arXiv subjects

L. J. Tacconi

Publications and source records attributed to L. J. Tacconi.

At least 199 records · Page 11Linked to original sources

arXiv2006

Hubble Space Telescope H-Band Imaging Survey of Massive Gas-Rich Mergers

We report the results from a deep HST NICMOS H-band imaging survey of a carefully selected sample of 33 luminous, late-stage galactic mergers at z < 0.3. Signs of a recent galactic interaction are seen in all of the objects in the HST sample, including all 7 IR-excess Palomar-Green (PG) QSOs in the sample. Unsuspected double nuclei are detected in 5 ULIRGs. A detailed two-dimensional analysis of the surface brightness distributions in these objects indicates that the great majority (81%) of the single-nucleus systems show a prominent early-type morphology. However, low-surface-brightness exponential disks are detected on large scale in at least 4 of these sources. The hosts of 'warm' AGN-like systems are of early type and have less pronounced merger-induced morphological anomalies than the hosts of cool systems with LINER or HII region-like nuclear optical spectral types. The host sizes and luminosities of the 7 PG~QSOs in our sample are statistically indistinguishable from those of the ULIRG hosts. In comparison, highly luminous quasars, such as those studied by Dunlop et al. (2003), have hosts which are larger and more luminous. The hosts of ULIRGs and PG QSOs lie close to the locations of intermediate-size (about 1 -- 2 L*) spheroids in the photometric projection of the fundamental plane of ellipticals, although there is a tendency in our sample for the ULIRGs with small hosts to be brighter than normal spheroids. Excess emission from a young stellar population in the ULIRG/QSO hosts may be at the origin of this difference. Our results provide support for a possible merger-driven evolutionary connection between cool ULIRGs, warm ULIRGs, and PG~QSOs although this sequence may break down at low luminosity. (abridged)

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arXiv2006

Mid-infrared Spitzer spectra of X-ray selected Type 2 QSOs: QSO2s are not ULIRGs

We have performed a spectroscopic study of 7 Type 2 QSOs using the mid-infrared spectrometer IRS on board the Spitzer Space Telescope. These are (to our knowledge) the first mid-IR spectra of X-ray selected QSO2s taken. The objects have been selected according to their high intrinsic luminosities and column densities in the X-rays. Their spectra strongly differ from template spectra of Type 2 AGN at lower luminosities. They do not exhibit strong PAH dust emission features from circum-nuclear star forming regions, typical for lower luminosity Type 2 Seyfert galaxies or other previously used QSO2 templates, such as the (Ultra)luminous Infrared Galaxy ((U)LIRG) NGC 6240. They also do not show the ice and silicate absorption features of highly luminous but deeply embedded compact nuclei seen in some ULIRGs. Instead they reveal a relatively featureless, rising continuum similar to luminous Type 1 AGN. We also find evidence for a 10 micron silicate feature in emission. Models of dusty tori in the AGN unification scenario predict this only for Type 1 AGN. The ratio of the AGN continuum luminosity at 6 micron to the absorption corrected 2-10keV X-ray AGN luminosity is very similar to that found in Seyfert galaxies. X-ray selected QSO2s are thus characterized by powerful AGN in hosts with a luminosity due to star formation < 1e11 L_sol. The dominance of the AGN light in the mid-IR spectra of QSO2s together with their flatter spectral energy distributions (SEDs) places important constraints on models of the cosmic infrared background and of the star formation history of the universe.

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arXiv2005

Probing for evolutionary links between local ULIRGs and QSOs from NIR spectroscopy

We present a study of the dynamical evolution of Ultraluminous Infrared Galaxies (ULIRGs), merging galaxies of infrared luminosity >10^12 L_sun. During our Very Large Telescope large program, we have obtained ISAAC near-infrared, high-resolution spectra of 54 ULIRGs (at several merger phases) and 12 local Palomar-Green QSOs to investigate whether ULIRGs go through a QSO phase during their evolution. One possible evolutionary scenario is that after nuclear coalescence, the black hole radiates close to Eddington to produce QSO luminosities. The mean stellar velocity dispersion that we measure from our spectra is similar (~160 km/s) for 30 post-coalescence ULIRGs and 7 IR-bright QSOs. The black holes in both populations have masses of order 10^7-10^8 M_sun (calculated from the relation to the host dispersion) and accrete at rates >0.5 Eddington. Placing ULIRGs and IR-bright QSOs on the fundamental plane of early-type galaxies shows that they are located on a similar region (that of moderate-mass ellipticals), in contrast to giant ellipticals and radio-loud QSOs. While this preliminary comparison of the ULIRG and QSO host kinematical properties indicates that (some) ULIRGs may undergo a QSO phase in their evolutionary history before they settle down as ellipticals, further data on non-IR excess QSOs are necessary to test this scenario.

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arXiv2005

High-resolution Millimeter Imaging of Submillimeter Galaxies

We present sub-arcsecond resolution IRAM PdBI interferometry of eight submillimeter galaxies at redshifts from 2 to 3.4, where we detect continuum at 1mm and/or CO lines at 3 and 1 mm. The CO 3-2/4-3 line profiles in five of the sources are double-peaked, indicative of orbital motion either in a single rotating disk or of a merger of two galaxies. The millimeter line and continuum emission is compact; we marginally resolve the sources or obtain tight upper limits to their intrinsic sizes in all cases. The median FWHM diameter for these sources and the previously resolved sources, SMMJ023952-0136 and SMMJ140104+0252 is less than or equal to 0.5" (4 kpc). The compactness of the sources does not support a scenario where the far-IR/submm emission comes from a cold, very extended dust distribution. These measurements clearly show that the submillimeter galaxies we have observed resemble scaled-up and more gas rich versions of the local Universe, ultra-luminous galaxy (ULIRG) population. Their central densities and potential well depths are much greater than in other redshift 2-3 galaxy samples studied so far. They are comparable to those of elliptical galaxies or massive bulges. The SMG properties fulfill the criteria of 'maximal' starbursts, in which most of the available initial gas reservoir of 10^10-10^11 solar masses is converted to stars on a few dynamical timescales.

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arXiv2005

Dynamical properties of Ultraluminous Infrared Galaxies I: Mass ratio conditions for ULIRG activity in interacting pairs

We present first results from our Very Large Telescope large program to study the dynamical evolution of Ultraluminous Infrared Galaxies (ULIRGs), which are the products of mergers of gas-rich galaxies. The full data set consists of high resolution, long-slit, H- and K-band spectra of 38 ULIRGs and 12 QSOs (between 0.042 3:1 typically do not force enough gas into the center to generate ULIRG luminosities.

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arXiv2005

Evolution of local Ultraluminous mergers from NIR spectroscopy

We present results from our VLT Large Program to study the dynamical evolution of Ultraluminous Infrared Galaxies (ULIRGs). We have so far obtained near-infrared high-resolution ISAAC spectra of 53 local ULIRGs at several merger timescales and 12 Palomar-Green QSOs (more than half of which are IR-bright sources). We have extracted the stellar velocity dispersion and rotational velocity along our slits to derive the kinematics of the merging galaxies. These quantities enable us to answer the following questions about the evolution of ULIRGs: 1) What are the progenitor mass ratios?, 2) How do the stellar kinematics evolve?, and, 3) Is there a connection between ULIRGs and QSOs? We find that the Ultraluminous phase is mainly triggered by mergers of approximately equal mass galaxies, however, less violent minor mergers (of progenitor mass ratio ~3:1) also exist in our sample. Dynamical heating of the merging hosts is observed as the stellar systematic rotation decreases with time in favour of the increase of random motions. The merger remnants, being dispersion-dominated systems with non-negligible rotation, resemble elliptical galaxies. Placing ULIRGs on the fundamental plane of early-type galaxies shows that they resemble intermediate-mass ellipticals. After the nuclear coalescence, the black hole masses of ULIRGs, calculated from their relation to the host dispersions, are of the order 10^7-10^8 M_sun. To investigate whether ULIRGs go through a QSO phase during their evolution, we perform a similar (preliminary) analysis of the IR-bright-QSO kinematics. We find that the average dispersion of the IR-bright QSOs is similar to that of the ULIRG remnants, indicating that evolutionary links between the two populations may exist.

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arXiv2005

Millimeter observations of obscured Spitzer 24 micron sources

We present MAMBO 1.2mm observations of 40 extragalactic sources from the Spitzer First Look Survey that are bright in the mid-infrared (S_24um>1mJy) but optically obscured (log_10 (nu F_nu (24um))/(nu F_nu (0.7um))>1). We use these observations to search for cold dust emission, probing the similarity of their spectral energy distributions to star forming infrared galaxies or obscured AGN. The sample as a whole is well detected at mean S_1.2mm=0.74+-0.09mJy and S_1.2mm/S_24um=0.15+-0.03. Seven (three) of the sources are individually detected at >3sigma (>5sigma) levels. Mean millimeter fluxes are higher for sources with the reddest mid-infrared/optical colors. Optically faint but with relatively low mm to mid-infrared ratio, the typical SEDs are inconsistent with redshifted SED shapes of local star-forming infrared galaxies. They also differ from SEDs of typical submillimeter selected galaxies, with the 24um sources that are individually detected by MAMBO possibly representing intermediate objects. Compared to star-forming galaxies, a stronger but optically obscured mid-infrared component without associated strong far-infrared emission has to be included. This component may be due to luminous optically obscured AGN, which would represent a significant part of the high redshift AGN population.

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arXiv2005

Near IR diffraction-limited integral-field SINFONI spectroscopy of the Circinus galaxy

Using the adaptive optics assisted near infrared integral field spectrometer SINFONI on the VLT, we have obtained observations of the Circinus galaxy on parsec scales. The morphologies of the H_2(1-0)S(1) 2.12um and Br_gamma 2.17um emission lines are only slightly different, but their velocity maps are similar and show a gradient along the major axis of the galaxy, consistent with rotation.Since V_rot/sigma is approximately 1 suggests that random motions are also important, it is likely that the lines arise in a rotating spheroid or thickened disk around the AGN. Comparing the Br_gamma flux to the stellar continuum indicates that the star formation in this region began almost 10^8 yr ago. We also detect the [SiVI] 1.96um,[AlIX] 2.04um and [CaVIII] 2.32um coronal lines. In all cases we observe a broad blue wing, indicating the presence of two or more components in the coronal line region. A correlation between the ionisation potential and the asymmetry of the profiles was found for these high excitation species.

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arXiv2005

Molecular Gas in NUclei of GAlaxies (NUGA) III. The warped LINER NGC3718

We present the first interferometric observations of CO(1-0) and CO(2-1) line emission from the warped LINER NGC3718, obtained with the IRAM PdBI. This L1.9 galaxy has a prominent dust lane and on kpc scales, a strongly warped atomic and molecular gas disk. The molecular gas is closely associated with the dust lane across the nucleus and its kinematic center is consistent with the mm continuum AGN. A comparison of our interferometric mosaic data, which fully cover the ~9kpc warped disk, with a previously obtained IRAM 30m single dish CO(1-0) map shows that the molecular gas distribution in the disk is heavily resolved by the PdBI map. After applying a short-spacing correction with the IRAM 30m data, we find in total six main source components within the dust lane: one associated with the nucleus, four symmetrically positioned on either side at galactocentric distances of about 1.3kpc and 4.0kpc from the center, and a sixth on the western side at ~3kpc with only a very weak eastern counterpart. In the framework of a kinematic model using tilted rings, we interpret the five symmetric source components as locations of strong orbital crowding. We further find indications that the warp appears not only on kpc scales, but continues down to 250pc. Besides the sixth feature on the western side, the lower flux of the eastern components compared to the western ones indicates an intrinsic large scale asymmetry in NGC3718 that cannot be explained by the warp. Indications for a small scale asymmetry are also seen in the central 600pc. These asymmetries might be evidence for a tidal interaction with a companion galaxy (large scales) and gas accretion onto the nucleus (small scales). Our study of NGC3718 is part of the NUGA project that aims at investigating the different processes of gas accretion onto AGN.

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arXiv2005

Silicate emissions in active galaxies - From LINERs to QSOs

We report the first detection of ~10 and ~18 micron silicate dust emissions in a low-luminosity active galactic nucleus (AGN), obtained in Spitzer-IRS 7-37 micron spectroscopy of the Type 1 LINER galaxy NGC3998. Silicate emissions in AGN have only recently been detected in several quasars. Our detection counters suggestions that silicate emissions are present only in the most luminous AGN. The silicate features may be signatures of a dusty ``obscuring torus'' viewed face-on as postulated for Type 1 AGN. However, the apparently cool (~200 K) dust is inconsistent with theoretical expectations of much hotter torus walls. Furthermore, not all Type 1 objects are silicate emission sources. Alternatively, the silicate emission may originate in dust not directly associated with a torus. We find that the long-wavelength (>20 micron) tail of the emission in NGC3998 is significantly weaker than in the sample of bright QSOs recently presented by Hao et al. The 10 micron profile in our NGC3998 spectrum is inconsistent with ``standard'' silicate ISM dust. This may indicate differences in the dust composition, grain size distribution, or degree of crystallization. The differences between NGC3998, QSOs, and Galactic templates suggest that there are significant environmental variations.

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arXiv2004

The Nuclear Gas Dynamics and Star Formation of Markarian 231

We report adaptive optics H- and K-band spectroscopy of the inner few arcsec of the luminous merger/ULIRG/QSO Mkn231, at spatial resolutions as small as 0.085". For the first time we have been able to resolve the active star forming region close to the AGN using stellar absorption features, finding that its luminosity profile is well represented by an exponential function with a disk scale length 0.18-0.24" (150-200pc), and implying that the stars exist in a disk rather than a spheroid. The stars in this region are also young (10-100Myr), and it therefore seems likely that they have formed in situ in the gas disk, which itself resulted from the merger. The value of the stellar velocity dispersion is a result of the large mass surface density of the disk. The stars in this region have a combined mass of at least 1.6x10^9M_sun, and account for 25-40% of the bolometric luminosity of the entire galaxy. We have detected the 2.12um 1-0S(1) H_2 and 1.64um [FeII] lines out to radii exceeding 0.5". The kinematics for the two lines are very similar to each other as well as to the stellar kinematics, and broadly consistent with the nearly face-on rotating disk reported in the literature and based on interferometric CO1-0 and CO2-1 measurements of the cold gas. However, they suggest a more complex situation in which the inner 0.2-0.3" (200pc) is warped out of its original disk plane. Such a scenario is supported by other observations.

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arXiv2004

Feeding AGN: new results from the NUGA survey

The NUGA project is a high-resolution (0.5''-1'') CO survey of low luminosity AGN including the full sequence of activity types (Seyferts, LINERs and transition objects). NUGA aims to systematically study the different mechanisms for gas fueling of AGNs in the Local Universe. In this paper we discuss the latest results of this recently completed survey, which now includes newly acquired subarcsec resolution observations for all targets of the sample. The large variety of circumnuclear disk morphologies found in NUGA galaxies (m=1, m=2 and stochastic instabilities) is a challenging result that urges the refinement of current dynamical models. In this paper we report on new results obtained in 4 study cases for NUGA: NGC4826, NGC7217, NGC4579 and NGC6951

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arXiv2004

Star Formation and Dynamics in the nuclei of AGN

Using adaptive optics on Keck and the VLT in the H- and K-bands, we have begun a project to probe the dynamics and star formation around AGN on scales of 0.1arcsec. The stellar content of the nucleus is traced through the 2.29micron CO2-0 and 1.62micron CO6-3 absorption bandheads. These features are directly spatially resolved, allowing us to measure the extent and distribution of the nuclear star forming region. The dynamics are traced through the 2.12micron H_2 1-0S(1) and 1.64micron [FeII] emission lines, as well as stellar absorption features. Matching disk models to the rotation curves at various position angles allows us to determine the mass of the stellar and gas components, and constrain the mass of the central black hole. In this contribution we summarise results for the two type~1 AGN Mkn231 and NGC7469.

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arXiv2004

SPIFFI observations of the starburst SMMJ14011+0252: Already old, fat, and rich by z = 2.565

Using the SPectrometer for Infrared Faint Field Imaging (SPIFFI) on the ESO VLT, we have obtained J, H, and K band integral field spectroscopy of the z = 2.565 luminous submillimeter galaxy SMMJ14011+0252. A global spectrum reveals the brighter of this spatially resolved system's two components as an intense starburst that is remarkably old, massive, and metal-rich for the early epoch at which it is observed. We see a strong Balmer break implying a > 100 Myr timescale for continuous star formation, as well as nebular emission line ratios implying a supersolar oxygen abundance on large spatial scales. Overall, the system is rapidly converting a large baryonic mass into stars over the course of only a few hundred Myr. Our study thus adds new arguments to the growing evidence that submillimeter galaxies are more massive than Lyman break galaxies, and more numerous at high redshift than predicted by current semi-analytic models of galaxy evolution.

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arXiv2004

Submillimeter Galaxies as Tracers of Mass Assembly at Large M

Luminous submillimeter galaxies (SMGs) have been known for some time to produce a substantial fraction of the far-IR/submillimeter background. New observations show that these systems are not only very luminous, but also very massive. Here we present results from (a) a survey of SMGs' molecular line emission, which has confirmed Keck optical redshifts and revealed their high gas and dynamical masses; and (b) a detailed case study of the rest-frame optical emission from one SMG, which has revealed its high metallicity and old stellar population. We infer a high z~2.7 comoving number density of galaxies with baryonic masses greater than 10^11 solar masses; this is not accounted for in current semi-analytic models of galaxy evolution.

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arXiv2003

NUclei of GAlaxies (NUGA): the IRAM Survey of Low Luminosity AGN

The NUGA project is the first high-resolution (0.5''-1'') CO survey of low luminosity AGN including the full sequence of activity types (Seyfert 1--2 and LINERs). NUGA aims to study systematically the different mechanisms for gas fueling of the AGN. In order to reach the critical spatial scales for AGN feeding (<100 pc), we are undertaking high-resolution observations, made at the Plateau de Bure Interferometer, to achieve a sharp view of the distribution and kinematics of molecular gas. In this talk we discuss the latest results of this survey which now includes newly acquired subarcsec resolution observations for 5 targets of our sample. The large variety of circumnuclear disk morphologies found in NUGA galaxies (m=1, m=2 and m=0-type gravitational instabilities) is a challenging result that urges the refinement of current dynamical models.

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arXiv2003

Molecular Gas in NUclei of GAlaxies (NUGA): II. The Ringed LINER NGC7217

We present CO(1-0) and CO(2-1) maps of the LINER galaxy NGC7217, obtained with the IRAM interferometer, at 2.4"x1.9" and 1.2"x0.8" resolution respectively. The nuclear ring (at r=12"=0.8kpc) dominates the CO maps, and has a remarkable sharp surface density gradient at its inner edge. The latter is the site of the stellar/Halpha ring, while the CO emission ring extends farther or is broader (500-600pc). This means that the star formation has been more intense toward the inner edge of the CO ring, in a thin layer, just at the location of the high gas density gradient. In the CO(2-1) map, a central concentration inside 3" coincides with a spiral dust lane, clearly seen in the HST V-I color image. N-body simulations including gas dissipation and star formation are performed to better understand the nature of the nuclear ring observed. In the best model, the gas content was higher in the recent past (having been consumed via star formation), and the oval/spiral formed were more self-gravitating (abridged).

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arXiv2003

Molecular Gas in NUclei of GAlaxies (NUGA) I.The counter-rotating LINER NGC4826

We present new high-resolution observations of the nucleus of the counter-rotating LINER NGC4826, made in the J=1-0 and J=2-1 lines of 12CO with the IRAM Plateau de Bure mm-interferometer(PdBI).The CO maps, which achieve 0.8''(16pc) resolution in the 2-1 line, fully resolve an inner molecular gas disk which is truncated at an outer radius of 700pc. The total molecular gas mass is distributed in a lopsided nuclear disk of 40pc radius and two one-arm spirals, which develop at different radii in the disk. The distribution and kinematics of molecular gas in the inner 1kpc of NGC4826 show the prevalence of different types of m=1 perturbations in the gas. Although dominated by rotation, the gas kinematics are perturbed by streaming motions related to the m=1 instabilities. The non-circular motions associated with the inner m=1 perturbations agree qualitatively with the pattern expected for a trailing wave developed outside corotation ('fast' wave). In contrast, the streaming motions in the outer m=1 spiral are better explained by a 'slow' wave. A paradoxical consequence is that the inner m=1 perturbations would not favour AGN feeding. An independent confirmation that the AGN is not being generously fueled at present is found in the low values of the gravitational torques exerted by the stellar potential for R<530pc. The distribution of star formation in the disk of NGC4826 is also strongly asymmetrical. Massive star formation is still vigorous, fed by the significant molecular gas reservoir at R<700pc. There is supporting evidence for a recent large mass inflow episode in NGC4826. These observations have been made in the context of the NUclei of GAlaxies (NUGA) project, aimed at the study of the different mechanisms for gas fueling of AGN.

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