surface brightness, magnitude) and their rotation speeds. Apart from the
Tully-Fisher relation, we derive the size-rotation velocity and surface
brightness-rotation velocity relations with unprecedentedly small scatters. We
show how the best size-rotation velocity relation is derived when size is
estimated not from disc scale lengths but from the isophotal diameter r23.5,
once these have been corrected for inclination and extinction effects.Comment: 14 pages and 10 figures. A&A submitte
The double RGB in M 2: C, N, Sr and Ba abundances
The globular cluster M 2 has a photometrically detected double red giant
branch (RGB) sequence. We investigate here the chemical differences between the
two RGBs in order to gain insight in the star formation history of this
cluster. The low-resolution spectra, covering the blue spectral range, were
collected with the MODS spectrograph on the LBT, and analyzed via spectrum
synthesis technique. The high quality of the spectra allows us to measure C, N,
Ba, and Sr abundances relative to iron for 15 RGB stars distributed along the
two sequences. We add to the MODS sample C and N measurements for 35 additional
stars belonging to the blue RGB sequence, presented in Lardo et al. (2012). We
find a clear separation between the two groups of stars in s-process elements
as well as C and N content. Both groups display a C-N anti-correlation and the
red RGB stars are on average richer in C and N with respect to the blue RGB.
Our results reinforce the suggestion that M2 belongs to the family of globular
clusters with complex star formation history, together with Omega Cen, NGC
1851, and M 22.Comment: 11 pages, 8 figures. Accepted for publication in MNRA
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