337 research outputs found
The evolution of the star formation activity per halo mass up to redshift ~1.6 as seen by Herschel
Aims. Star formation in massive galaxies is quenched at some point during hierarchical mass assembly. To understand where and when the quenching processes takes place, we study the evolution of the total star formation rate per unit total halo mass (ÎŁ(SFR)/M) in three different mass scales: low mass halos (field galaxies), groups, and clusters, up to a redshift z â 1.6.
Methods. We use deep far-infrared PACS data at 100 and 160 ÎŒmâto accurately estimate the total star formation rate of the luminous infrared galaxy population of 9 clusters with mass ~10^(15) M_â, and 9 groups/poor clusters with mass ~5 Ă 10^(13) M_â. Estimates of the field ÎŁ(SFR)/Mâare derived from the literature, by dividing the star formation rate density by the mean comoving matter density of the universe.
Results. The field ÎŁ(SFR)/Mâincreases with redshift up to z ~ 1 and it is constant thereafter. The evolution of the ÎŁ(SFR)/Mâââz relation in galaxy systems is much faster than in the field. Up to redshift z ~ 0.2, the field has a higher ÎŁ(SFR)/Mâthan galaxy groups and galaxy clusters. At higher redshifts, galaxy groups and the field have similar ÎŁ(SFR)/M, while massive clusters have significantly lower ÎŁ(SFR)/Mâthan both groups and the field. There is a hint of a reversal of the SFR activity vs. environment at z ~ 1.6, where the group ÎŁ(SFR)/Mâlies above the field ÎŁ(SFR)/M â z relation. We discuss possible interpretations of our results in terms of the processes of downsizing, and star-formation quenching
The RASS-SDSS galaxy cluster survey
Solid observational evidences indicate a strong dependence of the galaxy
formation and evolution on the environment. In order to study in particular the
interaction between the intracluster medium and the evolution of cluster
galaxies, we have created a large database of clusters of galaxies based on the
largest available X-ray and optical surveys: the ROSAT All Sky Survey (RASS),
and the Sloan Digital Sky Survey (SDSS). We analyzed the correlation between
the total optical and the X-ray cluster luminosity. The resulting correlation
of L_X and L_{op} shows a logarithmic slope of 0.6, a value close to the
self-similar correlation. We analysed also the cluster mass to light ratio, by
finding a significant dependence of M/L on the cluster mass with a logarithmic
slope ranging from 0.27 in the i and r bands to 0.22 in the z band.Comment: proceedings of 'Multiwavelength mapping of galaxy evolution'
conference held in Venice (Italy), October 2003, A. Renzini and R. Bender
(Eds.), 2 pages, 1 figur
Bulges and disks in the local Universe. Linking the galaxy structure to star formation activity
We use a sample built on the SDSS DR7 catalogue and the bulge-disc
decomposition of Simard et al. (2011) to study how the bulge and disc
components contribute to the parent galaxy's star formation activity, by
determining its position in the star formation rate (SFR) - stellar mass
(M) plane at 0.020.1. We use the bulge and disc colours as proxy
for their SFRs. We study the mean galaxy bulge-total mass ratio (B/T) as a
function of the residual from the MS () and find that the
B/T- relation exhibits a parabola-like shape with the peak of the
MS corresponding to the lowest B/Ts at any stellar mass. The lower and upper
envelop of the MS are populated by galaxies with similar B/T, velocity
dispersion and concentration () values. Bulges above the MS are
characterised by blue colours or, when red, by a high level of dust
obscuration, thus indicating that in both cases they are actively star forming.
When on the MS or below it, bulges are mostly red and dead. At stellar masses
above M, bulges on the MS or in the green valley tend to
be significantly redder than their counterparts in the quiescence region,
despite similar levels of dust obscuration. The disc color anti-correlates at
any mass with the distance from the MS, getting redder when approaching the MS
lower envelope and the quiescence region. We conclude that the position of a
galaxy in the LogSFR-LogM plane depends on the star formation
activity of its components: above the MS both bulge and disk are actively star
forming. The nuclear activity is the first to be suppressed, moving the
galaxies on the MS. Once the disk stops forming stars as well, the galaxy moves
below the MS and eventually to the quiescence region. This is confirmed by a
large fraction () of passive galaxies with a secure two component
morphology.Comment: Version modified after referee comment
RASS-SDSS Galaxy Cluster Survey. VII. On the Cluster Mass to Light ratio and the Halo Occupation Distribution
We explore the mass-to-light ratio in galaxy clusters and its relation to the
cluster mass. We study the relations among the optical luminosity (),
the cluster mass () and the number of cluster galaxies within
() in a sample of 217 galaxy clusters with confirmed 3D
overdensity. We correct for projection effects, by determining the galaxy
surface number density profile in our cluster sample. This is best fitted by a
cored King profile in low and intermediate mass systems. The core radius
decreases with cluster mass, and, for the highest mass clusters, the profile is
better represented by a generalized King profile or a cuspy Navarro, Frenk &
White profile. We find a very tight proportionality between and
, which, in turn, links the cluster mass-to-light ratio to the Halo
Occupation Distribution vs. . After correcting for
projection effects, the slope of the and
relations is found to be , close, but still significantly less
than unity. We show that the non-linearity of these relations cannot be
explained by variations of the galaxy luminosity distributions and of the
galaxy M/L with the cluster mass. We suggest that the nonlinear relation
between number of galaxies and cluster mass reflects an underlying nonlinear
relation between number of subhaloes and halo mass.Comment: 15 pages, 15 figures, accepted for publication in A&
Two-Face(s): ionized and neutral gas winds in the local Universe
We present a comprehensive study of the Na I 5890, 5895 (Na I D)
resonant lines in the Sloan Digital Sky Survey (SDSS, DR7) spectroscopic sample
to look for neutral gas outflows in the local galaxies. Individual galaxy
spectra are stacked in bins of M and SFR to investigate the dependence
of galactic wind occurrence and velocity as a function of the galaxy position
in the SFR- plane. In massive galaxies at the high SFR tail we find
evidence of a significant blue-shifted Na I D absorption, which we interpret as
evidence of neutral outflowing gas. The occurrence of the blue-shifted
absorption is observed at the same significance for purely SF galaxies, AGN and
composite systems at fixed SFR. In all classes of objects the blue-shift is the
largest and the Na I D equivalent width the smallest for face-on galaxies while
the absorption feature is at the systemic velocity for edge-on systems. This
indicates that the neutral outflow is mostly perpendicular or biconical with
respect to the galactic disk. We also compare the kinematics of the neutral gas
with the ionized gas phase as traced by the [OIII]5007, H,
[NII] and [NII] emission lines. Differently for the
neutral gas phase, all the emission lines show evidence of perturbed kinematics
only in galaxies with a significant level of nuclear activity and, they are
independent from the disk inclination. In conclusion, we find that, in the
local Universe, galactic winds show two faces which are related to two
different ejection mechanisms, namely the neutral outflowing gas phase related
to the SF activity along the galaxy disk and the ionized phase related to the
AGN feedback. In both the neutral and ionized gas phases, the observed wind
velocities suggest that the outflowing gas remains bound to the galaxy with no
definitive effect on the gas reservoir.Comment: Accepted to A&A, 13 pages, 9 figure
RASS-SDSS Galaxy Cluster Survey. VI. The dependence of the cluster SFR on the cluster global properties
Using a subsample of 79 nearby clusters from the RASS-SDSS galaxy cluster
catalogue of Popesso et al. (2005a), we perform a regression analysis between
the cluster integrated star formation rate (Sigma_SFR) the cluster total
stellar mass (M_star), the fractions of star forming (f_SF) and blue (f_b)
galaxies and other cluster global properties, namely its richness (N_gal, i.e.
the total number of cluster members within the cluster virial radius), velocity
dispersion (sigma_v), virial mass (M_200), and X-ray luminosity (L_X). All
cluster global quantities are corrected for projection effects before the
analysis. Galaxy SFRs and stellar masses are taken from the catalog of
Brinchmann et al. (2004), which is based on SDSS spectra. We only consider
galaxies with M_r <= -20.25 in our analysis, and exclude AGNs. We find that
both Sigma_SFR and M_star are correlated with all the cluster global
quantities. A partial correlation analysis show that all the correlations are
induced by the fundamental one between Sigma_SFR and N_gal, hence there is no
evidence that the cluster properties affect the mean SFR or M_star per galaxy.
The relations between Sigma_SFR and M_star, on one side, and both N_gal and
M_200, on the other side, are linear, i.e. we see no evidence that different
clusters have different SFR or different M_star per galaxy and per unit mass.
The fraction f_SF does not depend on any cluster property considered, while f_b
does depend on L_X. We note that a significant fraction of star-forming cluster
galaxies are red (~25% of the whole cluster galaxy population). We conclude
that the global cluster properties are unable to affect the SF properties of
cluster galaxies, but the presence of the X-ray luminous intra-cluster medium
can affect their colors, perhaps through the ram-pressure stripping mechanism.Comment: 14 pages, 12 figures, accepted for publication on A&A; corrected
coefficient in Tab.
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
The environmental properties of radio-emitting AGN
We study the environmental properties of z<1.2 radio-selected AGN belonging
to the ~2 square degrees of the COSMOS field, finding that about 20% of them
appear within overdense structures. AGN with are twice more likely to be found in clusters with respect to fainter
sources (~38% vs ~15%), just as radio-selected AGN with stellar masses
are twice more likely to be found in overdense
environments with respect to objects of lower mass (~24% vs ~11%). Comparisons
with galaxy samples further suggest that radio-selected AGN of large stellar
mass tend to avoid underdense environments more than normal galaxies with the
same stellar content. Stellar masses also seem to determine the location of
radio-active AGN within clusters: ~100% of the sources found as satellite
galaxies have , while ~100% of the AGN coinciding with a
cluster central galaxy have . No different location within
the cluster is instead observed for AGN of various radio luminosities. Radio
AGN which also emit in the MIR show a marked preference to be found as isolated
galaxies (~70%) at variance with those also active in the X-ray which all seem
to reside within overdensities. What emerges from our work is a scenario
whereby physical processes on sub-pc and kpc scales (e.g. emission respectively
related to the AGN and to star formation) are strongly interconnected with the
large-scale environment of the AGN itself.Comment: 8 pages, 6 figures, to appear on MNRA
Ways of knowing of farmers and scientists: tree and soil management in the Ethiopian Highlands
The Ethiopian Highlands have been studied extensively, hosting a large amount of research for development projects in agriculture and forestry over several decades. The encounters in these projects were also encounters of different ways of knowing that were negotiated by the actors meeting in the space provided by the projects. This research explores these encounters and the social worlds they are embedded in, drawing on actor-oriented approaches as well as theories of narratives and framing. Ways of knowing and citizen epistemologies are taken as a lens to understand the role of identities in knowledge production and use.
The two case studies were agroforestry research projects in the Ethiopian Highlands. The research followed a range of qualitative and ethnographic research methods. Different types of farmers and scientists meet in the case studies. I recognise that they all have individual agency, nevertheless I use the terms âscientistâ and âfarmerâ in this thesis. I use the terms to describe certain groups of actors who all draw on different ways of knowing, and different value systems, when interacting with each other and their environment.
The results indicate that the importance of social worlds at different scales and the contexts of research projects tend to be underestimated. In spite of good intentions scientific methodologies, terminologies and narratives tend to dominate. Scientists in the case studies acknowledged the existence of farmersâ âindigenousâ knowledge, but they determined the value of knowledge by its scientific applicability and the replicability of experiments. Research systems force the scientists into a certain modus operandi with limited possibilities to experiment and to respond to the complexities and diversities of people's social worlds.
Farmers in the case studies preferred observation from their parents, observing from others or the environment as a way of learning and gaining knowledge. Depending on their personalities and their life histories they also relied on alternative ways of knowing rooted in spirituality, emotions and memories. Powerful influences on ways of knowing resulted from the way languages and authority had been used. These often led to the exclusion of marginalised community members from access to knowledge and technologies.
Unfortunately, common narratives prevailed in the case studies, and alternative ways of knowing were often marginalised. By acknowledging different ways of knowing and the importance of different social worlds and different ways of doing research, both scientists and farmers could benefit and develop more sustainable pathways for agricultural and forestry land use
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