Home OALib Journal OALib PrePrints Submit Ranking News My Lib FAQ About Us Follow Us+
 Title Keywords Abstract Author All
Search Results: 1 - 10 of 100 matches for " "
 Page 1 /100 Display every page 5 10 20 Item
 Physics , 2012, DOI: 10.1088/0004-637X/768/2/168 Abstract: We analyze the link between active galactic nuclei (AGN) and mid-infrared flux using dust radiative transfer calculations of starbursts realized in hydrodynamical simulations. Focusing on the effects of galaxy dust, we evaluate diagnostics commonly used to disentangle AGN and star formation in ultraluminous infrared galaxies (ULIRGs). We examine these quantities as a function of time, viewing angle, dust model, AGN spectrum, and AGN strength in merger simulations representing two possible extremes of the ULIRG population: one is a typical gas-rich merger at z ~ 0, and the other is characteristic of extremely obscured starbursts at z ~ 2 to 4. This highly obscured burst begins star-formation-dominated with significant PAH emission, and ends with a ~10^9 yr period of red near-IR colors. At coalescence, when the AGN is most luminous, dust obscures the near-infrared AGN signature, reduces the relative emission from polycyclic aromatic hydrocarbons (PAHs), and enhances the 9.7 micron absorption by silicate grains. Although generally consistent with previous interpretations, our results imply none of these indicators can unambiguously estimate the AGN luminosity fraction in all cases. Motivated by the simulations, we show that a combination of the extinction feature at 9.7 micron, the PAH strength, and a near-infrared slope can simultaneously constrain the AGN fraction and dust grain distribution for a wide range of obscuration. We find that this indicator, accessible to the James Webb Space Telescope, may estimate the AGN power as tightly as the hard X-ray flux alone, thereby providing a valuable future cross-check and constraint for large samples of distant ULIRGs.
 Physics , 2012, DOI: 10.1088/0004-637X/757/1/13 Abstract: In this paper, we characterize the infrared spectral energy distributions (SEDs) of mid-IR selected z~0.3-3.0 and L_IR~10^11-10^13Lsun galaxies, and study how their SEDs differ from those of local and high-z analogs. Our mid-IR flux-limited sample of 191 sources is unique in size, and spectral coverage, including Spitzer mid-IR spectroscopy. Here we add Herschel photometry at 250um, 350um, and 500um, which allows us to obtain accurate total IR luminosities, as well as constrain the relative contributions of AGN and starbursts to those luminosities. Our sample constitutes ~23% AGN (i.e. where the AGN contributes >50% of L_IR), ~30% starbursts (where AGN contributes <20% of L_IR and the mid-IR spectra are starburst-like); and ~47% composites (which show both significant AGN and starburst activity). The AGN-dominated sources divide into ones that show a strong silicate 9.7um absorption feature, implying highly obscured systems, and ones that do not. The high-tau_9.7 sources are half of our z>1.2 AGN, but show SEDs that are extremely rare among local AGN. The SEDs of our z~2 starburst-dominated ULIRGs are much closer to those of local LIRGs than ULIRGs. This is consistent with our earlier finding that, unlike local ULIRGs, our high-z starbursts are typically only in the early stages of a merger. The SEDs of the composite sources are most similar to the local archetypal warm ULIRG, Mrk231. In summary, our results show that there is strong evolution in the SEDs between local and z~2 IR-luminous galaxies, as well as that there is a wide range of SEDs among high redshift IR-luminous sources. The publicly-available SED templates we derive from our sample will be particularly useful for infrared population synthesis models, as well as in the interpretation of other mid-IR high-z galaxies in particular those detected by the recent all sky WISE survey.
 Physics , 2002, DOI: 10.1086/346145 Abstract: Ultraluminous infrared galaxies (ULIRGs) are outstanding due to their huge luminosity output in the infrared, which is predominantly powered by super starbursts and/or hidden active galactic nuclei (AGN). NGC 6240 is one of the nearest ULIRGs and is considered a key representative of its class. Here, we report the first high-resolution imaging spectroscopy of NGC 6240 in X-rays. The observation, performed with the ACIS-S detector aboard the Chandra X-ray observatory, led to the discovery of two hard nuclei, coincident with the optical-IR nuclei of NGC 6240. The AGN character of both nuclei is revealed by the detection of absorbed hard, luminous X-ray emission and two strong neutral Fe_K_alpha lines. In addition, extended X-ray emission components are present, changing their rich structure in dependence of energy. The close correlation of the extended emission with the optical Halpha emission of NGC 6240, in combination with the softness of its spectrum, clearly indicates its relation to starburst-driven superwind activity.
 Physics , 2013, Abstract: We examine the use of submm recombination lines of H, He and He$^+$ to probe the extreme ultraviolet (EUV) luminosity of starbursts (SB) and AGN. We find that the submm recombination lines of H, He and He$^+$ are in fact extremely reliable and quantitative probes of the EUV continuum at 13.6 eV to above 54.6 eV. At submm wavelengths, the recombination lines originate from low energy levels (n = 20 -- 50). The maser amplification, which poses significant problems for quantitative interpretation of the higher n, radio frequency recombination lines, is insignificant. Lastly, at submm wavelengths the dust extinction is minimal. The submm line luminosities are therefore directly proportional to the emission measures ($EM_{ION} = n_e \times n_{ion} \times \rm{volume}$) of their ionized regions. We also find that the expected line fluxes are detectable with ALMA and can be imaged at $\sim0.1$\arcsec ~resolution in low redshift ULIRGs. Imaging of the HI lines will provide accurate spatial and kinematic mapping of the star formation distribution in low-z IR-luminous galaxies. And the relative fluxes of the HI and HeII recombination lines will strongly constrain the relative contributions of starbursts and AGN to the luminosity. The HI lines should also provide an avenue to constraining the submm dust extinction curve.
 Kimberly A. Weaver Physics , 2001, Abstract: The X-ray properties of 15 nearby (v<3,000 km/s) galaxies that possess AGN and/or starbursts are discussed. Two-thirds have nuclear extended emission on scales from ~0.5 to ~1.5 kpc that is either clearly associated with a nuclear outflow or morphologically resembles an outflow. Galaxies that are AGN-dominated tend to have linear structures while starburst-dominated galaxies tend to have plume-like structures. Significant X-ray absorption is present in the starburst regions, indicating that a circumnuclear starburst is sufficient to block an AGN at optical wavelengths. Galaxies with starburst activity possess more X-ray point sources within their central kpc than non-starbursts. Many of these sources are more luminous than typical X-ray binaries. The Chandra results are discussed in terms of the starburst-AGN connection, a revised unified model for AGN, and possible evolutionary scenarios.
 Physics , 2002, Abstract: Models for the origin of the hard X-ray background have suggested that sources with the most accretion activity lie hidden in highly obscured AGN. We report on our study of hard, serendipitous sources in the fields of Chandra clusters with fluxes close to the turn-over in the source-counts. These include two Type II quasars with measured X-ray luminosities >10^{45} ergs per second and column-densities >10^{23} cm^{-2}, one possibly being Compton-thick. Both show indications of redshifted Fe K\alpha line emission. Radiative transfer modelling of the broad-band spectrum of a highly-magnified source with deep ISOCAM detections implies the presence of warm-to-hot dust obscuring a quasar with L_{UV}>10^{45} ergs per second. Multi-wavelength spectroscopic and photometric follow-up of the optically-faint sources suggests that these objects are found in the centres of massive evolved galaxies at a range of redshifts, with red optical / near-infrared colours dominated by the host galaxy. Detailed source identification is difficult due to the paucity of strong emission features, especially in the infrared. We present the main results from a sample of near-infrared spectra of optically-faint sources obtained with 4 m and 8 m telescopes. Through the study of the harder and brighter X-ray background source population, we are likely to be viewing the most intense phase of the growth of supermassive black holes.
 Physics , 2002, DOI: 10.1086/340423 Abstract: Using the 1 Ms Chandra Deep Field North and 15 micron ISOCAM HDF-N surveys we find a tight correlation between the population of strongly evolving starbursts discovered in faint 15 micron ISOCAM surveys and the apparently normal galaxy population detected in deep X-ray surveys. Up to 100% of the X-ray detected emission-line galaxies (ELGs) have 15 micron counterparts, in contrast to 10-20% of the X-ray detected absorption-line galaxies and AGN-dominated sources. None of the X-ray detected ELGs is detected in the hard band (2-8 keV), and their stacked-average X-ray spectral slope of Gamma=2.0 suggests a low fraction of obscured AGN activity within the X-ray detected ELG population. The characteristics of the z=0.4-1.3 X-ray detected ELGs are consistent with those expected for M82 and NGC 3256-type starbursts; these X-ray detected ELGs contribute ~2% of the 0.5-8.0 keV X-ray background. The only statistical difference between the X-ray detected and X-ray undetected 15 micron selected ELGs is that a much larger fraction of the former have radio emission.
 Physics , 1999, DOI: 10.1086/301146 Abstract: We present a low resolution mid-infrared spectroscopic survey of an unbiased sample of 62 Ultraluminous Infrared Galaxies (ULIRGs) using ISOPHOT-S on board ISO. For comparison we also present ISOPHOT-S spectra for 23 AGN and 15 starburst and normal galaxies. The line-to-continuum ratio of the 7.7 $\mu$m PAH emission feature is used as a discriminator between starburst and AGN activity in ULIRGs. We find that the majority of ULIRGs are predominantly powered by starbursts. The ratio L$_{PAH}/$L$_{IR}$ for starburst dominated ULIRGs is similar to the one found for template starbursts.The shapes of the PAH features are sometimes unusual.Extinction has a noticeable effect on the PAH spectra of ULIRG starbursts. We have obtained high resolution near-infrared imaging for the Southern ISOPHOT-S ULIRGs to investigate their evolution stage. The majority (68%) of the ULIRGs imaged are double systems and all show distorted morphologies. Using separations for our work and from the literature, we have examined whether ULIRGs that are advanced mergers are more AGN-like. We have found no such evidence contrary to what is postulated by the classical evolutionary scenario. No correlation is found between the stage of merger in ULIRGs and their infrared luminosity.We also find that the total mass of interstellar gas, as estimated from the CO (1-0) luminosity, does not decrease with decreasing merger separation. When both an AGN and a starburst occur concurrently in ULIRGs, we find that the starburst dominates the luminosity output. We propose that the available gas reservoir and the individual structure of the interacting galaxies plays a major role in the evolution of the system.
 Physics , 2009, DOI: 10.1111/j.1365-2966.2009.15622.x Abstract: We present a quantitative estimate of the relative AGN/starburst content in a sample of 59 nearby (z<0.15) infrared bright ULIRGs taken from the 1 Jy sample, based on infrared L-band (3-4 micron) spectra. By using diagnostic diagrams and a simple deconvolution model, we show that at least 60% of local ULIRGs contain an active nucleus, but the AGN contribution to the bolometric luminosity is relevant only in ~15-20% of the sources. Overall, ULIRGs appear to be powered by the starburst process, responsible for >85% of the observed infrared luminosity. The subsample of sources optically classified as LINERs (31 objects) shows a similar AGN/starburst distribution as the whole sample, indicating a composite nature for this class of objects. We also show that a few ULIRGs, optically classified as starbursts, have L-band spectral features suggesting the presence of a buried AGN.
 Physics , 2009, DOI: 10.1051/0004-6361/200912261 Abstract: The co-evolution of host galaxies and the active black holes which reside in their centre is one of the most important topics in modern observational cosmology. Here we present a study of the properties of obscured Active Galactic Nuclei (AGN) detected in the CDFS 1Ms observation and their host galaxies. We limited the analysis to the MUSIC area, for which deep K-band observations obtained with ISAAC@VLT are available, ensuring accurate identifications of the counterparts of the X-ray sources as well as reliable determination of photometric redshifts and galaxy parameters, such as stellar masses and star formation rates. In particular, we: 1) refined the X-ray/infrared/optical association of 179 sources in the MUSIC area detected in the Chandra observation; 2) studied the host galaxies observed and rest frame colors and properties. We found that X-ray selected (L_X>10^{42} erg s^{-1}) AGN show Spitzer colors consistent with both AGN and starburst dominated infrared continuum; the latter would not have been selected as AGN from infrared diagnostics. The host galaxies of X-ray selected obscured AGN are all massive (M_*>10^{10} M_sun) and, in 50% of the cases, are also actively forming stars (1/SSFR1 and M_*>3x10^{11} M_sun, a fraction significantly higher than in the local Universe for AGN of similar luminosities.
 Page 1 /100 Display every page 5 10 20 Item