5,332 research outputs found
On Macroscopic Complexity and Perceptual Coding
The theoretical limits of 'lossy' data compression algorithms are considered.
The complexity of an object as seen by a macroscopic observer is the size of
the perceptual code which discards all information that can be lost without
altering the perception of the specified observer. The complexity of this
macroscopically observed state is the simplest description of any microstate
comprising that macrostate. Inference and pattern recognition based on
macrostate rather than microstate complexities will take advantage of the
complexity of the macroscopic observer to ignore irrelevant noise
Large Scale Structure in Dark Matter and Galaxies
Galaxy evolution and AGN growth in the early universe are believed to be strongly driven
by merging (hierarchical growth) and galaxy dynamical interactions. Thus, a fall exploration of the
environmental influences is absolutely essential to understanding this early evolution. The Cosmic
Evolution Survey (COSMOS, [Scoville et al. 2007a]) is specifically designed to probe the correlated
coevolution of galaxies, star formation, active galactic nuclei (AGN) and dark matter (DM) largescale
structures (LSS) over the redshift range z > 0.5 to 6. The survey includes midti-wavelength
imaging and spectroscopy from X-ray to radio wavelengths covering a 2 square degree equatorial
field. Photometric redshifts are derived using 34 photometric UV-IR bands for 800,000 galaxies
with accuracy reaching σ_z/(l +z) = 0.7 — 1.6% for bright galaxies (I_(AB) = 22 —24mag).
Large scale structures have been traced in COSMOS from z = 0.2 to 2.5 in the baryons (from
the galaxy density distribution) and in the dark matter to z = 1.1 (from weak tensing analysis of
HST ACS images). These LSS extend over 20 Mpc with total mass up to ~10^(15)_ ⊙. The overall
distribution of galaxy overdensities is similar with those predicted from the Millennium simulation.
A trend for an increasing high overdensities at low z is clearly apparent in these data. At higher
redshifts of z ~ 1, there appears to be a significant discrepancy between the observations and the
simulations - with the simidations exhibiting earUer development of high density structores than is
seen in the observed galaxy distributions. The observed galaxy spectral energy distributions (SEDs)
and star formation rates (SFRs) clearly depend systematically on both redshift and environmental
density - early SED types and lower SFRs in denser regions and at lower redshift. This evolution is
probably driven by the exhaustion of the ISM and by galaxy interactions, the latter being strongly
correlated with regions of highest dark matter density. Strong evolution is seen the frequency of
close pairs of galaxies - particidarly for lower mass companions at projected separations 10 - 20
kpc
Fast Autocorrelated Context Models for Data Compression
A method is presented to automatically generate context models of data by
calculating the data's autocorrelation function. The largest values of the
autocorrelation function occur at the offsets or lags in the bitstream which
tend to be the most highly correlated to any particular location. These offsets
are ideal for use in predictive coding, such as predictive partial match (PPM)
or context-mixing algorithms for data compression, making such algorithms more
efficient and more general by reducing or eliminating the need for ad-hoc
models based on particular types of data. Instead of using the definition of
the autocorrelation function, which considers the pairwise correlations of data
requiring O(n^2) time, the Weiner-Khinchin theorem is applied, quickly
obtaining the autocorrelation as the inverse Fast Fourier transform of the
data's power spectrum in O(n log n) time, making the technique practical for
the compression of large data objects. The method is shown to produce the
highest levels of performance obtained to date on a lossless image compression
benchmark.Comment: v2 includes bibliograph
Massive Accretion Disks
Recent high resolution near infrared (HST-NICMOS) and mm-interferometric
imaging have revealed dense gas and dust accretion disks in nearby
ultra-luminous galactic nuclei. In the best studied ultraluminous IR galaxy,
Arp 220, the 2 micron imaging shows dust disks in both of the merging galactic
nuclei and mm-CO line imaging indicates molecular gas masses approx. 10^9 M_sun
for each disk. The two gas disks in Arp 220 are counterrotating and their
dynamical masses are approx. 2x10^9 M_sun, that is, only slightly larger than
the gas masses. These disks have radii approx 100 pc and thickness 10-50 pc.
The high brightness temperatures of the CO lines indicate that the gas in the
disks has area filling factors of approx. 25-50% and mean densities of >~ 10^4
cm^(-3). Within these nuclear disks, the rate of massive star formation is
undoubtedly prodigious and, given the high viscosity of the gas, there will
also be high radial accretion rates, perhaps >~ 10 M_sun/yr. If this inflow
persists to very small radii, it is enough to feed even the highest luminosity
AGNs.Comment: LaTex, 6 pages with 1 postscript and 1 jpg figure, and 1 postscript
table, To appear in the proc. of the Ringberg workshop on "Ultraluminous
Galaxies: Monsters or Babies" (Ringberg castle, Sept. 1998), Ap&SS, in pres
The Extreme Nuclear Environments of Sgr A* and Arp 220
The dense ISM which is the fuel for both nuclear starbursts is believed to be accreted to the nucleus by stellar bars and galactic interactions. In this contribution, I summarize the observational results for two galactic nuclei at the extreme ends of starburst/AGN activity − our own Galactic nucleus with SgrA* and the ULIRG Arp 220. I discuss theoretical considerations for the properties of the ISM − its density and scale height, whether it is likely to clump into gravitational bound GMCs − and the self-regulation of SB and AGN fueling due to radiation pressure support of the ISM. The latter yields an Eddington-like limit on the activity for both SB and AGN, corresponding to approximately 500 L_ʘ/M_ʘ for optically thick regions in which the radiation has been degraded to the NIR
The zCOSMOS 10k-sample: the role of galaxy stellar mass in the colour-density relation up to z ~ 1
Aims. With the first ~10 000 spectra of the flux limited zCOSMOS sample (I_(AB) ≤ 22.5) we want to study the evolution of environmental effects
on galaxy properties since z ~ 1.0, and to disentangle the dependence among galaxy colour, stellar mass and local density.
Methods. We use our previously derived 3D local density contrast δ, computed with the 5th nearest neighbour approach, to study the evolution
with z of the environmental effects on galaxy U-B colour, D4000 Å break and [OII]λ3727 equivalent width (EW[OII]). We also analyze the implications
due to the use of different galaxy selections, using luminosity or stellar mass, and we disentangle the relations among colour, stellar mass
and δ studying the colour-density relation in narrow mass bins.
Results. We confirm that within a luminosity-limited sample (M_B ≤ −20.5 − z) the fraction of red (U − B ≥ 1) galaxies depends on δ at least
up to z ~ 1, with red galaxies residing mainly in high densities. This trend becomes weaker for increasing redshifts, and it is mirrored by the
behaviour of the fraction of galaxies with D4000 Å break ≥1.4. We also find that up to z ~ 1 the fraction of galaxies with log(EW[OII]) ≥ 1.15 is
higher for lower δ, and also this dependence weakens for increasing z. Given the triple dependence among galaxy colours, stellar mass and δ, the
colour-δ relation that we find in the luminosity-selected sample can be due to the broad range of stellar masses embedded in the sample. Thus, we
study the colour-δ relation in narrow mass bins within mass complete subsamples, defining red galaxies with a colour threshold roughly parallel
to the red sequence in the colour-mass plane. We find that once mass is fixed the colour-δ relation is globally flat up to z ~ 1 for galaxies with
log(M/M_⊙) ≳ 10.7. This means that for these masses any colour-δ relation found within a luminosity-selected sample is the result of the combined
colour-mass and mass-δ relations. On the contrary, even at fixed mass we observe that within 0.1 ≤ z ≤ 0.5 the fraction of red galaxies with
log(M/M_⊙) ≲ 10.7 depends on δ. For these mass and redshift ranges, environment affects directly also galaxy colours.
Conclusions. We suggest a scenario in which the colour depends primarily on stellar mass, but for an intermediate mass regime (10.2 ≲ log(M/M_⊙) ≲ 10.7) the local density modulates this dependence. These relatively low mass galaxies formed more recently, in an epoch when
more evolved structures were already in place, and their longer SFH allowed environment-driven physical processes to operate during longer
periods of time
A boost for the EW SUSY hunt: monojet-like search for compressed sleptons at LHC14 with 100 fb^-1
Current Large Hadron Collider (LHC) analyses are blind to compressed
supersymmetry (SUSY) models with sleptons near the lightest super partner (LSP)
in mass:
GeV. We present a search sensitive to the very compressed range using the channel with soft same-flavor leptons and one hard jet from initial state
radiation ( GeV). The sleptons recoil against the jet
boosting them and their decay products, making the leptons detectable and
providing substantial missing transverse momentum. We use the kinematic
variable along with a different-flavor control region to reduce
the large standard model backgrounds and control systematic uncertainty. We
find the analysis should allow LHC14 with to search for
degenerate left-handed selectrons and smuons in the compressed region up to
GeV. In addition, it should be sensitive to
GeV for the very challenging case of
auto-concealed SUSY, in which left-handed sleptons decay to the Kaluza-Klein
tower of a modulino LSP which lives in extra dimensions. In both the
compressed spectrum and auto-concealed SUSY scenarios this analysis will need
more data to improve on LEP2 limits for right-handed sleptons due to their
smaller cross sections.Comment: 15 pages, 3 figures, updated citations in v
Comparison Between Simulated and Observational Results of Galaxy Formation for Large Scale Structures
The Millennium simulation is the largest numerical simulation of how minor fluctuations in the density of the universe’s dark matter distribution are amplified by gravity to develop into the large scale structures(LSS) and galaxy clusters seen today(Springel et al. 2005). Although the simulations have been compared with the astronomical observations of the local universe, the simulations have not been widely compared with high redshift, early universe observations. In our study we compare the simulation data(Wang et al. 2008; Guo et al. 2008(in preparation)) for the first time with observations from the COSMOS survey(Scoville et al. 2006). Three quantities are proposed to characterize the structures and the structures distribution, namely the percent area occupied by LSS at each redshift, the average area of LSS and the shapes as characterized by the square root of the area divided by the circumference. We calculate these quantities for both the observations and the simulations, and quantify discrepancies between the existing simulations and observations. In particular, the simulations exhibit earlier development of dense structures than is seen in the observational data
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