511 research outputs found
Constraints on galaxy formation from alpha-enhancement in luminous elliptical galaxies
We explore the formation of alpha-enhanced and metal-rich stellar populations
in the nuclei of luminous ellipticals under the assumption of two extreme
galaxy formation scenarios based on hierarchical clustering, namely a fast
clumpy collapse and the merger of two spirals. We investigate the parameter
space of star formation time-scale, IMF slope, and stellar yields. In
particular, the latter add a huge uncertainty in constraining time-scales and
IMF slopes. We find that -- for Thielemann, Nomoto & Hashimoto nucleosynthesis
-- in a fast clumpy collapse scenario an [alpha/Fe] overabundance of approx.
0.2 dex in the high metallicity stars can be achieved with a Salpeter IMF and
star formation time-scales of the order 10^9 yr. The scenario of two merging
spirals which are similar to our Galaxy, instead, fails to reproduce
alpha-enhanced abundance ratios in the metal-rich stars, unless the IMF is
flattened during the burst ignited by the merger. This result is independent of
the burst time-scale. We suggest that abundance gradients give hints to
distinguish between the two extreme formation scenarios considered in this
paper.Comment: Accepted for publication in MNRAS, LaTex 2.09 with mn.sty, 13 pages,
5 figure
The star formation histories of early-type galaxies: insights from the rest-frame ultra-violet
Our current understanding of the star formation histories of early-type
galaxies is reviewed, in the context of recent observational studies of their
ultra-violet (UV) properties. Combination of UV and optical spectro-photometric
data indicates that the bulk of the stellar mass in the early-type population
forms at high redshift (z > 2), typically over short timescales (< 1 Gyr).
Nevertheless, early-types of all luminosities form stars over the lifetime of
the Universe, with most luminous (-23 < M(V) < -21) systems forming 10-15% of
their stellar mass after z = 1 (with a scatter to higher value), while their
less luminous (M(V) > -21) counterparts form 30-60% of their mass in the same
redshift range. The large scatter in the (rest-frame) UV colours in the
redshift range 0 < z < 0.7 indicates widespread low-level star formation in the
early-type population over the last 8 billion years. The mass fraction of young
(< 1 Gyr old) stars in luminous early-type galaxies varies between 1% and 6% at
z~0 and is in the range 5-13% at z~0.7. The intensity of recent star formation
and the bulk of the UV colour distribution is consistent with what might be
expected from minor mergers (mass ratios < 1:6) in an LCDM cosmology.Comment: Brief Review, Mod. Phys. Lett.
Simulating the recent star formation history in the halo of NGC 5128
Simulated color-magnitude diagrams (CMDs) are used to investigate the recent
star formation history in NGC 5128. The comparison of the simulations with the
observed UV CMD for a field in the NE shell, where recent star formation is
present, constrains the initial mass function (IMF) and the duration of the
star formation episode. The star formation burst is still on-going or at most
has stopped some 2 Myr ago. The look-back time on the main sequence is set by
the incompleteness of the U-band observations at V~26, and is of the order of
50 Myr. The post main sequence phases have a look-back time up to 100 Myr,
setting the maximum observable time for the star formation in this field. The
comparison of the observed and simulated luminosity functions and the number of
post main sequence vs. total number of stars favours models with active star
formation in the last 100 Myr. The data are best fitted by an IMF with Salpeter
slope (\alpha=2.35), and are also consistent with slopes in the range of
2<~\alpha<~2.6. They exclude steeper IMF slopes. The mean star formation rate
for a Salpeter IMF in the range of masses 0.6<~M<~100 M_sun within the last 100
Myr is 1.6x10^-4 M_sun yr^-1 kpc^-2.Comment: 16 pages, 10 figures; A&A in pres
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