53 research outputs found
Variability in Active Galactic Nuclei: Understanding Emission Mechanisms and Unification Models
Siirretty Doriast
A new extensive catalog of optically variable AGN in the GOODS Fields and a new statistical approach to variability selection
Variability is a property shared by practically all AGN. This makes
variability selection a possible technique for identifying AGN. Given that
variability selection makes no prior assumption about spectral properties, it
is a powerful technique for detecting both low-luminosity AGN in which the host
galaxy emission is dominating and AGN with unusual spectral properties. In this
paper, we will discuss and test different statistical methods for the detection
of variability in sparsely sampled data that allow full control over the false
positive rates. We will apply these methods to the GOODS North and South fields
and present a catalog of variable sources in the z band in both GOODS fields.
Out of 11931 objects checked, we find 155 variable sources at a significance
level of 99.9%, corresponding to about 1.3% of all objects. After rejection of
stars and supernovae, 139 variability selected AGN remain. Their magnitudes
reach down as faint as 25.5 mag in z. Spectroscopic redshifts are available for
22 of the variability selected AGN, ranging from 0.046 to 3.7. The absolute
magnitudes in the rest-frame z-band range from ~ -18 to -24, reaching
substantially fainter than the typical luminosities probed by traditional X-ray
and spectroscopic AGN selection in these fields. Therefore, this is a powerful
technique for future exploration of the evolution of the faint end of the AGN
luminosity function up to high redshifts.Comment: Accepted for publication in The Astrophysical Journal (version 2:
minor changes to text after receiving comments
The SEDs, Host Galaxies and Environments of Variability Selected AGN in GOODS-S
Variability selection has been proposed as a powerful tool for identifying
both low-luminosity AGN and those with unusual SEDs. However, a systematic
study of sources selected in such a way has been lacking. In this paper, we
present the multi-wavelength properties of the variability selected AGN in
GOODS South. We demonstrate that variability selection indeed reliably
identifies AGN, predominantly of low luminosity. We find contamination from
stars as well as a very small sample of sources that show no sign of AGN
activity, their number is consistent with the expected false positive rate. We
also study the host galaxies and environments of the AGN in the sample.
Disturbed host morphologies are relatively common. The host galaxies span a
wide range in the level of ongoing star-formation. However, massive star-bursts
are only present in the hosts of the most luminous AGN in the sample. There is
no clear environmental preference for the AGN sample in general but we find
that the most luminous AGN on average avoid dense regions while some
low-luminosity AGN hosted by late-type galaxies are found near the centres of
groups. AGN in our sample have closer nearest neighbours than the general
galaxy population. We find no indications that major mergers are a dominant
triggering process for the moderate to low luminosity AGN in this sample. The
environments and host galaxy properties instead suggest secular processes, in
particular tidal processes at first passage and minor mergers, as likely
triggers for the objects studied. This study demonstrates the strength of
variability selection for AGN and gives first hints at possibly triggering
mechanisms for high-redshift low luminosity AGN.Comment: Accepted for publication in MNRAS (missing references added in
version 2
Morphologies of low-redshift AGN host galaxies: what role does AGN luminosity play?
Mergers of galaxies have been suspected to be a major trigger of AGN activity
for many years. However, when compared to carefully matched control samples,
AGN host galaxies often show no enhanced signs of interaction. A common
explanation for this lack of observed association between AGN and mergers has
often been that while mergers are of importance for triggering AGN, they only
dominate at the very high luminosity end of the AGN population. In this study,
we compare the morphologies of AGN hosts to a carefully matched control sample
and particularly study the role of AGN luminosity. We find no enhanced merger
rates in AGN hosts and also find no trend for stronger signs of disturbance at
higher AGN luminosities. While this study does not cover very high luminosity
AGN, we can exclude a strong connection between AGN and mergers over a wide
range of AGN luminosities and therefore for a large part of the AGN population.Comment: Proceedings of the conference "Nuclei of Seyfert galaxies and QSOs -
Central engine & conditions of star formation" held in Bonn, Germany, 201
A simple non-parametric method for resolving merged doublet lines:Insights into complex kinematics and outflows
Doublet line emission and absorption is common in astronomical sources (e.g.
[OIII], [OII], NaD, MgII). In many cases, complex kinematics in the emitting
source can cause the doublet lines to merge, making characterisation of the
source kinematics challenging. Here, we present a non-parametric method for
resolving merged doublet emission when the line ratio and wavelength difference
is known. The method takes as input only the line ratio and wavelength
difference, using these quantities to resolve the components of the doublet
without resorting to fitting (e.g. using multiple Gaussians) or making any
assumptions about the components' line profiles (save that they are the same
for both components). The method is simple, fast and robust. It is also ideal
for visualisation. We show that the method recovers line profiles of merged
emission lines in simulated data. We also show, using simulated data and
mathematical analysis, that the method does not significantly increase noise
levels in the extracted lines, and is robust to background contamination. We
demonstrate the strength of the method by applying it to strongly merged [OIII]
5007/4959~{\AA} in Active Galactic Nuclei (AGN). A python implementation of the
method is provided in the Appendix.Comment: Accepted for publication in MNRAS (changes from v1: comparison of
method to fitting procedures and additional tests of method added
The redshift evolution of major merger triggering of luminous AGN:a slight enhancement at z∼2
TH was supported by STFC grant ST/M503812/1 during the course of this work. VW, JM-A, MP and KR acknowledge support from the European Research Council Starting Grant SEDmorph (P.I. V. Wild).Active galactic nuclei (AGN), particularly the most luminous AGN, are commonly assumed to be triggered through major mergers, however observational evidence for this scenario is mixed. To investigate any influence of galaxy mergers on AGN triggering and luminosities through cosmic time, we present a sample of 106 luminous X-ray selected type 1 AGN from the COSMOS survey. These AGN occupy a large redshift range (0.5 < z < 2.2) and two orders of magnitude in X-ray luminosity (∼1043 - 1045 erg s-1). AGN hosts are carefully mass and redshift matched to 486 control galaxies. A novel technique for identifying and quantifying merger features in galaxies is developed, subtracting galfit galaxy models and quantifying the residuals. Comparison to visual classification confirms this measure reliably picks out disturbance features in galaxies. No enhancement of merger features with increasing AGN luminosity is found with this metric, or by visual inspection. We analyse the redshift evolution of AGN associated with galaxy mergers and find no merger enhancement in lower redshift bins. Contrarily, in the highest redshift bin (z∼2) AGN are ∼4 times more likely to be in galaxies exhibiting evidence of morphological disturbance compared to control galaxies, at 99% confidence level (∼2.4σ) from visual inspection. Since only ∼15% of these AGN are found to be in morphologically disturbed galaxies, it is implied that major mergers at high redshift make a noticeable but subdominant contribution to AGN fuelling. At low redshifts other processes dominate and mergers become a less significant triggering mechanism.PostprintPeer reviewe
Incidence, scaling relations and physical conditions of ionized gas outflows in MaNGA
In this work, we investigate the strength and impact of ionised gas outflows
within MaNGA galaxies. We find evidence for outflows in 322
galaxies ( of the analysed line-emitting sample), 185 of which show
evidence for AGN activity. Most outflows are centrally concentrated with a
spatial extent that scales sublinearly with . The incidence of
outflows is enhanced at higher masses, central surface densities and deeper
gravitational potentials, as well as at higher SFR and AGN luminosity. We
quantify strong correlations between mass outflow rates and the mechanical
drivers of the outflow of the form
and . We derive a master scaling
relation describing the mass outflow rate of ionised gas as a function of
, SFR, and . Most of the observed winds are
anticipated to act as galactic fountains, with the fraction of galaxies with
escaping winds increasing with decreasing potential well depth. We further
investigate the physical properties of the outflowing gas finding evidence for
enhanced attenuation in the outflow, possibly due to metal-enriched winds, and
higher excitation compared to the gas in the galactic disk. Given that the
majority of previous studies have focused on more extreme systems with higher
SFRs and/or more luminous AGN, our study provides a unique view of the
non-gravitational gaseous motions within `typical' galaxies in the low-redshift
Universe, where low-luminosity AGN and star formation contribute jointly to the
observed outflow phenomenology.Comment: Accepted for publication in MNRAS, 27 pages, Fig 7 & 8 for scaling
wind strength with drivers, Fig 10 for master scalin
Cool outflows in MaNGA:a systematic study and comparison to the warm phase
This paper investigates the neutral gas phase of galactic winds via the Na I
D{\AA} feature within MaNGA galaxies,
and directly compares their incidence and strength to the ionized winds
detected within the same parent sample. We find evidence for neutral outflows
in 127 galaxies ( per cent of the analysed line-emitting sample). Na I
D winds are preferentially seen in galaxies with dustier central regions and
both wind phases are more often found in systems with elevated SFR surface
densities, especially when there has been a recent upturn in the star formation
activity according to the SFR/SFR parameter. We find the
ionized outflow kinematics to be in line with what we measure in the neutral
phase. This demonstrates that, despite their small contributions to the total
outflow mass budget, there is value to collecting empirical measurements of the
ionized wind phase to provide information on the bulk motion in the outflow.
Depending on dust corrections applied to the ionized gas diagnostics, the
neutral phase has dex higher mass outflow rates
(), on average, compared to the ionized phase. We quantify
scaling relations between and the strengths of the physical
wind drivers (SFR, ). Using a radial-azimuthal stacking method, and by
considering inclination dependencies, we find results consistent with biconical
outflows orthogonal to the disk plane. Our work complements other multi-phase
outflow studies in the literature which consider smaller samples, more extreme
objects, or proceed via stacking of larger samples.Comment: This is a pre-copyedited, author-produced PDF of an article accepted
for publication in Monthly Notices of the Royal Astronomical Society (MNRAS)
following peer revie
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