Browsing by Author "Alexander, D. M."

Now showing 1 - 18 of 18
  • Thumbnail Image
    Item
    A NuStar survey of nearby ultraluminous infrared galaxies
    (2015) Teng, Stacy H.; Rigby, Jane R.; Stern, Daniel; Ptak, Andrew; Alexander, D. M.; Bauer, Franz Erik; Boggs, Stephen E.; Brandt, W. Niel; Christensen, Finn E.; Comastri, Andrea
  • Thumbnail Image
    Item
    A NuSTAR survey of nearby ultraluminous infrared galaxies.
    (2015) Teng, Stacy H.; Bauer, Franz Erik; Treister, Ezequiel; Rigby, Jane R.; Stern, Daniel.; Ptak, Andrew.; Alexander, D. M.; Boggs, Stephen E.; Brandt, W N.; Christensen, Finn E.
  • No Thumbnail Available
    Item
    GOODS-Herschel: the far-infrared view of star formation in active galactic nucleus host galaxies since z ∼ 3
    (2012) Mullaney, J. R.; Pannella, M.; Daddi, E.; Alexander, D. M.; Elbaz, D.; Hickox, R. C.; Bournaud, F.; Altieri, B.; Aussel, H.; Coia, D.; Dannerbauer, H.; Dasyra, K.; Dickinson, M.; Hwang, H. S.; Kartaltepe, J.; Leiton, R.; Magdis, G.; Magnelli, B.; Popesso, P.; Valtchanov, I.; Bauer, F. E.; Brandt, W. N.; Del Moro, A.; Hanish, D. J.; Ivison, R. J.; Juneau, S.; Luo, B.; Lutz, D.; Sargent, M. T.; Scott, D.; Xue, Y. Q.
    We present a study of the infrared properties of X-ray selected, moderate-luminosity (i.e. L-X = 10(42)-10(44) erg s(-1)) active galactic nuclei (AGNs) up to z approximate to 3, in order to explore the links between star formation in galaxies and accretion on to their central black holes. We use 100 and 160 mu m fluxes from GOODS-Herschel - the deepest survey yet undertaken by the Herschel telescope - and show that in the vast majority of cases (i.e. > 94 per cent) these fluxes are dominated by emission from the host galaxy. As such, these far-infrared bands provide an uncontaminated view of star formation in the AGN host galaxies. We find no evidence of any correlation between the X-ray and infrared luminosities of moderate AGNs at any redshift, suggesting that global star formation is decoupled from nuclear (i.e. AGN) activity in these galaxies. On the other hand, we confirm that the star formation rates of AGN hosts increase strongly with redshift, by a factor of 43(-18)(+27) from z < 0.1 to z = 2-3 for AGNs with the same range of X-ray luminosities. This increase is entirely consistent with the factor of 25-50 increase in the specific star formation rates (SSFRs) of normal, star-forming (i.e. main-sequence) galaxies over the same redshift range. Indeed, the average SSFRs of AGN hosts are only marginally (i.e. approximate to 20 per cent) lower than those of main-sequence galaxies at all surveyed redshifts, with this small deficit being due to a fraction of AGNs residing in quiescent (i.e. low SSFR) galaxies. We estimate that 79 +/- 10 per cent of moderate-luminosity AGNs are hosted in main-sequence galaxies, 15 +/- 7 per cent in quiescent galaxies and < 10 per cent in strongly starbursting galaxies. We derive the fractions of all main-sequence galaxies at z < 2 that are experiencing a period of moderate nuclear activity, noting that it is strongly dependent on galaxy stellar mass (M-stars), rising from just a few per cent at M-stars similar to 10(10) M-circle dot to greater than or similar to 20 per cent at M-stars >= 10(11) M-circle dot. Our results indicate that it is galaxy stellar mass that is most important in dictating whether a galaxy hosts a moderate-luminosity AGN. We argue that the majority of moderate nuclear activity is fuelled by internal mechanisms rather than violent mergers, which suggests that high-redshift disc instabilities could be an important AGN feeding mechanism.
  • Thumbnail Image
    Item
    IC 3639—A NEW BONA FIDE COMPTON-THICK AGN UNVEILED BY NuSTAR
    (2016) Boorman, Peter G.; Gandhi, P.; Alexander, D. M.; Annuar, A.; Ballantyne, D. R.; Bauer, Franz Erik; Boggs, S. E.; Brandt, W. N.; Brightman, M.; Christensen, F. E.; Craig, W. W.; Farrah, D.; Hailey, C. J.; Harrison, F. A.; Hönig, S. F.; Koss, M.; LaMassa, S. M.; Masini, A. C.; Ricci, G.; Risaliti, D.; Stern, W.; Zhang, W.
  • Thumbnail Image
    Item
    LLAMA: normal star formation efficiencies of molecular gas in the centres of luminous Seyfert galaxies.
    (2018) Rosario, D.; Ricci, Claudio; Treister, Ezequiel; Burtscher, L.; Davies, R. I.; Koss, M.; Lutz, D.; Riffel, R.; Alexander, D. M.; Genzel, R.; Hicks, E. H.
  • No Thumbnail Available
    Item
    NuSTAR observations of four nearby X-ray faint AGNs: low luminosity or heavy obscuration?
    (OUP, 2020) Annuar, A.; Alexander, D. M.; Gandhi, P.; Lansbury, G. B.; Asmus, D.; Balokovic, M.; Ballantyne, D. R.; Bauer, Franz Erik; Boorman, P. G.; Brandt, W. N.; Brightman, M.; Chen, C. T. J.; Del Moro, A.; Farrah, D.; Harrison, F. A.; Koss, M. J.; Lanz, L.; Marchesi, S.; Masini, A.; Nardini, E.; Ricci, Claudio; Stern, D.; Zappacosta, L.
    We present NuSTAR (Nuclear Spectroscopic Telescope Array) observations of four active galactic nuclei (AGNs) located within 15 Mpc. These AGNs, namely ESO 121-G6, NGC 660, NGC 3486, and NGC 5195, have observed X-ray luminosities of L 2–10 keV,obs ≲ 10 39 erg s −1 , classifying them as low-luminosity AGN (LLAGN). We perform broad-band X-ray spectral analysis for the AGN by combining our NuSTAR data with Chandra or XMM–Newton observations to directly measure their column densities (N H ) and infer their intrinsic power. We complement our X-ray data with archival and new high-angular resolution mid-infrared (mid-IR) data for all objects, except NGC 5195. Based on our X-ray spectral analysis, we found that both ESO 121-G6 and NGC 660 are heavily obscured (N H > 10 23 cm −2 ; L 2–10 keV,int ∼ 10 41 erg s −1 ), and NGC 660 may be Compton thick. We also note that the X-ray flux and spectral slope for ESO 121-G6 have significantly changed over the last decade, indicating significant changes in the obscuration and potentially accretion rate. On the other hand, NGC 3486 and NGC 5195 appear to be unobscured and just mildly obscured, respectively, with L 2–10 keV,int < 10 39 erg s −1 , i.e. genuine LLAGN. Both of the heavily obscured AGNs have L bol > 10 41 erg s −1 and λ Edd ≳ 10 −3 , and are detected in high-angular resolution mid-IR imaging, indicating the presence of obscuring dust on nuclear scale. NGC 3486, however, is undetected in high-resolution mid-IR imaging, and the current data do not provide stringent constraints on the presence or absence of obscuring nuclear dust in the AGN.
  • Thumbnail Image
    Item
    NuSTAR Observations of WISE J1036+0449, a Galaxy at z~1 Obscured by Hot Dust.
    (2017) Ricci, Claudio; Bauer, Franz Erik; Treister, Ezequiel; Nikutta, R.; Assef T., Roberto; Stern, D.; Alexander, D. M.; Asmus, D.; Ballantyne, D. R.; Blain, A. W.; Boggs, Steven E.
  • No Thumbnail Available
    Item
    THE 4 Ms CHANDRA DEEP FIELD-SOUTH NUMBER COUNTS APPORTIONED BY SOURCE CLASS: PERVASIVE ACTIVE GALACTIC NUCLEI AND THE ASCENT OF NORMAL GALAXIES
    (2012) Lehmer, B. D.; Xue, Y. Q.; Brandt, W. N.; Alexander, D. M.; Bauer, F. E.; Brusa, M.; Comastri, A.; Gilli, R.; Hornschemeier, A. E.; Luo, B.; Paolillo, M.; Ptak, A.; Shemmer, O.; Schneider, D. P.; Tozzi, P.; Vignali, C.
    We present 0.5-2 keV, 2-8 keV, 4-8 keV, and 0.5-8 keV (hereafter soft, hard, ultra-hard, and full bands, respectively) cumulative and differential number-count (log N-log S) measurements for the recently completed approximate to 4 Ms Chandra Deep Field-South (CDF-S) survey, the deepest X-ray survey to date. We implement a new Bayesian approach, which allows reliable calculation of number counts down to flux limits that are factors of approximate to 1.9-4.3 times fainter than the previously deepest number-count investigations. In the soft band (SB), the most sensitive bandpass in our analysis, the approximate to 4 Ms CDF-S reaches a maximum source density of approximate to 27,800 deg(-2). By virtue of the exquisite X-ray and multiwavelength data available in the CDF-S, we are able to measure the number counts from a variety of source populations (active galactic nuclei (AGNs), normal galaxies, and Galactic stars) and subpopulations (as a function of redshift, AGN absorption, luminosity, and galaxy morphology) and test models that describe their evolution. We find that AGNs still dominate the X-ray number counts down to the faintest flux levels for all bands and reach a limiting SB source density of approximate to 14,900 deg(-2), the highest reliable AGN source density measured at any wavelength. We find that the normal-galaxy counts rise rapidly near the flux limits and, at the limiting SB flux, reach source densities of approximate to 12,700 deg(-2) and make up 46% +/- 5% of the total number counts. The rapid rise of the galaxy counts toward faint fluxes, as well as significant normal-galaxy contributions to the overall number counts, indicates that normal galaxies will overtake AGNs just below the approximate to 4 Ms SB flux limit and will provide a numerically significant new X-ray source population in future surveys that reach below the approximate to 4 Ms sensitivity limit. We show that a future approximate to 10 Ms CDF-S would allow for a significant increase in X-ray-detected sources, with many of the new sources being cosmologically distant (z greater than or similar to 0.6) normal galaxies.
  • Thumbnail Image
    Item
    THE CHANDRA DEEP FIELD-SOUTH SURVEY: 7 MS SOURCE CATALOGS
    (2017) Luo, B.; Brandt, W. N.; Xue, Y. Q.; Lehmer, B.; Alexander, D. M.; Bauer, Franz Erik; Vito, F.; Yang, G.; Basu-Zych, A. R.; Comastri, A.
  • Thumbnail Image
    Item
    The fall of active galactic nuclei and the rise of star-forming galaxies : a close look at the chandra deep field X-ray number counts.
    (2004) Bauer, Franz Erik; Treister, Ezequiel; Alexander, D. M.; Brandt, W N.; Schneider, D. P.; Hornschemeier, A.E.; Garmire, G. P.
  • Thumbnail Image
    Item
    The hidden side of cosmic star formation at z > 3 Bridging optically dark and Lyman-break galaxies with GOODS-ALMA
    (Wiley, 2023) Xiao, M-Y.; Elbaz, D.; Gomez-Guijarro, C.; Leroy, L.; Bing, L-J.; Daddi, E.; Magnelli, B.; Franco, M.; Zhou, L.; Dickinson, M.; Wang, T.; Rujopakarn, W.; Magdis, G. E.; Treister, Ezequiel; Inami, H.; Demarco, R.; Sargent, M. T.; Shu, X.; Kartaltepe, J. S.; Alexander, D. M.; Bethermin, M.; Bournaud, F.; Ciesla, L.; Ferguson, H. C.; Finkelstein, S. L.; Giavalisco, M.; Gu, Q-S.; Iono, D.; Juneau, S.; Lagache, G.; Leiton, R.; Messias, H.; Motohara, K.; Mullaney, J.; Nagar, N.; Pannella, M.; Papovich, C.; Pope, A.; Schreiber, C.; Silverman, J.
    Our current understanding of the cosmic star formation history at z > 3 is primarily based on UV-selected galaxies (Lyman-break galaxies, i.e., LBGs). Recent studies of H-dropouts (HST-dark galaxies) have revealed that we may be missing a large proportion of star formation that is taking place in massive galaxies at z > 3. In this work, we extend the H-dropout criterion to lower masses to select optically dark or faint galaxies (OFGs) at high redshifts in order to complete the census between LBGs and H-dropouts. Our criterion (H > 26.5 mag & [4.5] < 25 mag) combined with a de-blending technique is designed to select not only extremely dust-obscured massive galaxies but also normal star-forming galaxies (typically E(B - V) > 0.4) with lower stellar masses at high redshifts. In addition, with this criterion, our sample is not contaminated by massive passive or old galaxies. In total, we identified 27 OFGs at (Zphot) > 3 (with a median of z(med) = 4.1) in the GOODS-ALMA field, covering a wide distribution of stellar masses with log(M-star/M-circle dot) = 9.4-11.1 (with a median of log(M-star med/M-circle dot) = 10.3). We find that up to 75% of the OFGs with log(M-star/M-circle dot) = 9.5-10.5 were neglected by previous LBGs and H-dropout selection techniques. After performing an optical-to-millimeter stacking analysis of the OFGs, we find that rather than being limited to a rare population of extreme starbursts, these OFGs represent a normal population of dusty star-forming galaxies at z > 3. The OFGs exhibit shorter gas depletion timescales, slightly lower gas fractions, and lower dust temperatures than the scaling relation of typical star-forming galaxies. Additionally, the total star formation rate (SFRtot = SFRIR + SFRUV) of the stacked OFGs is much higher than the SFRUVcorr (SFRUV corrected for dust extinction), with an average SFRtot/SFRUVcorr = 8 +/- 1, which lies above (similar to 0.3 dex) the 16-84th percentile range of typical star-forming galaxies at 3 <= z <= 6. All of the above suggests the presence of hidden dust regions in the OFGs that absorb all UV photons, which cannot be reproduced with dust extinction corrections. The effective radius of the average dust size measured by a circular Gaussian model fit in the uv plane is R-e(1.13 mm) = 1.01 +/- 0.05 kpc. After excluding the five LBGs in the OFG sample, we investigated their contributions to the cosmic star formation rate density (SFRD). We found that the SFRD at z > 3 contributed by massive OFGs (log(M-star/M-circle dot) > 10.3) is at least two orders of magnitude higher than the one contributed by equivalently massive LBGs. Finally, we calculated the combined contribution of OFGs and LBGs to the cosmic SFRD at z = 4-5 to be 4 x 10(-2) M-circle dot yr(-1) Mpc(-3), which is about 0.15 dex (43%) higher than the SFRD derived from UV-selected samples alone at the same redshift. This value could be even larger, as our calculations were performed in a very conservative way.
  • Thumbnail Image
    Item
    The KMOS AGN Survey at High redshift (KASHz) : the prevalence and drivers of ionized outflows in the host galaxies of X-ray AGN
    (2016) Harrison, C. M.; Alexander, D. M.; Mullaney, J. R.; Stott, J. P.; Swinbank, A. M.; Arumugam, V.; Bauer, Franz Erik; Bower, R. G.; Bunker, A. J.; Sharples, R. M.
  • Thumbnail Image
    Item
    The NuSTAR Extragalactic Surveys: Initial Results and Catalog from the Extended Chandra Deep Field South
    (2015) Mullaney, J.; Bauer, Franz Erik; Treister, Ezequiel; Del Moro, A.; Aird, J. A.; Alexander, D. M.; Civano, Francesca; Hickox, R. C.; Lansbury, George B.; Ajello, Marco; Assef, R.
  • Thumbnail Image
    Item
    The NuSTAR Extragalactic Surveys: The Number Counts of Active Galactic Nuclei and the Resolved Fraction of the Cosmic X-Ray Background
    (2016) Harrison, F. A.; Bauer, Franz Erik; Treister, Ezequiel; Aird, J. A.; Civano, Francesca; Lansbury, George B.; Mullaney, J.; Ballantyne, D. R.; Alexander, D. M.; Stern, Daniel; Ajello, Marco
  • No Thumbnail Available
    Item
    The VST ATLAS Quasar Survey - II. Halo mass profiles of galaxies, LRGs and galaxy clusters via quasar and CMB lensing
    (2024) Eltvedt, Alice M.; Shanks, T.; Metcalfe, N.; Ansarinejad, B.; Barrientos, L. F.; Murphy, D. N. A.; Alexander, D. M.
    We cross-correlate a low-contamination subset of the VST ATLAS g < 22.5 quasar catalogue with g < 21.5 galaxy clusters, r < 21 galaxies and r < 19.5 luminous red galaxies (LRGs) to probe their halo mass profiles via quasar magnification bias caused by weak lensing. In the case of galaxy clusters, we find that at small scales their mass profiles are well fitted by Navarro, Frenk, and White models with masses within the expected range. For the galaxies, we find consistency with previous Sloan Digital Sky Survey-based results for the galaxy-quasar cross-correlation and the galaxy auto-correlation functions. Disagreement as to whether the cross-correlation results are in tension with Lambda cold dark matter appears due to different assumptions as to whether galaxies trace mass. We conclude that halo occupation distribution (HOD) models fit the galaxy-quasar lensing results better than models where galaxies trace the mass. We further test the cluster and galaxy HOD models in the 2-halo range using the Planck cosmic microwave background (CMB) lensing map, finding that the cross-correlation with both the poorest clusters and the galaxies may be marginally overpredicted by the above HOD models. Finally, we measure the magnification bias of LRGs using both quasar and CMB lensing and find that the observed quasar lensing amplitude may be approximate to 2x too high and, on larger scales, the CMB lensing amplitude may be too low to be explained by a standard LRG HOD model.
  • No Thumbnail Available
    Item
    The VST ATLAS quasar survey I: Catalogue of photometrically selected quasar candidates
    (2023) Eltvedt, Alice M.; Shanks, T.; Metcalfe, N.; Ansarinejad, B.; Barrientos, L. F.; Sharp, R.; Malik, U.; Murphy, D. N. A.; Irwin, M.; Wilson, M.; Alexander, D. M.; Kovacs, Andras; Garcia-Bellido, Juan; Ahlen, Steven; Brooks, David; de la Macorra, Axel; Font-Ribera, Andreu; Gontcho, Satya Gontcho a; Honscheid, Klaus; Meisner, Aaron; Miquel, Ramon; Nie, Jundan; Tarle, Gregory; Vargas-Magana, Mariana; Zhou, Zhimin
    We present the VST ATLAS Quasar Survey, consisting of similar to 1229 000 quasar (QSO) candidates with 16 < g < 22.5 over similar to 4700 deg(2). The catalogue is based on VST ATLAS+NEOWISE imaging surveys and aims to reach a QSO sky density of 130 deg-2 for z < 2.2 and similar to 30 deg(-2) for z > 2.2. To guide our selection, we use X-ray/UV/optical/MIR data in the extended William Herschel Deep Field (WHDF) where we find a g < 22.5 broad-line QSO density of 269 +/- 67 deg(-2), roughly consistent with the expected similar to 196 deg(-2). We find that similar to 25 per cent of our QSOs are morphologically classed as optically extended. Overall, we find that in these deep data, MIR, UV, and X-ray selections are similar to 70-90 per cent complete while X-ray suffers less contamination than MIR and UV. MIR is however more sensitive than X-ray or UV to z > 2.2 QSOs at g < 22.5 and the S-X (0.5 - 10 keV) > 1 x10(-14) ergs cm(-2) s(-1) limit of eROSITA. We adjust the selection criteria from our previous 2QDES pilot survey and prioritize VST ATLAS candidates that show both UV and MIR excess, also selecting candidates initially classified as extended. We test our selections using data from DESI (which will be released in DR1) and 2dF to estimate the efficiency and completeness, and we use ANNz2 to determine photometric redshifts. Applying over the similar to 4700 deg(2) ATLAS area gives us similar to 917 000 z < 2.2 QSO candidates of which 472 000 are likely to be z < 2.2 QSOs, implying a sky density of similar to 100 deg(-2), which our WHDF analysis suggests will rise to at least 130 deg(-2) when eROSITA X-ray candidates are included. At z > 2.2, we find similar to 310() 000 candidates, of which 169 000 are likely to be QSOs for a sky density of similar to 36 deg(-2).
  • Thumbnail Image
    Item
    The weak Fe fluorescence line and long-term X-ray evolution of the Compton-thick active galactic nucleus in NGC7674
    (2017) Gandhi, P.; Annuar, A.; Lansbury, G. B.; Stern, D.; Alexander, D. M.; Bauer, Franz Erik; Bianchi, S.; Boggs, S. E.; Boorman, P. G.; Ricci, Claudio
  • Thumbnail Image
    Item