313 research outputs found
Star formation properties of sub-mJy radio sources
We investigate the star formation properties of ~800 sources detected in one
of the deepest radio surveys at 1.4 GHz. Our sample spans a wide redshift range
(~0.1 - 4) and about four orders of magnitude in star formation rate (SFR). It
includes both star forming galaxies (SFGs) and active galactic nuclei (AGNs),
further divided into radio-quiet and radio-loud objects. We compare the SFR
derived from the far infrared luminosity, as traced by Herschel, with the SFR
computed from their radio emission. We find that the radio power is a good SFR
tracer not only for pure SFGs but also in the host galaxies of RQ AGNs, with no
significant deviation with redshift or specific SFR. Moreover, we quantify the
contribution of the starburst activity in the SFGs population and the
occurrence of AGNs in sources with different level of star formation. Finally
we discuss the possibility of using deep radio survey as a tool to study the
cosmic star formation history.Comment: 18 pages, 14 figures, 1 table (available in its entirety as ancillary
data
Comparative evaluation of synergy of combinations of \u3b2-lactams with fluoroquinolones or macrolides in Streptococcus pneumoniae
OBJECTIVES: Streptococcus pneumoniae has shown a great ability to develop
efficacious mechanisms of resistance to the main drugs for the treatment of
pneumonia, such as \u3b2-lactams, macrolides and fluoroquinolones. The present study
aimed to compare the antipneumococcal activity of combinations of respiratory
fluoroquinolones with cephalosporins (either parenteral or oral) or protected
penicillin versus the standard combinations (i.e. a macrolide with a protected
penicillin or cephalosporin) against 100 isolates with different susceptibilities
to macrolides and/or penicillin.
METHODS: Chequerboard assays for all isolates and time-kill curves for nine
isolates with different patterns of susceptibility were performed. Synergy
between antibiotics at serum peak concentrations was also determined.
RESULTS: The combination of levofloxacin with ceftriaxone produced the highest
rate of synergy (54/100), mainly against macrolide-resistant strains (22/30).
Antagonism was not observed for any tested combination apart from clarithromycin
with amoxicillin/clavulanic acid (22/100 isolates). Although the killing
activities of all antibiotics improved when they were tested in combination,
synergy was observed only for some combinations after 12 and/or 24 h. Serum
concentrations were effective in inhibiting the growth of the tested strains.
CONCLUSIONS: Combinations of levofloxacin with parenteral cephalosporins were the
most active among all the tested combinations, while antagonism occurred when
clarithromycin and amoxicillin/clavulanic acid were tested
An Eddington ratio-driven origin for the LX- Mârelation in quiescent and star-forming active galaxies
A mild correlation exists in active galaxies between the mean black hole accretion, as traced by the mean X-ray luminosity and the host galaxy stellar mass Mâ, characterised by a normalization steadily decreasing with cosmic time and lower in more quiescent galaxies. We create comprehensive semi-empirical mock catalogues of active black holes to pin down which parameters control the shape and evolution of the - Mâ relation of X-ray-detected active galaxies. We find that the normalization of the - Mâ relation is largely independent of the fraction of active galaxies (the duty cycle), but strongly dependent on the mean Eddington ratio, when adopting a constant underlying MBH - Mâ relation as suggested by observational studies. The data point to a decreasing mean Eddington ratio with cosmic time and with galaxy stellar mass at fixed redshift. Our data can be reproduced by black holes and galaxies evolving on similar MBH - Mâ relations but progressively decreasing their average Eddington ratios, mean X-ray luminosities, and specific star formation rates, when moving from the starburst to the quiescent phase. Models consistent with the observed - Mâ relation and independent measurements of the mean Eddington ratios are characterised by MBH - Mâ relations lower than those derived from dynamically measured local black holes. Our results point to the - Mâ relation as a powerful diagnostic to: (1) probe black hole-galaxy scaling relations and the level of accretion on to black holes; (2) efficiently break the degeneracies between duty cycles and accretion rates in cosmological models of black holes
The Bright and Dark Sides of High-Redshift starburst galaxies from {\it Herschel} and {\it Subaru} observations
We present rest-frame optical spectra from the FMOS-COSMOS survey of twelve
\textit{Herschel} starburst galaxies, with Star Formation Rate
(SFR) elevated by 8, on average, above the star-forming Main Sequence
(MS). Comparing the H to IR luminosity ratio and the Balmer Decrement
we find that the optically-thin regions of the sources contain on average only
percent of the total SFR whereas percent comes from an
extremely obscured component which is revealed only by far-IR observations and
is optically-thick even in H. We measure the [NII]/H
ratio, suggesting that the less obscured regions have a metal content similar
to that of the MS population at the same stellar masses and redshifts. However,
our objects appear to be metal-rich outliers from the metallicity-SFR
anticorrelation observed at fixed stellar mass for the MS population. The
[SII]/[SII] ratio from the average spectrum indicates an
electron density , larger than what
estimated for MS galaxies but only at the 1.5 level. Our results
provide supporting evidence that high- MS outliers are the analogous of
local ULIRGs, and are consistent with a major merger origin for the starburst
event.Comment: 6 pages, 4 figures, Accepted for publication in ApJ Letter
A 12um ISOCAM Survey of the ESO-Sculptor Field: Data Reduction and Analysis
We present a detailed reduction of a mid-infrared 12um (LW10 filter) ISOCAM
open time observation performed on the ESO-Sculptor Survey field (Arnouts et
al. 1997). A complete catalogue of 142 sources (120 galaxies and 22 stars),
detected with high significance (equivalent to 5sigma), is presented above an
integrated flux density of 0.24mJy. Star/galaxy separation is performed by a
detailed study of colour-colour diagrams. The catalogue is complete to 1mJy and
below this flux density the incompleteness is corrected using two independent
methods. The first method uses stars and the second uses optical counterparts
of the ISOCAM galaxies; these methods yield consistent results. We also apply
an empirical flux density calibration using stars in the field. For each star,
the 12um flux density is derived by fitting optical colours from a multi-band
chi^2 to stellar templates (BaSel-2.0) and using empirical optical-IR
colour-colour relations. This article is a companion analysis to
Rocca-Volmerange 2007 et al. where the 12um faint galaxy counts are presented
and analysed by galaxy type with the evolutionary code PEGASE.3.Comment: 12 pages, 7 figures, figure 1 modified from journal version for size,
accepted for publication in A&A, includes psfig.st
Dust temperature and CO-to-H2 conversion factor variations in the SFR-M* plane
Deep Herschel imaging and 12CO(2-1) line luminosities from the IRAM PdBI are
combined for a sample of 17 galaxies at z>1 from the GOODS-N field. The sample
includes galaxies both on and above the main sequence (MS) traced by
star-forming galaxies in the SFR-M* plane. The far-infrared data are used to
derive dust masses, Mdust. Combined with an empirical prescription for the
dependence of the gas-to-dust ratio on metallicity (GDR), the CO luminosities
and Mdust values are used to derive for each galaxy the CO-to-H2 conversion
factor, alpha_co. Like in the local Universe, the value of alpha_co is a factor
of ~5 smaller in starbursts compared to normal star-forming galaxies (SFGs). We
also uncover a relation between alpha_co and dust temperature (Tdust; alpha_co
decreasing with increasing Tdust) as obtained from modified blackbody fits to
the far-infrared data. While the absolute normalization of the alpha_co(Tdust)
relation is uncertain, the global trend is robust against possible systematic
biases in the determination of Mdust, GDR or metallicity. Although we cannot
formally distinguish between a step and a smooth evolution of alpha_co with the
dust temperature, we can conclude that in galaxies of near-solar metallicity, a
critical value of Tdust=30K can be used to determine whether the appropriate
alpha_co is closer to the starburst value (1.0 Msun(K kms pc^2)^-1, if
Tdust>30K) or closer to the Galactic value (4.35 Msun (K kms pc^2)^-1, if
Tdust<30K). This indicator has the great advantage of being less subjective
than visual morphological classifications of mergers/SFGs, which can be
difficult at high z because of the clumpy nature of SFGs. In the absence of
far-infrared data, the offset of a galaxy from the main sequence (i.e.,
log[SSFR(galaxy)/SSFR_MS(M*,z)]) can be used to identify galaxies requiring the
use of an alpha_co conversion factor lower than the Galactic value.Comment: Accepted for publication in Astronomy and Astrophysics (A&A); 15
pages, 6 figures; V2: updated reference lis
Eddington accreting black holes in the epoch of reionization
The evolution of the luminosity function (LF) of active galactic nuclei (AGNs) at redshift z âł 5 represents a key constraint to understand their contribution to the ionizing photon budget necessary to trigger the last phase transition in the Universe, i.e. the epoch of reionization. Recent searches for bright high-z AGNs suggest that the space densities of this population at z > 4 have to be revised upwards, and spark new questions about their evolutionary paths. Gas accretion is the key physical mechanism to understand both the distribution of luminous sources and the growth of central supermassive black holes (SMBHs). In this work, we model the high-z AGN-LF assuming that high-z luminous AGNs shine at their Eddington limit: We derive the expected evolution as a function of the âduty cycleâ (fdc), i.e. the fraction of lifetime that a given SMBH spends accreting at the Eddington rate. Our results show that intermediate values (fdc â 0.1) predict the best agreement with the ionizing background and photoionization rate, but do not provide enough ionizing photons to account for the observed evolution of the hydrogen neutral fraction. Smaller values (fdcâČ 0.05) are required for AGNs to be the dominant population responsible for hydrogen reionization in the early Universe. We then show that this low-fdc evolution can be reconciled with the current constraints on helium reionization, although it implies a relatively large number of inactive SMBHs at z âł 5, in tension with SMBH growth models based on heavy seeding
Dust properties of Lyman break galaxies at
We explore from a statistical point of view the far-infrared (far-IR) and
sub-millimeter (sub-mm) properties of a large sample of LBGs (22,000) at z~3 in
the COSMOS field. The large number of galaxies allows us to split it in several
bins as a function of UV luminosity, UV slope, and stellar mass to better
sample their variety. We perform stacking analysis in PACS (100 and 160 um),
SPIRE (250, 350 and 500 um) and AzTEC (1.1 mm) images. Our stacking procedure
corrects the biases induced by galaxy clustering and incompleteness of our
input catalogue in dense regions. We obtain the full IR spectral energy
distributions (SED) of subsamples of LBGs and derive the mean IR luminosity as
a function of UV luminosity, UV slope, and stellar mass. The average IRX is
roughly constant over the UV luminosity range, with a mean of 7.9 (1.8 mag).
However, it is correlated with UV slope, and stellar mass. We investigate using
a statistically-controlled stacking analysis as a function of (stellar mass, UV
slope) the dispersion of the IRX-UVslope and IRX-M* plane. Our results enable
us to study the average relation between star-formation rate (SFR) and stellar
mass, and we show that our LBG sample lies on the main sequence of star
formation at z~3.Comment: Accepted to A&A, 17 Pages, 14 Figures, 2 Table
A panchromatic spatially-resolved analysis of nearby galaxies -- I. Sub-kpc scale Main Sequence in grand-design spirals
We analyse the spatially resolved relation between stellar mass (M)
and star formation rate (SFR) in disk galaxies (i.e. the Main Sequence, MS).
The studied sample includes eight nearby face-on grand-design spirals, e.g. the
descendant of high-redshift, rotationally-supported star-forming galaxies. We
exploit photometric information over 23 bands, from the UV to the far-IR, from
the publicly available DustPedia database to build spatially resolved maps of
stellar mass and star formation rates on sub-galactic scales of 0.5-1.5 kpc, by
performing a spectral energy distribution fitting procedure that accounts for
both the observed and the obscured star formation processes, over a wide range
of internal galaxy environments (bulges, spiral arms, outskirts). With more
than 30 thousands physical cells, we have derived a definition of the local
spatially resolved MS per unit area for disks,
=0.82log-8.69. This is consistent with the
bulk of recent results based on optical IFU, using the H line emission
as a SFR tracer. Our work extends the analysis at lower sensitivities in both
M and SFR surface densities, up to a factor 10. The self
consistency of the MS relation over different spatial scales, from sub-galactic
to galactic, as well as with a rescaled correlation obtained for high redshift
galaxies, clearly proves its universality.Comment: 21 pages, 15 figures. Accepted for publication in MNRA
The FMOS-COSMOS survey of star-forming galaxies at z ~ 1.6. I. H\alpha -based star formation rates and dust extinction
We present the first results from a near-IR spectroscopic survey of the
COSMOS field, using the Fiber Multi-Object Spectrograph on the Subaru
telescope, designed to characterize the star-forming galaxy population at
. The high-resolution mode is implemented to detect H in
emission between with erg cm s. Here, we specifically
focus on 271 sBzK-selected galaxies that yield a H detection thus
providing a redshift and emission line luminosity to establish the relation
between star formation rate and stellar mass. With further -band
spectroscopy for 89 of these, the level of dust extinction is assessed by
measuring the Balmer decrement using co-added spectra. We find that the
extinction () rises with stellar
mass and is elevated at high masses compared to low-redshift galaxies. Using
this subset of the spectroscopic sample, we further find that the differential
extinction between stellar and nebular emission
\hbox{} is 0.7--0.8, dissimilar to
that typically seen at low redshift. After correcting for extinction, we derive
an H-based main sequence with a slope () and normalization
similar to previous studies at these redshifts.Comment: 6 pages, 4 figures, and 1 table. Published in ApJ Letter
- âŠ