European Southern Observatory Very Large Telescope Research Articles

The galaxy cluster AC114 – II. Stellar populations and the mass–metallicity relation

ABSTRACT We investigate the mass–metallicity relation (MZR)for galaxies in the Abell cluster AC114 from 7 h of VIMOS/MR data collected at the European Southern Observatory-Very Large Telescope in 2009. The dynamical analysis completed in our previous paper allowed us to select cluster members, whose spectra are here analysed with stellar population synthesis models. Active and passive galaxies are identified based on the presence/absence of the [O ii] λ3727, [O iii] λλ4959, 5007 and/or Hβ emission lines, depending on the galaxy redshift. We find that active galaxies have lower average masses than passive ones, and have lower average metallicities. The MZR of the cluster is found to be steeper than that for galaxies in the local universe.

Evolution of Hen 3-1357, the Stingray Nebula

ABSTRACTThe spectroscopic evolution of Hen 3-1357, the Stingray Nebula, is presented by analysing data from 1990 to 2021. High-resolution data obtained in 2021 with South African Large Telescope High Resolution Spectrograph and in 2009 with European Southern Observatory-Very Large Telescope UVES spectrograph are used to determine physical conditions and chemical abundances in the nebula. From comparison of these data with data from different epochs it is found that the intensity of highly ionized emission lines has been decreasing with time, while the emission of low-ionization lines has been increasing, confirming that the nebula is recombining, lowering its excitation class, as a consequence of the changes in the central star which in 2002 had an effective temperature of 60 000 K and from then it has been getting colder. The present effective temperature of the central star is about 40 000 K. It has been suggested that the central star has suffered a late thermal pulse and it is returning to the AGB phase. The nebular chemistry of Hen 3-1357 indicates that all the elements, except He and Ne, present subsolar abundances. The comparison of the nebular abundances with the values predicted by stellar nucleosynthesis models at the end of the AGB phase shows that the central star had an initial mass lower than 1.5 M⊙. We estimated the ADF(O+2) to be between 2.6 and 3.5.

A detailed study of X-ray cavities in the intracluster environment of the cool core cluster Abell 3017

ABSTRACT We present a detailed analysis of a cool-core galaxy cluster Abell 3017, at a redshift of z = 0.219, which has been identified to be merging with its companion cluster Abell 3016. This study has made use of X-ray (Chandra), ultraviolet (UV) [Galaxy Evolution Explorer(GALEX)], optical [European Southern Observatory (ESO)/very large telescope (VLT)], mid-infrared [(Wide-field Infrared Survey Explorer (WISE)], and radio upgraded Giant Metrewave radio telescope (uGMRT) observations of this cluster. Using various image processing techniques, such as unsharp masking, 2D fits using β models, contour binning and the use of surface brightness profiles, we show the existence of a pair of X-ray cavities, at a projected distance of ∼20 arcsec (70 kpc) and ∼16 arcsec (57 kpc), respectively, from the core of Abell 3017. We also detect an excess of X-ray emission located at ∼25 arcsec (88 kpc) south of the centre of Abell 3017, is likely due to the bulk motions in the intracluster medium either by gas sloshing or ram-pressure striping due to a merger. We find that the radio lobes are responsible for the observed X-ray cavities detected in this system. The lower values of mid-infrared WISE colour [W1–W2] and [W2–W3] imply that the central BCG of Abell 3017 is a star-forming galaxy. The current star formation rate of the central BCG, estimated from the H α and GALEX far-ultraviolet (FUV) luminosities, are equal to be ∼5.06 ± 0.78 and ∼9.20 ± 0.81 $\rm M_{\odot }$ yr−1, respectively. We detect, for the first time, a radio phoenix ∼150 kpc away from the radio core, with a spectral index of (α ≤ −1.8). We also report the detection of $\rm ~Pa\,\alpha$ emission in this cluster using ESO VLT SINFONI imaging data.

Near-infrared spectroscopic observations of high redshift C I absorbers

We study a sample of 17 z > 1.5 absorbers selected based on the presence of strong C I absorption lines in Sloan Digital Sky Survey (SDSS) spectra and observed with the European Southern Observatory Very Large Telescope (ESO-VLT) spectrograph X-shooter. We derive metallicities, depletion onto dust, and extinction by dust, and analyse the absorption from Mg II, Mg I, Ca II, and Na I that are redshifted into the near infrared wavelength range. We show that most of these C I absorbers have high metallicity and dust content. We detect nine Ca II absorptions with W(Ca IIλ3934) > 0.23 Å out of 14 systems where we have appropriate wavelength coverage. The observed equivalent widths are similar to what has been measured in other lower redshift surveys of Ca II systems. We detect ten Na I absorptions in the 11 systems where we could observe this absorption. The median equivalent width (W(Na Iλ5891) = 0.68 Å) is larger than what is observed in local clouds with similar H I column densities but also in z < 0.7 Ca II systems detected in the SDSS. The systematic presence of Na I absorption in these C I systems strongly suggests that the gas is neutral and cold, maybe part of the diffuse molecular gas in the interstellar medium of high-redshift galaxies. Most of the systems (12 out of 17) have W(Mg IIλ2796) > 2.5 Å and six of them have log N(H I) < 20.3, with the extreme case of J1341+1852 that has log N(H I) = 18.18. The Mg II absorptions are spread over more than Δυ ~ 400 km s−1 for half of the systems; three absorbers have Δυ > 500 km s−1. The kinematics are strongly perturbed for most of these systems, which indicates that these systems probably do not arise in quiet disks and must be close to regions with intense star-formation activity and/or are part of interacting objects. All this suggests that a large fraction of the cold gas at high redshift arises in disturbed environments.

Low-mass variable stars in the globular cluster NGC 6397

We have conducted a photometric survey of the globular cluster NGC 6397 in a search for variable stars. We obtained ~11h of time-resolved photometric images with one ne European Southern Observatory-Very Large Telescope using the FOcal Reducer and low dispersion Spectrograph imager distributed over two consecutive nights. We analyzed 8391 light curves of stars brighter than magnitude 23 with the 465 nm-filter, and we identified 412 variable stars, reaching ~ 4.8 +- 0.2 per cent of variability with timescales between 0.004 and 2d, with amplitudes variation greater than +- 0.2 mag.

High-resolution spectroscopy of the 3D cosmic web with close QSO groups★

We study the three-dimensional distribution of matter at z~2 using high resolution spectra of QSO pairs and simulated spectra drawn from cosmological hydro-dynamical simulations. We present a sample of 15 QSOs, corresponding to 21 baselines of angular separations evenly distributed between ~1 and 14 arcmin, observed with the Ultraviolet and Visual Echelle Spectrograph (UVES) at the European Southern Observatory-Very Large Telescope (ESO-VLT). The observed correlation functions of the transmitted flux in the HI Lya forest transverse to and along the line of sight are in agreement, implying that the distortions in redshift space due to peculiar velocities are relatively small and - within the relatively large error bars - not significant. The clustering signal is significant up to velocity separations of ~300 km/s, corresponding to about 5 h^{-1} comoving Mpc. Compatibility at the 2 sigma level has been found both for the Auto- and Cross-correlation functions and for the set of the Cross correlation coefficients. The analysis focuses in particular on two QSO groups of the sample. Searching for alignments in the redshift space between Lya absorption lines belonging to different lines of sight, it has been possible to discover the presence of a wide HI structures extending over about ten Mpc in comoving space, and give constraints on the sizes of two cosmic under-dense regions in the intergalactic medium.

The FALCON concept: multi-object adaptive optics and atmospheric tomography for integral field spectroscopy - principles and performance on an 8-m telescope

Integral field spectrographs are major instruments with which to study the mechanisms involved in the formation and the evolution of early galaxies. When combined with multi-object spectroscopy, those spectrographs can behave as machines used to derive physical parameters of galaxies during their formation process. Up to now, there has been only one available spectrograph with multiple integral field units, i.e. FLAMES/GIRAFFE on the European Southern Observatory (ESO) Very Large Telescope (VLT). However, current ground-based instruments suffer from a degradation of their spatial resolution due to atmospheric turbulence. In this article we describe the performance of FALCON, an original concept of a new-generation multi-object integral field spectrograph with adaptive optics for the ESO VLT. The goal of FALCON is to combine high angular resolution (0.25 arcsec) and high spectral resolution (R > 5000) in the J and H bands over a wide field of view (10 x 10 arcmin 2 ) in the VLT Nasmyth focal plane. However, instead of correcting the whole field, FALCON will use multi-object adaptive optics (MOAO) to perform the adaptive optics correction locally on each scientific target. This requires us then to use atmospheric tomography in order to use suitable natural guide stars for wavefront sensing. We will show that merging MOAO and atmospheric tomography allows us to determine the internal kinematics of distant galaxies up to z ≈ 2 with a sky coverage of 50 per cent, even for objects observed near the Galactic pole. The application of such a concept to extremely large telescopes seems therefore to be a very promising way to study galaxy evolution from z = 1 to redshifts as high as z = 7.

Tomography of the intergalactic medium with Ly forests in close QSO pairs

We study the three-dimensional distribution of non-virialized matter at z ∼ 2 using high-resolution spectra of quasi-stellar object (QSO) pairs and simulated spectra drawn from cosmological hydrodynamical simulations. We have collected the largest sample of QSO pairs ever observed with Ultraviolet and Visual Echelle Spectrograph (UVES) at the European Southern Observatory-Very Large Telescope (ESO-VLT), with angular separations between ∼1 and 14 arcmin. The observed correlation functions of the transmitted flux in the H i Lyman a forest along and transverse to the lines of sight are in good agreement implying that the distortions in redshift space due to peculiar velocities are small. The clustering signal is significant up to velocity separations of ∼200 km s -1 , or about 3 h -1 comoving Mpc. The regions at lower overdensity (ρ/ρ ≤ 6.5) are still clustered but on smaller scales (Δu ≤ 100km s -1 ). The observed and simulated correlation functions are compatible at the 3σ level. A better concordance is obtained when only the low overdensity regions are selected for the analysis or when the effective optical depth of the simulated spectra is increased artificially, suggesting a deficiency of strong lines in the simulated spectra. We found that also a lower value of the power-law index of the temperature-density relation for the Lyman a forest gas improves the agreement between observed and simulated results. If confirmed, this would be consistent with other observations favouring a late He n reionization epoch (at z ∼3). We remark the detection of a significant clustering signal in the cross-correlation coefficient at a transverse velocity separation Δν ⊥ ∼ 500 km s -1 whose origin needs further investigation.

Aqueous altered silicates at the surface of two Plutinos?

In April 2001 we obtained visible and near-infrared spectra, as well as photometric data, for two Trans-Neptunian Objects (TNOs), 2000 GN171 (now numbered 47932) and 2000 EB173 (now designated 38628 Huya), which belong to the dynamical class of Plutinos. These observations were made with the FORS1 and ISAAC instruments at the European Southern Observatory-Very Large Telescope (ESO-VLT). The detection of weak absorption features in the visible spectra of these two Trans-Neptunians has been reported elsewhere (Lazzarin et al. 2003). In this paper, we discuss the interpretation of the features, which are different for the two objects, and we present some complementary observations in the near-infrared, as well as more recent (May 2002) visible spectra in which the features are absent. Although the visible colors of the two objects are comparable, the near-infrared spectra are different: (47932) 2000 GN171 shows a nearly flat spectrum, except in the H band where a broad absorption appears, whereas the spectrum of (38628) Huya (2000 EB173) has a red slope in the J band and some absorption beyond 2 micron. The features detected in the visible spectra of the two objects are tentatively attributed to the presence of iron oxides or phyllosilicates at the surfaces of the two objects. The differences between the April 2001 and May 2002 visible spectra are attributed to spatial variations at the surfaces of the objects. We briefly discuss possibilities for aqueous alteration in TNOs, after reviewing what we know about the presence of aqueously altered minerals in other small bodies of the solar system. Further studies monitoring the rotation of these two objects are highly desirable.