New quasars behind the Magellanic Clouds. Spectroscopic confirmation of near-infrared selected candidates

Context. Quasi-stellar objects (quasars) located behind nearby galaxies provide an excellent absolute reference system for astrometric studies, but they are difficult to identify because of fore- and background contamination. Deep wide-field, high angular resolution surveys spanning the entire area of nearby galaxies are needed to obtain a complete census of such quasars. Aims. We embarked on a program to expand the quasar reference system behind the Large and the Small Magellanic Clouds, the Magellanic Bridge, and the Magellanic Stream that connects the Clouds with the Milky Way. Methods. Hundreds of quasar candidates were selected based on their near-infrared colors and variability properties from the ongoing public ESO VISTA Magellanic Clouds survey. A subset of 49 objects was followed up with optical spectroscopy. Results. We confirmed the quasar nature of 37 objects (34 new identifications): four are low redshift objects, three are probably stars, and the remaining three lack prominent spectral features for a secure classification. The bona fide quasars, identified from their broad emisison lines, are located as follows: 10 behind the LMC, 13 behind the SMC, and 14 behind the Bridge. The quasars span a redshift range from z ~ 0.5 to z ~ 4.1. Conclusions. Upon completion the VMC survey is expected to yield a total of ~1500 quasars with Y< 19.32 mag, J< 19.09 mag, and Ks< 18.04 mag

Towards a fundamental astrometric reference system behind the Malleganic clouds : spectroscopic confirmation of new quasar candidates selected in the near-infrared

Quasi-stellar objects (quasars) located behind nearby galaxies provide an excellent absolute reference system for astrometric studies, but they are difficult to identify because of fore- and background contamination. We have embarked on a programme to expand the quasar reference system behind the Large and Small Magellanic Clouds, the Magellanic Bridge and Magellanic Stream. Hundreds of quasar candidates were selected, based on their near-infrared colours and variability properties from the ESO VISTA Magellanic Clouds (VMC) Public Survey. A subset of 49 objects was followed up with optical spectroscopy with FORS2. We confirmed the quasar nature of 37 objects (34 new identifications) that span a redshift range from z ~ 0.5 to 4.1

The VMC Survey - VI. Quasars behind the Magellanic system

The number and spatial distribution of confirmed quasi-stellar objects (QSOs) behind the Magellanic system is limited. This undermines their use as astrometric reference objects for different types of studies. We have searched for criteria to identify candidate QSOs using observations from the VISTA survey of the Magellanic Clouds system (VMC) that provides photometry in the YJKs bands and 12 epochs in the Ks band. The (Y-J) versus (J-Ks) diagram has been used to distinguish QSO candidates from Milky Way stars and stars of the Magellanic Clouds. Then, the slope of variation in the Ks band has been used to identify a sample of high confidence candidates. These criteria were developed based on the properties of 117 known QSOs presently observed by the VMC survey. VMC YJKs magnitudes and Ks light-curves of known QSOs behind the Magellanic system are presented. About 75% of them show a slope of variation in Ks>10^-4 mag/day and the shape of the light-curve is in general irregular and without any clear periodicity. The number of QSO candidates found in tiles including the South Ecliptic Pole and the 30 Doradus regions is 22 and 26, respectively, with a ~20% contamination by young stellar objects, planetary nebulae, stars and normal galaxies. By extrapolating the number of QSO candidates to the entire VMC survey area we expect to find about 1200 QSOs behind the LMC, 400 behind the SMC, 200 behind the Bridge and 30 behind the Stream areas, but not all will be suitable for astrometry. Further, the Ks band light-curves can help support investigations of the mechanism responsible for the variations.Comment: 17 pages, 15 figures, replaced with accepted version by Astronomy & Astrophysic

Magellanic Cloud Structure from Near-IR Surveys I: The Viewing Angles of the LMC

We present a detailed study of the viewing angles of the LMC disk plane. We find that our viewing direction differs considerably from the commonly accepted values, which has important implications for the structure of the LMC. The discussion is based on an analysis of spatial variations in the apparent magnitude of features in the near-IR color-magnitude diagrams extracted from the DENIS and 2MASS surveys. Sinusoidal brightness variations with a peak-to-peak amplitude of approximately 0.25 mag are detected as function of position angle, for both AGB and RGB stars. This is naturally interpreted as the result of distance variations, due to one side of the LMC plane being closer to us than the opposite side. The best fitting geometric model of an inclined plane yields an inclination angle i = 34.7 +/- 6.2 degrees and line-of-nodes position angle Theta = 122.5 +/- 8.3 degrees. There is tentative evidence that the LMC disk plane may be warped. Traditional methods to estimate the position angle of the line of nodes have used either the major axis position angle Theta_maj of the spatial distribution of tracers on the sky, or the position angle Theta_max of the line of maximum gradient in the velocity field, given that for a circular disk Theta_maj = Theta_max = Theta. The present study does not rely on the assumption of circular symmetry, and is considerably more accurate than previous studies of its kind. We find that the actual position angle of the line of nodes differs considerably from both Theta_maj and Theta_max, for which measurements have fallen in the range 140-190 degrees. This indicates that the intrinsic shape of the LMC disk is not circular, but elliptical, as discussed further in Paper II. [Abridged]Comment: Astronomical Journal, in press. 44 pages, LaTeX, with 8 PostScript figures. Contains minor revisions with respect to previously posted version. Check out http://www.stsci.edu/~marel/lmc.html for a large scale (23x21 degree) stellar number-density image of the LMC constructed from RGB and AGB stars in the 2MASS and DENIS surveys. The paper is available with higher resolution figures from http://www.stsci.edu/~marel/abstracts/abs_R31.htm

Magellanic Cloud Structure from Near-IR Surveys II: Star Count Maps and the Intrinsic Elongation of the LMC

I construct a near-IR star count map of the LMC and demonstrate, using the viewing angles derived in Paper I, that the LMC is intrinsically elongated. I argue that this is due to the tidal force from the Milky Way. The near-IR data from the 2MASS and DENIS surveys are used to create a star count map of RGB and AGB stars, which is interpreted through ellipse fitting. The radial number density profile is approximately exponential with a scale-length 1.3-1.5 kpc. However, there is an excess density at large radii that may be due to the tidal effect of the Milky Way. The position angle and ellipticity profile converge to PA_maj = 189.3 +/- 1.4 degrees and epsilon = 0.199 +/- 0.008 for r > 5 deg. At large radii there is a drift of the center of the star count contours towards the near side of the plane, which can be undrestood as due to viewing perspective. The fact that PA_maj differes from the line of nodes position angle Theta = 122.5 +/- 8.3 (cf. Paper I) indicates that the LMC disk is not circular, but has an intrinsic ellipticity of 0.31. The LMC is elongated in the general direction of the Galactic center, and is elongated perpendicular to the Magellanic Stream and the velocity vector of the LMC center of mass. This suggests that the elongation of the LMC has been induced by the tidal force of the Milky Way. The position angle of the line of nodes differs from the position angle Theta_max of the line of maximum line of sight velocity gradient: Theta_max - Theta = 20-60 degrees. This could be due to: (a) streaming along non-circular orbits in the elongated disk; (b) uncertainties in the transverse motion of the LMC center of mass; (c) precession and nutation of the LMC disk as it orbits the Milky Way (expected on theoretical grounds). [Abridged]Comment: Astronomical Journal, in press. 34 pages, LaTeX, with 7 PostScript figures. Contains minor revisions with respect to previously posted version. Check out http://www.stsci.edu/~marel/lmc.html for a large scale (23x21 degree) stellar number-density image of the LMC constructed from RGB and AGB stars in the 2MASS and DENIS surveys. The paper is available with higher resolution color figures from http://www.stsci.edu/~marel/abstracts/abs_R32.htm

The global gas and dust budget of the Large Magellanic Cloud: AGB stars and supernovae and the impact on the ISM evolution

‘The definitive version is available at: www3.interscience.wiley.com '. Copyright Blackwell / Royal Astronomical Society. DOI: 10.1111/j.1365-2966.2009.14743.xWe report on an analysis of the gas and dust budget in the interstellar medium (ISM) of the Large Magellanic Cloud (LMC). Recent observations from the Spitzer Space Telescope enable us to study the mid-infrared dust excess of asymptotic giant branch (AGB) stars in the LMC. This is the first time we can quantitatively assess the gas and dust input from AGB stars over a complete galaxy, fully based on observations. The integrated mass-loss rate over all intermediate and high mass-loss rate carbon-rich AGB candidates in the LMC is 8.5 × 10−3 M⊙ yr−1 , up to 2.1 × 10−2 M⊙ yr−1 . This number could be increased up to 2.7 × 10−2 M⊙ yr−1 if oxygen-rich stars are included. This is overall consistent with theoretical expectations, considering the star formation rate (SFR) when these low- and intermediate-mass stars where formed, and the initial mass functions. AGB stars are one of the most important gas sources in the LMC, with supernovae (SNe), which produces about 2–4 × 10−2 M⊙ yr−1 . At the moment, the SFR exceeds the gas feedback from AGB stars and SNe in the LMC, and the current star formation depends on gas already present in the ISM. This suggests that as the gas in the ISM is exhausted, the SFR will eventually decline in the LMC, unless gas is supplied externally. Our estimates suggest 'a missing dust-mass problem' in the LMC, which is similarly found in high-z galaxies: the accumulated dust mass from AGB stars and possibly SNe over the dust lifetime (400–800 Myr) is significant less than the dust mass in the ISM. Another dust source is required, possibly related to star-forming regions.Peer reviewe

The Frequency of Active and Quiescent Galaxies with Companions: Implications for the Feeding of the Nucleus

We analyze the idea that nuclear activity, either AGN or star formation, can be triggered by interactions, studying the percentage of active, HII and quiescent galaxies with companions. Our sample was selected from the Palomar survey, and avoids selection biases faced by previous studies. The comparison between the local galaxy density distributions showed that in most cases there is no statistically significant difference among galaxies of different activity types. The comparison of the percentage of galaxies with nearby companions showed that there is a higher percentage of LINERs, transition, and absorption line galaxies with companions than Seyferts and HII galaxies. However, we find that when we consider only galaxies of similar morphological types (ellipticals or spirals), there is no difference in the percentage of galaxies with companions among different activity types, indicating that the former result was due to the morphology-density effect. Also, only small differences are found when we consider galaxies with similar Halpha luminosities. The comparison between HII galaxies of different Halpha luminosities shows that there is a significantly higher percentage of galaxies with companions among the higher luminosity HII galaxies, indicating that interactions increase the amount of circumnuclear star formation, in agreement with previous results. The fact that we find that galaxies of different activity types have the same percentage of companions, suggests that interactions between galaxies is not a necessary condition to trigger the nuclear activity in AGNs. We compare our results with previous ones and discuss their implications. (abridged)Comment: 30 pages, including 6 figures and 3 tables. To appear in The Astronomical Journal, November issu

Spatial distribution of stellar populations in the Magellanic Clouds: Implementation to Gaia

The main goal of our project is to investigate the spatial distribution of different stellar populations in the Magellanic Clouds. The results from modelling the Magellanic Clouds can be useful, among others, for simulations during the Gaia mission preparation. Isodensity contour maps have been used in order to trace the morphology of the different stellar populations and estimate the size of these structures. Moreover, star density maps are constructed through star counts and projected radial density profiles are obtained. Fitting exponential disk and King law curves to the spatial distribution allows us to derive the structural parameters that describe these profiles. The morphological structure and spatial distributions of various stellar components in the Magellanic Clouds (young and intermediate age stars, carbon stars) along with the overall spatial distribution in both Clouds are provided.Comment: 12 pages, 9 figures, to be published in Astronomy & Astrophysics; typos and language correcte

The VMC survey - XIV : First results on the look-back time star formation rate tomography of the Small Magellanic Cloud

Date of Acceptance: 20/01/2015We analyse deep images from the VISTA survey of the Magellanic Clouds in the YJKs filters, covering 14 deg2 (10 tiles), split into 120 subregions, and comprising the main body and Wing of the Small Magellanic Cloud (SMC). We apply a colour-magnitude diagram reconstruction method that returns their best-fitting star formation rate SFR(t), age-metallicity relation (AMR), distance and mean reddening, together with 68 per cent confidence intervals. The distance data can be approximated by a plane tilted in the East-West direction with a mean inclination of 39°, although deviations of up to ±3 kpc suggest a distorted and warped disc. After assigning to every observed star a probability of belonging to a given age-metallicity interval, we build high-resolution population maps. These dramatically reveal the flocculent nature of the young star-forming regions and the nearly smooth features traced by older stellar generations. They document the formation of the SMC Wing at ages <0.2 Gyr and the peak of star formation in the SMC Bar at ~40 Myr. We clearly detect periods of enhanced star formation at 1.5 and 5 Gyr. The former is possibly related to a new feature found in the AMR, which suggests ingestion of metal-poor gas at ages slightly larger than 1 Gyr. The latter constitutes a major period of stellar mass formation. We confirm that the SFR(t) was moderately low at even older ages.Peer reviewe