2,489 research outputs found
Star formation in cooling flow galaxies
Spectroscopic observations of central dominant galaxies are reviewed. Through the analysis of absorption spectral features (mainly the strength of the Mg triplet at 5175 A and the break in 4000 A), both in the galaxy centers and along the radii, we will be able to impose limits on the ongoing star formation as the ultimate fate for the large amounts of accreted gas. With the same aim we will carry out a dynamical study based on velocity dispersion measurements
Spectral gradients in central cluster galaxies: further evidence of star formation in cooling flows
We have obtained radial gradients in the spectral features D4000 and Mg2 for
a sample of 11 central cluster galaxies (CCGs). The new data strongly confirm
the correlations between line-strength indices and the cooling flow phenomenon
found in our earlier study. We find that such correlations depend on the
presence and characteristics of emission lines in the inner regions of the
CCGs. CCGs in cooling flow clusters exhibit a clear sequence in the D4000-Mg2
plane, with a neat segregation depending on emission-line types and blue
morphology. This sequence can be modelled, using stellar population models with
a normal IMF, by a recent burst of star formation. In CCGs with emission lines,
the gradients in the spectral indices are flat or positive inside the
emission-line regions, suggesting the presence of young stars. Outside the
emission-line regions, and in cooling flow galaxies without emission lines,
gradients are negative and consistent with those measured in CCGs in clusters
without cooling flows and giant elliptical galaxies. Index gradients measured
exclusively in the emission-line region correlate with mass deposition rate. We
have also estimated the radial profiles of the mass transformed into new stars
which are remarkably parallel to the radial behaviour of the mass deposition
rate. A large fraction (probably most) of the cooling flow gas accreted into
the emission-line region is converted into stars. We discuss the evolutionary
sequence suggested by McNamara (1997), in which radio triggered star formation
bursts take place several times during the lifetime of the cooling flow. This
scenario is consistent with the available observations.Comment: 19 pages, 18 PostScript figures, accepted for publication in MNRA
Kinematic Properties and Dark Matter Fraction of Virgo Dwarf Early-Type Galaxies
What happens to dwarf galaxies as they enter the cluster potential well is
one of the main unknowns in studies of galaxy evolution. Several evidence
suggests that late-type galaxies enter the cluster and are transformed to dwarf
early-type galaxies (dEs). We study the Virgo cluster to understand which
mechanisms are involved in this transformation. We find that the dEs in the
outer parts of Virgo have rotation curves with shapes and amplitudes similar to
late-type galaxies of the same luminosity. These dEs are rotationally
supported, have disky isophotes, and younger ages than those dEs in the center
of Virgo, which are pressure supported, often have boxy isophotes and are
older. Ram pressure stripping, thus, explains the properties of the dEs located
in the outskirts of Virgo. However, the dEs in the central cluster regions,
which have lost their angular momentum, must have suffered a more violent
transformation. A combination of ram pressure stripping and harassment is not
enough to remove the rotation and the spiral/disky structures of these
galaxies. We find that on the the Faber-Jackson and the Fundamental Plane
relations dEs deviate from the trends of massive elliptical galaxies towards
the position of dark matter dominated systems such as the dwarf spheroidal
satellites of the Milky Way and M31. Both, rotationally and pressure supported
dEs, however, populate the same region in these diagrams. This indicates that
dEs have a non-negligible dark matter fraction within their half light radius.Comment: Proceeding of the XXVIII IAU General Assembly, Special Session 3:
Galaxy Evolution through Secular Processes. Edts: R. Buta and D. Pfennige
Reliable random error estimation in the measurement of line-strength indices
We present a new set of accurate formulae for the computation of random
errors in the measurement of atomic and molecular indices. The new expressions
are in excellent agreement with numerical simulations. We have found that, in
some cases, the use of approximated equations can give misleading line-strength
index errors. It is important to note that accurate errors can only be achieved
after a full control of the error propagation throughout the data reduction
with a parallel processing of data and error frames. Finally, simple recipes
for the estimation of the required signal-to-noise ratio to achieve a fixed
index error are presented.Comment: 9 pages, LaTeX file + 5 PostScript figures, psfig.sty and laa-s.sty
required, to be published in Astronomy & Astrophysics Supplement Serie
Line-Strength Indices in Bright Spheroidals: Evidence for a Stellar Population Dichotomy between Spheroidal and Elliptical Galaxies
We present new measurements of central line-strength indices (namely Mg2,
, and Hbeta and gradients for a sample of 6 bright spheroidal galaxies
(Sph's) in the Virgo cluster. Comparison with similar measurements for
elliptical galaxies (E's), galactic globular clusters (GGC's), and stellar
population models yield the following results: (1) In contrast with bright E's,
bright Sph's are consistent with solar abundance [Mg/Fe] ratios; (2) Bright
Sph's exhibit metallicities ranging from values typical for metal-rich GGC's to
those for E's; (3) Although absolute mean ages are quite model dependent, we
find evidence that the stellar populations of some (if not all) Sph's look
significantly younger than GGC's; and (4) Mg2 gradients of bright Sph's are
significantly shallower than those of E galaxies. We conclude that the
dichotomy found in the structural properties of Sph and E galaxies is also
observed in their stellar populations. A tentative interpretation in terms of
differences in star formation histories is suggested.Comment: 14 pages, LaTeX file + 2 PostScript figures, aasms4.sty require
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