1,103 research outputs found
The PEP Survey: Infrared Properties of Radio-Selected AGN
By exploiting the VLA-COSMOS and the Herschel-PEP surveys, we investigate the
Far Infrared (FIR) properties of radio-selected AGN. To this purpose, from
VLA-COSMOS we considered the 1537, F[1.4 GHz]>0.06 mJy sources with a reliable
redshift estimate, and sub-divided them into star-forming galaxies and AGN
solely on the basis of their radio luminosity. The AGN sample is complete with
respect to radio selection at all z<~3.5. 832 radio sources have a counterpart
in the PEP catalogue. 175 are AGN. Their redshift distribution closely
resembles that of the total radio-selected AGN population, and exhibits two
marked peaks at z~0.9 and z~2.5. We find that the probability for a
radio-selected AGN to be detected at FIR wavelengths is both a function of
radio power and redshift, whereby powerful sources are more likely to be FIR
emitters at earlier epochs. This is due to two distinct effects: 1) at all
radio luminosities, FIR activity monotonically increases with look-back time
and 2) radio activity of AGN origin is increasingly less effective at
inhibiting FIR emission. Radio-selected AGN with FIR emission are
preferentially located in galaxies which are smaller than those hosting
FIR-inactive sources. Furthermore, at all z<~2, there seems to be a
preferential (stellar) mass scale M ~[10^{10}-10^{11}] Msun which maximizes the
chances for FIR emission. We find such FIR (and MIR) emission to be due to
processes indistinguishable from those which power star-forming galaxies. It
follows that radio emission in at least 35% of the entire AGN population is the
sum of two contributions: AGN accretion and star-forming processes within the
host galaxy.Comment: 13 pages, 14 figures, to appear in MNRA
The PEP survey: clustering of infrared-selected galaxies and structure formation at z~2 in the GOODS South
ABRIDGED-This paper presents the first direct estimate of the 3D clustering
properties of far-infrared sources up to z~3. This has been possible thanks to
the Pacs Evolutionary Probe (PEP) survey of the GOODS South field performed
with the PACS instrument onboard the Herschel Satellite. An analysis of the
two-point correlation function over the whole redshift range spanned by the
data reports for the correlation length, r_0~6.3 Mpc and r_0~6.7 Mpc,
respectively at 100um and 160um, corresponding to dark matter halo masses
M>~10^{12.4} M_sun. Objects at z~2 instead seem to be more strongly clustered,
with r_0~19 Mpc and r_0~17 Mpc in the two considered PACS channels. This
dramatic increase of the correlation length between z~1 and z~2 is connected
with the presence of a wide, M>~10^{14} M_sun, filamentary structure which
includes more than 50% of the sources detected at z~2. An investigation of the
properties of such sources indicates the possibility for boosted star-forming
activity in those which reside within the overdense environment with respect of
more isolated galaxies found in the same redshift range. Lastly, we also
present our results on the evolution of the relationship between luminous and
dark matter in star-forming galaxies between z~1 and z~2. We find that the
increase of (average) stellar mass in galaxies between z~1 and z~2 is
about a factor 10 lower than that of the dark matter haloes hosting such
objects ([z~1]/[z~2] ~ 0.4 vs M_{halo}[z~1]/M_{halo}[z~2] ~ 0.04). Our
findings agree with the evolutionary picture of downsizing whereby massive
galaxies at z~2 were more actively forming stars than their z~1 counterparts,
while at the same time contained a lower fraction of their mass in the form of
luminous matter.Comment: 14 pages, 8 figures, MNRAS accepte
A highly obscured and strongly clustered galaxy population discovered with the Spitzer Space Telescope
The ~800 optically unseen (R>25.5) 24mum-selected sources in the complete
Spitzer First Look Survey sample (Fadda et al. 2006) with F[24mum]>0.35 mJy are
found to be very strongly clustered. If, as indicated by several lines of
circumstantial evidence, they are ultraluminous far-IR galaxies at z ~
[1.6-2.7], the amplitude of their spatial correlation function is very high.
The associated comoving clustering length is estimated to be
r_0=14.0_{-2.4}^{+2.1} Mpc, value which puts these sources amongst the most
strongly clustered populations of our known universe. Their 8mum-24mum colours
suggest that the AGN contribution dominates above F[24mum] ~ 0.8 mJy,
consistent with earlier analyses. The properties of these objects (number
counts, redshift distribution, clustering amplitude) are fully consistent with
those of proto-spheroidal galaxies in the process of forming most of their
stars and of growing their active nucleus, as described by the Granato et al.
(2004) model. In particular, the inferred space density of such galaxies at z ~
2 is much higher than what expected from most semi-analytic models. Matches of
the observed projected correlation function w(\theta) with models derived
within the so-called Halo Occupation Scenario show that these sources have to
be hosted by haloes more massive than ~10^{13.4} M_\odot. This value is
significantly higher than that for the typical galactic haloes hosting massive
elliptical galaxies, suggesting a duration of the starburst phase of massive
high-redshift dusty galaxies of T_B ~ 0.5 Gyr.Comment: 14 pages, 10 figures, minor revisions, to appear on MNRA
Separating the BL Lac and Cluster X-ray Emissions in Abell 689 with Chandra
We present the results of a Chandra observation of the galaxy cluster Abell
689 (z=0.279). Abell 689 is one of the most luminous clusters detected in the
ROSAT All Sky Survey (RASS), but was flagged as possibly including significant
point source contamination. The small PSF of the Chandra telescope allows us to
confirm this and separate the point source from the extended cluster X-ray
emission. For the cluster we determine a bolometric luminosity of
L_{bol}=(3.3+/-0.3)x10^{44} erg s-1 and a temperature of kT=5.1^{+2.2}_{-1.3}
keV when including a physically motivated background model. We compare our
measured luminosity for A689 to that quoted in the Rosat All Sky Survey (RASS)
and find L_{0.1-2.4,keV}=2.8x10^{44} erg s-1, a value \sim10 times lower than
the ROSAT measurement. Our analysis of the point source shows evidence for
significant pileup, with a pile-up fraction of ~60%. SDSS spectra and HST
images lead us to the conclusion that the point source within Abell 689 is a BL
Lac object. Using radio and optical observations from the VLA and HST archives,
we determine {\alpha}_{ro}=0.50, {\alpha}_{ox}=0.77 and {\alpha}_{rx}=0.58 for
the BL Lac, which would classify it as being of 'High-energy peak BL Lac' (HBL)
type. Spectra extracted of A689 show a hard X-ray excess at energies above 6
keV that we interpret as inverse Compton emission from aged electrons that may
have been transported into the cluster from the BL Lac.Comment: 11 pages, 15 figures, MNRAS in pres
The dust content of high-z submillimeter galaxies revealed by Herschel
We use deep observations taken with the Photodetector Array Camera and
Spectrometer (PACS), on board the Herschel satellite as part of the PACS
evolutionary probe (PEP) guaranteed project along with submm ground-based
observations to measure the dust mass of a sample of high-z submillimeter
galaxies (SMGs). We investigate their dust content relative to their stellar
and gas masses, and compare them with local star-forming galaxies. High-z SMGs
are dust rich, i.e. they have higher dust-to-stellar mass ratios compared to
local spiral galaxies (by a factor of 30) and also compared to local
ultraluminous infrared galaxies (ULIRGs, by a factor of 6). This indicates that
the large masses of gas typically hosted in SMGs have already been highly
enriched with metals and dust. Indeed, for those SMGs whose gas mass is
measured, we infer dust-to-gas ratios similar or higher than local spirals and
ULIRGs. However, similarly to other strongly star-forming galaxies in the local
Universe and at high-z, SMGs are characterized by gas metalicities lower (by a
factor of a few) than local spirals, as inferred from their optical nebular
lines, which are generally ascribed to infall of metal-poor gas. This is in
contrast with the large dust content inferred from the far-IR and submm data.
In short, the metalicity inferred from the dust mass is much higher (by more
than an order of magnitude) than that inferred from the optical nebular lines.
We discuss the possible explanations of this discrepancy and the possible
implications for the investigation of the metalicity evolution at high-z.Comment: Accepted for publication in Astronomy & Astrophysics Letters. One
reference update
The lesser role of starbursts for star formation at z=2
Two main modes of star formation are know to control the growth of galaxies:
a relatively steady one in disk-like galaxies, defining a tight star formation
rate (SFR)-stellar mass sequence, and a starburst mode in outliers to such a
sequence which is generally interpreted as driven by merging. Such starburst
galaxies are rare but have much higher SFRs, and it is of interest to establish
the relative importance of these two modes. PACS/Herschel observations over the
whole COSMOS and GOODS-South fields, in conjunction with previous
optical/near-IR data, have allowed us to accurately quantify for the first time
the relative contribution of the two modes to the global SFR density in the
redshift interval 1.5<z<2.5, i.e., at the cosmic peak of the star formation
activity. The logarithmic distributions of galaxy SFRs at fixed stellar mass
are well described by Gaussians, with starburst galaxies representing only a
relatively minor deviation that becomes apparent for SFRs more than 4 times
higher than on the main sequence. Such starburst galaxies represent only 2% of
mass-selected star forming galaxies and account for only 10% of the cosmic SFR
density at z~2. Only when limited to SFR>1000M(sun)/yr, off-sequence sources
significantly contribute to the SFR density (46+/-20%). We conclude that
merger-driven starbursts play a relatively minor role for the formation of
stars in galaxies, whereas they may represent a critical phase towards the
quenching of star formation and morphological transformation in galaxies.Comment: Accepted for publication in ApJ Letter
Constraints on the Clustering, Biasing and Redshift Distribution of Radio Sources
We discuss how different theoretical predictions for the variance
of the distribution of radio sources can be matched to measurements from the
FIRST survey at different flux limits. The predictions are given by the
integration of models for the angular correlation function for
three different functional forms of the redshift distribution , different
spatial correlation functions and by different evolutions of the bias
with redshift. We also consider the two cases of open and flat Universes.
Although the predicted show substantial differences due to
differences in the 's, these differences are not significant compared to
the uncertainties in the current observations. It turns out that the best fit
is provided by models with constant biasing at all times, although the
difference between models with epoch-independent bias and models with bias that
evolves linearly with redshift is not very large. All models with strong
evolution of bias with epoch are ruled out. As a further step we directly
calculated at 3mJy from the catalogue and matched it with our
models for the angular correlation function in the hypothesis that the
clustering signal comes from two different populations, namely AGN-powered
sources and starbursting galaxies. The results are consistent with a scenario
for hierarchical clustering where the fainter starbursting galaxies trace the
mass at all epochs, while brighter AGN's are strongly biased, with
evolving linearly with redshift, as suggested by some theories of galaxy
formation and evolution.Comment: 14 pages, 12 figures, version to appear on MNRA
The ATESP 5 GHz radio survey. III. 4.8, 8.6 and 19 GHz follow-up observations of radio galaxies
[Abridged] Physical and evolutionary properties of the sub-mJy radio
population are not entirely known. The radio/optical analysis of the ATESP 5
GHz sample has revealed a significant class of compact flat/inverted
radio-spectrum sources associated to early-type galaxies up to redshift 2. Such
sources are most plausibly triggered by an AGN, but their observational
properties are not entirely consistent with those of standard radio galaxy
populations. In the present work we aim at a better understanding of the radio
spectra of such sources and ultimately of the nature of AGNs at sub-mJy flux
levels. We used the ATCA to get multi-frequency (4.8, 8.6 and 19 GHz)
quasi-simultaneous observations for a representative sub-sample of ATESP radio
sources associated with early-type galaxies (26 objects with S>0.6 mJy). This
can give us insight into the accretion/radiative mechanism that is at work,
since different regimes display different spectral signatures in the radio
domain. From the analysis of the radio spectra, we find that our sources are
most probably jet-dominated systems. ADAF models are ruled out by the high
frequency data, while ADAF+jet scenarios are still consistent with
flat/moderately inverted-spectrum sources, but are not required to explain the
data. We compared our sample with high (>20 GHz) frequency selected surveys,
finding spectral properties very similar to the ones of much brighter (S>500
mJy) radio galaxies extracted from the Massardi et al. (2008) sample. Linear
sizes of ATESP 5 GHz sources associated with early type galaxies are also often
consistent with the ones of brighter B2 and 3C radio galaxies, with possibly a
very compact component that could be associated at least in part to (obscured)
radio-quiet quasar-like objects and/or low power BL Lacs.Comment: Accepted for publication in Astronomy & Astrophysic
PEP: first Herschel probe of dusty galaxy evolution up to z~3
We exploit the deepest existing far-infrared (FIR) data obtained so far by
Herschel at 100 and 160 um in the GOODS-N, as part of the PACS Evolutionary
Probe (PEP) survey, to derive for the first time the evolution of the
rest-frame 60-um, 90-um, and total IR luminosity functions (LFs) of galaxies
and AGNs from z=0 to unprecedented high redshifts (z~2-3). The PEP LFs were
computed using the 1/Vmax method. The FIR sources were classified by means of a
detailed broad- band SED-fitting analysis and spectral characterisation. Based
on the best-fit model results, k-correction and total IR (8-1000 um) luminosity
were obtained for each source. LFs (monochromatic and total) were then derived
for various IR populations separately in different redshift bins and compared
to backward evolution model predictions. We detect strong evolution in the LF
to at least z~2. Objects with SEDs similar to local spiral galaxies are the
major contributors to the star formation density (SFD) at z< 0.3, then, as
redshift increases, moderate SF galaxies - most likely containing a
low-luminosity AGN - start dominating up to z ~= 1.5. At >1.5 the SFD is
dominated by the contributions of starburst galaxies. In agreement with
previous findings, the comoving IR LD derived from our data evolves
approximately as (1 + z)^(3.8+/-0.3) up to z~1, there being some evidence of
flattening up to z~2.Comment: Accepted for publication in the A&A Herschel first results Special
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