Knud Jahnke

Pennsylvania State University, University Park, Maryland, United States

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Publications (58)266.94 Total impact

  • Mauricio Cisternas · Knud Jahnke
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    ABSTRACT: According to the current co-evolution picture, most present-day galaxies experienced at least one phase of vigorous black hole (BH) activity in the past, during which a tight link between galaxy and BH gets established. While during the last two decades we have witnessed tremendous progress in the field, additional robust observational constraints are required on how galaxy and BH related at earlier times, and which mechanisms are responsible for triggering these BH growth phases. In our recent studies, we analyzed a large sample of active galactic nuclei (AGN) out to z ~ 1 from the COSMOS survey (Scoville et al. 2007), allowing us to study in detail growing BHs together with their host galaxies. In Cisternas et al. (2011b) we found that, for a sample of 32 active galaxies at z ~ 0.7, BH mass scales with total galaxy stellar mass in the same way as it does locally, at z = 0, with galactic bulge mass. I will argue that for these galaxies to obey the local relation only a disk-to-bulge stellar mass redistribution is needed, likely driven by passive secular evolution. I will also present the results from Cisternas et al. (2011a), aiming to understand the relevance of major mergers as AGN activity triggering mechanisms. By looking for merging signatures on the morphologies of 140 AGN (some examples shown in Figure 1), and comparing them with a sample of over 1200 matched inactive galaxies, we found that the merger fraction between samples is statistically the same, at roughly 15%. Together with the fact that the majority of the AGN host galaxies are disk-dominated, unlikely relics of a recent major merger, these results are the strongest evidence to date that secular evolution rather than major merging has dominated BH fueling at least since z ~ 1.
    No preview · Article · Aug 2015 · Proceedings of the International Astronomical Union
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    ABSTRACT: We measure the location and evolutionary vectors of 69 Herschel-detected broad-line active galactic nuclei (BLAGNs) in the M_BH-M_* plane. BLAGNs are selected from the COSMOS and CDF-S fields, and span the redshift range 0.2< z<2.1. Black-hole masses are calculated using archival spectroscopy and single-epoch virial mass estimators, and galaxy total stellar masses are calculated by fitting the spectral energy distribution (subtracting the BLAGN component). The mass-growth rates of both the black hole and galaxy are calculated using Chandra/XMM-Newton X-ray and Herschel far-infrared data, reliable measures of the BLAGN accretion and galaxy star formation rates, respectively. We use Monte Carlo simulations to account for biases in our sample, due to both selection limits and the steep slope of the massive end of the galaxy stellar-mass distribution. We find our sample is consistent with no evolution in the M_BH-M_* relation from z~2 to z~0. BLAGNs and their host galaxies which lie off the black hole mass galaxy total stellar mass relation tend to have evolutionary vectors anti-correlated with their mass ratios: that is, galaxies with over-massive (under-massive) black holes tend to have a low (high) ratio of the specific accretion rate to the specific star formation rate. We also use the measured growth rates to estimate the preferred AGN duty cycle for our galaxies to evolve most consistently onto the local M_BH-M_Bul relation. Under reasonable assumptions of exponentially declining star formation histories, the data suggest a non-evolving (no more than a factor of a few) BLAGN duty cycle among star-forming galaxies of 10% (1sigma range of 1-42% at z<1 and 2-34% at z>1).
    Preview · Article · Feb 2015 · The Astrophysical Journal
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    ABSTRACT: Limited studies have been performed on the radio-loud fraction in X-ray selected type 1 AGN samples. The consistency between various radio-loudness definitions also needs to be checked. We measure the radio-loudness of the 407 type 1 AGNs in the XMM-COSMOS quasar sample using nine criteria from the literature (six defined in the rest-frame and three defined in the observed frame): $R_L=\log(L_{5GHz}/L_B)$, $q_{24}=\log(L_{24\mu m}/L_{1.4GHz})$, $R_{uv}=\log(L_{5GHz}/L_{2500\AA})$, $R_{i}=\log(L_{1.4GHz}/L_i)$, $R_X=\log(\nu L_{\nu}(5GHz)/L_X)$, $P_{5GHz}=\log(P_{5GHz}(W/Hz/Sr))$, $R_{L,obs}=\log(f_{1.4GHz}/f_B)$ (observed frame), $R_{i,obs}=\log(f_{1.4GHz}/f_i)$ (observed frame), and $q_{24, obs}=\log(f_{24\mu m}/f_{1.4GHz})$ (observed frame). Using any single criterion defined in the rest-frame, we find a low radio-loud fraction of $\lesssim 5\%$ in the XMM-COSMOS type 1 AGN sample, except for $R_{uv}$. Requiring that any two criteria agree reduces the radio-loud fraction to $\lesssim 2\%$ for about 3/4 of the cases. The low radio-loud fraction cannot be simply explained by the contribution of the host galaxy luminosity and reddening. The $P_{5GHz}=\log(P_{5GHz}(W/Hz/Sr))$ gives the smallest radio-loud fraction. Two of the three radio-loud fractions from the criteria defined in the observed frame without k-correction ($R_{L,obs}$ and $R_{i,obs}$) are much larger than the radio-loud fractions from other criteria.
    Full-text · Article · Aug 2014
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    ABSTRACT: We present optical integral field spectroscopy (IFS) observations of the Mice, a major merger between two massive (>10^11Msol) gas-rich spirals NGC4676A and B, observed between first passage and final coalescence. The spectra provide stellar and gas kinematics, ionised gas properties and stellar population diagnostics, over the full optical extent of both galaxies. The Mice provide a perfect case study highlighting the importance of IFS data for improving our understanding of local galaxies. The impact of first passage on the kinematics of the stars and gas has been significant, with strong bars likely induced in both galaxies. The barred spiral NGC4676B exhibits a strong twist in both its stellar and ionised gas disk. On the other hand, the impact of the merger on the stellar populations has been minimal thus far: star formation induced by the recent close passage has not contributed significantly to the global star formation rate or stellar mass of the galaxies. Both galaxies show bicones of high ionisation gas extending along their minor axes. In NGC4676A the high gas velocity dispersion and Seyfert-like line ratios at large scaleheight indicate a powerful outflow. Fast shocks extend to ~6.6kpc above the disk plane. The measured ram pressure and mass outflow rate (~8-20Msol/yr) are similar to superwinds from local ULIRGs, although NGC4676A has only a moderate infrared luminosity of 3x10^10Lsol. Energy beyond that provided by the mechanical energy of the starburst appears to be required to drive the outflow. We compare the observations to mock kinematic and stellar population maps from a merger simulation. The models show little enhancement in star formation during and following first passage, in agreement with the observations. We highlight areas where IFS data could help further constrain the models.
    Full-text · Article · Jul 2014 · Astronomy and Astrophysics
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    ABSTRACT: We perform a quantitative morphological comparison between the hosts of Active Galactic Nuclei (AGN) and quiescent galaxies at intermediate redshifts (z~0.7). The imaging data are taken from the large HST/ACS mosaics of the GEMS and STAGES surveys. Our main aim is to test whether nuclear activity at this cosmic epoch is triggered by major mergers. Using images of quiescent galaxies and stars, we create synthetic AGN images to investigate the impact of an optical nucleus on the morphological analysis of AGN hosts. Galaxy morphologies are parameterized using the asymmetry index A, concentration index C, Gini coefficient G and M20 index. A sample of ~200 synthetic AGN is matched to 21 real AGN in terms of redshift, host brightness and host-to-nucleus ratio to ensure a reliable comparison between active and quiescent galaxies. The optical nuclei strongly affect the morphological parameters of the underlying host galaxy. Taking these effects into account, we find that the morphologies of the AGN hosts are clearly distinct from galaxies undergoing violent gravitational interactions. In fact, the host galaxies' distributions in morphological descriptor space are more similar to undisturbed galaxies than major mergers. Intermediate-luminosity (Lx < 10^44 erg/s) AGN hosts at z~0.7 show morphologies similar to the general population of massive galaxies with significant bulges at the same redshifts. If major mergers are the driver of nuclear activity at this epoch, the signatures of gravitational interactions fade rapidly before the optical AGN phase starts, making them undetectable on single-orbit HST images, at least with usual morphological descriptors. This could be investigated in future synthetic observations created from numerical simulations of galaxy-galaxy interactions.
    Full-text · Article · Oct 2012 · Astronomy and Astrophysics
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    ABSTRACT: We define a quasar-galaxy mixing diagram using the slopes of their spectral energy distributions (SEDs) from 1\mu m to 3000\AA\ and from 1\mu m to 3\mu m in the rest frame. The mixing diagram can easily distinguish among quasar-dominated, galaxy-dominated and reddening-dominated SED shapes. By studying the position of the 413 XMM selected Type 1 AGN in the wide-field "Cosmic Evolution Survey" (COSMOS) in the mixing diagram, we find that a combination of the Elvis et al. (1994, hereafter E94) quasar SED with various contributions from galaxy emission and some dust reddening is remarkably effective in describing the SED shape from 0.3-3\mu m for large ranges of redshift, luminosity, black hole mass and Eddington ratio of type 1 AGN. In particular, the location in the mixing diagram of the highest luminosity AGN is very close (within 1\sigma) to that of the E94 SED. The mixing diagram can also be used to estimate the host galaxy fraction and reddening in quasar. We also show examples of some outliers which might be AGN in different evolutionary stages compared to the majority of AGN in the quasar-host galaxy co-evolution cycle.
    Full-text · Article · Oct 2012 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: The mid-infrared-to-ultraviolet (0.1–10 μm) spectral energy distribution (SED) shapes of 407 X-ray-selected radio-quiet type 1 active galactic nuclei (AGN) in the wide-field ‘Cosmic Evolution Survey’ (COSMOS) have been studied for signs of evolution. For a sub-sample of 200 radio-quiet quasars with black hole mass estimates and host galaxy corrections, we studied their mean SEDs as a function of a broad range of redshift, bolometric luminosity, black hole mass and Eddington ratio, and compared them with the Elvis et al. (E94) type 1 AGN mean SED. We found that the mean SEDs in each bin are closely similar to each other, showing no statistical significant evidence of dependence on any of the analysed parameters. We also measured the SED dispersion as a function of these four parameters, and found no significant dependences. The dispersion of the XMM-COSMOS SEDs is generally larger than E94 SED dispersion in the ultraviolet, which might be due to the broader ‘window function’ for COSMOS quasars, and their X-ray-based selection.
    Full-text · Article · Oct 2012 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: Euclid is a European Space Agency medium class mission selected for launch in 2019 within the Cosmic Vision 2015-2025 programme. The main goal of Euclid is to understand the origin of the accelerated expansion of the Universe. Euclid will explore the expansion history of the Universe and the evolution of cosmic structures by measuring shapes and redshifts of galaxies as well as the distribution of clusters of galaxies over a large fraction of the sky. Although the main driver for Euclid is the nature of dark energy, Euclid science covers a vast range of topics, from cosmology to galaxy evolution to planetary research. In this review we focus on cosmology and fundamental physics, with a strong emphasis on science beyond the current standard models. We discuss five broad topics: dark energy and modified gravity, dark matter, initial conditions, basic assumptions and questions of methodology in the data analysis. This review has been planned and carried out within Euclid's Theory Working Group and is meant to provide a guide to the scientific themes that will underlie the activity of the group during the preparation of the Euclid mission.
    Full-text · Article · Jun 2012 · Living Reviews in Relativity
  • Marco Barden · Knud Jahnke · Boris Häußler
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    ABSTRACT: Bandpass shifting and the (1+z)5 surface brightness dimming (for a fixed width filter) make standard tools for the extraction of structural parameters of galaxies wavelength dependent. If only few (or one) observed high-res bands exist, this dependence has to be corrected to make unbiased statements on the evolution of structural parameters or on galaxy subsamples defined by morphology. FERENGI artificially redshifts low-redshift galaxy images to different redshifts by applying the correct cosmological corrections for size, surface brightness and bandpass shifting. A set of artificially redshifted galaxies in the range 0.1<z<1.1 using a set of ~100 SDSS low-redshift (v<7000 km s-1) images as input has been created to use as a training set of realistic images of galaxies of diverse morphologies and a large range of redshifts for the GEMS and COSMOS galaxy evolution projects. This training set allows other studies to investigate and quantify the effects of cosmological redshift on the determination of galaxy morphologies, distortions, and other galaxy properties that are potentially sensitive to resolution, surface brightness, and bandpass issues. The data sets are also available for download from the FERENGI website.
    No preview · Article · Mar 2012
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    ABSTRACT: We present a new automatic method to identify galaxy mergers using the morphological information contained in the residual images of galaxies after the subtraction of a smooth Sérsic model. The removal of the bulk signal from the host galaxy light is done with the aim of detecting the much fainter and elusive minor mergers. The specific morphological parameters that are used in the merger diagnostic suggested here are the residual flux fraction (RFF) and the asymmetry of the residuals [A(Res)]. The new diagnostic has been calibrated and optimized so that the resulting merger sample is very complete. However, the contamination by non-mergers is also high. If the same optimization method is adopted for combinations of other structural parameters such as the Concentration, Asymmetry, clumpineSs (CAS) system, the merger indicator we introduce yields merger samples of equal or higher statistical quality than the samples obtained through the use of other structural parameters. We investigate the ability of the method presented here to select minor mergers by identifying a sample of visually classified mergers that would not have been picked up by the use of the CAS system, when using its usual limits. However, given the low prevalence of mergers among the general population of galaxies and the optimization used here, we find that the merger diagnostic introduced in this work is best used as a negative merger test, that is, it is very effective at selecting non-merging galaxies. In common with all the currently available automatic methods, the sample of merger candidates selected is heavily contaminated by non-mergers, and further steps are needed to produce a clean merger sample. This merger diagnostic has been developed using the Hubble Space Telescope/ACS F606W images of the A901/902 multiple cluster system (z= 0.165) obtained by the Space Telescope A901/902 Galaxy Evolution Survey team. In particular, we have focused on a mass- and magnitude-limited sample (log M/M⊙ > 9.0, RVega, Total≤ 23.5 mag) which includes 905 cluster galaxies and 655 field galaxies of all morphological types.
    Preview · Article · Nov 2011 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We present new constraints on the ratio of black hole (BH) mass to total galaxy stellar mass at 0.3 < z < 0.9 for a sample of 32 type-1 active galactic nuclei (AGNs) from the XMM-COSMOS survey covering the range M BH ~ 107.2 – 8.7 M ☉. Virial M BH estimates based on Hβ are available from the COSMOS Magellan/IMACS survey. We use high-resolution Hubble Space Telescope (HST) imaging to decompose the light of each type-1 AGN and host galaxy, and employ a specially built mass-to-light ratio to estimate the stellar masses (M *). The M BH-M * ratio shows a zero offset with respect to the local relation for galactic bulge masses, and we also find no evolution in the mass ratio M BH/M *(1 + z)0.02 ± 0.34 up to z ~ 0.9. Interestingly, at the high-M BH end there is a positive offset from the z = 0 relation, which can be fully explained by a mass function bias with a cosmic scatter of σμ = 0.3, reaffirming that the intrinsic distribution is consistent with zero evolution. From our results we conclude that since z ~ 0.9 no substantial addition of stellar mass is required: the decline in star formation rates and merger activity at z < 1 support this scenario. Nevertheless, given that a significant fraction of these galaxies show a disk component, their bulges are indeed undermassive. This is a direct indication that for the last 7 Gyr the only essential mechanism required for these galaxies to obey the z = 0 relation is a redistribution of stellar mass to the bulge, likely driven by secular processes, i.e., internal instabilities and minor merging.
    Full-text · Article · Oct 2011 · The Astrophysical Journal Letters
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    ABSTRACT: We present an automatic method to identify galaxy mergers using the morphological information contained in the residual images of galaxies after the subtraction of a Sersic model. The removal of the bulk signal from the host galaxy light is done with the aim of detecting the fainter minor mergers. The specific morphological parameters that are used in the merger diagnostic suggested here are the Residual Flux Fraction and the asymmetry of the residuals. The new diagnostic has been calibrated and optimized so that the resulting merger sample is very complete. However, the contamination by non-mergers is also high. If the same optimization method is adopted for combinations of other structural parameters such as the CAS system, the merger indicator we introduce yields merger samples of equal or higher statistical quality than the samples obtained through the use of other structural parameters. We explore the ability of the method presented here to select minor mergers by identifying a sample of visually classified mergers that would not have been picked up by the use of the CAS system, when using its usual limits. Given the low prevalence of mergers among the general population of galaxies and the optimization used here, we find that the merger diagnostic introduced in this work is best used as a negative merger test, i.e., it is very effective at selecting non-merging galaxies. As with all the currently available automatic methods, the sample of merger candidates selected is contaminated by non-mergers, and further steps are needed to produce a clean sample. This merger diagnostic has been developed using the HST/ACS F606W images of the A901/02 cluster (z=0.165) obtained by the STAGES team. In particular, we have focused on a mass and magnitude limited sample (log M/M_{O}>9.0, R_{Vega}<23.5mag)) which includes 905 cluster galaxies and 655 field galaxies of all morphological types.
    Full-text · Article · Sep 2011
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    ABSTRACT: We present an analysis of V-band radial surface brightness profiles for spiral galaxies from the field and cluster environments using Hubble Space Telescope/Advanced Camera for Surveys imaging and data from the Space Telescope A901/2 Galaxy Evolution Survey (STAGES). We use a large sample of ∼330 face-on to intermediately inclined spiral galaxies and assess the effect of the galaxy environment on the azimuthally averaged radial surface brightness μ profiles for each galaxy in the outer stellar disc (24 < μ < 26.5 mag arcsec−2). For galaxies with a purely exponential outer disc (∼50 per cent), we determine the significance of an environmental dependence on the outer disc scalelength hout. For galaxies with a broken exponential in their outer disc, either down-bending (truncation, ∼10 per cent) or up-bending (antitruncation, ∼40 per cent), we measure the strength T (outer-to-inner scalelength ratio, log10 hout/hin) of the μ breaks and determine the significance of an environmental dependence on break strength T. Surprisingly, we find no evidence to suggest any such environmental dependence on either outer disc scalelength hout or break strength T, implying that the galaxy environment is not affecting the stellar distribution in the outer stellar disc. We also find that for galaxies with small effective radii (re < 3 kpc) there is a lack of outer disc truncations in both the field and cluster environments. Our results suggest that the stellar distribution in the outer disc of spiral galaxies is not significantly affected by the galaxy environment.
    Full-text · Article · Aug 2011 · Monthly Notices of the Royal Astronomical Society
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    Rene Andrae · Knud Jahnke
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    ABSTRACT: Testing theories of angular-momentum acquisition of rotationally supported disc galaxies is the key to understand the formation of this type of galaxies. The tidal-torque theory tries to explain this acquisition process in a cosmological framework and predicts positive autocorrelations of angular-momentum orientation and spiral-arm handedness on distances of 1Mpc/h. This disc alignment can also cause systematic effects in weak-lensing measurements. Previous observations claimed discovering such correlations but did not account for errors in redshift, ellipticity and morphological classifications. We explain how to rigorously propagate all important errors. Analysing disc galaxies in the SDSS database, we find that positive autocorrelations of spiral-arm handedness and angular-momentum orientations on distances of 1Mpc/h are plausible but not statistically significant. This result agrees with a simple hypothesis test in the Local Group, where we find no evidence for disc alignment. Moreover, we demonstrate that ellipticity estimates based on second moments are strongly biased by galactic bulges, thereby corrupting correlation estimates and overestimating the impact of disc alignment on weak-lensing studies. Finally, we discuss the potential of future sky surveys. We argue that photometric redshifts have too large errors, i.e., PanSTARRS and LSST cannot be used. We also discuss potentials and problems of front-edge classifications of galaxy discs in order to improve estimates of angular-momentum orientation.
    Preview · Article · Aug 2011 · Monthly Notices of the Royal Astronomical Society
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    Rene Andrae · Peter Melchior · Knud Jahnke
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    ABSTRACT: Parametrising galaxy morphologies is a challenging task, e.g., in shear measurements of weak lensing or investigations of galaxy evolution. The huge variety of morphologies requires an approach that is highly flexible, e.g., accounting for azimuthal structure. We revisit the method of sersiclets, where galaxy morphologies are decomposed into basis functions based on the Sersic profile. This approach is justified by the fact that the Sersic profile is the first-order Taylor expansion of any real light profile. We show that sersiclets overcome the modelling failures of shapelets. However, sersiclets implicate an unphysical relation between the steepness of the light profile and the spatial scale of azimuthal structures, which is not obeyed by real galaxy morphologies and can therefore give rise to modelling failures. Moreover, we demonstrate that sersiclets are prone to undersampling, which restricts sersiclet modelling to highly resolved galaxy images. Analysing data from the Great08 challenge, we demonstrate that sersiclets should not be used in weak-lensing studies. We conclude that although the sersiclet approach appears very promising at first glance, it suffers from conceptual and practical problems that severly limit its usefulness. The Sersic profile can be enhanced by higher-order terms in the Taylor expansion, which can drastically improve model reconstructions of galaxy images. If orthonormalised, these higher-order profiles can overcome the problems of sersiclets while preserving their mathematical justification.
    Full-text · Article · Jun 2011 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: Recent studies of the tight scaling relations between the masses of supermassive black holes and their host galaxies have suggested that in the past black holes constituted a larger fraction of their host galaxies' mass. However, these arguments are limited by selection effects and difficulties in determining robust host galaxy masses at high redshifts. Here we report the first results of a new, complementary diagnostic route: we directly determine a dynamical host galaxy mass for the z=1.3 luminous quasar J090543.56+043347.3 through high-spatial-resolution (0.47", 4kpc FWHM) observations of the host galaxy gas kinematics over 30x40 kpc using ESO/VLT/SINFONI with LGS/AO. Combining our result of M_dyn = 2.05+1.68_0.74 x 10^11 M_sun (within a radius 5.25 +- 1.05 kpc) with M_BH,MgII = 9.02 \pm 1.43 x 10^8 M_sun, M_BH,Halpha = 2.83 +1.93-1.13 x 10^8 M_sun, we find that the ratio of black hole mass to host galaxy dynamical mass for J090543.56+043347.3 matches the present-day relation for M_BH vs. M_Bulge,Dyn, well within the IR scatter, deviating at most a factor of two from the mean. J090543.56+043347.3 displays clear signs of an ongoing tidal interaction and of spatially extended star formation at a rate of 50-100 M_sun/yr, above the cosmic average for a galaxy of this mass and redshift. We argue that its subsequent evolution may move J090543.56+043347.3 even closer to the z=0 relation for M_BH vs. M_Bulge,Dyn. Our results support the picture where any substantive evolution in these relations must occur prior to z~1.3. Having demonstrated the power of this modelling approach we are currently analyzing similar data on seven further objects to better constrain such evolution.
    Full-text · Article · Mar 2011
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    ABSTRACT: We show how accretion rate governs the physical properties of a sample of unobscured broad-line, narrow-line, and lineless active galactic nuclei (AGNs). We avoid the systematic errors plaguing previous studies of AGN accretion rate by using accurate accretion luminosities (L_int) from well-sampled multiwavelength SEDs from the Cosmic Evolution Survey (COSMOS), and accurate black hole masses derived from virial scaling relations (for broad-line AGNs) or host-AGN relations (for narrow-line and lineless AGNs). In general, broad emission lines are present only at the highest accretion rates (L_int/L_Edd > 0.01), and these rapidly accreting AGNs are observed as broad-line AGNs or possibly as obscured narrow-line AGNs. Narrow-line and lineless AGNs at lower specific accretion rates (L_int/L_Edd < 0.01) are unobscured and yet lack a broad line region. The disappearance of the broad emission lines is caused by an expanding radiatively inefficient accretion flow (RIAF) at the inner radius of the accretion disk. The presence of the RIAF also drives L_int/L_Edd < 10^-2 narrow-line and lineless AGNs to 10 times higher ratios of radio to optical/UV emission than L_int/L_Edd > 0.01 broad-line AGNs, since the unbound nature of the RIAF means it is easier to form a radio outflow. The IR torus signature also tends to become weaker or disappear from L_int/L_Edd < 0.01 AGNs, although there may be additional mid-IR synchrotron emission associated with the RIAF. Together these results suggest that specific accretion rate is an important physical "axis" of AGN unification, described by a simple model.
    Full-text · Article · Mar 2011 · The Astrophysical Journal
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    ABSTRACT: We present Subaru/FOCAS spectropolarimetry of two active galaxies in the Cosmic Evolution Survey. These objects were selected to be optically dull, with the bright X-ray emission of an active galactic nucleus (AGN) but missing optical emission lines in our previous spectroscopy. Our new observations show that one target has very weak emission lines consistent with an optically dull AGN, while the other object has strong emission lines typical of a host-diluted Type 2 Seyfert galaxy. In neither source do we observe polarized emission lines, with 3σ upper limits of P BLR 2%. This means that the missing broad emission lines (and weaker narrow emission lines) are not due to simple anisotropic obscuration, e.g., by the canonical AGN torus. The weak-lined optically dull AGN exhibits a blue polarized continuum with P = 0.78% ± 0.07% at 4400 Å < λrest < 7200 Å (P = 1.37% ± 0.16% at 4400 Å < λrest < 5050 Å). The wavelength dependence of this polarized flux is similar to that of an unobscured AGN continuum and represents the intrinsic AGN emission, either as synchrotron emission or the outer part of an accretion disk reflected by a clumpy dust scatterer. Because this intrinsic AGN emission lacks emission lines, this source is likely to have a radiatively inefficient accretion flow.
    Full-text · Article · Feb 2011 · The Astrophysical Journal
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    René Andrae · Knud Jahnke · Peter Melchior
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    ABSTRACT: Given the enormous galaxy data bases of modern sky surveys, parametrizing galaxy morphologies is a very challenging task due to the huge number and variety of objects. We assess the different problems faced by existing parametrization schemes (CAS, Gini, M20, Sérsic profile, shapelets) in an attempt to understand why parametrization is so difficult and in order to suggest improvements for future parametrization schemes. We demonstrate that morphological observables (e.g. steepness of the radial light profile, ellipticity, asymmetry) are intertwined and cannot be measured independently of each other. We present strong arguments in favour of model-based parametrization schemes, namely reliability assessment, disentanglement of morphological observables and point spread function modelling. Furthermore, we demonstrate that estimates of the concentration and Sérsic index obtained from the Zurich Structure & Morphology catalogue are in excellent agreement with theoretical predictions. We also demonstrate that the incautious use of the concentration index for classification purposes can cause a severe loss of the discriminative information contained in a given data sample. Moreover, we show that, for poorly resolved galaxies, concentration index and M20 suffer from strong discontinuities, i.e. similar morphologies are not necessarily mapped to neighbouring points in the parameter space. This limits the reliability of these parameters for classification purposes. Two-dimensional Sérsic profiles accounting for centroid and ellipticity are identified as the currently most reliable parametrization scheme in the regime of intermediate signal-to-noise ratios and resolutions, where asymmetries and substructures do not play an important role. We argue that basis functions provide good parametrization schemes in the regimes of high signal-to-noise ratios and resolutions. Concerning Sérsic profiles, we show that scale radii cannot be compared directly for profiles of different Sérsic indices. Furthermore, we show that parameter spaces are typically highly non-linear. This implies that significant caution is required when distance-based classification methods are used.
    Full-text · Article · Feb 2011 · Monthly Notices of the Royal Astronomical Society
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    Rene Andrae · Knud Jahnke · Peter Melchior
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    ABSTRACT: We demonstrate that morphological observables (e.g. steepness of the radial light profile, ellipticity, asymmetry) are intertwined and cannot be measured independently of each other. We present strong arguments in favour of model-based parametrisation schemes, namely reliability assessment, disentanglement of morphological observables, and PSF modelling. Furthermore, we demonstrate that estimates of the concentration and Sersic index obtained from the Zurich Structure & Morphology catalogue are in excellent agreement with theoretical predictions. We also demonstrate that the incautious use of the concentration index for classification purposes can cause a severe loss of the discriminative information contained in a given data sample. Moreover, we show that, for poorly resolved galaxies, concentration index and M_20 suffer from strong discontinuities, i.e. similar morphologies are not necessarily mapped to neighbouring points in the parameter space. This limits the reliability of these parameters for classification purposes. Two-dimensional Sersic profiles accounting for centroid and ellipticity are identified as the currently most reliable parametrisation scheme in the regime of intermediate signal-to-noise ratios and resolutions, where asymmetries and substructures do not play an important role. We argue that basis functions provide good parametrisation schemes in the regimes of high signal-to-noise ratios and resolutions. Concerning Sersic profiles, we show that scale radii cannot be compared directly for profiles of different Sersic indices. Furthermore, we show that parameter spaces are typically highly nonlinear. This implies that significant caution is required when distance-based classificaton methods are used. Comment: 18 pages, 13 figures
    Full-text · Article · Sep 2010

Publication Stats

2k Citations
266.94 Total Impact Points

Institutions

  • 2015
    • Pennsylvania State University
      University Park, Maryland, United States
  • 2006-2015
    • Max Planck Institute for Astronomy
      Heidelburg, Baden-Württemberg, Germany
  • 2011
    • Harvard-Smithsonian Center for Astrophysics
      • Smithsonian Astrophysical Observatory
      Cambridge, Massachusetts, United States
  • 2008
    • French National Centre for Scientific Research
      • Institut d'astrophysique spatiale (IAS)
      Lutetia Parisorum, Île-de-France, France
    • University of British Columbia - Vancouver
      • Department of Physics and Astronomy
      Vancouver, British Columbia, Canada