571 research outputs found
The radio AGN population dichotomy: Green valley Seyferts versus red sequence low-excitation AGN
Radio outflows of active galactic nuclei (AGN) are invoked in cosmological
models as a key feedback mechanism in the latest phases of massive galaxy
formation. Recently it has been suggested that the two major radio AGN
populations -- the powerful high-excitation, and the weak low-excitation radio
AGN (HERAGN and LERAGN, resp.) -- represent two earlier and later stages of
massive galaxy build-up. To test this, here we make use of a local (0.04<z<0.1)
sample of ~500 radio AGN with available optical spectroscopy, drawn from the
FIRST, NVSS, SDSS, and 3CRR surveys. A clear dichotomy is found between the
properties of low-excitation (absorption line AGN, and LINERs) and
high-excitation (Seyferts) radio AGN. The hosts of the first have the highest
stellar masses, reddest optical colors, and highest mass black holes but
accrete inefficiently (at low rates). On the other hand, the high-excitation
radio AGN have lower stellar masses, bluer optical colors (consistent with the
`green valley'), and lower mass black holes that accrete efficiently (at high
rates). Such properties can be explained if these two radio AGN populations
represent different stages in the formation of massive galaxies, and thus are
also linked to different phases of the `AGN feedback'.Comment: 5 pages, 4 figures, 1 table, to appear in ApJ
Nuclear activity and star formation properties of Seyfert 2 galaxies
In order to characterize the amount of recent or ongoing stellar formation in
the circumnuclear region of active galaxies on a statistically sound basis, we
have studied the stellar component of the nuclear spectra in three different
samples of galaxies, namely Seyfert 2 galaxies (hereafter S2G), star-forming
galaxies (SFG) and passive normal galaxies (NG), i.e., no emission lines
observed, using Sloan Digital Sky Survey data (SDSS) (Adelman-McCarthy, 2008).
The stellar component of the observed spectra has been extracted using
STARLIGHT (Cid Fernandes et al., 2004), which fits an observed spectrum with a
model (template) spectrum obtained by combining a library of pre-defined simple
stellar populations spectra, with distinct ages and metallicities. The
resulting template spectra for the different samples of galaxies have been
compared to determine the features of the stellar emission component and to
evaluate the presence and intensity of the star formation in the nuclear
regions of different families of galaxies. From a first qualitative analysis it
results that the shape of the Spectral Energy Distribution (SED) of S2G and NG
is very similar, while that of SFG is characterized by a strong blue excess.
The presence of the 4000 A break in the spectra of S2G and NG together with the
lack of a strong blue continuum clearly indicate the absence of ongoing star
formation in the circumnuclear regions of S2G and obviously of NG. Anyway
traces of a recent star formation history are evident in the spectra of S2G
galaxies, which show a 4000 A break systematically shallower than in NG.Comment: Proceeding of the VII Serbian Conference on Spectral Line Shapes in
Astrophysic
The H-alpha and Infrared Star Formation Rates for the Nearby Field Galaxy Survey
We investigate the H-alpha and infrared star formation rate (SFR) diagnostics
for galaxies in the Nearby Field Galaxy Survey (NFGS). For the 81 galaxies in
our sample, we derive H-alpha fluxes (included here) from integrated spectra.
There is a strong correlation between the ratio of far-infrared to optical
luminosities L(FIR)/L(H-alpha) and the extinction E(B-V) measured with the
Balmer decrement. Before reddening correction, the SFR(IR) and SFR(H-alpha) are
related to each other by a power-law. Correction of the SFR(H-alpha) for
extinction using the Balmer decrement and a classical reddening curve both
reduces the scatter in the SFR(IR)-SFR(H-alpha) correlation and results in a
much closer agreement (within ~10%) between the two SFR indicators. This SFR
relationship spans 4 orders of magnitude and holds for all Hubble types with
IRAS detections in the NFGS. A constant ratio between the SFR(IR) and
SFR(H-alpha) for all Hubble types, including early types (S0-Sab), suggests
that the IR emission in all of these objects results from a young stellar
population.Comment: 23 pages, 5 figures, 1 table. Accepted for publication in the
Astronomical Journal. V2: Important changes: IRAS fluxes updated. Only
moderate and good quality IRAS FIR fluxes are now used, resulting in slight
changes to the equations and figures. The IR and H-alpha SFRs now agree to
within ~10%, rather than ~30% as quoted previousl
The Molecular Gas Content of z<0.1 Radio Galaxies: Linking the AGN Accretion Mode to Host Galaxy Properties
One of the main achievements in modern cosmology is the so-called `unified
model', which successfully describes most classes of active galactic nuclei
(AGN) within a single physical scheme. However, there is a particular class of
radio-luminous AGN that presently cannot be explained within this framework --
the `low-excitation' radio AGN (LERAGN). Recently, a scenario has been put
forward which predicts that LERAGN, and their regular `high-excitation' radio
AGN (HERAGN) counterparts represent different (red sequence vs. green valley)
phases of galaxy evolution. These different evolutionary states are also
expected to be reflected in their host galaxy properties, in particular their
cold gas content. To test this, here we present CO(1-0) observations toward a
sample of 11 of these systems conducted with CARMA. Combining our observations
with literature data, we derive molecular gas masses (or upper limits) for a
complete, representative, sample of 21 z<0.1 radio AGN. Our results yield that
HERAGN on average have a factor of ~7 higher gas masses than LERAGN. We also
infer younger stellar ages, lower stellar, halo, and central supermassive black
masses, as well as higher black hole accretion efficiencies in HERAGN relative
to LERAGN. These findings support the idea that high- and low-excitation radio
AGN form two physically distinct populations of galaxies that reflect different
stages of massive galaxy build-up.Comment: 8 pages, 4 figures, 4 tables; accepted for publication in Ap
Chemical pre-processing of cluster galaxies over the past 10 billion years in the IllustrisTNG simulations
We use the IllustrisTNG simulations to investigate the evolution of the
mass-metallicity relation (MZR) for star-forming cluster galaxies as a function
of the formation history of their cluster host. The simulations predict an
enhancement in the gas-phase metallicities of star-forming cluster galaxies
(10^9< M_star<10^10 M_sun) at z<1.0 in comparisons to field galaxies. This is
qualitatively consistent with observations. We find that the metallicity
enhancement of cluster galaxies appears prior to their infall into the central
cluster potential, indicating for the first time a systematic "chemical
pre-processing" signature for {\it infalling} cluster galaxies. Namely,
galaxies which will fall into a cluster by z=0 show a ~0.05 dex enhancement in
the MZR compared to field galaxies at z<0.5. Based on the inflow rate of gas
into cluster galaxies and its metallicity, we identify that the accretion of
pre-enriched gas is the key driver of the chemical evolution of such galaxies,
particularly in the stellar mass range (10^9< M_star<10^10 M_sun). We see
signatures of an environmental dependence of the ambient/inflowing gas
metallicity which extends well outside the nominal virial radius of clusters.
Our results motivate future observations looking for pre-enrichment signatures
in dense environments.Comment: 5 pages, 4 figures, accepted for publication in MNRAS Letter
Spitzer observations of Abell 1763 - II: Constraining the nature of activity in the cluster-feeding filament with VLA and XMM-Newton data
The Abell 1763 superstructure at z=0.23 contains the first galaxy filament to
be directly detected using mid-infrared observations. Our previous work has
shown that the frequency of starbursting galaxies, as characterized by 24{\mu}m
emission is much higher within the filament than at either the center of the
rich galaxy cluster, or the field surrounding the system. New VLA and
XMM-Newton data are presented here. We use the radio and X-ray data to examine
the fraction and location of active galaxies, both active galactic nuclei (AGN)
and starbursts. The radio far-infrared correlation, X-ray point source
location, IRAC colors, and quasar positions are all used to gain an
understanding of the presence of dominant AGN. We find very few MIPS-selected
galaxies that are clearly dominated by AGN activity. Most radio selected
members within the filament are starbursts. Within the supercluster, 3 of 8
spectroscopic members detected both in the radio and in the mid-infrared are
radio-bright AGN. They are found at or near the core of Abell 1763. The five
starbursts are located further along the filament. We calculate the physical
properties of the known wide angle tail (WAT) source which is the brightest
cluster galaxy (BCG) of Abell 1763. A second double lobe source is found along
the filament well outside of the virial radius of either cluster. The velocity
offset of the WAT from the X-ray centroid, and the bend of the WAT in the
intracluster medium (ICM) are both consistent with ram pressure stripping,
indicative of streaming motions along the direction of the filament. We
consider this as further evidence of the cluster-feeding nature of the galaxy
filament.Comment: 26 pages, 13 figures AJ, accepte
The High-Metallicity Explosion Environment of the Relativistic Supernova 2009bb
We investigate the environment of the nearby (d ~ 40Mpc) broad-lined Type Ic
supernova SN 2009bb. This event was observed to produce a relativistic outflow
likely powered by a central accreting compact object. While such a phenomenon
was previously observed only in long-duration gamma-ray bursts (LGRBs), no LGRB
was detected in association with SN 2009bb. Using an optical spectrum of the SN
2009bb explosion site, we determine a variety of ISM properties for the host
environment, including metallicity, young stellar population age, and star
formation rate. We compare the SN explosion site properties to observations of
LGRB and broad-lined SN Ic host environments on optical emission line ratio
diagnostic diagrams. Based on these analyses, we find that the SN 2009bb
explosion site has a very high metallicity of ~2x solar, in agreement with
other broad-lined SN Ic host environments and at odds with the low-redshift
LGRB host environments and recently proposed maximum metallicity limits for
relativistic explosions. We consider the implications of these findings and the
impact that SN 2009bb's unusual explosive properties and environment have on
our understanding of the key physical ingredient that enables some SNe to
produce a relativistic outflow.Comment: 7 pages, 4 figures, 1 table; accepted for publication in ApJ Letters
(replaced to include missing figure
A Puzzling X-Ray Source Found in the chandra Deep Field South
In this letter we report the detection of an extremely strong X-ray emission
line in the 940ks chandra ACIS-I spectrum of CXO CDFS J033225.3-274219. The
source was identified as a Type1 AGN at redshift of z = 1.617, with 2.0 -- 10.0
keV rest frame X-ray luminosity of ~ 10^44 ergs s^-1. The emission line was
detected at 6.2^{+0.2}_{-0.1} keV, with an equivalent width (EW) of
4.4^{+3.2}_{-1.4} keV, both quantities referring to the observed frame. In the
rest frame, the line is at 16.2^{+0.4}_{-0.3} keV with an EW of
11.5^{+8.3}_{-3.7} keV. An X-ray emission line at similar energy (~ 17 keV,
rest frame) in QSO PKS 2149-306 was discovered before using ASCA data. We
reject the possibility that the line is due to a statistical or instrumental
artifact. The line is most likely due to blueshifted Fe-K emission from an
relativistic outflow, probably an inner X-ray jet, with velocities of the order
of ~ 0.6-0.7c. Other possible explanations are also discussed
A closer view of the radio-FIR correlation: disentangling the contributions of star formation and AGN activity
We extend the Unified Radio Catalog, a catalog of sources detected by various
(NVSS, FIRST, WENSS, GB6) radio surveys, and SDSS, to IR wavelengths by
matching it to the IRAS Point and Faint Source catalogs. By fitting each
NVSS-selected galaxy's NUV-NIR spectral energy distribution (SED) with stellar
population synthesis models we add to the catalog star formation rates, stellar
masses, and attenuations.We further add information about optical emission line
properties for NVSS-selected galaxies with available SDSS spectroscopy. Using
an NVSS 20cm (F_{1.4GHz} ge 2.5mJy) selected sample, matched to the SDSS
spectroscopic ("main" galaxy and quasar) catalogs and IRAS data (0.04<z le 0.2)
we perform an in depth analysis of the radio-FIR correlation for various types
of galaxies, separated into i) quasars, ii) star forming, iii) composite, iv)
Seyfert, v) LINER and vi) absorption line galaxies using the standard optical
spectroscopic diagnostic tools. We utilize SED-based star formation rates to
independently quantify the source of radio and FIR emission in our galaxies.
Our results show that Seyfert galaxies have FIR/radio ratios lower than, but
still within the scatter of, the canonical value due to an additional (likely
AGN) contribution to their radio continuum emission. Furthermore, IR-detected
absorption and LINER galaxies are on average strongly dominated by AGN activity
in both their FIR and radio emission; however their average FIR/radio ratio is
consistent with that expected for star forming galaxies. In summary, we find
that most AGN-containing galaxies in our NVSS-IRAS-SDSS sample have FIR/radio
flux ratios indistinguishable from those of the star-forming galaxies that
define the radio-FIR correlation. Thus, attempts to separate AGNs from
star-forming galaxies by their FIR/radio flux ratios alone can separate only a
small fraction of the AGNs, such as the radio-loud quasars.Comment: 22 pages, 12 figures, 2 tables. Accepted for publication in Ap
Environmental Effects on the Metal Enrichment of Low Mass Galaxies in Nearby Clusters
In this paper we study the chemical history of low-mass star-forming (SF)
galaxies in the local Universe clusters Coma, A1367, A779, and A634. The aim of
this work is to search for the imprint of the environment on the chemical
evolution of these galaxies. Galaxy chemical evolution is linked to the star
formation history (SFH), as well as to the gas interchange with the
environment, and low-mass galaxies are well known to be vulnerable systems to
environmental processes affecting both these parameters. For our study we have
used spectra from the SDSS-III DR8. We have examined the mass-metallicity
relation of cluster galaxies finding well defined sequences. The slope of these
sequences, for galaxies in low-mass clusters and galaxies at large
cluster-centric distances, follows the predictions of recent hydrodynamic
models. A flattening of this slope has been observed for galaxies located in
the core of the two more massive clusters of the sample, principally in Coma,
suggesting that the imprint of the cluster environment on the chemical
evolution of SF galaxies should be sensitive to both the galaxy mass and the
host cluster mass. The HI gas content of Coma and A1367 galaxies indicate that
low-mass SF galaxies, located at the core of these clusters, have been severely
affected by ram-pressure stripping. The observed mass-dependent enhancement of
the metal content of low-mass galaxies in dense environments seems plausible,
according to hydrodynamic simulations. This enhanced metal enrichment could be
produced by the combination of effects such as wind reaccretion, due to
pressure cofinement by the intra-cluster medium (ICM), and the truncation of
gas infall, as a result of the ram-pressure stripping. Thus, the properties of
the ICM should play an important role in the chemical evolution of low-mass
galaxies in clusters.Comment: ApJ accepted, 31 pages, 13 figure
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