280 research outputs found
What Determines the Incidence and Extent of MgII Absorbing Gas Around Galaxies?
We study the connections between on-going star formation, galaxy mass, and
extended halo gas, in order to distinguish between starburst-driven outflows
and infalling clouds that produce the majority of observed MgII absorbers at
large galactic radii (>~ 10 h^{-1} kpc) and to gain insights into halo gas
contents around galaxies. We present new measurements of total stellar mass
(M_star), H-alpha emission line strength (EW(H-alpha)), and specific star
formation rate (sSFR) for the 94 galaxies published in H.-W. Chen et al.
(2010). We find that the extent of MgII absorbing gas, R_MgII, scales with
M_star and sSFR, following R_MgII \propto M_star^{0.28}\times sSFR^{0.11}. The
strong dependence of R_MgII on M_star is most naturally explained, if more
massive galaxies possess more extended halos of cool gas and the observed MgII
absorbers arise in infalling clouds which will subsequently fuel star formation
in the galaxies. The additional scaling relation of R_MgII with sSFR can be
understood either as accounting for extra gas supplies due to starburst
outflows or as correcting for suppressed cool gas content in high-mass halos.
The latter is motivated by the well-known sSFR--M_star} inverse correlation in
field galaxies. Our analysis shows that a joint study of galaxies and MgII
absorbers along common sightlines provides an empirical characterization of
halo gaseous radius versus halo mass. A comparison study of R_MgII around red-
and blue-sequence galaxies may provide the first empirical constraint for
resolving the physical origin of the observed sSFR--M_star} relation in
galaxies.Comment: 6 pages, 3 figures; ApJL in pres
First results from the IllustrisTNG simulations: the galaxy color bimodality
We introduce the first two simulations of the IllustrisTNG project, a next
generation of cosmological magnetohydrodynamical simulations, focusing on the
optical colors of galaxies. We explore TNG100, a rerun of the original
Illustris box, and TNG300, which includes 2x2500^3 resolution elements in a
volume twenty times larger. Here we present first results on the galaxy color
bimodality at low redshift. Accounting for the attenuation of stellar light by
dust, we compare the simulated (g-r) colors of 10^9 < M*/Msun < 10^12.5
galaxies to the observed distribution from the Sloan Digital Sky Survey (SDSS).
We find a striking improvement with respect to the original Illustris
simulation, as well as excellent quantitative agreement in comparison to the
observations, with a sharp transition in median color from blue to red at a
characteristic M* ~ 10^10.5 Msun. Investigating the build-up of the color-mass
plane and the formation of the red sequence, we demonstrate that the primary
driver of galaxy color transition in the TNG model is supermassive blackhole
feedback in its low-accretion state. Across the entire population we measure a
median color transition timescale dt_green of ~1.6 Gyr, a value which drops for
increasingly massive galaxies. We find signatures of the physical process of
quenching: at fixed stellar mass, redder galaxies have lower SFRs, gas
fractions, and gas metallicities; their stellar populations are also older and
their large-scale interstellar magnetic fields weaker than in bluer galaxies.
Finally, we measure the amount of stellar mass growth on the red sequence.
Galaxies with M* > 10^11 Msun which redden at z<1 accumulate on average ~25% of
their final z=0 mass post-reddening; at the same time, ~18% of such massive
galaxies acquire half or more of their final stellar mass while on the red
sequence.Comment: The IllustrisTNG project website is http://www.tng-project.or
Feedback and metal enrichment in cosmological SPH simulations I. A model for chemical enrichment
We discuss a model for treating chemical enrichment by SNII and SNIa
explosions in simulations of cosmological structure formation. Our model
includes metal-dependent radiative cooling and star formation in dense
collapsed gas clumps. Metals are returned into the diffuse interstellar medium
by star particles using a local SPH smoothing kernel. A variety of chemical
abundance patterns in enriched gas arise in our treatment owing to the
different yields and lifetimes of SNII and SNIa progenitor stars. In the case
of SNII chemical production, we adopt metal-dependent yields. Because of the
sensitive dependence of cooling rates on metallicity, enrichment of galactic
haloes with metals can in principle significantly alter subsequent gas infall
and the build up of the stellar components. Indeed, in simulations of isolated
galaxies we find that a consistent treatment of metal-dependent cooling
produces 25% more stars outside the central region than simulations with a
primordial cooling function. In the highly-enriched central regions, the
evolution of baryons is however not affected by metal cooling, because here the
gas is always dense enough to cool. A similar situation is found in
cosmological simulations because we include no strong feedback processes which
could spread metals over large distances and mix them into unenriched diffuse
gas. We demonstrate this explicitly with test simulations which adopt
super-solar cooling functions leading to large changes both in the stellar mass
and in the metal distributions. We also find that the impact of metallicity on
the star formation histories of galaxies may depend on their particular
evolutionary history. Our results hence emphasise the importance of feedback
processes for interpreting the cosmic metal enrichment.Comment: 15 pages, 15 figures, MNRAS, modified to match published versio
The Arecibo Galaxy Environment Survey: II. A HI view of the Abell cluster 1367 and its outskirts
We present 21 cm HI line observations of 5x1 square degrees centered on the
local Abell cluster 1367 obtained as part of the Arecibo Galaxy Environment
Survey. One hundred sources are detected (79 new HI measurements and 50 new
redshifts), more than half belonging to the cluster core and its infalling
region. Combining the HI data with SDSS optical imaging we show that our HI
selected sample follows scaling relations similar to the ones usually observed
in optically selected samples. Interestingly all galaxies in our sample appear
to have nearly the same baryon fraction independently of their size, surface
brightness and luminosity. The most striking difference between HI and
optically selected samples resides in their large scale distribution: whereas
optical and X-ray observations trace the cluster core very well, in HI there is
almost no evidence of the presence of the cluster. Some implications on the
determination of the cluster luminosity function and HI distribution for
samples selected at different wavelength are also discussed.Comment: 22 pages, 15 figures, 4 tables. Accepted for publication on MNRAS
Main Journal. High resolution version of this paper can be downloaded at
http://www.astro.cf.ac.uk/pub/Luca.Cortese/papers/ages_a1367.pdf . Datacubes
and catalogs can be downloaded at http://www.naic.edu/~ages/public_data.htm
Dwarf galaxy populations in present-day galaxy clusters: I. Abundances and red fractions
We compare the galaxy population in the Virgo, Fornax, Coma and Perseus
cluster to a state-of-the-art semi-analytic model, focusing on the regime of
dwarf galaxies with luminosities from approximately 10^8 L_sun to 10^9 L_sun.
We find that the number density profiles of dwarfs in observed clusters are
reproduced reasonably well, and that the red fractions of model clusters
provide a good match to Coma and Perseus. On the other hand, the red fraction
among dwarf galaxies in Virgo is clearly lower than in model clusters. We argue
that this is mainly caused by the treatment of environmental effects in the
model. This explanation is supported by our finding that the colours of central
("field") dwarf galaxies are reproduced well, in contrast to previous claims.
Finally, we find that the dwarf-to-giant ratio in model clusters is too high.
This may indicate that the current model prescription for tidal disruption of
faint galaxies is still not efficient enough.Comment: 20 pages, 10 figures. Accepted by MNRAS. Includes the modifications
after referee report. Main results unchanged, interpretation slightly change
A complete spectroscopic catalogue of local galaxies in the Northern spring sky -- Gas properties and nuclear activity in different environments
With the aim of providing the complete demography of galaxies in the local
Universe, including their nuclear properties, we present SPRING, a complete
census of local galaxies limited to the spring quarter of the Northern sky
(10h< RA <16h; 0< Dec <65). The SPRING catalogue is a flux- and volume-limited
sample (r < 17.7 mag, cz < 10000 km/s) of 30597 galaxies, including the Virgo,
Coma and A1367 clusters. To inspect possible secular and environmental
dependencies of the various nuclear excitation properties (SF vs. AGN), we
perform a multidimensional analysis by dividing the sample according to (i)
their position in the (NUV-i) vs. M* diagram,(ii) local galaxy density, (iii)
stellar-mass, (iv) halo-mass of the group to which galaxies belong, and (v)
neutral Hydrogen content. We present a new calibration of the optical
diameter-based HI-deficiency parameter employing a reference sample of isolated
galaxies. At intermediate distances between Virgo and Coma, we identify a
ring-like structure of galaxies constituted by three large filaments. The
fraction of HI-deficient galaxies within the filament suggests that filaments
are a transitioning environment between field and cluster in terms of HI
content. We classify the nuclear spectra according to the four-line BPT and the
two-line WHAN diagrams, and investigate the variation in the fraction of AGN
with stellar-mass, as well as their colours and environments. In general, we
observe that the mass-dependency of the fraction of Seyfert nuclei is little
sensitive to the environment, whereas the fraction of star-forming nuclei is a
steeper function of M* in lower-density environments and in blue-cloud
galaxies. We find that the fraction of LINERs depends on galaxy colour and, for
logM* > 9.5-10, increases in galaxies belonging to the green valley.Comment: 26 pages, 22 figures. Accepted for publication in Astronomy &
Astrophysic
ADVANCED RANDOM TIME QUEUE BLOCKING WITH TRAFFIC PREDICTION FOR DEFENSE OF LOW-RATE DOS ATTACKS AGAINST APPLICATION SERVERS
Among many strategies of Denial of Services, low-rate traffic denial-of-service (DoS) attacks are more significant. This strategy denies the services of a network by detection of the vulnerabilities in performance of the application. In this research, an efficient defence methodology is developed against low-rate DoS attack in the application servers. Though, the Improved Random Time Queue Blocking (IRTQB) technique can eliminate the vulnerabilities in the network and also avoiding the attacker from capturing all the server queue positions by defining a spatial similarity metric (SSM). However, the differentiation of the attack requests from the legitimate users’ is not always efficient since only the source IP addresses and the record timestamp are considered in the SSM. It was improved by using Advanced Random Time Queue Blocking (ARTQB) scheme that employed Bandwidth utilization of attacker in IRTQB to detect the DoS attack that normally consumes a huge number of resources of the server. However, this method becomes ineffective when the attack consumes more network traffic. In this paper, an efficient detection technique called Advanced Random Time Queue Blocking with Traffic Prediction (ARTQB-TP) is proposed for defining SSM which contains, Source IP, timestamp, Bandwidth between two requests and the difference between the attack traffic and legitimate traffic. The ARTQB-TP technique is utilized to reduce the attack efficiency in 18 different server configurations which are more vulnerable to the DoS attacks and where the attacks may also have a chance to improve its effectiveness. Experimental results show that the proposed system performs better protection of application servers against the LRDoS attacks by solving its impacts on any kind of server architectures and reduced the attack efficiencies of all the types of attack strategies
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