396 research outputs found
Massive spheroids can form in single minor mergers
Accepted for publication in MNRAS, 12 pages, 6 figuresUnderstanding how rotationally supported discs transform into dispersion-dominated spheroids is central to our comprehension of galaxy evolution. Morphological transformation is largely merger-driven. While major mergers can efficiently create spheroids, recent work has highlighted the significant role of other processes, like minor mergers, in driving morphological change. Given their rich merger histories, spheroids typically exhibit large fractions of âex situâ stellar mass, i.e. mass that is accreted, via mergers, from external objects. This is particularly true for the most massive galaxies, whose stellar masses typically cannot be attained without a large number of mergers. Here, we explore an unusual population of extremely massive (M â > 10 11M) spheroids, in the Horizon-AGN simulation, which exhibit anomalously low ex situ mass fractions, indicating that they form without recourse to significant merging. These systems form in a single minor-merger event (with typical merger mass ratios of 0.11â0.33), with a specific orbital configuration, where the satellite orbit is virtually co-planar with the disc of the massive galaxy. The merger triggers a catastrophic change in morphology, over only a few hundred Myr, coupled with strong in situ star formation. While this channel produces a minority (âŒ5 per cent) of such galaxies, our study demonstrates that the formation of at least some of the most massive spheroids need not involve major mergers â or any significant merging at all â contrary to what is classically believed.Peer reviewedFinal Accepted Versio
The COSMOS2015 Catalog: Exploring the 1 < z < 6 Universe with Half a Million Galaxies
We present the COSMOS2015 catalog, which contains precise photometric redshifts and stellar masses for more than half a million objects over the 2deg^2 COSMOS field. Including new YJHK_s images from the UltraVISTA-DR2 survey, Y-band images from Subaru/Hyper-Suprime-Cam, and infrared data from the Spitzer Large Area Survey with the Hyper-Suprime-Cam Spitzer legacy program, this near-infrared-selected catalog is highly optimized for the study of galaxy evolution and environments in the early universe. To maximize catalog completeness for bluer objects and at higher redshifts, objects have been detected on a Ï^2 sum of the YJHK_s and z^(++) images. The catalog contains ~6 x 10^5 objects in the 1.5 deg^2 UltraVISTA-DR2 region and ~ 1.5 x 10^5 objects are detected in the "ultra-deep stripes" (0.62 deg^2) at K_s â©œ 24.7 (3Ï, 3", AB magnitude). Through a comparison with the zCOSMOS-bright spectroscopic redshifts, we measure a photometric redshift precision of Ï_Îz/(1+_s) = 0.007 and a catastrophic failure fraction of η = 0.5%. At 3 < z < 6, using the unique database of spectroscopic redshifts in COSMOS, we find Ï_Îz/(1+z_s) = 0.021 and η = 13.2%. The deepest regions reach a 90% completeness limit of 10^10 M_â to z = 4. Detailed comparisons of the color distributions, number counts, and clustering show excellent agreement with the literature in the same mass ranges. COSMOS2015 represents a unique, publicly available, valuable resource with which to investigate the evolution of galaxies within their environment back to the earliest stages of the history of the universe. The COSMOS2015 catalog is distributed via anonymous ftp and through the usual astronomical archive systems (CDS, ESO Phase 3, IRSA)
The clustering properties of radio-selected AGN and star-forming galaxies up to redshifts z~3
We present the clustering properties of a complete sample of 968 radio
sources detected at 1.4 GHz by the VLA-COSMOS survey with radio fluxes brighter
than 0.15 mJy. 92% have redshift determinations from the Laigle et al. (2016)
catalogue. Based on their radio-luminosity, these objects have been divided
into two populations of 644 AGN and 247 star-forming galaxies. By fixing the
slope of the auto-correlation function to gamma=2, we find
r_0=11.7^{+1.0}_{-1.1} Mpc for the clustering length of the whole sample, while
r_0=11.2^{+2.5}_{-3.3} Mpc and r_0=7.8^{+1.6}_{-2.1} Mpc (r_0=6.8^{+1.4}_{-1.8}
Mpc if we restrict our analysis to z<0.9) are respectively obtained for AGN and
star-forming galaxies. These values correspond to minimum masses for dark
matter haloes of M_min=10^[13.6^{+0.3}_{-0.6}] M_sun for radio-selected AGN and
M_min=10^[13.1^{+0.4}_{-1.6}] M_sun for radio-emitting star-forming galaxies
(M_min=10^[12.7^{+0.7}_{-2.2}] M_sun for z<0.9). Comparisons with previous
works imply an independence of the clustering properties of the AGN population
with respect to both radio luminosity and redshift. We also investigate the
relationship between dark and luminous matter in both populations. We obtain
/M_halo/M_halo<~10^{-2.4} in the case of
star-forming galaxies. Furthermore, if we restrict to z<~0.9 star-forming
galaxies, we derive /M_halo<~10^{-2.1}, result which clearly indicates the
cosmic process of stellar build-up as one moves towards the more local
universe. Comparisons between the observed space density of radio-selected AGN
and that of dark matter haloes shows that about one in two haloes is associated
with a black hole in its radio-active phase. This suggests that the
radio-active phase is a recurrent phenomenon.Comment: 11 pages, 7 figures, minor changes to match published version on
MNRA
Unravelling the consequences of ultra-fine milling on physical and chemical characteristics of flax fibres
In recent years, lignocellulosic biomass has been increasingly used in various applications, often replacing petro-sourced materials. While for many of these applications the plant materials require coarse milling, some new applications for green chemistry, bio-energy and bio-packaging necessitate comminution to obtain very finely calibrated particles (below 200 m in size). This milling step is not inconsequential for lignocellulosic materials and can influence the physical (size, shape) and chemical characteristics (cellulose crystallinity, composition) of the powder. However, these different effects are still poorly understood. In this work, we study and elucidate the impact of intense and ultra-fine milling on the physico-chemical properties of plant fibres. Flax was chosen for this study because of its well-described hierarchical structure and biochemical composition in literature, making it a model material. Our main results evidence a strong impact of 0 to 23hrs ball milling on flax fibre morphology, especially on fibre aspect ratio falling from 20 to 5 but also on cell wall ultrastructure and composition. Cellulose content and crystallinity significantly decrease with milling time, leading to higher water sorption and lower thermal stability.The authors also thank the French national research Network âGDR 3710 INRA/ CNRS SYMBIOSE â Synthons et matĂ©riaux biosourcĂ©sâ, for its financial suppor
Spitzer bright, UltraVISTA faint sources in COSMOS: the contribution to the overall population of massive galaxies at z=3-7
We have analysed a sample of 574 Spitzer 4.5 micron-selected galaxies with
[4.5]24 (AB) over the UltraVISTA ultra-deep COSMOS field. Our
aim is to investigate whether these mid-IR bright, near-IR faint sources
contribute significantly to the overall population of massive galaxies at
redshifts z>=3. By performing a spectral energy distribution (SED) analysis
using up to 30 photometric bands, we have determined that the redshift
distribution of our sample peaks at redshifts z~2.5-3.0, and ~32% of the
galaxies lie at z>=3. We have studied the contribution of these sources to the
galaxy stellar mass function (GSMF) at high redshifts. We found that the
[4.5]24 galaxies produce a negligible change to the GSMF
previously determined for Ks_auto<24 sources at 3=<z<4, but their contribution
is more important at 4=~50% of the galaxies with stellar
masses Mst>~6 x 10^10 Msun. We also constrained the GSMF at the highest-mass
end (Mst>~2 x 10^11 Msun) at z>=5. From their presence at 5=<z<6, and virtual
absence at higher redshifts, we can pinpoint quite precisely the moment of
appearance of the first most massive galaxies as taking place in the ~0.2 Gyr
of elapsed time between z~6 and z~5. Alternatively, if very massive galaxies
existed earlier in cosmic time, they should have been significantly
dust-obscured to lie beyond the detection limits of current, large-area, deep
near-IR surveys.Comment: 18 pages, 15 figures, 4 tables. Updated to match version in press at
the Ap
Deletion of TRAAK Potassium Channel Affects Brain Metabolism and Protects against Ischemia
Cerebral stroke is a worldwide leading cause of disability. The two-pore domain K(+) channels identified as background channels are involved in many functions in brain under physiological and pathological conditions. We addressed the hypothesis that TRAAK, a mechano-gated and lipid-sensitive two-pore domain K(+) channel, is involved in the pathophysiology of brain ischemia. We studied the effects of TRAAK deletion on brain morphology and metabolism under physiological conditions, and during temporary focal cerebral ischemia in Traak(-/-) mice using a combination of in vivo magnetic resonance imaging (MRI) techniques and multinuclear magnetic resonance spectroscopy (MRS) methods. We provide the first in vivo evidence establishing a link between TRAAK and neurometabolism. Under physiological conditions, Traak(-/-) mice showed a particular metabolic phenotype characterized by higher levels of taurine and myo-inositol than Traak(+/+) mice. Upon ischemia, Traak(-/-) mice had a smaller infarcted volume, with lower contribution of cellular edema than Traak(+/+) mice. Moreover, brain microcirculation was less damaged, and brain metabolism and pH were preserved. Our results show that expression of TRAAK strongly influences tissue levels of organic osmolytes. Traak(-/-) mice resilience to cellular edema under ischemia appears related to their physiologically high levels of myo-inositol and of taurine, an aminoacid involved in the modulation of mitochondrial activity and cell death. The beneficial effects of TRAAK deletion designate this channel as a promising pharmacological target for the treatment against stroke
A Coherent Study of Emission Lines from Broad-Band Photometry: Specific Star-Formation Rates and [OIII]/H{\beta} Ratio at 3 < z < 6
We measure the H{\alpha} and [OIII] emission line properties as well as
specific star-formation rates (sSFR) of spectroscopically confirmed 3<z<6
galaxies in COSMOS from their observed colors vs. redshift evolution. Our model
describes consistently the ensemble of galaxies including intrinsic properties
(age, metallicity, star-formation history), dust-attenuation, and optical
emission lines. We forward-model the measured H{\alpha} equivalent-widths (EW)
to obtain the sSFR out to z~6 without stellar mass fitting. We find a strongly
increasing rest-frame H{\alpha} EW that is flattening off above z~2.5 with
average EWs of 300-600A at z~6. The sSFR is increasing proportional to
(1+z)^2.4 at z<2.2 and (1+z)^1.5 at higher redshifts, indicative of a fast mass
build-up in high-z galaxies within e-folding times of 100-200Myr at z~6. The
redshift evolution at z>3 cannot be fully explained in a picture of cold
accretion driven growth. We find a progressively increasing
[OIII]{\lambda}5007/H{\beta} ratio out to z~6, consistent with the ratios in
local galaxies selected by increasing H{\alpha} EW (i.e., sSFR). This
demonstrates the potential of using "local high-z analogs" to investigate the
spectroscopic properties and relations of galaxies in the re-ionization epoch.Comment: 18 pages, 11 figures, 3 table
Kinematic unrest of low mass galaxy groups
In an effort to better understand the formation of galaxy groups, we examine the kinematics of a large sample of spectroscopically confirmed X-ray galaxy groups in the Cosmic Evolution Survey with a high sampling of galaxy group members up to z & x2004;=& x2004;1. We compare our results with predictions from the cosmological hydrodynamical simulation of HORIZON-AGN. Using a phase-space analysis of dynamics of groups with halo masses of M-200c & x2004;similar to & x2004;10(12.6) - 10(14.50)& x2006;M-circle dot, we show that the brightest group galaxies (BGG) in low mass galaxy groups (M-200c & x2004;<& x2004;2 x 10(13)& x2006;M-circle dot) have larger proper motions relative to the group velocity dispersion than high mass groups. The dispersion in the ratio of the BGG proper velocity to the velocity dispersion of the group, sigma(BGG)/sigma(group), is on average 1.48 +/- 0.13 for low mass groups and 1.01 +/- 0.09 for high mass groups. A comparative analysis of the HORIZON-AGN simulation reveals a similar increase in the spread of peculiar velocities of BGGs with decreasing group mass, though consistency in the amplitude, shape, and mode of the BGG peculiar velocity distribution is only achieved for high mass groups. The groups hosting a BGG with a large peculiar velocity are more likely to be offset from the L-x - sigma(v) relation; this is probably because the peculiar motion of the BGG is influenced by the accretion of new members.Peer reviewe
Rest-UV Absorption Lines as Metallicity Estimator: the Metal Content of Star-Forming Galaxies at z~5
We measure a relation between the depth of four prominent rest-UV absorption
complexes and metallicity for local galaxies and verify it up to z~3. We then
apply this relation to a sample of 224 galaxies at 3.5 = 4.8) in
COSMOS, for which unique UV spectra from DEIMOS and accurate stellar masses
from SPLASH are available. The average galaxy population at z~5 and log(M/Msun)
> 9 is characterized by 0.3-0.4 dex (in units of 12+log(O/H)) lower
metallicities than at z~2, but comparable to z~3.5. We find galaxies with
weak/no Ly-alpha emission to have metallicities comparable to z~2 galaxies and
therefore may represent an evolved sub-population of z~5 galaxies. We find a
correlation between metallicity and dust in good agreement with local galaxies
and an inverse trend between metallicity and star-formation rate (SFR)
consistent with observations at z~2. The relation between stellar mass and
metallicity (MZ relation) is similar to z~3.5, however, there are indications
of it being slightly shallower, in particular for the young, Ly-alpha emitting
galaxies. We show that, within a "bathtub" approach, a shallower MZ relation is
expected in the case of a fast (exponential) build-up of stellar mass with an
e-folding time of 100-200 Myr. Due to this fast evolution, the process of dust
production and metal enrichment as a function of mass could be more stochastic
in the first billion years of galaxy formation compared to later times.Comment: 20 pages, 13 figures, 4 tables; Submitted to Ap
Star-forming galaxies versus low- and high-excitation radio AGN in the VLA-COSMOS 3GHz Large Project
We study the composition of the faint radio population selected from the
VLA-COSMOS 3GHz Large Project, a radio continuum survey performed at 10 cm
wavelength. The survey covers the full 2 square degree COSMOS field with mean
Jy/beam, cataloging 10,899 source components above . By combining these radio data with UltraVISTA, optical, near-infrared,
and Spitzer/IRAC mid-infrared data, as well as X-ray data from the Chandra
Legacy, and Chandra COSMOS surveys, we gain insight into the emission
mechanisms within our radio sources out to redshifts of . From these
emission characteristics we classify our souces as star forming galaxies or
AGN. Using their multi-wavelength properties we further separate the AGN into
sub-samples dominated by radiatively efficient and inefficient AGN, often
referred to as high- and low-excitation emission line AGN. We compare our
method with other results based on fitting of the sources' spectral energy
distributions using both galaxy and AGN spectral models, and those based on the
infrared-radio correlation. We study the fractional contributions of these
sub-populations down to radio flux levels of 10 Jy. We find that at
3 GHz flux densities above 400 Jy quiescent, red galaxies,
consistent with the low-excitation radio AGN class constitute the dominant
fraction. Below densities of 200 Jy star-forming galaxies begin to
constitute the largest fraction, followed by the low-excitation, and X-ray- and
IR-identified high-excitation radio AGN.Comment: 7 pages, 3 figures, The many facets of extragalactic radio surveys:
towards new scientific challenges, Bologna 20-23 October 201
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