80 research outputs found
Kinematic Evolution of Simulated Star-Forming Galaxies
Recent observations have shown that star-forming galaxies like our own Milky
Way evolve kinematically into ordered thin disks over the last ~8 billion years
since z=1.2, undergoing a process of "disk settling." For the first time, we
study the kinematic evolution of a suite of four state of the art "zoom in"
hydrodynamic simulations of galaxy formation and evolution in a fully
cosmological context and compare with these observations. Until now, robust
measurements of the internal kinematics of simulated galaxies were lacking as
the simulations suffered from low resolution, overproduction of stars, and
overly massive bulges. The current generation of simulations has made great
progress in overcoming these difficulties and is ready for a kinematic
analysis. We show that simulated galaxies follow the same kinematic trends as
real galaxies: they progressively decrease in disordered motions (sigma_g) and
increase in ordered rotation (Vrot) with time. The slopes of the relations
between both sigma_g and Vrot with redshift are consistent between the
simulations and the observations. In addition, the morphologies of the
simulated galaxies become less disturbed with time, also consistent with
observations, and they both have similarly large scatter. This match between
the simulated and observed trends is a significant success for the current
generation of simulations, and a first step in determining the physical
processes behind disk settling.Comment: ApJ accepted; 6 pages; A pdf with full resolution figures can be
found at https://db.tt/8y4Vzaff (2.8M
The Rise and Fall of Star Formation Histories of Blue Galaxies at Redshifts 0.2 < z < 1.4
Popular cosmological scenarios predict that galaxies form hierarchically from the merger of many progenitor, each with their own unique star formation history (SFH). We use the approach recently developed by Pacifici et al. to constrain the SFHs of 4517 blue (presumably star-forming) galaxies with spectroscopic redshifts in the range O.2 < z < 1:4 from the All-Wavelength Extended Groth Strip International Survey (AEGIS). This consists in the Bayesian analysis of the observed galaxy spectral ' energy distributions with a comprehensive library of synthetic spectra assembled using state-of-the-art models of star formation and chemical enrichment histories, stellar population synthesis, nebular emission and attenuation by dust. We constrain the SFH of each galaxy in our sample by comparing the observed fluxes in the B, R,l and K(sub s) bands and rest-frame optical emission-line luminosities with those of one million model spectral energy distributions. We explore the dependence of the resulting SFH on galaxy stellar mass and redshift. We find that the average SFHs of high-mass galaxies rise and fall in a roughly symmetric bell-shaped manner, while those of low-mass galaxies rise progressively in time, consistent with the typically stronger activity of star formation in low-mass compared to high-mass galaxies. For galaxies of all masses, the star formation activity rises more rapidly at high than at low redshift. These findings imply that the standard approximation of exponentially declining SFHs wIdely used to interpret observed galaxy spectral energy distributions is not appropriate to constrain the physical parameters of star-forming galaxies at intermediate redshifts
Dark and Baryonic Matter in Bright Spiral Galaxies: I.Near-infrared and Optical Broadband Surface Photometry of 30 Galaxies
We present photometrically calibrated images and surface photometry in the B,
V, R, J, H, and K-bands of 25, and in the g, r, and K-bands of 5 nearby bright
(Bo_T<12.5 mag) spiral galaxies with inclinations between 30-65 degrees
spanning the Hubble Sequence from Sa to Scd. Data are from The Ohio State
University Bright Spiral Galaxy Survey, the Two Micron All Sky Survey, and the
Sloan Digital Sky Survey Second Data Release. Radial surface brightness
profiles are extracted, and integrated magnitudes are measured from the
profiles. Axis ratios, position angles, and scale lengths are measured from the
near-infrared images. A 1-dimensional bulge/disk decomposition is performed on
the near-infrared images of galaxies with a non-negligible bulge component, and
an exponential disk is fit to the radial surface brightness profiles of the
remaining galaxies.Comment: 28 page
The angular momentum of baryons and dark matter halos revisited
Recent theoretical studies have shown that galaxies at high redshift are fed
by cold, dense gas filaments, suggesting angular momentum transport by gas
differs from that by dark matter. Revisiting this issue using high-resolution
cosmological hydrodynamics simulations with adaptive mesh refinement, we find
that at the time of accretion, gas and dark matter do carry a similar amount of
specific angular momentum, but that it is systematically higher than that of
the dark matter halo as a whole. At high redshift, freshly accreted gas rapidly
streams into the central region of the halo, directly depositing this large
amount of angular momentum within a sphere of radius r=0.1rvir. In contrast,
dark matter particles pass through the central region unscathed, and a fraction
of them ends up populating the outer regions of the halo (r/rvir>0.1),
redistributing angular momentum in the process. As a result, large-scale
motions of the cosmic web have to be considered as the origin of gas angular
momentum rather than its virialised dark matter halo host. This generic result
holds for halos of all masses at all redshifts, as radiative cooling ensures
that a significant fraction of baryons remain trapped at the centre of the
halos. Despite this injection of angular momentum enriched gas, we predict an
amount for stellar discs which is in fair agreement with observations at z=0.
This arises because the total specific angular momentum of the baryons remains
close to that of dark matter halos. We propose a new scenario where gas
efficiently carries the angular momentum generated by large-scale structure
motions deep inside dark matter halos, redistributing it only in the vicinity
of the disc.Comment: 15 pages,12 figures, submitted to MNRA
On the Evolution of the Velocity-Mass-Size Relations of Disk-Dominated Galaxies over the Past 10 Billion Years
We study the evolution of the scaling relations between maximum circular
velocity, stellar mass and optical half-light radius of star-forming
disk-dominated galaxies in the context of LCDM-based galaxy formation models.
Using data from the literature combined with new data from the DEEP2 and AEGIS
surveys we show that there is a consistent observational and theoretical
picture for the evolution of these scaling relations from z\sim 2 to z=0. The
evolution of the observed stellar scaling relations is weaker than that of the
virial scaling relations of dark matter haloes, which can be reproduced, both
qualitatively and quantitatively, with a simple, cosmologically-motivated model
for disk evolution inside growing NFW dark matter haloes. In this model optical
half-light radii are smaller, both at fixed stellar mass and maximum circular
velocity, at higher redshifts. This model also predicts that the scaling
relations between baryonic quantities evolve even more weakly than the
corresponding stellar relations. We emphasize, though, that this weak evolution
does not imply that individual galaxies evolve weakly. On the contrary,
individual galaxies grow strongly in mass, size and velocity, but in such a way
that they move largely along the scaling relations. Finally, recent
observations have claimed surprisingly large sizes for a number of star-forming
disk galaxies at z \sim 2, which has caused some authors to suggest that high
redshift disk galaxies have abnormally high spin parameters. However, we argue
that the disk scale lengths in question have been systematically overestimated
by a factor \sim 2, and that there is an offset of a factor \sim 1.4 between
H\alpha sizes and optical sizes. Taking these effects into account, there is no
indication that star forming galaxies at high redshifts (z\sim 2) have
abnormally high spin parameters.Comment: 19 pages, 10 figures, accepted to MNRAS, minor changes to previous
versio
z~2: An Epoch of Disk Assembly
We explore the evolution of the internal gas kinematics of star-forming
galaxies from the peak of cosmic star-formation at to today.
Measurements of galaxy rotation velocity , which quantify ordered
motions, and gas velocity dispersion , which quantify disordered
motions, are adopted from the DEEP2 and SIGMA surveys. This sample covers a
continuous baseline in redshift from to , spanning 10 Gyrs. At
low redshift, nearly all sufficiently massive star-forming galaxies are
rotationally supported (). By , the percentage of
galaxies with rotational support has declined to 50 at low stellar mass
() and 70 at high stellar mass
(). For , the percentage
drops below 35 for all masses. From to now, galaxies exhibit
remarkably smooth kinematic evolution on average. All galaxies tend towards
rotational support with time, and it is reached earlier in higher mass systems.
This is mostly due to an average decline in by a factor of 3 since a
redshift of 2, which is independent of mass. Over the same time period,
increases by a factor of 1.5 for low mass systems, but does not
evolve for high mass systems. These trends in and with
time are at a fixed stellar mass and should not be interpreted as evolutionary
tracks for galaxy populations. When galaxy populations are linked in time with
abundance matching, not only does decline with time as before, but
strongly increases with time for all galaxy masses. This enhances the
evolution in . These results indicate that is a
period of disk assembly, during which the strong rotational support present in
today's massive disk galaxies is only just beginning to emerge.Comment: 12 pages, 8 figures, submitted to Ap
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