13,103 research outputs found
The diverse evolutionary paths of simulated high-z massive, compact galaxies to z=0
Massive quiescent galaxies have much smaller physical sizes at high redshift
than today. The strong evolution of galaxy size may be caused by progenitor
bias, major and minor mergers, adiabatic expansion, and/or renewed star
formation, but it is difficult to test these theories observationally. Herein,
we select a sample of 35 massive, compact galaxies (
M, M/kpc) at in the
cosmological hydrodynamical simulation Illustris and trace them forward to
to uncover their evolution and identify their descendants. By , the
original factor of 3 difference in stellar mass spreads to a factor of 20. The
dark matter halo masses similarly spread from a factor of 5 to 40. The
galaxies' evolutionary paths are diverse: about half acquire an ex-situ
envelope and are the core of a more massive descendant, a third survive
undisturbed and gain very little mass, 15% are consumed in a merger with a more
massive galaxy, and a small remainder are thoroughly mixed by major mergers.
The galaxies grow in size as well as mass, and only 10% remain compact by
. The majority of the size growth is driven by the acquisition of ex-situ
mass. The most massive galaxies at are the most likely to have compact
progenitors, but this trend possesses significant dispersion which precludes a
direct linkage to compact galaxies at . The compact galaxies' merger rates
are influenced by their environments, so that isolated or satellite
compact galaxies (which are protected from mergers) are the most likely to
survive to the present day.Comment: 19 pages, 10 figures, MNRAS accepted version including 2 new figure
Triggering Active Galactic Nuclei in Hierarchical Galaxy Formation: Disk instability vs. Interactions
Using a semi analytic model for galaxy formation we investigate the effects
of Black Hole accretion triggered by disk instabilities (DI) in isolated
galaxies on the evolution of AGN. Specifically, we took on, developed and
expanded the Hopkins & Quataert (2011) model for the mass inflow following disk
perturbations, and compare the corresponding evolution of the AGN population
with that arising in a scenario where galaxy interactions trigger AGN (IT
mode). We extended and developed the DI model by including different disk
surface density profiles, to study the maximal contribution of DI to the
evolution of the AGN population. We obtained the following results: i) for
luminosities corresponding to the DI mode can provide the
BH accretion needed to match the observed AGN luminosity functions up to ; in such a luminosity range and redshift, it can compete with the
IT scenario as the main driver of cosmological evolution of AGN; ii) The DI
scenario cannot provide the observed abundance of high-luminosity QSO with
AGN, as well as the abundance of high-redhshift QSOs with , while the IT scenario provides
an acceptable match up to , as found in our earliest works; iii)
The dispersion of the distributions of Eddington ratio for low- and
intermediate-luminosity AGN (bolometric = -
erg/s) is predicted to be much smaller in the DI scenario compared to the IT
mode; iv) The above conclusions are robust with respect to the explored
variants of the Hopkins & Quataert (2011) model. We discuss the physical origin
of our findings, and how it is possible to pin down the dominant fueling
mechanism in the low-intermediate luminosity range where
both the DI and the IT modes are viable candidates as drivers for the AGN
evolution.Comment: Accepted for publication in Astronomy & Astrophysics, 24 pages, 8
figures; updated reference
Decision Tree Classifiers for Star/Galaxy Separation
We study the star/galaxy classification efficiency of 13 different decision
tree algorithms applied to photometric objects in the Sloan Digital Sky Survey
Data Release Seven (SDSS DR7). Each algorithm is defined by a set of parameters
which, when varied, produce different final classification trees. We
extensively explore the parameter space of each algorithm, using the set of
SDSS objects with spectroscopic data as the training set. The
efficiency of star-galaxy separation is measured using the completeness
function. We find that the Functional Tree algorithm (FT) yields the best
results as measured by the mean completeness in two magnitude intervals: () and (). We compare the performance of the
tree generated with the optimal FT configuration to the classifications
provided by the SDSS parametric classifier, 2DPHOT and Ball et al. (2006). We
find that our FT classifier is comparable or better in completeness over the
full magnitude range , with much lower contamination than all but
the Ball et al. classifier. At the faintest magnitudes (), our classifier
is the only one able to maintain high completeness (80%) while still
achieving low contamination (). Finally, we apply our FT classifier
to separate stars from galaxies in the full set of SDSS
photometric objects in the magnitude range .Comment: Submitted to A
Merger-Induced Metallicity Dilution in Cosmological Galaxy Formation Simulations
Observational studies have revealed that galaxy pairs tend to have lower
gas-phase metallicity than isolated galaxies. This metallicity deficiency can
be caused by inflows of low-metallicity gas due to the tidal forces and
gravitational torques associated with galaxy mergers, diluting the metal
content of the central region. In this work we demonstrate that such
metallicity dilution occurs in state-of-the-art cosmological simulations of
galaxy formation. We find that the dilution is typically 0.1 dex for major
mergers, and is noticeable at projected separations smaller than kpc. For
minor mergers the metallicity dilution is still present, even though the
amplitude is significantly smaller. Consistent with previous analysis of
observed galaxies we find that mergers are outliers from the \emph{fundamental
metallicity relation}, with deviations being larger than expected for a
Gaussian distribution of residuals. Our large sample of mergers within full
cosmological simulations also makes it possible to estimate how the star
formation rate enhancement and gas consumption timescale behave as a function
of the merger mass ratio. We confirm that strong starbursts are likely to occur
in major mergers, but they can also arise in minor mergers if more than two
galaxies are participating in the interaction, a scenario that has largely been
ignored in previous work based on idealised isolated merger simulations.Comment: Submitted to MNRA
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