Large Telescope Observations Research Articles

The stellar populations in the low-luminosity, early-type galaxy NGC 59

Low-luminosity galaxies may be the building blocks of more luminous systems. Southern African Large Telescope observations of the low-luminosity, early-type galaxy NGC 59 are obtained and analysed. These data are used to measure the stellar population parameters in the centre and off-centre regions of this galaxy, in order to uncover its likely star formation history. We find evidence of older stars, in addition to young stars in the emission-line regions. The metallicity of the stellar population is constrained to be [Z/H] ∼ −1.1 to −1.6, which is extremely low, even for this low-luminosity galaxy, since it is not classed as a dwarf spheroidal galaxy. The measured [α/Fe] ratio is subsolar, which indicates an extended star formation history in NGC 59. If such objects formed the building blocks of more massive, early-type galaxies, then they must have been gaseous mergers, rather than dry mergers, in order to increase the metals to observed levels in luminous, early-type galaxies.

Supersolar Ni/Fe production in the Type IIP SN 2012ec

SN 2012ec is a Type IIP supernova (SN) with a progenitor detection and comprehensive photospheric-phase observational coverage. Here, we present Very Large Telescope and PESSTO observations of this SN in the nebular phase. We model the nebular [O I] 6300, 6364 lines and find their strength to suggest a progenitor main-sequence mass of 13-15 Msun. SN 2012ec is unique among hydrogen-rich SNe in showing a distinct and unblended line of stable nickel [Ni II] 7378. This line is produced by 58Ni, a nuclear burning ash whose abundance is a sensitive tracer of explosive burning conditions. Using spectral synthesis modelling, we use the relative strengths of [Ni II] 7378 and [Fe II] 7155 (the progenitor of which is 56Ni) to derive a Ni/Fe production ratio of 0.20pm0.07 (by mass), which is a factor 3.4pm1.2 times the solar value. High production of stable nickel is confirmed by a strong [Ni II] 1.939 micron line. This is the third reported case of a core-collapse supernova producing a Ni/Fe ratio far above the solar value, which has implications for core-collapse explosion theory and galactic chemical evolution models.

X-shooter reveals powerful outflows in z ∼ 1.5 X-ray selected obscured quasi-stellar objects

We present X-shooter at the Vewry Large Telescope observations of a sample of 10 luminous, X-ray obscured QSOs at z$\sim1.5$ from the XMM-COSMOS survey, expected to be caught in the transitioning phase from starburst to active galactic nucleus (AGN)-dominated systems. The main selection criterion is X-ray detection at bright fluxes (L$>=10^{44}$ erg s$^{-1}$) coupled to red optical-to-NIR-to-MIR colors. Thanks to its large wavelength coverage, X-shooter allowed us to determine accurate redshifts from the presence of multiple emission lines for five out of six targets for which we had only a photometric redshift estimate, with an 80 percent success rate, significantly larger than what is observed in similar programs of spectroscopic follow-up of red QSOs. We report the detection of broad and shifted components in the [OIII]$\lambda\lambda$5007,4959 complexes for six out of eight sources with these lines observable in regions free from strong atmospheric absorptions. The full width half-maximum (FWHM) associated with the broad components are in the range FWHM$\sim900-1600$ km s$^{-1}$, larger than the average value observed in SDSS Type 2 AGN samples at similar observed [OIII] luminosity, but comparable to those observed for QSO/ultraluminous infrared galaxies systems for which the presence of kpc scale outflows have been revealed through integral field unit spectroscopy. Although the total outflow energetics (inferred under reasonable assumptions) may be consistent with winds accelerated by stellar processes, we favour an AGN origin for the outflows given the high outflow velocities oberved (v$>1000$ km s$^{-1}$) and the presence of strong winds also in objects undetected in the far infrared.

Comparison of fringe-tracking algorithms for single-mode near-infrared long-baseline interferometers

To enable optical long baseline interferometry toward faint objects, long integrations are necessary despite atmospheric turbulence. Fringe trackers are needed to stabilize the fringes and thus increase the fringe visibility and phase signal-to-noise ratio (SNR), with efficient controllers robust to instrumental vibrations, and to subsequent path fluctuations and flux drop-outs. We report on simulations, analysis and comparison of the performances of a classical integrator controller and of a Kalman controller, both optimized to track fringes under realistic observing conditions for different source magnitudes, disturbance conditions, and sampling frequencies. The key parameters of our simulations (instrument photometric performance, detection noise, turbulence and vibrations statistics) are based on typical observing conditions at the Very Large Telescope observatory and on the design of the GRAVITY instrument, a 4-telescope single-mode long baseline interferometer in the near-infrared, next in line to be installed at VLT Interferometer. We find that both controller performances follow a two-regime law with the star magnitude, a constant disturbance limited regime, and a diverging detector and photon noise limited regime. Moreover, we find that the Kalman controller is optimal in the high and medium SNR regime due to its predictive commands based on an accurate disturbance model. In the low SNR regime, the model is not accurate enough to be more robust than an integrator controller. Identifying the disturbances from high SNR measurements improves the Kalman performances in case of strong optical path difference disturbances.

PRECISION EPHEMERIDES FOR GRAVITATIONAL-WAVE SEARCHES. I. Sco X-1

Rapidly-rotating neutron stars are the only candidates for persistent high-frequency gravitational wave emission, for which a targeted search can be performed based on the spin period measured from electromagnetic (e.g. radio and X-ray) observations. The principal factor determining the sensitivity of such searches is the measurement precision of the physical parameters of the system. Neutron stars in X-ray binaries present additional computational demands for searches due to the uncertainty in the binary parameters. We present the results of a pilot study with the goal of improving the measurement precision of binary orbital parameters for candidate gravitational wave sources. We observed the optical counterpart of Sco X-1 in 2011 June with the William Herschel Telescope, and also made use of Very Large Telescope observations in 2011, to provide an additional epoch of radial-velocity measurements to earlier measurements in 1999. From a circular orbit fit to the combined dataset, we obtained an improvement of a factor of two in the orbital period precision, and a factor of 2.5 in the epoch of inferior conjunction $T_0$. While the new orbital period is consistent with the previous value of Gottllieb et al. (1975), the new $T_0$ (and the amplitude of variation of the Bowen line velocities) exhibited a significant shift, which we attribute to variations in the emission geometry with epoch. We propagate the uncertainties on these parameters through to the expected Advanced LIGO & VIRGO detector network observation epochs, and quantify the improvement obtained with additional optical observations.

Determining the Hubble constant using giant extragalactic H ii regions and H ii galaxies

Abstract We report the first results of a long-term programme aiming to provide accurate independent estimates of the Hubble constant (H0) using the L(Hβ)-σ distance estimator for giant extragalactic H ii regions (GEHR) and H ii galaxies. We have used Very Large Telescope and Subaru high-dispersion spectroscopic observations of a local sample of H ii galaxies, identified in the Sloan Digital Sky Survey Data Release 7 (SDSS DR7) catalogue in order to redefine and improve the L(Hβ)-σ distance indicator and to determine the Hubble constant. To this end, we utilized as local calibration or ‘anchor’ of this correlation GEHR in nearby galaxies which have accurate distance measurements determined via primary indicators. Using our best sample of 69 nearby H ii galaxies and 23 GEHR in nine galaxies, we obtain H0 = 74.3 ± 3.1 (statistical) ± 2.9 (systematic) km s−1 Mpc−1, in excellent agreement with, and independently confirming, the most recent Type Ia supernovae based results.

A new analysis of the ESO Very Large Telescope (VLT) observations of Titan at 2 μm

We have performed an analysis of ESO Very Large Telescope (VLT) observations of Titan at 2 μm. The data were acquired with the Nasmyth Adaptative Optics System Near-Infrared Imager and Spectrograph (NAOS/CONICA), on the 16th of January 2005, that is 2 days after the landing of the Huygens probe ( Hirtzig et al., 2007). The data consist in 21 spectra taken along two diameters of Titan’s disk at wavelengths between 2.03 and 2.5 μm. This range covers a part of the 2 μm methane window and the adjacent band. The data received a preliminary analysis in a recent paper ( Negrão et al., 2007), essentially focused on the surface albedo near Huygens landing site. In this work, we perform an in-depth analysis to retrieve information about several aspects: the latitude haze distribution in the stratosphere and in the low atmosphere, the latitudinal variation of the surface albedo and its spectral behaviour. Also, this analysis allowed us to make sensitivity tests on the influence of the scatterer profiles on the retrieved surface albedo and its spectral slope. The news analysis confirms that, as was the case with VIMS observations at the same epoch, the Northern (currently winter) Hemisphere contains more haze than the southern one (Summer Hemisphere). The sensitivity tests show that the scatterer profiles have just a little impact on the surface albedo and its spectral slope. The analysis seems to confirm the presence of H 2O and CH 4 ices.

A SENSITIVE SEARCH FOR DEUTERATED WATER IN COMET 8P/TUTTLE

We report a sensitive search for deuterated water (HDO) in comet 8P/Tuttle using high-resolution spectroscopy at infrared (IR) wavelengths. The deuterium enrichment of cometary water is one of the most important cosmogonic indicators in comets. The ratio preserves information about the conditions under which comet material formed, and tests the possible contribution of comets in delivering water for Earth's oceans. Water (H2O) and HDO were sampled in comet 8P/Tuttle from 2008 January 27 to 2008 February 3 using the new IR spectrometer (Cryogenic Infrared Echelle Spectrograph) at the 8.2 m Antu telescope of the Very Large Telescope Observatory atop Cerro Paranal, Chile. Twenty-three lines of HDO were sampled near 3.7 μm, leading to a production rate of 4.73 ± 1.68 × 1025 s–1. Combining this value with the H2O production rate of 5790 ± 250 × 1025 s–1 provides a formal value of = 4.09 ± 1.45 × 10–4 in comet 8P/Tuttle. This value is larger by a factor of 2.62 ± 0.93 than Vienna Standard Mean Ocean Water, and is comparable to enrichment factors measured for three other Oort cloud comets. The technique described here provides unprecedented sensitivities, ultimately permitting us to routinely measure this prime cosmogonic indicator, even in comets having relatively modest gas production rate like 8P/Tuttle.

Near-infrared/optical counterparts of hotspots in radio galaxies

We present new high spatial resolution Very Large Telescope (VLT) and Very Large Array (VLA) observations of a sample of nine low-power (P 1.4GHz ≤ 10 25 W Hz -1 ) radio hotspots. Infrared/optical emission is definitely detected in four of the nine observed objects, resulting in a detection rate of at least 45 per cent. This emission is interpreted as synchrotron radiation from the electrons accelerated in the hotspots. The integrated spectra of these hotspots reveal typical break frequencies between 10 5 and 10 6 GHz, two orders of magnitude higher than typically found in high-power hotspots. This supports the idea that in low-power hotspots with their relatively low magnetic field strengths, electrons emit most of their energy at higher frequencies. A simple spectral ageing analysis would imply that the emitting electrons have been injected into the hotspot volume less than ∼ 10 3 years ago. We discuss possible scenarios to explain the lack of older electrons in the hotspot region. In particular, the extended morphology of the near-infrared/optical emission would suggest that efficient re-acceleration mechanisms rejuvenate the electron populations.

A possible optical counterpart to the old nearby pulsar J0108-1431

Context. The multi-wavelength study of old (>100 Myr) radio pulsars holds the key to understanding the long-term evolution of neutron stars, including the advanced stages of neutron star cooling and the evolution of the magnetosphere. Optical/UV observations are particularly useful for such studies because they allow one to explore both thermal and non-thermal emission processes. In particular, studying the optical/UV emission constrains the temperature of the bulk of the neutron star surface, too cold to be measured in X-ray observations. Aims. Aim of this work is to identify the optical counterpart of the very old (166 Myr) radio pulsar J0108−1431. Methods. We have re-analyzed our original Very Large Telescope (VLT) observations, where a very faint object was tentatively detected close to the radio position, near the edge of a field galaxy. Results. We found that the backward extrapolation of the PSR J0108−1431 proper motion recently measured by Chandra fits the position of this object. Based on that, we propose it as a viable candidate for the optical counterpart to PSR J0108−1431. The object fluxes (U = 26.4 ± 0.3; B ≈ 27.9; V ≥ 27.8) are consistent with a thermal spectrum with a brightness temperature of ∼9 × 10 4 K (for R = 13 km at a distance of 130 pc), emitted from the bulk of the neutron star surface. Conclusions. New optical observations are required to confirm the optical identification of PSR J0108−1431 and measure its spectrum.

Physical Conditions in Quasar Outflows: Very Large Telescope Observations of QSO 2359–1241

We analyze the physical conditions of the outflow seen in QSO 2359-1241 (NVSS J235953–124148), based on high-resolution spectroscopic VLT observations. This object was previously studied using Keck HIRES data. The main improvement over the HIRES results is our ability to accurately determine the number density of the outflow. For the major absorption component, the populations from five different Fe II excited levels yield a gas density nH = 104.4 cm−3 with less than 20% scatter. We find that the Fe II absorption arises from a region with roughly constant conditions and temperature greater than 9000 K, before the ionization front where temperature and electron density drop. Further, we model the observed spectra and investigate the effects of varying gas metallicities and the spectral energy distribution of the incident ionizing radiation field. The accurately measured column densities allow us to determine the ionization parameter (log UH ≈ − 2.4) and total column density of the outflow [log NH(cm −2) ≈ 20.6]. Combined with the number density finding, these are stepping stones toward determining the mass flux and kinetic luminosity of the outflow, and therefore its importance to AGN feedback processes.

Diverse young stellar populations in the intermediate-redshift radio galaxies 3C 213.1 and 3C 459: implications for the evolution of the host galaxies

We present European Southern Observatory Very Large Telescope (ESO VLT) and William Herschel Telescope (WHT) spectroscopic observations of two powerful radio galaxies at intermediate redshifts (3C 213.1 and 3C 459), obtained with the aim of establishing the nature and evolutionary status of the host galaxies. Spectral synthesis modelling has been used to demonstrate that young stellar populations (YSP) make a major contribution to the integrated optical light in both sources, contributing at least 37 and 80 per cent of the integrated B-band light in 3C 213.1 and 3C 459, respectively. While in the case of 3C 213.1 the YSP has an intermediate age (0.4–0.8 Gyr) and comprises 3–30 per cent of the total stellar mass, adequate fits to the spectra of 3C 459 require a combination of younger (<0.1 Gyr) and intermediate (0.2–1.2 Gyr) age YSP components that together comprise 5–100 per cent of the total stellar mass. Both the optical and far-IR properties of 3C 459 are consistent with its status as an ultraluminous infrared galaxy (ULIRG), and a scenario in which the observed radio source has been triggered close to the peak of star formation activity in a major gas-rich merger. On the other hand, in 3C 213.1 it is likely that the radio source has been triggered (or re-triggered) a significant period after the starburst peak, and the far-IR luminosity of the source is substantially lower. Following correction for the contributions of the YSP, the absolute magnitudes of the host galaxies of both sources are relatively modest (≤2L*), thus demonstrating that powerful radio activity is not always associated with the most massive and luminous elliptical galaxies. These results serve to emphasize that the radio source population is diverse, in terms of both the host galaxy properties and the triggering mechanism(s) for the radio source activity.

The Variable Radio–to–X‐Ray Spectrum of the Magnetar XTE J1810−197

We have observed the 5.54 s anomalous X-ray pulsar XTE J1810-197 at radio, millimeter, and infrared (IR) wavelengths, with the aim of learning about its broadband spectrum. At the IRAM 30 m telescope, we have detected the magnetar at ν = 88 and 144 GHz, the highest radio-frequency emission ever seen from a pulsar. At 88 GHz we detected numerous individual pulses, with typical widths ~2 ms and peak flux densities up to 45 Jy. Together with nearly contemporaneous observations with the Parkes, Nancay, and Green Bank telescopes, we find that in late 2006 July the spectral index of the pulsar was -0.5 α 0 (with flux density Sν ∝ να) over the range 1.4-144 GHz. Nine dual-frequency Very Large Array and Australia Telescope Compact Array observations in 2006 May-September are consistent with this finding, while showing variability of α with time. We infer from the IRAM observations that XTE J1810-197 remains highly linearly polarized at millimeter wavelengths. Also, toward this pulsar, the transition frequency between strong and weak scattering in the interstellar medium may be near 50 GHz. At Gemini, we detected the pulsar at 2.2 μm in 2006 September, at the faintest level yet observed, Ks = 21.89 ± 0.15. We have also analyzed four archival IR Very Large Telescope observations (two unpublished), finding that the brightness fluctuated within a factor of 2-3 over a span of 3 years, unlike the monotonic decay of the X-ray flux. Thus, there is no correlation between IR and X-ray flux, and it remains uncertain whether there is any correlation between IR and radio flux.

Giant Ringlike Radio Structures Around Galaxy Cluster Abell 3376

In the current paradigm of cold dark matter cosmology, large-scale structures are assembling through hierarchical clustering of matter. In this process, an important role is played by megaparsec (Mpc)-scale cosmic shock waves, arising in gravity-driven supersonic flows of intergalactic matter onto dark matter-dominated collapsing structures such as pancakes, filaments, and clusters of galaxies. Here, we report Very Large Array telescope observations of giant ( approximately 2 Mpc by 1.6 Mpc), ring-shaped nonthermal radio-emitting structures, found at the outskirts of the rich cluster of galaxies Abell 3376. These structures may trace the elusive shock waves of cosmological large-scale matter flows, which are energetic enough to power them. These radio sources may also be the acceleration sites where magnetic shocks are possibly boosting cosmic-ray particles with energies of up to 10(18) to 10(19) electron volts.

X-Ray and Infrared Point Source Identification and Characteristics in the Embedded, Massive Star-Forming Region RCW 38

We report on results of a 96.7 ks Chandra observation of one of the youngest, most embedded, and most massive young stellar clusters studied in X-rays: RCW 38. We detect 460 sources in the field, of which 360 are confirmed to be associated with the RCW 38 cluster. The cluster members range in luminosity from 1030 to 1033.5 ergs s-1. Over 10% of the cluster members with over 100 counts exhibit flares, while about 15% of the cluster members with over 30 counts are variable. Of the sources identified as cluster members, 160 have near-infrared (NIR) counterparts either in the Two Micron All Sky Survey database or detected via Very Large Telescope observations. Of these, about 20% appear to have optically thick disks. An additional 353 members are identified through NIR observations, of which at least 50% possess optically thick disks. We fit over 100 X-ray sources as absorbed Raymond-Smith-type plasmas and find that the column to the cluster members varies from 1021.5 to 1023 cm-2. We compare the gas to dust absorption signatures in these stars and find NH = AV cm-2. We find that the cluster contains 31 candidate OB stars and is centered about 10'' (0.1 pc) west of the primary source of the ionization, the O5 star IRS 2. The cluster has a peak central density of about 400 X-ray sources pc-2. We estimate that the total cluster membership exceeds 2000 stars.

High-resolution spectroscopy of flat-spectrum quasars

We report Very Large Telescope (VLT) observations of the quasar PKS 2145+067. We obtained high-resolution (R ≈ 100 000 or 3 km s −1 ) optical spectra with the UV-Visual Echelle Spectrograph (UVES) at the VLT UT2 telescope. The emission redshift measured from (O  )i szem = 1.0003. In our spectra, the well-known intervening metal-rich absorption system at zabs = 0.79089 shows Mg  absorption lines resolved into 15 velocity components with a total spread of about 206 km s −1 . This absorption is consistent with a line of sight passing through the extended gaseous halo of a disk galaxy. The derived Doppler line broadening parameters are in the range b = 2t o 5k m s −1 . We also detected absorption lines of Mg  ,F e ,a nd Ca in the same system. The ionization state of the absorbing gas is analyzed and compared with previous studies of PKS 2145+067. Finally, we discuss the implications of high-resolution data for proposed cosmological variations of the fine-structure constant.

Linking Very Large Telescope asteroid observations

A novel method for the preliminary identification of asteroids at discovery and a few days thereafter is being developed in Helsinki. Having two different sets of asteroid observations, the goal is to identify all possible pairs of objects between the sets. An arbitrary asteroid can either remain unidentified, or be preliminary linked to one or more asteroids. In the case of ambiguity, the final decision must usually be based on additional observations. We use a multistep approach, during which possible pairs of objects are first selected by comparing ephemerides that have been generated for three common epochs. The method has been successfully tested using both Very Large Telescope observations, and simulated observations of near-Earth and main-belt objects. Identification results of simulated observations indicate that the observing strategy promoted by the Minor Planet Center might not be the best one, at least for the purposes of identification. The ultimate goal is to produce a real-time asteroid identification tool for ESA's astrometric space observatory Gaia, the Lowell Observatory Near-Earth-Object Search, the Near-Earth Space Surveillance mission, and the Nordic Near-Earth Object Network. The tool could also benefit large-scale surveys done with the Large Synoptic Survey Telescope, and the Discovery Channel Telescope.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Color-selected Galaxies at z 6 in the Great Observatories Origins Deep Survey

We report early results on galaxies at z ~ 6 selected from Hubble Space Telescope imaging for the Great Observatories Origins Deep Survey. Spectroscopy of one object with the Advanced Camera for Surveys grism and from the Keck and Very Large Telescope observatories shows a strong continuum break and asymmetric line emission, identified as Lyα at z = 5.83. We find only five spatially extended candidates with signal-to-noise ratios greater than 10, two of which have spectroscopic confirmation. This is much fewer than would be expected if galaxies at z = 6 had the same luminosity function as those at z = 3. There are many fainter candidates, but we expect substantial contamination from foreground interlopers and spurious detections. Our best estimates favor a z = 6 galaxy population with fainter luminosities, higher space density, and similar comoving ultraviolet emissivity to that at z = 3, but this depends critically on counts at fluxes fainter than those reliably probed by the current data.

Large Velocity Gradients in the Tidal Tails of the Interacting Galaxy AM 1353−272 (“The Dentist’s Chair”)

We present Very Large Telescope observations of the interacting system AM 1353-272. Using the FORS2 instrument, we studied the kinematics of the ionized gas along its prominent tidal tails and discovered strikingly large velocity gradients associated with seven luminous tidal knots. These kinematical structures cannot be caused by streaming motion and most likely do not result from projection effects. More probably, instabilities in the tidal tails have led to the formation of kinematically decoupled objects that could be the progenitors of self-gravitating tidal dwarf galaxies.

Testing Intermediate-Age Stellar Evolution Models with VLT Photometry of Large Magellanic Cloud Clusters. I. The Data

This is the first of a series of three papers devoted to the calibration of a few parameters of crucial importance in the modeling of the evolution of intermediate-mass stars, giving special attention to the amount of convective core overshoot. To this end we acquired deep V and R photometry for three globular clusters of the Large Magellanic Cloud, namely, NGC 2173, SL 556, and NGC 2155, in the age interval 1–3 Gyr. In this first paper, we describe the aim of the project and Very Large Telescope observations and data reduction and we make preliminary comparisons of the color-magnitude diagrams with both the Padova and Yonsei-Yale isochrones. Two following papers in this series present the results of a detailed analysis of these data, independently carried out by members of the Yale and Padova stellar evolution groups. This allows us to compare both sets of models and discuss their main differences, as well as the systematic effects that they would have on the determination of the ages and metallicities of intermediate-age single-stellar populations.