arXiv subjects

S. Leon

Publications and source records attributed to S. Leon.

At least 73 records · Page 4Linked to original sources

arXiv2005

A Study of the Type II-P Supernova 2003gd in M74

We present photometric and spectroscopic data of the type II-P supernova 2003gd, which was discovered in M74 close to the end of its plateau phase. SN 2003gd is the first type II supernova to have a directly confirmed red supergiant progenitor. We compare SN 2003gd with SN 1999em, a similar type II-P supernova, and estimate an explosion date of 18th March 2003. We determine a reddening towards the supernova of E(B-V) = 0.14+/-0.06, using three different methods. We also calculate three new distances to M74 of 9.6+/-2.8 Mpc, 7.7+/-1.7 Mpc and 9.6+/-2.2 Mpc. The former was estimated using the Standardised Candle Method (SCM), for type II supernovae, and the latter two using the Brightest Supergiants Method (BSM). When combined with existing kinematic and BSM distance estimates, we derive a mean value of 9.3+/-1.8 Mpc. SN 2003gd was found to have a lower tail luminosity compared to other ``normal'' type II-P SNe bringing into question the nature of this supernova. We present a discussion concluding that this is a ``normal'' type II-P supernova which is consistent with the observed progenitor mass of 8(+4/-2) Mo.

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arXiv2004

The role of environment in triggering starburst galaxies: A sample of isolated galaxies

The project AMIGA (Analysis of the interstellar Medium of Isolated GAlaxies) will provide a statistically significant sample of the most isolated galaxies in the northern sky. Such a control sample is necessary to assess the role of the environment in galaxy properties and evolution. The sample is based on the Catalogue of Isolated Galaxies (CIG, Karachentseva, 1973) and the database will include blue and near-infrared luminosities, far-infrared (FIR) emission, atomic gas (HI) emission, radio continuum, and, for a redshift limited subsample of about 200 galaxies, CO and H$α$ emission. The data will be released and periodically updated at http://www.iaa.csic.es/AMIGA.html. Here, we present the project and its status, as well as a preliminary analysis of the relation between star formation activity and the environment. We found a trend that the galaxies with asymmetric HI spectrum also tend to show an enhanced star formation, traced by a high value of $L_{FIR}/M(HI)$, indicating that a perturbed gas kinematics and enhanced SF are related.

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arXiv2004

Molecular and ionized gas in the tidal tail in Stephan's Quintet

We have mapped with the IRAM interferometer at Plateau de Bure (PdBI) the 12CO emission towards intergalactic star forming regions located in the tidal tail stemming from NGC7319, in the Stephan's Quintet compact group of galaxies. The 13CO emission of the same region was observed with the IRAM 30m telescope and optical spectroscopy of several HII regions in the area were obtained with the Calar Alto 3.5m telescope. We recovered with the interferometer about 50% of the CO(1-0) total emission that had been earlier measured with single dish observations. We find two main areas of CO emission: (i) an elongated region towards the area known as SQ B where a Tidal Dwarf Galaxy could currently be forming (ii) a barely resolved area at the tip of the optical tidal arm. Both regions follow dust lanes visible on HST images and their CO peak coincides spatially exactly with the maximum of the Halpha line emission. In SQ B, there is furthermore very good kinematical agreement between the CO, Halpha and HI components. We conclude from these coincidences that the gaseous matter found in quantities in the area is physically associated to the optical tidal tail and thus that the intergalactic atomic hydrogen there was expelled from NGC7319. Furthermore, the relatively high oxygen abundances (about solar) estimated from the optical spectra of the HII regions imply that the gas feeding the star formation originated from the inner regions of the parent galaxy. In SQ B, we derive from different tracers a star formation rate of 0.5 \msun/yr. The inferred molecular gas consumption time of 0.5 Gyr lies in the range of values found for spiral and starburst galaxies. On the other hand, the ratio of 12CO/13CO>25 is much higher than the values found in disks of spiral galaxies. A relatively low opacity for the 12CO gas is the most likely reason.

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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

Molecular Gas in Tidal Dwarf Galaxies: On-going Galaxy Formation

We investigate the process of galaxy formation as can be observed in the only currently forming galaxies -- the so-called Tidal Dwarf Galaxies, hereafter TDGs -- through observations of the molecular gas detected via its CO (Carbon Monoxide) emission. Molecular gas is a key element in the galaxy formation process, providing the link between a cloud of gas and a {\it bona fide} galaxy. We have now detected CO in 9 TDGs with an overall detection rate of 80%, showing that molecular gas is abundant in TDGs, up to a few $10^8 M_\odot$. The CO emission coincides both spatially and kinematically with the HI emission, indicating that the molecular gas forms from the atomic hydrogen where the HI column density is high. A possible trend of more evolved TDGs having greater molecular gas masses is observed, in accord with the transformation of HI into H$_2$. Although uncertainties are still large for individual objects as the geometry is unknown, we find that the "dynamical" masses of TDGs, estimated from the CO line widths, do not seem to be greater than the "visible" masses (HI + H$_2$ + a stellar component), i.e., TDGs require no dark matter. We provide evidence that TDGs are self-gravitating entities, implying that we are witnessing the ensemble of processes in galaxy formation: concentration of large amounts of gas in a bound object, condensation of the gas, which is atomic at this point, to form molecular gas and the subsequent star formation from the dense molecular component.

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arXiv2004

Colliding molecular clouds in head-on galaxy collisions

We present further observations of molecular gas in head-on collisions of spiral galaxies, this time of the CO(J=1-0) and CO(J=2-1) lines in the UGC 813 -- UGC 816 system. UGC 813/6 are only the second known example of head-on spiral-spiral collisions, the first example being the UGC 12914/5 pair. Strong CO emission is present in the bridge between UGC 813 and 816, unassociated with stellar emission, just as in UGC 12914/5. The CO emission from the UGC 813/6 bridge, not counting the emission from the galaxies themselves, is at least that of the entire Milky Way. Collisions of gas-rich spirals are really collisions between the interstellar media (ISMs) of the galaxies. We show that collisions between molecular clouds bring H2 into the bridge region. Although the dense clouds are ionized by the collisions, they cool and recombine very quickly and become molecular again even before the galactic disks separate. Because the clouds acquire an intermediate velocity post-collision, they are left in the bridge between the separating galaxies. The star formation efficiency appears low in the molecular clouds in the bridges. We speculate that the pre-stellar cores in the molecular clouds may expand during the cloud collisions, thus retarding future star formation. Because the ISM-ISM collisions discussed here require a very small impact parameter, they are rare among field spirals. In clusters, however, these collisions should be an important means of ejecting enriched gas from the inner parts of spirals.

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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

The Warped Gas and Dust Lane in NGC 3718

We present the first observations of molecular line emission in NGC 3718 with the IRAM 30m and the Plateau de Bure Interferometer. This galaxy is an impressive example for a strongly warped gas disk harboring an active galactic nucleus (AGN). An impressive dust lane is crossing the nucleus and a warp is developing into a polar ring. The molecular gas content is found to be typical of an elliptical galaxy with a relatively low molecular gas mass content (~4*10^8 Msun). The molecular gas distribution is found to warp from the inner disk together with the HI distribution. The CO data were also used to improve the kinematic modeling in the inner part of the galaxy, based on the so-called "tilted ring"-model. The nature of NGC 3718 is compared with its northern sky ``twin'' Centaurus A and the possible recent swallowing of a small-size gas-rich spiral is discussed.

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arXiv2003

Revised positions for the CIG galaxies

We present revised positions for the 1051 galaxies belonging to the Karachentseva Catalog of Isolated Galaxies (CIG). New positions were calculated by applying SExtractor to the Digitized Sky Survey CIG fields with a spatial resolution of 1.2''. We visually checked the results and for 118 galaxies had to recompute the assigned positions due to complex morphologies (e.g. distorted isophotes, undefined nuclei, knotty galaxies) or the presence of bright stars. We found differences between older and newer positions of up to 38'' with a mean value of 2.96'' relative to SIMBAD and up to 38'' and 2.42'' respectively relative to UZC. Based on star positions from the APM catalog we determined that the DSS astrometry of five CIG fields has a mean offset in (RA, Dec) of (-0.90'',0.93'') with a dispersion of 0.4''. These results have been confirmed using the 2MASS All-Sky Catalog of Point Sources. The intrinsic errors of our method combined with the astrometric ones are of the order of 0.5''.

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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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arXiv2002

NUGA: the IRAM survey of AGN spiral hosts

The NUclei of GAlaxies (NUGA) project is a combined effort to carry out a high-resolution (<1'') interferometer CO survey of a sample of 12 nearby AGN spiral hosts, using the IRAM array. We map the distribution and dynamics of molecular gas in the inner 1 kpc of the nuclei with resolutions of 10-50 pc, and study the mechanisms for gas fueling of the different low-luminosity AGN. First results show evidence for the occurrence of strong m=1 gas instabilities in Seyferts. NUGA maps allow us to address the origin/nature of m=1 modes and their link with m=2 modes and acoustic instabilities, present in other targets.

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arXiv2002

Study of the Interstellar Medium and Star Formation of a complete sample of 760 isolated galaxies

A key problem in astronomy involves the role of the environment in the formation and evolution of galaxies. In order to answer this question it is necessary to characterize a reference sample with minimum influence from the environment, so that its evolution is completeley determined by nature. The aim of this project is to provide such reference, quantifying the ISM properties of a well defined and statistically significant sample of 760 isolated galaxies. Based on optical, Halpha and infrared luminosities, radiocontinuum emission, molecular and atomic gas content, compiled from the bibliography or observed by ourselves, together with POSS-II digitized images, we will perform a statistical study of the Interstellar Medium (ISM) properties as a function of isolation, and its relation to star formation, morphology and luminosities, as well as nuclear activity frequency. This sample will be different from previous studies by three essential characteristics: a) strict definition of isolation, b) statistical significance, and c) complete multiwavelength information concerning the ISM. It will serve to evaluate the properties of interacting galaxies, which will be of special interest to analyse the large amount of data that will be generated during the next decade for high z galaxies, with the new instruments to come. Once the analysis will be finished, the data will become public by means of a database with free internet acces via a simple and efficient WEB interface.

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arXiv2002

Molecular gas in the 3-ringed Seyfert/Liner galaxy NGC 7217

We present CO(1-0) and CO(2-1) maps of the Seyfert galaxy NGC 7217, obtained with the IRAM interferometer, at 3" and 1.5" resolution respectively. The nuclear ring (at r=12"=0.8kpc) is predominant in the CO maps, with a remarkable surface density gradient between the depleted region inside the ring and the inner border of the ring. The CO nuclear ring is significantly broader (500-600pc) than the dust lane ring. The CO(2-1)/CO(1-0) ratio is around 1, typical of optically thick gas with high density. The overall morphology of the ring is quite circular, with no evidence of non-circular velocities. In the CO(2-1) map, a central concentration might be associated with the circumnuclear ring of ionised gas detected inside r=3" and interpreted as a polar ring by Sil'chenko and Afanasiev (2000). Our interpretation is more in terms of a bar/spiral structure, in the same plane as the global galaxy but affected by non-circular motions, which results in a characteristic S-shape of the isovels. This nuclear bar/spiral structure, clearly seen in a V-I HST colour image, is essentially gaseous and might be explained with acoustic waves.

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arXiv2002

Molecular gas in the double-barred Seyfert 2 galaxy NGC 5728

We present CO(1-0) and CO(2-1) maps of the Seyfert galaxy NGC 5728. Although a stellar nuclear bar structure is clearly identified in the near-infrared images in the central 10", inside the ring identified as the ILR of the primary bar, there is no nuclear bar structure in the molecular gas. Instead, the CO emission reveals an elongated structure, 15" in length, beginning at the nucleus (defined by the radio center) aligned with the jet/ionisation cone, at a PA of 127 degrees. This morphology, not frequently observed in Seyfert galaxies, may be interpreted in terms of enhanced CO excitation along the walls of the cone. Kinematical perturbations along the cone support this scenario. At larger-scale, CO emission is tracing the primary bar, and outer ring structure. The total molecular mass, estimated from the CO emission, is M(H2) = 3.1 10^9 Msun.

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arXiv2002

Discovery of High-Latitude CO in a HI Supershell in NGC 5775

We report the discovery of very high latitude molecular gas in the edge-on spiral galaxy, NGC 5775. Emission from both the J=1-0 and 2-1 lines of 12CO is detected up to 4.8 kpc away from the mid-plane of the galaxy. NGC 5775 is known to host a number of HI supershells. The association of the molecular gas M(H2,F2) = 3.1x10^7 solar masses reported here with one of the HI supershells (labeled F2) is clear, which suggests that molecular gas may have survived the process which originally formed the supershell. Alternatively, part of the gas could have been formed in situ at high latitude from shock-compression of pre-existing HI gas. The CO J=2-1/J=1-0 line ratio of 0.34+-40% is significantly lower than unity, which suggests that the gas is excited subthermally, with gas density a few times 100 cubic cm. The molecular gas is likely in the form of cloudlets which are confined by magnetic and cosmic rays pressure. The potential energy of the gas at high latitude is found to be 2x10^56 ergs and the total (HI + H2) kinetic energy is 9x10^53 ergs. Based on the energetics of the supershell, we suggest that most of the energy in the supershell is in the form of potential energy and that the supershell is on the verge of falling and returning the gas to the disk of the galaxy.

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arXiv2001

Abundant Molecular Gas in Tidal Dwarf Galaxies: On-going Galaxy Formation

[Abridged...] We investigate the process of galaxy formation as can be observed in the only currently forming galaxies -- the so-called Tidal Dwarf Galaxies, hereafter TDGs -- through observations of the molecular gas detected via its CO emission. These objects are formed of material torn off of the outer parts of a spiral disk due to tidal forces in a collision between two massive galaxies. Molecular gas is a key element in the galaxy formation process, providing the link between a cloud of gas and a bona fide galaxy. We have detected CO in 8 TDGs (two of them have already been published in Braine et al. 2000), with an overall detection rate of 80%, showing that molecular gas is abundant in TDGs, up to a few 10^8 M_sun. The CO emission coincides both spatially and kinematically with the HI emission, indicating that the molecular gas forms from the atomic hydrogen where the HI column density is high. A possible trend of more evolved TDGs having greater molecular gas masses is observed, in accord with the transformation of HI into H2.... Although higher spatial resolution CO (and HI) observations would help reduce the uncertainties, we find that TDGs require no dark matter, which would make them the only galaxy-sized systems where this is the case. Dark matter in spirals should then be in a halo and not a rotating disk. Most dwarf galaxies are dark matter-rich, implying that they are not of tidal origin....

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arXiv2001

Molecular gas in nearby powerful radio galaxies

We report the detection of CO(1-0) and CO(2-1) emission from the central region of nearby 3CR radio galaxies (z$<$ 0.03). Out of 21 galaxies, 8 have been detected in, at least, one of the two CO transitions. The total molecular gas content is below 10$^9$ \msun. Their individual CO emission exhibit, for 5 cases, a double-horned line profile that is characteristic of an inclined rotating disk with a central depression at the rising part of its rotation curve. The inferred disk or ring distributions of the molecular gas is consistent with the observed presence of dust disks or rings detected optically in the cores of the galaxies. We reason that if their gas originates from the mergers of two gas-rich disk galaxies, as has been invoked to explain the molecular gas in other radio galaxies, then these galaxies must have merged a long time ago (few Gyr or more) but their remnant elliptical galaxies only recently (last 10$^7$ years or less) become active radio galaxies. Instead, we argue the the cannibalism of gas-rich galaxies provide a simpler explanation for the origin of molecular gas in the elliptical hosts of radio galaxies (Lim et al. 2000). Given the transient nature of their observed disturbances, these galaxies probably become active in radio soon after the accretion event when sufficient molecular gas agglomerates in their nuclei.

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