716 research outputs found
Highly Ionised Gas as a Diagnostic of the Inner NLR
The spectra of AGN from the ultraviolet to the near infrared, exhibit
emission lines covering a wide range of ionisation states, from neutral species
such as [O I] 6300A, up to [Fe XIV] 5303A. Here we report on some recent
studies of the properties of highly ionised lines (HILs), plus two case studies
of individual objects. Future IFU observations at high spatial and good
spectral resolution, will probe the excitation and kinematics of the gas in the
zone between the extended NLR and unresolved BLR. Multi-component SED fitting
can be used to link the source of photoionisation with the strengths and ratios
of the HILs.Comment: Proceedings of the IAU Symposium: Co-evolution of Central Black Holes
and Galaxie
Modeling the connection between ultraviolet and infrared galaxy populations across cosmic times
Using a phenomenological approach, we self-consistently model the redshift evolution of the ultraviolet (UV) and infrared (IR) luminosity functions across cosmic time, as well as a range of observed IR properties of UV-selected galaxy population. This model is an extension of the 2SFM (2 star-formation modes) formalism, which is based on the observed "main-sequence" of star-forming galaxies, i.e. a strong correlation between their stellar mass and their star formation rate (SFR), and a secondary population of starbursts with an excess of star formation. The balance between the UV light from young, massive stars and the dust-reprocessed IR emission is modeled following the empirical relation between the attenuation (IRX for IR excess hereafter) and the stellar mass, assuming a scatter of 0.4\,dex around this relation. We obtain a good overall agreement with the measurements of the IR luminosity function up to z~3 and the UV luminosity functions up to z~6, and show that a scatter on the IRX-M relation is mandatory to reproduce these observables. We also naturally reproduce the observed, flat relation between the mean IRX and the UV luminosity at LUV>109.5 L⊙. Finally, we perform predictions of the UV properties and detectability of IR-selected samples and the vice versa, and discuss the results in the context of the UV-rest-frame and sub-millimeter surveys of the next decade
An enhanced fraction of starbursting galaxies among high Eddington ratio AGNs
We investigate the star-forming properties of 1620 X-ray selected active galactic nuclei (AGN) host galaxies as a function of their specific X-ray luminosity (i.e. X-ray luminosity per unit host stellar mass) – a proxy of the Eddington ratio. Our motivation is to determine whether there is any evidence of a suppression of star formation at high Eddington ratios, which may hint towards ‘AGN feedback’ effects. Star formation rates (SFRs) are derived from fits to Herschel-measured far-infrared spectral energy distributions, taking into account any contamination from the AGN. Herschel-undetected AGNs are included via stacking analyses to provide average SFRs in bins of redshift and specific X-ray luminosity (spanning 0.01≲LX/M∗≲100L⊙M−1. After normalizing for the effects of mass and redshift arising from the evolving galaxy main sequence, we find that the SFRs of high specific luminosity AGNs are slightly enhanced compared to their lower specific luminosity counterparts. This suggests that the SFR distribution of AGN hosts changes with specific X-ray luminosity, a result reinforced by our finding of a significantly higher fraction of starbursting hosts among high specific luminosity AGNs compared to that of the general star-forming galaxy population (i.e. 8–10 per cent versus 3 per cent). Contrary to our original motivation, our findings suggest that high specific luminosity AGNs are more likely to reside in galaxies with enhanced levels of star formation
A tidal disruption event in the nearby ultra-luminous infrared galaxy F01004-2237
Tidal disruption events (TDEs), in which stars are gravitationally disrupted
as they pass close to the supermassive black holes in the centres of galaxies,
are potentially important probes of strong gravity and accretion physics. Most
TDEs have been discovered in large-area monitoring surveys of many 1000s of
galaxies, and the rate deduced for such events is relatively low: one event
every 10 - 10 years per galaxy. However, given the selection effects
inherent in such surveys, considerable uncertainties remain about the
conditions that favour TDEs. Here we report the detection of unusually strong
and broad helium emission lines following a luminous optical flare (Mv < -20.1
mag) in the nucleus of the nearby ultra-luminous infrared galaxy F01004-2237.
The particular combination of variability and post-flare emission line spectrum
observed in F01004-2237 is unlike any known supernova or active galactic
nucleus. Therefore, the most plausible explanation for this phenomenon is a TDE
-- the first detected in a galaxy with an ongoing massive starburst. The fact
that this event has been detected in repeat spectroscopic observations of a
sample of 15 ultra-luminous infrared galaxies over a period of just 10 years
suggests that the rate of TDEs is much higher in such objects than in the
general galaxy population.Comment: 17 pages, 4 figures, accepted for publication in Nature Astronom
Using Gaussian Processes to detect AGN flares
A key feature of active galactic nuclei (AGN) is their variability across all
wavelengths. Typically, AGN vary by a few tenths of a magnitude or more over
periods lasting from hours to years. By contrast, extreme variability of AGN --
large luminosity changes that are a significant departure from the baseline
variability -- are known as AGN flares. These events are rare and their
timescales poorly constrained, with most of the literature focusing on
individual events. It has been suggested that extreme AGN variability including
flares can provide insights into the accretion processes in the disk. With
surveys such as the Legacy Survey of Space and Time (LSST) promising millions
of transient detections per night in the coming decade, there is a need for
fast and efficient classification of AGN flares. The problem with the
systematic detection of AGN flares is the requirement to detect them against a
stochastically variable baseline; the ability to define a signal as a
significant departure from the ever-present variability is a statistical
challenge. Recently, Gaussian Processes (GPs) have revolutionised the analysis
of time-series data in many areas of astronomical research. They have, however,
seen limited uptake within the field of transient detection and classification.
Here we investigate the efficacy of Gaussian Processes to detect AGN flares in
both simulated and real optical light curves. We show that GP analysis can
successfully detect AGN flares with a false-positive rate of less than seven
per cent, and we present examples of AGN light curves that show extreme
variability.Comment: 14 pages, 25 figures, accepted for publication in MNRA
The Location and Kinematics of the Coronal-Line Emitting Regions in Active Galactic Nuclei
We use the photoionization code CLOUDY to determine both the location and the kinematics of the optical forbidden, high-ionization line (hereafter, FHIL) emitting gas in the narrow line Seyfert 1 galaxy Ark 564. The results of our models are compared with the observed properties of these emission lines to produce a physical model that is used to explain both the kinematics and the source of this gas. The main features of this model are that the FHIL emitting gas is launched from the putative dusty torus and is quickly accelerated to its terminal velocity of a few hundred km s-1. Iron-carrying grains are destroyed during this initial acceleration. This velocity is maintained by a balance between radiative forces and gravity in this super-Eddington source. Eventually the outflow is slowed at large radii by the gravitational forces of and interactions with the host galaxy. In this model, FHIL emission traces the transition between the active galactic nucleus (AGN) and bulge zones of influence
GOODS-Herschel: the far-infrared view of star formation in active galactic nucleus host galaxies since z ≈ 3
We present a study of the infrared properties of X-ray selected, moderate-luminosity (i.e. L_X= 10^(42)–10^(44) erg s^(−1)) active galactic nuclei (AGNs) up to z ≈ 3, in order to explore the links between star formation in galaxies and accretion on to their central black holes. We use 100 and 160 μ m fluxes from GOODS-Herschel – the deepest survey yet undertaken by the Herschel telescope – and show that in the vast majority of cases (i.e. >94 per cent) these fluxes are dominated by emission from the host galaxy. As such, these far-infrared bands provide an uncontaminated view of star formation in the AGN host galaxies. We find no evidence of any correlation between the X-ray and infrared luminosities of moderate AGNs at any redshift, suggesting that global star formation is decoupled from nuclear (i.e. AGN) activity in these galaxies. On the other hand, we confirm that the star formation rates of AGN hosts increase strongly with redshift, by a factor of 43^(+27)_(−18) from z < 0.1 to z = 2–3 for AGNs with the same range of X-ray luminosities. This increase is entirely consistent with the factor of 25–50 increase in the specific star formation rates (SSFRs) of normal, star-forming (i.e. main-sequence) galaxies over the same redshift range. Indeed, the average SSFRs of AGN hosts are only marginally (i.e. ≈20 per cent) lower than those of main-sequence galaxies at all surveyed redshifts, with this small deficit being due to a fraction of AGNs residing in quiescent (i.e. low SSFR) galaxies. We estimate that 79 ± 10 per cent of moderate-luminosity AGNs are hosted in main-sequence galaxies, 15 ± 7 per cent in quiescent galaxies and <10 per cent in strongly starbursting galaxies. We derive the fractions of all main-sequence galaxies at z < 2 that are experiencing a period of moderate nuclear activity, noting that it is strongly dependent on galaxy stellar mass (M_(stars)), rising from just a few per cent at M_(stars) ∼ 10^(10) M_⊙ to ≳20 per cent at M_(stars)≥ 10^(11) M_⊙. Our results indicate that it is galaxy stellar mass that is most important in dictating whether a galaxy hosts a moderate-luminosity AGN. We argue that the majority of moderate nuclear activity is fuelled by internal mechanisms rather than violent mergers, which suggests that high-redshift disc instabilities could be an important AGN feeding mechanism
Characterizing the far-infrared properties of distant X-ray detected AGNs: evidence for evolution in the infrared–X-ray luminosity ratio
We investigate the far-infrared (FIR) properties of X-ray sources detected in the Chandra Deep Field-South (CDF-S) survey using the ultradeep 70 and 24 μm Spitzer observations taken in this field. Since only 30 (i.e. ≈ 10 per cent) of the 266 X-ray sources in the region of the 70 μm observations are detected at 70 μm, we rely on stacking analyses of the 70 μm data to characterize the average 70 μm properties of the X-ray sources as a function of redshift, X-ray luminosity and X-ray absorption. Using Spitzer-IRS data of the Swift-Burst Alert Telescope (BAT) sample of z ≈ 0 active galactic nuclei (AGNs), we show that the 70/24 μm flux ratio can distinguish between AGN-dominated and starburst-dominated systems out to z ≈ 1.5 . Among the X-ray sources detected at 70 μm, we note a large scatter in the observed 70/24 μm flux ratios, spanning almost a factor of 10 at similar redshifts, irrespective of object classification, suggesting a range of AGN:starburst ratios. From stacking analyses we find that the average observed 70/24 μm flux ratios of AGNs out to an average redshift of 1.5 are similar to z ≈ 0 AGNs with similar X-ray luminosities (L_X = 10^(42-44) erg s^(−1)) and absorbing column densities (N_H ≤ 10^(23) cm^(−2)) . Furthermore, both high-redshift and z ≈ 0 AGNs follow the same tendency towards warmer 70/24 μm colours with increasing X-ray luminosity (LX). From analyses of the Swift-BAT sample of z ≈ 0 AGNs, we note that the 70 μm flux can be used to determine the IR (8–1000 μm) luminosities of high-redshift AGNs. We use this information to show that L_X = 10^(42-43) erg s^(−1) AGNs at high redshifts (z = 1–2) have IR to X-ray luminosity ratios (L_(IR)/L_X) that are, on average, 4.7^(+10.2)_(−2.0) and 12.7+7.1−2.6 times higher than AGNs with similar X-ray luminosities at z = 0.5–1 and ≈0, respectively. By comparison, we find that the L_(IR)/L_X ratios of L_X= 10^(43-44) erg s^(−1) AGNs remain largely unchanged across this same redshift interval. We explore the consequences that these results may have on the identification of distant, potentially Compton-thick AGNs using L_(IR)/L_X ratios. In addition, we discuss possible scenarios for the observed increase in the L_(IR)/L_X ratio with redshift, including changes in the dust covering factor of AGNs and/or the star formation rates of their host galaxies. Finally, we show how deep observations to be undertaken by the Herschel Space Observatory will enable us to discriminate between these proposed scenarios and also identify Compton-thick AGNs at high redshifts
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