962 research outputs found
Building Blocks in Hierarchical Clustering Scenarios and their Connection with Damped Ly Systems
We carried out a comprehensive analysis of the chemical properties of the
interstellar medium (ISM) and the stellar population (SP) of current normal
galaxies and their progenitors in a hierarchical clustering scenario. We
compared the results with observations of Damped Lyman- systems (DLAs)
under the hypothesis that, at least, part of the observed DLAs could originate
in the building blocks of today normal galaxies. We used a hydrodynamical
cosmological code which includes star formation and chemical enrichment.
Galaxy-like objects are identified at and then followed back in time.
Random line-of-sights (LOS) are drawn through these structures in order to
mimic Damped Lyman systems. We then analysed the chemical properties
of the ISM and SP along the LOS. We found that the progenitors of current
galaxies in the field with mean and virial circular velocity of
could be the associated DLA galaxies. For these systems
we detected a trend for to increase with redshift.(Abridged)Comment: 15 pages, 11 Postscript figures. Accepted to MNRA
Chemo-Archaeological Downsizing in a Hierarchical Universe: Impact of a Top Heavy IGIMF
We make use of a semi-analytical model of galaxy formation to investigate the
origin of the observed correlation between [a/Fe] abundance ratios and stellar
mass in elliptical galaxies. We implement a new galaxy-wide stellar initial
mass function (Top Heavy Integrated Galaxy Initial Mass Function, TH-IGIMF) in
the semi-analytic model SAG and evaluate its impact on the chemical evolution
of galaxies. The SFR-dependence of the slope of the TH-IGIMF is found to be key
to reproducing the correct [a/Fe]-stellar mass relation. Massive galaxies reach
higher [a/Fe] abundance ratios because they are characterized by more top-heavy
IMFs as a result of their higher SFR. As a consequence of our analysis, the
value of the minimum embedded star cluster mass and of the slope of the
embedded cluster mass function, which are free parameters involved in the
TH-IGIMF theory, are found to be as low as 5 solar masses and 2, respectively.
A mild downsizing trend is present for galaxies generated assuming either a
universal IMF or a variable TH-IGIMF. We find that, regardless of galaxy mass,
older galaxies (with formation redshifts > 2) are formed in shorter time-scales
(< 2 Gyr), thus achieving larger [a/Fe] values. Hence, the time-scale of galaxy
formation alone cannot explain the slope of the [a/Fe]-galaxy mass relation,
but is responsible for the big dispersion of [a/Fe] abundance ratios at fixed
stellar mass.We further test the hyphothesis of a TH-IGIMF in elliptical
galaxies by looking into mass-to-light ratios, and luminosity functions. Models
with a TH-IGIMF are also favoured by these constraints. In particular,
mass-to-light ratios agree with observed values for massive galaxies while
being overpredicted for less massive ones; this overprediction is present
regardless of the IMF considered.Comment: 24 pages, 15 figures, 2 tables. (Comments most welcome). Summited to
MNRA
Electrical Control of Linear Dichroism in Black Phosphorus from the Visible to Mid-Infrared
The incorporation of electrically tunable materials into photonic structures
such as waveguides and metasurfaces enables dynamic control of light
propagation by an applied potential. While many materials have been shown to
exhibit electrically tunable permittivity and dispersion, including transparent
conducting oxides (TCOs) and III-V semiconductors and quantum wells, these
materials are all optically isotropic in the propagation plane. In this work,
we report the first known example of electrically tunable linear dichroism,
observed here in few-layer black phosphorus (BP), which is a promising
candidate for multi-functional, broadband, tunable photonic elements. We
measure active modulation of the linear dichroism from the mid-infrared to
visible frequency range, which is driven by anisotropic quantum-confined Stark
and Burstein-Moss effects, and field-induced forbidden-to-allowed optical
transitions. Moreover, we observe high BP absorption modulation strengths,
approaching unity for certain thicknesses and photon energies
Bulge formation from SSCs in a responding cuspy dark matter halo
We simulate the bulge formation in very late-type dwarf galaxies from
circumnuclear super star clusters (SSCs) moving in a responding cuspy dark
matter halo (DMH). The simulations show that (1) the response of DMH to sinking
of SSCs is detectable only in the region interior to about 200 pc. The mean
logarithmic slope of the responding DM density profile over that area displays
two different phases: the very early descent followed by ascent till
approaching to 1.2 at the age of 2 Gyrs. (2) the detectable feedbacks of the
DMH response on the bulge formation turned out to be very small, in the sense
that the formed bulges and their paired nuclear cusps in the fixed and the
responding DMH are basically the same, both are consistent with
observations. (3) the yielded mass correlation of bulges to their nuclear
(stellar) cusps and the time evolution of cusps' mass are accordance with
recent findings on relevant relations. In combination with the consistent
effective radii of nuclear cusps with observed quantities of nuclear clusters,
we believe that the bulge formation scenario that we proposed could be a very
promising mechanism to form nuclear clusters.Comment: 27 pages, 11 figures, accepted for publication in Ap
A fitting formula for the merger timescale of galaxies in hierarchical clustering
We study galaxy mergers using a high-resolution cosmological hydro/N-body
simulation with star formation, and compare the measured merger timescales with
theoretical predictions based on the Chandrasekhar formula. In contrast to
Navarro et al., our numerical results indicate, that the commonly used equation
for the merger timescale given by Lacey and Cole, systematically underestimates
the merger timescales for minor mergers and overestimates those for major
mergers. This behavior is partly explained by the poor performance of their
expression for the Coulomb logarithm, \ln (m_pri/m_sat). The two alternative
forms \ln (1+m_pri/m_sat) and 1/2\ln [1+(m_pri/m_sat)^2] for the Coulomb
logarithm can account for the mass dependence of merger timescale successfully,
but both of them underestimate the merger time scale by a factor 2. Since \ln
(1+m_pri/m_sat) represents the mass dependence slightly better we adopt this
expression for the Coulomb logarithm. Furthermore, we find that the dependence
of the merger timescale on the circularity parameter \epsilon is much weaker
than the widely adopted power-law \epsilon^{0.78}, whereas
0.94*{\epsilon}^{0.60}+0.60 provides a good match to the data. Based on these
findings, we present an accurate and convenient fitting formula for the merger
timescale of galaxies in cold dark matter models.Comment: 16 pages, 14 figures, accepted for publication in ApJ, minor changes
in the last few sentences of the discussio
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