132 research outputs found
Testing galaxy formation simulations with damped Lyman-α abundance and metallicity evolution
We examine the properties of damped Lyman- absorbers (DLAs) emerging
from a single set of cosmological initial conditions in two state-of-the-art
cosmological hydrodynamic simulations: {\sc Simba} and {\sc Technicolor Dawn}.
The former includes star formation and black hole feedback treatments that
yield a good match with low-redshift galaxy properties, while the latter uses
multi-frequency radiative transfer to model an inhomogeneous ultraviolet
background (UVB) self-consistently and is calibrated to match the Thomson
scattering optical depth, UVB amplitude, and Ly- forest mean
transmission at . Both simulations are in reasonable agreement with the
measured stellar mass and star formation rate functions at , and both
reproduce the observed neutral hydrogen cosmological mass density, . However, the DLA abundance and metallicity distribution are sensitive
to the galactic outflows' feedback and the UVB amplitude. Adopting a strong UVB
and/or slow outflows under-produces the observed DLA abundance, but yields
broad agreement with the observed DLA metallicity distribution. By contrast,
faster outflows eject metals to larger distances, yielding more metal-rich DLAs
whose observational selection may be more sensitive to dust bias. The DLA
metallicity distribution in models adopting an -regulated star
formation recipe includes a tail extending to , lower than any
DLA observed to date, owing to curtailed star formation in low-metallicity
galaxies. Our results show that DLA observations play an imporant role in
constraining key physical ingredients in galaxy formation models, complementing
traditional ensemble statistics such as the stellar mass and star formation
rate functions.Comment: Accepted for publication in MNRA
Color-Induced Displacement double stars in SDSS
We report the first successful application of the astrometric color-induced
displacement technique (CID, the displacement of the photocenter between
different bandpasses due to a varying contribution of differently colored
components to the total light), originally proposed by Wielen (1996) for
discovering unresolved binary stars. Using the Sloan Digital Sky Survey (SDSS)
Data Release 1 with 2.5 million stars brighter than 21m in the u and g bands,
we select 419 candidate binary stars with CID greater than 0.5 arcsec. The SDSS
colors of the majority of these candidates are consistent with binary systems
including a white dwarf and any main sequence star with spectral type later
than ~K7. The astrometric CID method discussed here is complementary to the
photometric selection of binary stars in SDSS discussed by Smolcic et al.
(2004), but there is considerable overlap (15%) between the two samples of
selected candidates. This overlap testifies both to the physical soundness of
both methods, as well as to the astrometric and photometric quality of SDSS
data.Comment: submitted to A&A, 13 pages, 6 figure
The Physical and Photometric Properties of High-Redshift Galaxies in Cosmological Hydrodynamic Simulations
We study the physical and photometric properties of galaxies at z=4 in
cosmological hydrodynamic simulations of a lambda-CDM universe. We focus on
galaxies satisfying the GOODS "B-dropout" criteria. Our goals are: (1) to study
the nature of high-redshift galaxies; (2) to test the simulations against
published measurements of high-redshift galaxies; (3) to find relations between
photometric measurements by HST/ACS (0.4 -- 1 micron) and Spitzer/IRAC (3.6 --
8 micron) and the intrinsic physical properties of GOODS "B-dropouts" such as
stellar mass, stellar age, dust reddening, and star-formation rate; and (4) to
assess how representative the GOODS survey is at this epoch. Our simulations
predict that high-redshift galaxies show strong correlations in star formation
rate versus stellar mass, and weaker correlations versus environment and age,
such that GOODS galaxies are predicted to be the most massive, most rapidly
star-forming galaxies at that epoch, living preferentially in dense regions.
The simulated rest-frame UV luminosity function (LF) and integrated luminosity
density are in broad agreement with observations at z~4. The predicted
rest-frame optical (observed 3.6 micron) LF is similar to the rest-frame UV LF,
shifted roughly one magnitude brighter. We predict that GOODS detects less than
50% of the total stellar mass density formed in galaxies more massive than
10^8.7 M_sun by z=4, mainly because of brightness limits in the HST/ACS bands.
The most rapidly star forming galaxies in our simulations have rates exceeding
1000 M_sun yr^-1, similar to observed sub-mm galaxies. The star formation rates
of these galaxies show at most a mild excess (2--3x) over the rates that would
be expected for their stellar mass. Whether these bright galaxies would be
observable as LBGs depends on the uncertain effects of dust reddening.Comment: 27 pages, 15 figures, submitted to Ap
2MASS Galaxies in the Fornax Cluster Spectroscopic Survey
The Fornax Cluster Spectroscopic Survey (FCSS) is an all-object survey of a
region around the Fornax Cluster of galaxies undertaken using the 2dF
multi-object spectrograph on the Anglo-Australian Telescope. Its aim was to
obtain spectra for a complete sample of all objects with 16.5 < b_j < 19.7
irrespective of their morphology (i.e. including `stars', `galaxies' and
`merged' images). We explore the extent to which (nearby) cluster galaxies are
present in 2MASS. We consider the reasons for the omission of 2MASS galaxies
from the FCSS and vice versa. We consider the intersection (2.9 square degrees
on the sky) of our data set with the infra-red 2 Micron All-Sky Survey (2MASS),
using both the 2MASS Extended Source Catalogue (XSC) and the Point Source
Catalogue (PSC). We match all the XSC objects to FCSS counterparts by position
and also extract a sample of galaxies, selected by their FCSS redshifts, from
the PSC. We confirm that all 114 XSC objects in the overlap sample are
galaxies, on the basis of their FCSS velocities. A total of 23 Fornax Cluster
galaxies appear in the matched data, while, as expected, the remainder of the
sample lie at redshifts out to z = 0.2 (the spectra show that 61% are early
type galaxies, 18% are intermediate types and 21% are strongly star
forming).The PSC sample turns out to contain twice as many galaxies as does the
XSC. However, only one of these 225 galaxies is a (dwarf) cluster member. On
the other hand, galaxies which are unresolved in the 2MASS data (though almost
all are resolved in the optical) amount to 71% of the non-cluster galaxies with
2MASS detections and have redshifts out to z=0.32.Comment: 5 pages, accepted by A&A, resubmitted due to missing reference
The Optical, Infrared and Radio Properties of Extragalactic Sources Observed by SDSS, 2MASS and FIRST Surveys
We positionally match sources observed by the Sloan Digital Sky Survey
(SDSS), the Two Micron All Sky Survey (2MASS), and the Faint Images of the
Radio Sky at Twenty-cm (FIRST) survey. Practically all 2MASS sources are
matched to an SDSS source within 2 arcsec; ~11% of them are optically resolved
galaxies and the rest are dominated by stars. About 1/3 of FIRST sources are
matched to an SDSS source within 2 arcsec; ~80% of these are galaxies and the
rest are dominated by quasars. Based on these results, we project that by the
completion of these surveys the matched samples will include about 10^7 stars
and 10^6 galaxies observed by both SDSS and 2MASS, and about 250,000 galaxies
and 50,000 quasars observed by both SDSS and FIRST. Here we present a
preliminary analysis of the optical, infrared and radio properties for the
extragalactic sources from the matched samples. In particular, we find that the
fraction of quasars with stellar colors missed by the SDSS spectroscopic survey
is probably not larger than ~10%, and that the optical colors of radio-loud
quasars are ~0.05 mag. redder (with 4-sigma significance) than the colors of
radio-quiet quasars.Comment: 10 pages, 6 color figures, presented at IAU Colloquium 184. AGN
Survey
The Opacity of the Intergalactic Medium During Reionization: Resolving Small-Scale Structure
Early in the reionization process, the intergalactic medium (IGM) would have
been quite inhomogeneous on small scales, due to the low Jeans mass in the
neutral IGM and the hierarchical growth of structure in a cold dark matter
Universe. This small-scale structure acted as an important sink during the
epoch of reionization, impeding the progress of the ionization fronts that
swept out from the first sources of ionizing radiation. Here we present results
of high-resolution cosmological hydrodynamics simulations that resolve the
cosmological Jeans mass of the neutral IGM in representative volumes several
Mpc across. The adiabatic hydrodynamics we follow are appropriate in an
unheated IGM, before the gas has had a chance to respond to the photoionization
heating. Our focus is determination of the resolution required in cosmological
simulations in order to sufficiently sample and resolve small-scale structure
regulating the opacity of an unheated IGM. We find that a dark matter particle
mass of m_dm 1 Mpc are required. With our
converged results we show how the mean free path of ionizing radiation and
clumping factor of ionized hydrogen depends upon the ultraviolet background
(UVB) flux and redshift. We find, for example at z = 10, clumping factors
typically of 10 to 20 for an ionization rate of Gamma ~ 0.3 - 3 x 1e-12 s^-1,
with corresponding mean free paths of ~ 3 - 15 Mpc, extending previous work on
the evolving mean free path to considerably smaller scales and earlier times.Comment: Accepted for publication in the Astrophysical Journa
Understanding the Observed Evolution of the Galaxy Luminosity Function from z=6-10 in the Context of Hierarchical Structure Formation
Recent observations of the Lyman-break galaxy (LBG) luminosity function (LF)
from z~6-10 show a steep decline in abundance with increasing redshift.
However, the LF is a convolution of the mass function of dark matter halos
(HMF)--which also declines sharply over this redshift range--and the
galaxy-formation physics that maps halo mass to galaxy luminosity. We consider
the strong observed evolution in the LF from z~6-10 in this context and
determine whether it can be explained solely by the behavior of the HMF. From
z~6-8, we find a residual change in the physics of galaxy formation
corresponding to a ~0.5 dex increase in the average luminosity of a halo of
fixed mass. On the other hand, our analysis of recent LF measurements at z~10
shows that the paucity of detected galaxies is consistent with almost no change
in the average luminosity at fixed halo mass from z~8. The LF slope also
constrains the variation about this mean such that the luminosity of galaxies
hosted by halos of the same mass are all within about an order-of-magnitude of
each other. We show that these results are well-described by a simple model of
galaxy formation in which cold-flow accretion is balanced by star formation and
momentum-driven outflows. If galaxy formation proceeds in halos with masses
down to 10^8 Msun, then such a model predicts that LBGs at z~10 should be able
to maintain an ionized intergalactic medium as long as the ratio of the
clumping factor to the ionizing escape fraction is C/f_esc < 10.Comment: 15 pages, 2 figures; results unchanged; accepted by JCA
Quenching Massive Galaxies with On-the-fly Feedback in Cosmological Hydrodynamic Simulations
Massive galaxies today typically are not forming stars despite being
surrounded by hot gaseous halos with short central cooling times. This likely
owes to some form of "quenching feedback" such as merger-driven quasar activity
or radio jets emerging from central black holes. Here we implement heuristic
prescriptions for these phenomena on-the-fly within cosmological hydrodynamic
simulations. We constrain them by comparing to observed luminosity functions
and color-magnitude diagrams from SDSS. We find that quenching from mergers
alone does not produce a realistic red sequence, because 1 - 2 Gyr after a
merger the remnant accretes new fuel and star formation reignites. In contrast,
quenching by continuously adding thermal energy to hot gaseous halos
quantitatively matches the red galaxy luminosity function and produces a
reasonable red sequence. Small discrepancies remain - a shallow red sequence
slope suggests that our models underestimate metal production or retention in
massive red galaxies, while a deficit of massive blue galaxies may reflect the
fact that observed heating is intermittent rather than continuous. Overall,
injection of energy into hot halo gas appears to be a necessary and sufficient
condition to broadly produce red and dead massive galaxies as observed.Comment: 23 pages, 14 figures. MNRAS accepted. Added Sec. 4.4 and
significantly modified the Discussion at the suggestion of the refere
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