Compact Nuclei in Galaxies at Moderate Redshift

Near-infrared long-slit spectroscopy has been used to study the stellar population (SP) of the low luminosity active galactic nuclei (AGNs) and matched analogues (LLAMA) sample. To perform the SP fits we have employed the X-shooter simple stellar population models together with the starlight code. Our main conclusions are: The star formation history of the AGNs is very complex, presenting many episodes of star formation during their lifetimes. In general, AGN hosts have higher fractions of intermediate-age SP (light-weighted mean ages, <t > L ≲ 4.5 Gyr) when compared with their analogues (<t > L ≲ 8.0 Gyr). AGNs are more affected by reddening and require significant fractions of featureless continuum and hot dust components. The ratio between the AGN radiated energy and the gravitational potential energy of the molecular gas (ERad/EPG) for the AGN is compared with the <t > L and a possible anticorrelation is observed. This suggests that the AGN is affecting the star formation in these galaxies, in the sense that more energetic AGN [log(ERad/EPG) ≳ 3] tend to host nuclear younger SP (<t > L ≲4 Gyr). We found that the recent (t <2 Gyr) returned (recycled) stellar mass is higher in AGN than in the controls. We also provide evidence that the mass-loss of stars would be enough to feed the AGN, thus providing observational constraints for models that predict that AGN feeding is partially due to the recycled gas from dying stars.

Recent surveys have detected Lya emission from z = 4.5 to 6.5 at luminosities as low as 10 41 erg s -1 . There is good evidence that low numbers of active galactic nuclei (AGN) are among observed faint Lya emitters. Combining these observations with an empirical relation between the intrinsic Lya and B-band luminosities of AGN, we obtain an upper limit on the number density of AGN with absolute magnitudes M B ∈ [-16, -19] at z = 4.5-6.5. These AGN are up to two orders of magnitude fainter than those discovered in the Chandra Deep Field, resulting in the faintest observational constraints to date at these redshifts. At z = 4.5, the power-law slope of the very faint end of the luminosity function of AGN is shallower than the slope observed at lower redshifts, β 1 < 1.6, at the 98 per cent confidence level. In fact, we find marginal evidence that the luminosity function rises with luminosity, corresponding to a power-law slope β 1 < 0, at magnitudes fainter than M B ∼ -20 (75 per cent confidence level). These results suggest either that accretion on to lower mass black holes is less efficient than on to their more massive counterparts, or that the number of black holes powering AGN with M B ≥ -20 is lower than expected from the M BH -σ relation by one-two orders of magnitude. Extrapolating from reverberation-mapping studies suggests that these black holes would have M BH = 10 6 -10 7 M ⊙ . To facilitate the identification of AGN among observed Lya emitters, we derive observational properties of faint AGN in the Lya line, as well as in the X-ray and optical bands.

view Abstract Citations (262) References (82) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Nuclear Activity of Interacting Galaxies Dahari, O. Abstract A search for active galactic nuclei among interacting galaxies is reported. A sample of 167 systems of interacting and asymmetric galaxies was observed spectrophotometrically in the spectral range 4700-7100 A. The results are compared with a sample of isolated galaxies. It is found that (1) there are no Seyfert nuclei in elliptical or dwarf irregular galaxies of the sample; (2) there is an excess of Seyfert nuclei among interacting spirals, but it is only at the 90 percent confidence level; (3) this excess becomes statistically significant (98 percent) when only strongly interacting spirals are included (four new Seyfert nuclei are presented); (4) in the subgroup of galaxies with extreme tidal distortions, no Seyfert nuclei were found. Publication: The Astrophysical Journal Supplement Series Pub Date: April 1985 DOI: 10.1086/191021 Bibcode: 1985ApJS...57..643D Keywords: Astronomical Photometry; Astronomical Spectroscopy; Galactic Clusters; Galactic Nuclei; Interacting Galaxies; Seyfert Galaxies; Active Galactic Nuclei; Astronomical Catalogs; Data Reduction; Gravitational Effects; Interactions; Spectrophotometry; Spiral Galaxies; Astrophysics full text sources ADS | data products NED (249) SIMBAD (235) Related Materials (1) Erratum: 1986ApJS...60..601D

We present Spitzer 3.6-160 micron images, Spitzer mid-infrared spectra, and JCMT SCUBA 850 micron images of the Sombrero Galaxy (NGC 4594), an Sa galaxy with a 10^9 M_solar low luminosity active galactic nucleus (AGN). The brightest infrared sources in the galaxy are the nucleus and the dust ring. The spectral energy distribution of the AGN demonstrates that, while the environment around the AGN is a prominent source of mid-infrared emission, it is a relatively weak source of far-infrared emission, as had been inferred for AGN in previous research. The weak nuclear 160 micron emission and the negligible polycyclic aromatic hydrocarbon emission from the nucleus also implies that the nucleus is a site of only weak star formation activity and the nucleus contains relatively little cool interstellar gas needed to fuel such activity. We propose that this galaxy may be representative of a subset of low ionization nuclear emission region galaxies that are in a quiescent AGN phase because of the lack of gas needed to fuel circumnuclear star formation and Seyfert-like AGN activity. Surprisingly, the AGN is the predominant source of 850 micron emission. We examine the possible emission mechanisms that could give rise to the 850 micron emission and find that neither thermal dust emission, CO line emission, bremsstrahlung emission, nor the synchrotron emission observed at radio wavelengths can adequately explain the measured 850 micron flux density by themselves. The remaining possibilities for the source of the 850 micron emission include a combination of known emission mechanisms, synchrotron emission that is self-absorbed at wavelengths longer than 850 microns, or unidentified spectral lines in the 850 micron band.

We discuss the efficiency of stellar gravity torques as a mechanism to account for the feeding of the central engines of four low luminosity Active Galactic Nuclei (AGN): NGC 4321 (HII nucleus/LINER), NGC 4826 (HII nucleus/LINER), NGC 4579 (LINER 1.9/Seyfert 1.9) and NGC 6951 (Seyfert 2). These galaxies have been observed as part of the NUclei of GAlaxies–(NUGA) CO project, aimed at the study of AGN fueling mechanisms. Our calculations allow us to derive the characteristic time-scales for gas flows and discuss whether torques from the stellar potentials are efficient enough to drain the gas angular momentum in the inner 1 kpc of these galaxies. The stellar potentials are derived using high-resolution near infrared (NIR) images and the averaged effective torques on the gas are estimated using the high-resolution (~0.5´´–2´´) CO maps of the galaxies. Results indicate paradoxically that feeding should be thwarted close to the AGNs: in the four cases analyzed, gravity torques are mostly positive inside pc, resulting in no inflow on these scales. As a possible solution for the paradox, we speculate that the agent responsible for driving inflow to still smaller radii is transient and thus presently absent in the stellar potential. Alternatively, the gravity torque barrier associated with the Inner Lindblad Resonance of the bars in these galaxies could be overcome by other mechanisms that become competitive in due time against gravity torques. In particular, we estimate on a case-by-case basis the efficiency of viscosity versus gravity torques to drive AGN fueling. We find that viscosity can counteract moderate-to-low gravity torques on the gas if it acts on a nuclear ring of high gas surface density contrast and ~a few 100 pc size. We propose an evolutionary scenario in which gravity torques and viscosity act in concert to produce recurrent episodes of activity during the typical lifetime of any galaxy. In this scenario the recurrence of activity in galaxies is indirectly related to that of the bar instabilities although the active phases are not necessarily coincident with the maximum strength of a single bar episode. The general implications of these results for the current understanding of fueling of low-luminosity AGN are discussed.

The 4 Ms Chandra Deep Field-South (CDF-S) and other deep X-ray surveys have been highly effective at selecting active galactic nuclei (AGN). However, cosmologically distant low-luminosity AGN (LLAGN) have remained a challenge to identify due to significant contribution from the host galaxy. We identify long-term X ray variability (approx. month years, observed frame) in 20 of 92 CDF-S galaxies spanning redshifts approx equals 00.8 - 1.02 that do not meet other AGN selection criteria. We show that the observed variability cannot be explained by X-ray binary populations or ultraluminous X-ray sources, so the variability is most likely caused by accretion onto a supermassive black hole. The variable galaxies are not heavily obscured in general, with a stacked effective power-law photon index of Gamma(sub Stack) approx equals 1.93 +/- 0.13, and arc therefore likely LLAGN. The LLAGN tend to lie it factor of approx equal 6-89 below the extrapolated linear variability-luminosity relation measured for luminous AGN. This may he explained by their lower accretion rates. Variability-independent black-hole mass and accretion-rate estimates for variable galaxies show that they sample a significantly different black hole mass-accretion-rate space, with masses a factor of 2.4 lower and accretion rates a factor of 22.5 lower than variable luminous AGNs at the same redshift. We find that an empirical model based on a universal broken power-law power spectral density function, where the break frequency depends on SMBH mass and accretion rate, roughly reproduces the shape, but not the normalization, of the variability-luminosity trends measured for variable galaxies and more luminous AGNs.

Context. Active galactic nuclei (AGNs) of galaxies play an important role in the life and evolution of galaxies through the impact they exert on certain properties and on the evolutionary path of galaxies. It is well known that infrared (IR) emission is useful for selecting galaxies with AGNs, although it has been observed that there is contamination by star-forming galaxies. Aims. We investigate the properties of galaxies that host AGNs that are identified at mid- (MIR) and near-IR wavelengths. The sample of AGNs selected at IR wavelengths was confirmed using optical spectroscopy and X-ray photometry. We study the near-UV, optical, near-IR and MIR properties, as well as the [O III] λ5007 luminosity, black hole mass, and morphology properties of optical and IR colour-selected AGNs. Methods. We selected AGN candidates using two MIR colour selection techniques: a power-law emission method, and a combination of MIR and near-IR selection techniques. We confirmed the AGN selection with two line diagnostic diagrams that use the ratio [O III]/Hβ and the emission line width σ[O III] (kinematics–excitation diagram, KEx) and the host galaxy stellar mass (mass–excitation diagram, MEx), as well as X-ray photometry. Results. According to the diagnostic diagrams, the methods with the greatest success in selecting AGNs are those that use a combination of a mid- and near-IR selection technique and a power-law emission. The method that uses a combination of MIR and near-IR observations selects a large number of AGNs and is reasonably efficient in the success rate (61%) and total number of AGNs recovered. We also find that the KEx method presents contamination of star-forming galaxies within the AGN selection box. According to morphological studies based on the Sérsic index, AGN samples have higher percentages of galaxy morphologies with bulge+disk components than galaxies without AGNs.

We present the clustering properties and halo occupation distribution (HOD) modelling of very low redshift, hard X-ray-detected active galactic nuclei (AGN) using cross-correlation function measurements with Two-Micron All Sky Survey galaxies. Spanning a redshift range of $0.007 < z < 0.037$, with a median $z=0.024$, we present a precise AGN clustering study of the most local AGN in the Universe. The AGN sample is drawn from the SWIFT/BAT 70-month and INTEGRAL/IBIS eight year all-sky X-ray surveys and contains both type I and type II AGN. We find a large-scale bias for the full AGN sample of $b=1.04^{+0.10}_{-0.11}$, which corresponds to a typical host dark matter halo mass of $M_{\rm h}^{\rm typ}=12.84^{+0.22}_{-0.30}\,h^{-1} M_{\odot}$. When split into low and high X-ray luminosity and type I and type II AGN subsamples, we detect no statistically significant differences in the large-scale bias parameters. However, there are differences in the small-scale clustering which are reflected in the full HOD model results. We find that low and high X-ray luminosity AGN, as well as type I and type II AGN, occupy dark matter haloes differently, with 3.4$\sigma$ and 4.0$\sigma$ differences in their mean halo masses, respectively, when split by luminosity and type. The latter finding contradicts a simple orientation-based AGN unification model. As a by-product of our cross-correlation approach, we also present the first HOD model of 2MASS galaxies.

We present an X-ray stacking analysis of ∼75,000 star-forming galaxies between 0.1 &lt; z &lt; 5.0 using the Chandra COSMOS-Legacy survey to study the X-ray emission of low-luminosity active galactic nuclei (AGN) and its connection to host galaxy properties. The stacks at z &lt; 0.9 have luminosity limits as low as 1040–1041 erg s−1, a regime in which X-ray binaries (XRBs) can dominate the X-ray emission. Comparing the measured luminosities to established XRB scaling relations, we find that the redshift evolution of the luminosity per star formation rate (SFR) of XRBs depends sensitively on the assumed obscuration and may be weaker than previously found. The XRB scaling relation based on stacks from the Chandra Deep Field South overestimates the XRB contribution to the COSMOS high specific SFR stacks, possibly due to a bias affecting the CDF-S stacks because of their small galaxy samples. After subtracting the estimated XRB contribution from the stacks, we find that most stacks at z &gt; 1.3 exhibit a significant X-ray excess indicating nuclear emission. The AGN emission is strongly correlated with stellar mass but does not exhibit an additional correlation with SFR. The hardness ratios of the high-redshift stacks indicate that the AGN are substantially obscured (N H ∼ 1023 cm−2). These obscured AGN are not identified by IRAC color selection and have L X ∼ 1041–1043 erg s−1, consistent with accretion at an Eddington rate of ∼10−3 onto 107–108 M ⊙ black holes. Combining our results with other X-ray studies suggests that AGN obscuration depends on stellar mass and an additional variable, possibly the Eddington rate.

Context.Understanding the physics and geometry of accretion and ejection around super massive black holes (SMBHs) is important to understand the evolution of active galactic nuclei (AGN) and therefore of the large scale structures of the Universe.Aims.We aim at providing a simple, coherent, and global view of the sub-parsec accretion and ejection flow in AGN with varying Eddington ratio,ṁ, and black hole mass,MBH.Methods.We made use of theoretical insights, results of numerical simulations, as well as UV and X-ray observations to review the inner regions of AGN by including different accretion and ejection modes, with special emphasis on the role of radiation in driving powerful accretion disk winds from the inner regions around the central SMBH.Results.We propose fiveṁregimes where the physics of the inner accretion and ejection flow around SMBHs is expected to change, and that correspond observationally to quiescent and inactive galaxies; low luminosity AGN (LLAGN); Seyferts and mini-broad absorption line quasars (mini-BAL QSOs); narrow line Seyfert 1 galaxies (NLS1s) and broad absorption line quasars (BAL QSOs); and super-Eddington sources. We include in this scenario radiation-driven disk winds, which are strong in the highṁ, largeMBHregime, and possibly present but likely weak in the moderateṁ, smallMBHregime.Conclusions.A great diversity of the accretion/ejection flows in AGN can be explained to a good degree by varying just two fundamental properties: the Eddington ratioṁand the black hole massMBH, and by the inclusion of accretion disk winds that can naturally be launched by the radiation emitted from luminous accretion disks.

With the aim of assessing the effects of bars on active galactic nuclei (AGN), we present an analysis of host characteristics and nuclear activity of AGN galaxies with and without bars selected from the Sloan Digital Sky Survey Data Release 7 (SDSS-DR7). By visual inspection of SDSS images we classified the hosts of face-on AGN spiral galaxies brighter than g < 16.5 into barred or unbarred. With the purpose of providing an appropriate quantification of the effects of bars, we also constructed a suitable control sample of unbarred AGN galaxies with similar redshift, magnitude, morphology, bulge sizes and local environment distributions. We find that the bar fraction, with respect to the full sample of spiral face-on AGN host galaxies, is 28.5%, in good agreement with previous works. Barred AGN host galaxies show an excess of young stellar populations dominated by red u-r and g-r colors, with respect to the control sample, suggesting that bars produce an important effect on galaxy properties of AGN hosts. Regarding the nuclear activity distribution, we find that barred AGN galaxies show a shift toward higher Lum[OIII] values with respect to AGN without bars. In addition, we also find that this trend is more significant in less massive, younger stellar population and bluer AGN host galaxies. We found that the fraction of powerful AGN increase towards more massive hosts with bluer colors and younger stellar populations residing in denser environments. However, barred host AGN systematically show a higher fraction of powerful active nuclei galaxies with respect to the control sample. We also explored the accretion rate onto the central black holes finding that barred AGN show an excess of objects with high accretion rate values with respect to unbarred AGN galaxies.

We present an analysis of 10 ks snapshot Chandra observations of 12 shocked post-starburst galaxies, which provide a window into the unresolved question of active galactic nuclei (AGN) activity in post-starburst galaxies and its role in the transition of galaxies from active star formation to quiescence. While seven of the 12 galaxies have statistically significant detections (with two more marginal detections), the brightest only obtained 10 photons. Given the wide variety of hardness ratios in this sample, we chose to pursue a forward-modeling approach to constrain the intrinsic luminosity and obscuration of these galaxies, rather than stacking. We constrain the intrinsic luminosity of obscured power laws based on the total number of counts and spectral shape, itself mostly set by the obscuration, with hardness ratios consistent with the data. We also tested thermal models. While all the galaxies have power-law models consistent with their observations, a third of the galaxies are better fit as an obscured power law and another third are better fit as thermal emission. If these post-starburst galaxies, early in their transition, contain AGNs, then these are mostly confined to lower obscuration (N H ≤ 1023 cm−2) and lower luminosity (L 2−10 keV ≤ 1042 erg s−1). Two galaxies, however, are clearly best fit as significantly obscured AGNs. At least half of this sample shows evidence of at least low-luminosity AGN activity, though none could radiatively drive out the remaining molecular gas reservoirs. Therefore, these AGNs are more likely along for the ride, having been fed gas by the same processes driving the transition.

Galaxy major mergers are indicated as one of the principal pathways to trigger active galactic nuclei (AGN). We present the first statistical analysis of the major merger and AGN connection in the Euclid Deep Fields, and showcase the statistical power of the Euclid data. We constructed a stellar-mass-complete (M_⋆&gt;10^ M_⊙) sample of galaxies from the quick data release (Q1) in the redshift range z=0.5--2. We selected AGN using X-ray detections, optical spectroscopy, and mid-infrared (MIR) colours, and by processing observations with an image decomposition algorithm. We used convolutional neural networks trained on cosmological hydrodynamic simulations to classify galaxies as mergers and non-mergers. We found a larger fraction of AGN in mergers compared to the non-merger controls for all AGN selections, with AGN excess factors ranging from two to six. The largest excess we observed was in the MIR AGN. Likewise, a generally larger merger fraction (f_̊m merg) was seen in active galaxies than in the non-active controls, with the excess depending on the AGN selection method. Furthermore, we analysed f_̊m merg as a function of the AGN bolometric luminosity (L_̊m bol) and the contribution of the point-source component to the total galaxy light in the IE-band (f_ PSF ) as a proxy for the relative AGN contribution fraction. We uncovered a rising f_̊m merg, with an increasing f_ PSF up to f_ PSF ≃ 0.55, after which we observed a decreasing trend. In the range f_ PSF = 0.3--0.7, mergers appear to be the dominant AGN fuelling mechanism. We then derived the point-source luminosity (L_ PSF ) and showed that f_̊m merg monotonically increases as a function of L_ PSF at z&lt;0.9, with f_ ̊m merg ≥50% for L_ PSF ≃ 2 . Similarly, at $0.9łeq z łeq 2$, f_̊m merg rises as a function of L_ PSF though mergers do not dominate until L_ PSF ≃ 10^ . For the X-ray and spectroscopically detected AGN, we derived the bolometric luminosity, L_ bol, which has a positive correlation with f_ merg for X-ray AGN, while there is a less pronounced trend for spectroscopically selected AGN due to the smaller sample size. At L_ bol AGN mostly reside in mergers. We conclude that mergers are most strongly associated with the most powerful and dust-obscured AGN, which are typically linked to a fast-growing phase of the supermassive black hole, while other mechanisms, such as secular processes, might be the trigger of less luminous and dominant AGN.

We present near- and mid-infrared spectra of eight low-luminosity active galactic nuclei (LLAGN), spanning nearly 4 orders of magnitude in black hole mass and Eddington ratio, obtained with JWST/NIRSpec and MIRI as part of the ReveaLLAGN program, along with identical archival data of Cen A. The high spatial resolution of JWST cleanly separates active galactic nuclei (AGN) emission from host-galaxy contamination, enabling detections of high-ionization-potential lines more than an order of magnitude fainter than previously measured. Emission-line diagnostics reveal a transition at log( L bol / L Edd ) ∼−3.5, where the spectral energy distribution becomes increasingly deficient in ultraviolet photons. We find that rotational H 2 excitation temperatures are elevated (∼500 K higher) compared to both higher-luminosity AGN and star-forming galaxies, while the H 2 (0–0)S(3)/PAH 11.3 μ m ratios are consistent with those observed in the AGN population. We discuss the possible roles of outflows, jets, and X-ray-dominated regions in shaping the interstellar medium surrounding LLAGN. Silicate emission at ∼10 μ m, localized to the nuclear region, is detected in most ReveaLLAGN targets. This dataset offers the first comprehensive JWST-based characterization of infrared emission lines in the nuclear regions of LLAGN.

We present the first sub-arcsecond resolution two-dimensional stellar kinematics and X-ray observations of the prototypical starburst galaxy NGC253 which define the position and nature of the galactic nucleus. We get an estimate of the stellar kinematic center location corresponding to an area of r~1.2" centered 0.7" southwest from the radio core (TH2). Newly processed Chandra data reveal a central hard X-ray source (X-1) lying 0.4" southwest from the kinematic center. Very accurate alignment between radio, infrared and X-ray sources shows that TH2, the IR photometric center and X-1 are not associated with each other. As the kinematic center is consistent with TH2 and X-1, we consider the two as possible galactic nucleus candidates. Although TH2 is the strongest radio source in the nuclear region, it does not have any infrared, optical or X-ray counterparts. If the kinematic center is associated with this source, by analogy we suggest that the nucleus of NGC253 resembles our Galactic Center SgrA*. On the other hand, X-1 is a heavily absorbed object only detected at energies >2 keV. If X-1 is instead associated with the kinematic center, the nucleus of NGC253 is compatible with an obscured low luminosity active galactic nucleus (AGN) or a spatially resolved super star cluster (SSC) brightening up in X-rays most probably due to young supernovae or supernova remnants. If no SSC is associated with the kinematic center, we conclude that NGC253 is a galaxy in which a strong starburst and a weak AGN (either TH2 or X-1) coexist. Results from few other high resolution studies of nearby starburst galaxies indicate that the AGN in these systems, if present, is always in the low luminosity regime. This may indicate that the onset of nuclear activity in galaxies is closely related with the occurrence of star formation, and that we are witnessing the emergence or disappearance of an AGN.