arXiv subjects

Kirpal Nandra

Publications and source records attributed to Kirpal Nandra.

At least 127 records · Page 7Linked to original sources

arXiv2010

On the co-evolution of supermassive black holes and their host galaxies since z = 3

[Abridged] To investigate the evolution in the relation between galaxy stellar and central black hole mass we construct a volume limited complete sample of 85 AGN with host galaxy stellar masses M_{*} > 10^{10.5} M_{sol}, and specific X-ray luminosities L_{X} > 2.35 x 10^{43} erg s^{-1} at 0.4 < z < 3. We calculate the Eddington limiting masses of the supermassive black holes residing at the centre of these galaxies, and observe an increase in the average Eddington limiting black hole mass with redshift. By assuming that there is no evolution in the Eddington ratio (μ) and then that there is maximum possible evolution to the Eddington limit, we quantify the maximum possible evolution in the M_{*} / M_{BH} ratio as lying in the range 700 < M_{*}/M_{BH} < 10000, compared with the local value of M_{*}/M_{BH} ~ 1000. We furthermore find that the fraction of galaxies which are AGN (with L_{X} > 2.35 x 10^{43} erg s^{-1}) rises with redshift from 1.2 +/- 0.2 % at z = 0.7 to 7.4 +/- 2.0 % at z = 2.5. We use our results to calculate the maximum timescales for which our sample of AGN can continue to accrete at their observed rates before surpassing the local galaxy-black hole mass relation. We use these timescales to calculate the total fraction of massive galaxies which will be active (with L_{X} > 2.35 x 10^{43} erg s^{-1}) since z = 3, finding that at least ~ 40% of all massive galaxies will be Seyfert luminosity AGN or brighter during this epoch. Further, we calculate the energy density due to AGN activity in the Universe as 1.0 (+/- 0.3) x 10^{57} erg Mpc^{-3} Gyr^{-1}, potentially providing a significant source of energy for AGN feedback on star formation. We also use this method to compute the evolution in the X-ray luminosity density of AGN with redshift, finding that massive galaxy Seyfert luminosity AGN are the dominant source of X-ray emission in the Universe at z < 3.

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arXiv2010

Rms variability properties of the iron K alpha line in Seyfert galaxies

We present an analysis of the rms variability spectra of a sample of 18 observations of 14 Seyfert galaxies observed by XMM-Newton, which exhibit sufficient variability and signal-to-noise ratio to examine the variations in the iron K-band. The narrow core of the K alpha line at 6.4 keV, seen universally in Seyferts, shows minimal evidence for variability and is always less variable than the continuum, supporting an origin in distant material such as the torus. At least half the observations do show evidence for variations in the wider iron K-band, however, and in at least 5 cases the excess line variations appear to be broad. The simplest prediction -- that the broad emission line is as variable as the continuum -- is generally not confirmed as only two observations show this type of behaviour. In four cases, the red wing of the line is more variable than the power-law continuum and extends down to energies of ~ 5 keV. Three observations show strong variability blueward of the line core that could also be from the disk, but alternatively might be due to emission or absorption by other hot or photoionised gas close to the nucleus. In cases where this excess blue variability is present, it is not always seen in the time-averaged spectrum. Six observations possess a broad iron line in the time-averaged spectra but with an invariant red wing, and three of these six show no variability across the entire iron line region. This suggests a decoupling of the continuum and reflection component, perhaps due to light bending or other anisotropic effects as has been suggested for MCG-6-30-15 and other narrow-line Seyfert 1s. A key result is that the rms spectra of objects such as NGC 3516 do not agree with complex absorption effects mimicking the broad red wing.

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arXiv2010

The International X-ray Observatory

The International X-ray Observatory (IXO) is a joint ESA-JAXA-NASA effort to address fundamental and timely questions in astrophysics: What happens close to a black hole? How did supermassive black holes grow? How does large scale structure form? What is the connection between these processes? To address these questions IXO will employ optics with 3 sq m collecting area and 5 arc sec angular resolution - 20 times more collecting area at 1 keV than any previous X-ray observatory. Focal plane instruments will deliver a 100-fold increase in effective area for high-resolution spectroscopy, deep spectral imaging over a wide field of view, unprecedented polarimetric sensitivity, microsecond spectroscopic timing, and high count rate capability. The mission is being planned for launch in 2021 to an L2 orbit, with a five-year lifetime and consumables for 10 years.

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arXiv2009

On the X-ray properties of submm-selected galaxies

We present an analysis of the X-ray properties of 35 submm galaxies(SMGs) in the CDF-N. Using a sample of robust 850micron-selected galaxies, with accurate positions from Spitzer and/or radio counterparts, we find 16 objects (45+/-8%) with significant X-ray detections in the 2Ms Chandra data. 6 of these SMGs (~17+/-6%) have measured X-ray luminosities or upper limits consistent with those expected based on the far-infrared (FIR) or radio-derived star formation rate (SFR), and hence with the X-rays coming solely from star formation. In another 7 sources (20+/-7%) a dominant AGN contribution to the X-ray emission is required, while in 3 more it is unclear whether stellar process or accretion are responsible. Stacking of the X-ray undetected SMGs reveals a highly significant detection. If due to star formation, this corresponds to an average X-ray derived SFR of ~150 Msun/yr. We deduce that the AGN fraction in SMGs based on X-ray observations is 20-29 (+/-7) %, which is towards the lower limit of previous estimates. Spectral analysis shows that in general the SMGs are not heavily obscured in the X-ray but most of the SMGs classfied as AGN show absorption with N_H in excess of 10^22 cm^-2. Of the secure AGN, the bolometric luminosity appears to be dominated by the AGN in only 3 cases. In ~85% of the SMGs, the X-ray spectrum effectively rules out an AGN contribution that dominates the bolometric emission, even if the AGN is Compton thick. The evidence therefore suggests that intense star formation accounts for both the FIR and X-ray emission in most SMGs. We argue that, rather than having an especially high AGN fraction or duty cycle, SMGs have a high X-ray detection rate at very faint fluxes partly because of their high star formation rates and, in rarer cases, because the submm emission is from an AGN.(abridged)

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arXiv2009

Groups of Galaxies in AEGIS: The 200 ksec Chandra Extended X-ray Source catalogue

We present the discovery of seven X-ray emitting groups of galaxies selected as extended X-ray sources in the 200 ksec Chandra coverage of the All-wavelength Extended Groth Strip International Survey (AEGIS). In addition, we report on AGN activity associated to these systems. Using the DEEP2 Galaxy Redshift Survey coverage, we identify optical counterparts and determine velocity dispersions. In particular, we find three massive high-redshift groups at z>0.7, one of which is at z=1.13, the first X-ray detections of spectroscopically selected DEEP2 groups. We also present a first look at the the L_X-T, L_X-sigma, and sigma-T scaling relations for high-redshift massive groups. We find that the properties of these X-ray selected systems agree well with the scaling relations of similar systems at low redshift, although there are X-ray undetected groups in the DEEP2 catalogue with similar velocity dispersions. The other three X-ray groups with identified redshifts are associated with lower mass groups at z~0.07 and together form part of a large structure or "supergroup" in the southern portion of the AEGIS field. All of the low-redshift systems are centred on massive elliptical galaxies, and all of the high-redshift groups have likely central galaxies or galaxy pairs. All of the central group galaxies host X-ray point sources, radio sources, and/or show optical AGN emission. Particularly interesting examples of central AGN activity include a bent-double radio source plus X-ray point source at the center of a group at z=0.74, extended radio and double X-ray point sources associated to the central galaxy in the lowest-redshift group at z=0.066, and a bright green valley galaxy (part of a pair) in the z=1.13 group which shows optical AGN emission lines.

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arXiv2009

AGN dust tori: the X-ray-infrared connection

We have combined the CLASXS Chandra survey in Lockman with the Spitzer SWIRE survey data to study the X-ray-infrared connection for AGN. The sample consists of 401 X-ray-sources, of which 306 are detected by Spitzer, and a further 257 AGN candidates detected through their dust torus, but not by Chandra. For X-ray sources the X-ray hardness ratio has been modelled in terms of a power-law with absorption N(H). The optical and infrared data have been modelled in terms of our well-established optical galaxy and QSO templates, and infrared templates based on radiative transfer models. Our estimate of the N(H) distribution is consistent with other studies, but we do find a higher proportion of low absorption objects at z < 0.5 than at z > 0.5. While we find only one X-ray AGN with N(H) > 10^{24} cm^{-2}, we argue that 10 objects with torus luminosity apparently exceeding the bolometric X-ray to 3 μm luminosity are strong candidates for being heavily absorbed in X-rays. We also estimate that at least half of the infrared-detected AGN dust tori which are undetected in X-rays are likely to be Compton thick. Our estimate of the total number of Compton-thick objects corresponds to > 20%$ of the combined SWIRE-CLASXS sample (and with an upper limit of 39 %). The range of dust covering factors is 1-100 %, with a mean of 40 %, ie a Type 2 fraction of 40 %. Measured by the ratio of dust torus luminosity to X-ray or (for Type 1 objects) optical luminosity, the covering factor appears to decrease towards intermediate AGN luminosity, in contradiction to estimates based on ratios of narrow-line and broad-line spectra, but may increase again at low AGN luminosity.

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arXiv2009

Understanding the Astrophysics of Galaxy Evolution: the role of spectroscopic surveys in the next decade

Over the last decade optical spectroscopic surveys have characterized the low redshift galaxy population and uncovered populations of star-forming galaxies back to z ~ 7. This work has shown that the primary epoch of galaxy building and black hole growth occurs at redshifts of 2 to 3. The establishment of the concordance LCDM cosmology shifted the focus of galaxy population studies from constraining cosmological parameters to characterizing the processes which regulate the formation and evolution of galaxies.In the next decade, high redshift observers will attempt to formulate a coherent evolutionary picture connecting galaxies in the high redshift Universe to galaxies today. In order to link galaxy populations at different redshifts, we must not only characterize their evolution in a systematic way, we must establish which physical processes are responsible for it. Considerable progress has already been made in understanding how galaxies evolved from z ~ 1 to the present day. Large spectroscopic surveys in the near infrared are required to push these studies back towards the main epoch of galaxy building. Only then will we understand the full story of the formation of L* galaxies like our own Milky Way. A large near-IR spectroscopic survey will also provide the calibration needed to avoid systematics in the large photometric programs proposed to study the nature of dark matter and dark energy. We provide an outline design for a multi-object 0.4 to 1.8 micron spectrograph, which could be placed on an existing telescope, and which would allow a full characterization of the galaxy population out to z ~ 2. We strongly recommend a serious further study to design a real instrument, which will be required for galaxy formation studies to advance to the next frontier.

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arXiv2008

On the nature of unabsorbed Seyfert 2 galaxies

We present an analysis of six 12 um selected Seyfert 2 galaxies that have been reported to be unabsorbed in the X-ray. By comparing the luminosities of these galaxies in the mid-IR (12um), optical ([O III]) and hard X-ray (2-10 keV), we show that they are all under-luminous in the 2-10 keV X-ray band. Four of the objects exhibit X-ray spectra indicative of a hard excess, consistent with a heavily obscured X-ray component and hence a hidden nucleus. In these objects the softer X-rays may be dominated by a strong soft scattered continuum or contamination from the host galaxy, which is responsible for the unabsorbed X-ray spectra observed, and accounts for the anomalously low 2-10 keV X-ray luminosity. We confirm this assertion in NGC4501 with a Chandra observation, which shows hard X-ray emission coincident with the nucleus, consistent with heavy absorption, and a number of contaminating softer sources which account for the bulk of the softer emission. We point out that such 'Compton thick' sources need not necessarily present iron Ka emission of high EW. An example in our sample is IRASF01475-0740, which we know must host an obscured AGN as it hosts a HBLR seen in scattered light (Tran 2003). The X-ray spectrum is nonetheless relatively unobscured and the iron Ka line only moderate in strength. These observations can be reconciled if the hidden nuclear emission is dominated by transmitted, rather than reflected X-rays, which can then be weak compared to the soft scattered light or galactic emission. Despite these considerations, we conclude that NGC3147 & 3660 may intrinsically lack a BLR, confirming the recent results of Bianchi et al. (2008) for NGC3147. Neither X-ray spectrum shows signs of hidden hard emission and both sources exhibit X-ray variability leading us to believe we are viewing the nucleus directly.

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arXiv2008

The X-ray luminosity function of AGN at z~3

We combine Lyman-break colour selection with ultradeep (> 200 ks) Chandra X-ray imaging over a survey area of ~0.35 deg^2 to select high redshift AGN. Applying careful corrections for both the optical and X-ray selection functions, the data allow us to make the most accurate determination to date of the faint end of the X-ray luminosity function (XLF) at z~3. Our methodology recovers a number density of X-ray sources at this redshift which is at least as high as previous surveys, demonstrating that it is an effective way of selecting high z AGN. Comparing to results at z=1, we find no evidence that the faint slope of the XLF flattens at high z, but we do find significant (factor ~3.6) negative evolution of the space density of low luminosity AGN. Combining with bright end data from very wide surveys we also see marginal evidence for continued positive evolution of the characteristic break luminosity L*. Our data therefore support models of luminosity-dependent density evolution between z=1 and z=3. A sharp upturn in the the XLF is seen at the very lowest luminosities (Lx < 10^42.5 erg s^-1), most likely due to the contribution of pure X-ray starburst galaxies at very faint fluxes.

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arXiv2008

The DEEP2 Galaxy Redshift Survey: The Red Sequence AGN Fraction and its Environment and Redshift Dependence

We measure the dependence of the AGN fraction on local environment at z~1, using spectroscopic data taken from the DEEP2 Galaxy Redshift Survey, and Chandra X-ray data from the All-Wavelength Extended Groth Strip International Survey (AEGIS). To provide a clean sample of AGN we restrict our analysis to the red sequence population; this also reduces additional colour-environment correlations. We find evidence that high redshift LINERs in DEEP2 tend to favour higher density environments relative to the red population from which they are drawn. In contrast, Seyferts and X-ray selected AGN at z~1 show little (or no) environmental dependencies within the same underlying population. We compare these results with a sample of local AGN drawn from the SDSS. Contrary to the high redshift behaviour, we find that both LINERs and Seyferts in the SDSS show a slowly declining red sequence AGN fraction towards high density environments. Interestingly, at z~1 red sequence Seyferts and LINERs are approximately equally abundant. By z~0, however, the red Seyfert population has declined relative to the LINER population by over a factor of 7. We speculate on possible interpretations of our results.

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arXiv2007

AEGIS: New Evidence Linking Active Galactic Nuclei to the Quenching of Star Formation

Utilizing Chandra X-ray observations in the All-wavelength Extended Groth Strip International Survey (AEGIS) we identify 241 X-ray selected Active Galactic Nuclei (AGNs, L > 10^{42} ergs/s) and study the properties of their host galaxies in the range 0.4 < z < 1.4. By making use of infrared photometry from Palomar Observatory and BRI imaging from the Canada-France-Hawaii Telescope, we estimate AGN host galaxy stellar masses and show that both stellar mass and photometric redshift estimates (where necessary) are robust to the possible contamination from AGNs in our X-ray selected sample. Accounting for the photometric and X-ray sensitivity limits of the survey, we construct the stellar mass function of X-ray selected AGN host galaxies and find that their abundance decreases by a factor of ~2 since z~1, but remains roughly flat as a function of stellar mass. We compare the abundance of AGN hosts to the rate of star formation quenching observed in the total galaxy population. If the timescale for X-ray detectable AGN activity is roughly 0.5-1 Gyr--as suggested by black hole demographics and recent simulations--then we deduce that the inferred AGN "trigger" rate matches the star formation quenching rate, suggesting a link between these phenomena. However, given the large range of nuclear accretion rates we infer for the most massive and red hosts, X-ray selected AGNs may not be directly responsible for quenching star formation.

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arXiv2007

On the relativistic iron line and soft excess in the Seyfert 1 galaxy Markarian 335

We report on a 133 ks XMM-Newton observation of the Seyfert 1 galaxy Markarian 335. The 0.4-12 keV spectrum contains an underlying power law continuum, a soft excess below 2 keV, and a double-peaked iron emission feature in the 6-7 keV range. We investigate the possibility that the double-peaked emission might represent the characteristic signature of the accretion disc. Detailed investigations show that a moderately broad, accretion disc line is most likely present, but that the peaks may be owing to narrower components from more distant material. The peaks at 6.4 and 7 keV can be identified, respectively, with the molecular torus in active galactic nucleus unification schemes, and very highly ionized, optically thin gas filling the torus. The X-ray variability spectra on both long (~100 ks) and short (~1 ks) timescales disfavour the recent suggestion that the soft excess is an artifact of variable, moderately ionized absorption.

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arXiv2007

Multiwavelength Study of X-ray Selected Star Forming Galaxies within the Chandra Deep Field South

We have combined multi-wavelength observations of a selected sample of starforming galaxies with galaxy evolution models in order to compare the results obtained for different SFR tracers and to study the effect that the evolution of the starforming regions has on them. We also aimed at obtaining a better understanding of the corrections due to extinction and nuclear activity on the derivation of the SFR. We selected the sample from Chandra data for the well studied region Chandra Deep Field South (CDFS) and chose the objects that also have UV and IR data from GALEX and GOODS-Spitzer respectively. Our main finding is that there is good agreement between the extinction corrected SFR(UV) and the SFR(X), and we confirm the use of X-ray luminosities as a trustful tracer of recent star formation activity. Nevertheless, at SFR(UV) larger than about 5Msol/year there are several galaxies with an excess of SFR(X) suggesting the presence of an obscured AGN not detected in the optical spectra. We conclude that the IR luminosity is driven by recent star formation even in those galaxies where the SFR(X) is an order of magnitude higher than the SFR(UV) and therefore may harbour an AGN. One object shows SFR(X) much lower than expected based on the SFR(UV); this SFR(X) `deficit' may be due to an early transient phase before most of the massive X-ray binaries were formed. An X-ray deficit could be used to select extremely young bursts in an early phase just after the explosion of the first supernovae associated with massive stars and before the onset of massive X-ray binaries.

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arXiv2007

The DEEP2 Galaxy Redshift Survey: AEGIS Observations of a Dual AGN at z=0.7

We present evidence for a dual Active Galactic Nucleus (AGN) within an early-type galaxy at z=0.709 in the Extended Groth Strip. The galaxy lies on the red sequence, with absolute magnitude M_B=-21.0 (AB, with h=0.7) and rest-frame color U-B=1.38. Its optical spectrum shows strong, double-peaked [OIII] emission lines and weak Hbeta emission, with Seyfert-like line ratios. The two narrow peaks are separated by 630 km/s in velocity and arise from two distinct regions, spatially resolved in the DEIMOS spectrum, with a projected physical separation of 1.2 kpc. HST/ACS imaging shows an early-type (E/S0) galaxy with hints of disturbed structure, consistent with the remnant of a dissipationless merger. Multiwavelength photometric information from the AEGIS consortium confirms the identification of a dust-obscured AGN in an early-type galaxy, with detections in X-ray, optical, infrared and radio wavebands. These data are most readily explained as a single galaxy harboring two AGN--the first such system to be observed in an otherwise typical early-type galaxy.

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arXiv2006

Evidence for relativistic features in the X-ray spectrum of Mrk 335

We present an analysis of hard X-ray features in the spectrum of the bright Sy 1 galaxy Mrk 335 observed by the XMM-Newton satellite. Our analysis confi rms the presence of a broad, ionised iron Kalpha emission line in the spectrum, first found by Gondoin et al. The broad line can be modeled successfully by relativistic accretion disc reflection models. This interpretation is unusually robust in the case of Mrk 335 because of the lack of any ionised (`` warm'') absorber and the absence a clear narrow core to the line. Partial covering by neutral gas cannot, however, be ruled out statistically as the origin of the broad residuals. Regardless of the underlyin g continuum we report, for the first time in this source, the detection of a narrow absorption feature at the rest frame energy of ~5.9 keV. If the feature is identified with a resonance absorption line of iron in a highly ionised medium, the redshift of the line corresponds to an inflow velocity of ~0.11 -0.15 c. We present a simple model for the inflow, accounting approximate ly for relativistic and radiation pressure effects, and use Monte Carlo methods to compute synthetic spectra for qualitative comparison with the data. This mode ling shows that the absorption feature can plausibly be reproduced by infalling gas providing that the feature is identified with Fe xxvi. We require the inflowing gas to extend over a limited range of radii at a few tens of rg to match the observed feature. The mass accretion rate in the flow correspond s to 60% of the Eddington limit, in remarkable agreement with the observed rate . The narrowness of the absorption line tends to argue against a purely gravitational origin for the redshift of the line, but given the current data quality we stress that such an interpretation cannot be ruled out.

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arXiv2006

Multiwavelength Monitoring of the Dwarf Seyfert 1 Galaxy NGC 4395. III. Optical Variability and X-ray/UV/Optical Correlations

We present optical observations of the low-luminosity Seyfert 1 nucleus of NGC 4395, as part of a multiwavelength reverberation-mapping program. Observations were carried out over two nights in 2004 April at Lick, Wise, and Kitt Peak Observatories. We obtained V-band and B-band photometry, and spectra over the range 3500-6800 Angstroms. Simultaneous Hubble Space Telescope UV and Chandra X-ray observations are presented in companion papers. Even though NGC 4395 was in an extremely low state of activity, we detect significant continuum variability of 2-10%, increasing toward shorter wavelengths. The continuum light curves, both spectroscopic and photometric, are qualitatively similar to the simultaneous UV and X-ray light curves. Inter-band cross-correlations suggest that the optical continuum emission lags behind the UV continuum emission by 24 +7/-9 min, and that the optical continuum emission lags behind the X-ray continuum emission by 44 +/- 13 min, consistent with a reprocessing model for active galactic nucleus emission. There are also hints of Balmer emission lines lagging behind the optical continuum by an amount slightly larger than the emission-line lag detected in the UV. These results are all similar to those of other Seyfert 1 nuclei. The emission-line lag yields a mass measurement of the central black hole, which although not very significant, is consistent with the value derived from the simultaneous UV data.

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arXiv2006

Multiwavelength Monitoring of the Dwarf Seyfert 1 Galaxy NGC 4395. II. X-ray and Ultraviolet Continuum Variability

We report on two Chandra observations, and a simultaneous Hubble Space Telescope ultraviolet observation, of the dwarf Seyfert 1 galaxy NGC 4395. Each Chandra observation had a duration of ~30 ks, with a separation of ~50 ks. The spectrum was observed to harden between these observations via a scaling down of the soft-band flux. The inter-observation variability is in a different sense to the observed variability within each observation and is most likely the result of increased absorption. Spectral variations were seen during the first observation suggesting that the X-ray emission is produced in more than one disconnected region. We have also re-analyzed a ~17 ks Chandra observation conducted in 2000. During the three Chandra observations the 2-10 keV flux is about a factor of 2 lower than seen during an XMM-Newton observation conducted in 2003. Moreover, the fractional variability amplitude exhibited during the XMM-Newton observation is significantly softer than seen during the Chandra observations. A power-spectral analysis of the first of the two new Chandra observations revealed a peak at 341s with a formal detection significance of 99%. A similar peak was seen previously in the 2000 Chandra data. However, the detection of this feature is tentative given that it was found in neither the second of our two new Chandra observations nor the XMM-Newton data, and it is much narrower than expected. The Hubble Space Telescope observation was conducted during part of the second Chandra visit. A zero-lag correlation between the ultraviolet and X-ray fluxes was detected with a significance of about 99.5%, consistent with the predictions of the two-phase model for the X-ray emission from active galactic nuclei.

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arXiv2005

The relationship between X-ray variability amplitude and black hole mass in active galactic nuclei

We have investigated the relationship between the 2-10 keV X-ray variability amplitude and black hole mass for a sample of 46 radio-quiet active galactic nuclei observed by ASCA. Thirty-three of the objects in our sample exhibited variability over a time-scale of ~40 ks, and we found a significant anti-correlation between excess variance and mass. Unlike most previous studies, we have quantified the variability using nearly the same time-scale for all objects. Moreover, we provide a prescription for estimating the uncertainties in excess variance which accounts both for measurement uncertainties and for the stochastic nature of the variability. We also present an analytical method to predict the excess variance from a model power spectrum accounting for binning, sampling and windowing effects. Using this, we modelled the variance-mass relation assuming all objects have a universal twice-broken power spectrum, with the position of the breaks being dependent on mass. This accounts for the general form of the relationship but there is considerable scatter. We investigated this scatter as a function of the X-ray photon index, luminosity and Eddington ratio. After accounting for the dependence of excess variance on mass, we find no significant correlation with either luminosity or X-ray spectral slope. We do find an anti-correlation between excess variance and the Eddington ratio, although this relation might be an artifact owing to the uncertainties in the mass measurements. It remains to be established that enhanced X-ray variability is a property of objects with steep X-ray slopes or large Eddington ratios.

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