European Southern Observatory

About: European Southern Observatory is a facility organization based out in Garching bei München, Germany. It is known for research contribution in the topics: Galaxy & Stars. The organization has 3594 authors who have published 16157 publications receiving 823095 citations. The organization is also known as: The European Southern Observatory，ESO & ESO.

The VLT-FLAMES Tarantula Survey: I. Introduction and observational overview⋆

Abstract: The VLT-FLAMES Tarantula Survey (VFTS) is an ESO Large Programme that has obtained multi-epoch optical spectroscopy of over 800 massive stars in the 30 Doradus region of the Large Magellanic Cloud (LMC). Here we introduce our scientific motivations and give an overview of the survey targets, including optical and near-infrared photometry and comprehensive details of the data reduction. One of the principal objectives was to detect massive binary systems via variations in their radial velocities, thus shaping the multi-epoch observing strategy. Spectral classifications are given for the massive emission-line stars observed by the survey, including the discovery of a new Wolf-Rayet star (VFTS 682, classified as WN5h), 2' to the northeast of R136. To illustrate the diversity of objects encompassed by the survey, we investigate the spectral properties of sixteen targets identified by Gruendl & Chu from Spitzer photometry as candidate young stellar objects or stars with notable mid-infrared excesses. Detailed spectral classification and quantitative analysis of the O- and B-type stars in the VFTS sample, paying particular attention to the effects of rotational mixing and binarity, will be presented in a series of future articles to address fundamental questions in both stellar and cluster evolution.

Dissecting photometric redshift for active galactic nucleus using XMM- and Chandra-COSMOS samples

Abstract: In this paper, we release accurate photometric redshifts for 1692 counterparts to Chandra sources in the central square degree of the Cosmic Evolution Survey (COSMOS) field. The availability of a large training set of spectroscopic redshifts that extends to faint magnitudes enabled photometric redshifts comparable to the highest quality results presently available for normal galaxies. We demonstrate that morphologically extended, faint X-ray sources without optical variability are more accurately described by a library of normal galaxies (corrected for emission lines) than by active galactic nucleus (AGN) dominated templates, even if these sources have AGN-like X-ray luminosities. Preselecting the library on the bases of the source properties allowed us to reach an accuracy σ_(Δz/(1+z(spec))~0.015 with a fraction of outliers of 5.8% for the entire Chandra-COSMOS sample. In addition, we release revised photometric redshifts for the 1735 optical counterparts of the XMM-detected sources over the entire 2 deg^2 of COSMOS. For 248 sources, our updated photometric redshift differs from the previous release by Δz > 0.2. These changes are predominantly due to the inclusion of newly available deep H-band photometry (H_(AB) = 24 mag). We illustrate once again the importance of a spectroscopic training sample and how an assumption about the nature of a source together, with the number and the depth of the available bands, influences the accuracy of the photometric redshifts determined for AGN. These considerations should be kept in mind when defining the observational strategies of upcoming large surveys targeting AGNs, such as eROSITA at X-ray energies and the Australian Square Kilometre Array Pathfinder Evolutionary Map of the Universe in the radio band.

The Nature and Evolution of Ultraluminous Infrared Galaxies:A Mid-Infrared Spectroscopic Survey

Abstract: We report the first results of a low-resolution mid-infrared spectroscopic survey of an unbiased, far-infrared-selected sample of 60 ultraluminous infrared galaxies (ULIRGs) (LIR${r IR}$ -->$t SUBgt {r IR}t/SUBgt $ --> > 10 -->12 L?) using ISOPHOT-S on board the Infrared Space Observatory (ISO). We use the ratio of the 7.7 ?m polycyclic aromatic hydrocarbon (PAH) emission feature to the local continuum as a discriminator between starburst and active galactic nucleus (AGN) activity. About 80% of all ULIRGs are found to be predominantly powered by star formation, but the fraction of AGN-powered objects increases with luminosity. Whereas only about 15% of ULIRGs at luminosities below 2 ? 10 -->12 L? are AGN powered, this fraction increases to about half at higher luminosity. Observed ratios of the PAH features in ULIRGs differ slightly from those in lower luminosity starbursts. This can be plausibly explained by the higher extinction and/or different physical conditions in the interstellar medium of ULIRGs. The PAH feature-to-continuum ratio is anticorrelated with the ratio of feature-free 5.9 ?m continuum to the IRAS 60 ?m continuum, confirming suggestions that strong mid-infrared continuum is a prime AGN signature. The location of starburst-dominated ULIRGs in such a diagram is consistent with previous ISO-Short Wavelength Spectrograph spectroscopy, which implies significant extinction even in the mid-infrared. We have searched for indications that ULIRGs that are advanced mergers might be more AGN-like, as postulated by the classical evolutionary scenario. No such trend has been found among those objects for which near-infrared images are available to assess their likely merger status.

The role of stellar feedback and dark matter in the evolution of dwarf galaxies

Abstract: Supernova and multiple supernova events regulate several structural properties of dwarf galaxies. In particular, they govern the metal enrichment and the energy budget of the ISM; they might induce partial (blowout) or total (blowaway) gas removal from the galaxy; they also regulate the pressure of the ISM, and consequently the morphology of the galactic gaseous body. Significant amounts of dark matter may play an equally important role: the dark matter gravitational potential tends to concentrate baryons towards the centre, thus enhancing both the star formation rate and the metal production. Also, the dynamical properties of the ISM, and the occurrence of a blowout or blowaway, are shown to be determined by the dark matter content. We present detailed analytical/numerical models describing the evolution of dwarf irregular galaxies (dIs), including the above and other effects. The main results are: (i) dwarfs with total masses M & 5 � 10 6 M( are blown away; those with gas masses up to .10 9 M( lose mass in an outflow; (ii) metallicities are found to correlate tightly with dark matter content, and are consistent with a range of dark-to-visible mass ratios f < 0±30; with about 65per cent of the dwarfs in the sample having f < 0±10; (iii) we predict a lower limit to the oxygen abundance in dIs of 12 1 log…O=H† < 7:2; (iv) outflows are not particularly important for the metallicity evolution of dwarf galaxies, and certainly less important than star formation for gas consumption; however, dwarfs with gas masses of a few �10 8 M( are shown to be the major pollutants of the IGM; (v) the ISM Hi velocity dispersion correlates with metallicity and, independently of dark matter, scales as Z 3.5 . Specific comparisons with well-studied dIs, such as for example Leo A, yield excellent agreement with the data. Based on our results, we discuss a scenario in which late-type and early-type dwarfs had common progenitors in the past, but differences in their total mass forced these objects to follow different evolutionary paths. We therefore consider dI ! dE transitions occurring at present cosmic times as very unlikely.

A simplified view of blazars: clearing the fog around long‐standing selection effects

Abstract: We propose a scenario where blazars are classified as flat-spectrum radio quasars (FSRQs), BL Lacs, low synchrotron, or high synchrotron peaked objects according to a varying mix of the Doppler boosted radiation from the jet, the emission from the accretion disk, the broad line region, and the light from the host galaxy. In this framework the peak energy of the synchrotron power (ν S ) in blazars is independent of source type and of radio luminosity. We test this new approach, which builds upon unified schemes, using extensive Monte Carlo simulations and show that it can provide simple answers to a number of long-standing issues including, amongst others, the different cosmological evolution of BL Lacs selected in the radio and X-ray bands, the larger ν S peak values observed in BL Lacs, the fact that high synchrotron peaked blazars are always of the BL Lac type, and the existence of FSRQ/BL Lac transition objects. Objects so far classified as BL Lacs on the basis of their observed weak, or undetectable, emission lines are of two physically different classes: intrinsically weak lined objects, more common in X-ray selected samples, and heavily diluted broad lined sources, more frequent in radio selected samples, which explains some of the confusion in the literature. We also show that strong selection effects are the main cause of the diversity observed in radio and X-ray samples, and that the correlation between luminosity and ν S , that led to the proposal of the “blazar sequence”, is also a selection effect arising from the comparison of shallow radio and X-ray surveys, and to the fact that high ν S peak - high radio power objects have never been considered because their redshift is not measurable.