6,731 research outputs found
Optical spectroscopic observations of gamma-ray blazar candidates II. The 2013 KPNO campaign in the Northern Hemisphere
We recently started a systematic search of low-energy counterparts of the
unidentified gamma-ray sources (UGSs) listed in the Fermi-Large Area Telescope
(LAT) First Source Catalog (1FGL) and the Fermi-LAT 2-Year Source Catalog
(2FGL).} The main goal of our investigation is to find active galaxies
belonging to the blazar class that lie within the positional uncertainty region
of the UGSs and thus could be their potential low-energy counterparts. To
achieve our aims, we first adopted several procedures based on the peculiar
observational properties of blazars in the radio and in the IR. Then we carried
out a follow-up spectroscopic campaign in the optical band to verify the nature
of the candidates selected as potential counterparts of the UGSs. Here we
present the results of the observations carried out in 2013 in the Northern
Hemisphere at Kitt Peak National Observatory (KPNO). Optical spectroscopy is
crucial to confirm the nature of the sources and can be used to estimate their
redshifts; it will also allow us to test the robustness of our methods when the
whole campaign is completed. Here we present the optical spectroscopic
observations of 39 sources. Within our sample we found that 6 sources are
blazars, candidates to be low-energy counterparts of the UGSs listed in the
2FGL. We confirm that an additional 8 sources, previously classified as active
galaxies of uncertain type and associated in the 2FGL, are also all BL Lac
objects. Moreover, we also present 20 new spectra for known blazars listed in
the Multi-frequency Catalogue of Blazars as having an uncertain redshift and/or
being classified as BL Lac candidates. We conclude that our methods for
selecting gamma-ray blazar candidates allows us to discover new blazars and
increase the list of potential low-energy counterparts for the Fermi UGSs.Comment: 27 pages, 39 figures, 1 table, A&A accepted for publication
(pre-proof version
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Simulated Galaxy Interactions as Probes of Merger Spectral Energy Distributions
We present the first systematic comparison of ultraviolet-millimeter spectral
energy distributions (SEDs) of observed and simulated interacting galaxies. Our
sample is drawn from the Spitzer Interacting Galaxy Survey, and probes a range
of galaxy interaction parameters. We use 31 galaxies in 14 systems which have
been observed with Herschel, Spitzer, GALEX, and 2MASS. We create a suite of
GADGET-3 hydrodynamic simulations of isolated and interacting galaxies with
stellar masses comparable to those in our sample of interacting galaxies.
Photometry for the simulated systems is then calculated with the SUNRISE
radiative transfer code for comparison with the observed systems. For most of
the observed systems, one or more of the simulated SEDs match reasonably well.
The best matches recover the infrared luminosity and the star formation rate of
the observed systems, and the more massive systems preferentially match SEDs
from simulations of more massive galaxies. The most morphologically distorted
systems in our sample are best matched to simulated SEDs close to coalescence,
while less evolved systems match well with SEDs over a wide range of
interaction stages, suggesting that an SED alone is insufficient to identify
interaction stage except during the most active phases in strongly interacting
systems. This result is supported by our finding that the SEDs calculated for
simulated systems vary little over the interaction sequence.Comment: 24 pages, 16 figures, 2 tables, accepted for publication in ApJ.
Animations of the evolution of the simulated SEDs can be found at
http://www.cfa.harvard.edu/~llanz/sigs_sim.htm
Optical archival spectra of blazar candidates of uncertain type in the 3 Fermi Large Area Telescope Catalog
Despite the fact that blazars constitute the rarest class among active
galactic nuclei (AGNs) they are the largest known population of associated
-ray sources. Many of the -ray objects listed in the
Fermi-Large Area Telescope Third Source catalog (3FGL) are classified as blazar
candidates of uncertain type (BCUs), either because they show multifrequency
behaviour similar to blazars but lacking optical spectra in the literature, or
because the quality of such spectra is too low to confirm their nature. Here we
select, out of 585 BCUs in the 3FGL, 42 BCUs which we identify as probable
blazars by their WISE infrared colors and which also have optical spectra that
are available in the Sloan Digital Sky Survey (SDSS) and/or Six-Degree Field
Galaxy Survey Database (6dFGS). We confirm the blazar nature of all of the
sources. We furthermore conclude that 28 of them are BL Lacs, 8 are radio-loud
quasars with flat radio spectrum and 6 are BL Lac whose emission is dominated
by their host galaxy
Variations of the ISM Compactness Across the Main Sequence of Star-Forming Galaxies: Observations and Simulations
(abridged) The majority of star-forming galaxies follow a simple empirical
correlation in the star formation rate (SFR) versus stellar mass () plane,
usually referred to as the star formation Main Sequence (MS). Here we combine a
set of hydro-dynamical simulations of interacting galactic disks with
state-of-the-art radiative transfer codes to analyze how the evolution of
mergers is reflected upon the properties of the MS. We present
\textsc{Chiburst}, a Markov Chain Monte Carlo (MCMC) Spectral Energy
Distribution (SED) code that fits the multi-wavelength, broad-band photometry
of galaxies and derives stellar masses, star formation rates, and geometrical
properties of the dust distribution. We apply this tool to the SEDs of
simulated mergers and compare the derived results with the reference output
from the simulations. Our results indicate that changes in the SEDs of mergers
as they approach coalescence and depart from the MS are related to an evolution
of dust geometry in scales larger than a few hundred parsecs. This is reflected
in a correlation between the specific star formation rate (sSFR), and the
compactness parameter , that parametrizes this geometry and hence
the evolution of dust temperature () with time. As mergers
approach coalescence, they depart from the MS and increase their compactness,
which implies that moderate outliers of the MS are consistent with late-type
mergers. By further applying our method to real observations of Luminous
Infrared Galaxies (LIRGs), we show that the merger scenario is unable to
explain these extreme outliers of the MS. Only by significantly increasing the
gas fraction in the simulations are we able to reproduce the SEDs of LIRGs.Comment: 18 pages, 10 figures, accepted in Ap
The AGN Luminosity Fraction in Merging Galaxies
Galaxy mergers are key events in galaxy evolution, often causing massive
starbursts and fueling active galactic nuclei (AGN). In these highly dynamic
systems, it is not yet precisely known how much starbursts and AGN respectively
contribute to the total luminosity, at what interaction stages they occur, and
how long they persist. Here we estimate the fraction of the bolometric infrared
(IR) luminosity that can be attributed to AGN by measuring and modeling the
full ultraviolet to far-infrared spectral energy distributions (SEDs) in up to
33 broad bands for 24 merging galaxies with the Code for Investigating Galaxy
Emission. In addition to a sample of 12 confirmed AGN in late-stage mergers,
found in the Revised Bright Galaxy Sample or
Faint Source Catalog, our sample includes a comparison sample of 12 galaxy
mergers from the Interacting Galaxies Survey, mostly early-stage. We
perform identical SED modeling of simulated mergers to validate our methods,
and we supplement the SED data with mid-IR spectra of diagnostic lines obtained
with InfraRed Spectrograph. The estimated AGN contributions to the IR
luminosities vary from system to system from 0% up to 91% but are significantly
greater in the later-stage, more luminous mergers, consistent with what is
known about galaxy evolution and AGN triggering.Comment: 26 pages, 10 figure
Radio weak BL Lac objects in the Fermi era
The existence of âradio weak BL Lac objectsâ (RWBLs) has been an open question, still unsolved, since the discovery that quasars could be radio-quiet or radio-loud. Recently several groups identified RWBL candidates, mostly found while searching for low energy counterparts of the unidentified/ unassociated gamma-ray sources listed in the Fermi catalogs. Confirming RWBLs is a challenging task since they could be confused with white dwarfs or weak emission line quasars when there are not sufficient data to precisely draw their broad band spectral energy distribution and their classification is mainly based on a featureless optical spectra. Motivated by the recent discovery that Fermi BL Lacs appear to have very peculiar mid-IR emission, we show that it is possible to distinguish between WDs, WELQs and BL Lacs using the [3.4]-[4.6]-[12]ÎŒm color-color plot built using the WISE magnitudes when the optical spectrum is available. On the basis of this analysis, we identify WISE J064459.38+603131 and WISE J141046.00+740511.2 as the first two genuine RWBLs, both potentially associated with Fermi sources. Finally, to strengthen our identification of these objects as true RWBLs, we present multifrequency observations for these two candidates to show that their spectral behavior is indeed consistent with those of the BL Lac population.Facultad de Ciencias AstronĂłmicas y GeofĂsica
The Gamma-ray Blazar Quest: new optical spectra, state of art and future perspectives
We recently developed a procedure to recognize gamma-ray blazar candidates
within the positional uncertainty regions of the unidentified/unassociated
gamma-ray sources (UGSs). Such procedure was based on the discovery that Fermi
blazars show peculiar infrared colors. However, to confirm the real nature of
the selected candidates, optical spectroscopic data are necessary. Thus, we
performed an extensive archival search for spectra available in the literature
in parallel with an optical spectroscopic campaign aimed to reveal and confirm
the nature of the selected gamma-ray blazar candidates. Here, we first search
for optical spectra of a selected sample of gamma-ray blazar candidates that
can be potential counterparts of UGSs using the Sloan Digital Sky Survey (SDSS
DR12). This search enables us to update the archival search carried out to
date. We also describe the state-of-art and the future perspectives of our
campaign to discover previously unknown gamma-ray blazars.Comment: 11 pages, 2 figures, 5 tables, pre-proof version, accepted for
publication of Astrophysics and Space Scienc
The Spitzer Interacting Galaxies Survey: A Mid-infrared Atlas of Star Formation
The Spitzer Interacting Galaxies Survey is a sample of 103 nearby galaxies in 48 systems, selected using association likelihoods and therefore free from disturbed morphology biases. All galaxies have been observed with Infrared Array Camera and MIPS 24 ÎŒm bands from the Spitzer Space Telescope. This catalog presents the global flux densities and colors of all systems and correlations between the interacting systems and their specific star formation rate (sSFR). This sample contains a wide variety of galaxy interactions with systems ranging in mass, mass ratios, and gas-content as well as interaction strength. This study seeks to identify the process of triggering star formation in galaxy interactions, therefore, we focus on the non-active galactic nucleus spiral galaxies only. From this subset of 70 spiral galaxies we have determined that this sample has enhanced sSFR compared to a sample of non-interacting field galaxies. Through optical data we have classified each system by "interaction strength"; the strongly interacting (Stage 4) galaxies have higher sSFR values than the weakly (Stage 2) and moderately (Stage 3) interacting systems. However, the Stage 2 and 3 systems have statistically identical sSFR properties, despite the lack of optical interaction signatures exhibited by the Stage 2 galaxies. We suggest that the similarity of sSFR in these stages could be a consequence of some of these Stage 2 systems actually being post-perigalactic and having had sufficient time for their tidal features to fade to undetectable levels. This interpretation is consistent with the correlation of sSFR with separation, which we have determined to have little variation up to 100 kpc
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