A calibration map for Wide Field Imager photometry

Koch, Andreas; Odenkirchen, M.; Grebel, Eva K.; Caldwell, J. A. R.

doi:10.1002/asna.200310176

Cited by 32 publications

(34 citation statements)

References 23 publications

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“…The de-reddening procedure also corrects for residual variations in the photometry that result from the variation of the point spread function or illumination (e.g. Koch et al 2004) across the combined mosaics. We find a mean E(B − V) ∼ 0.5 mag towards NGC 4372 in a good agreement with the Schlafly & Finkbeiner (2011) re-calibration of the Schlegel et al (1998) extinction map, with a significant variation between 0.3 and 0.8 mag across the field of the GC.…”

Section: Photometrymentioning

confidence: 99%

A study of rotating globular clusters

Kacharov

Bianchini

Koch

et al. 2014

A&A

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A study of rotating globular clusters ABSTRACTContext. NGC 4372 is a poorly studied old, very metal-poor globular cluster (GC) located in the inner Milky Way halo. Aims. We present the first in-depth study of the kinematic properties and derive the structural parameters of NGC 4372 based on the fit of a Plummer profile and a rotating, physical model. We explore the link between internal rotation to different cluster properties and together with similar studies of more GCs, we put these in the context of globular cluster formation and evolution. Methods. We present radial velocities for 131 cluster member stars measured from high-resolution FLAMES/GIRAFFE observations. Their membership to the GC is additionally confirmed from precise metallicity estimates. We build a velocity dispersion profile and a systemic rotation curve using this kinematic data set. Additionally, we obtain an elliptical number density profile of NGC 4372 based on optical images using a Markov chain Monte Carlo fitting algorithm. From this, we derive the cluster's half-light radius and ellipticity as r h = 3.44 ± 0.04 and = 0.08 ± 0.01. Finally, we give a physical interpretation of the observed morphological and kinematic properties of this GC by fitting an axisymmetric, differentially rotating, dynamical model. Results. Our results show that NGC 4372 has an unusually high ratio of rotation amplitude to velocity dispersion (1.2 vs. 4.5 km s −1 ) for its metallicity. This puts it in line, however, with two other exceptional, very metal-poor GCs: M 15 and NGC 4590. We also find a mild flattening of NGC 4372 in the direction of its rotation. Given its old age, this suggests that the flattening is indeed caused by the systemic rotation rather than tidal interactions with the Galaxy. Additionally, we estimate the dynamical mass of the GC M dyn = 2.0 ± 0.5 × 10 5 M based on the dynamical model, which constrains the mass-to-light ratio of NGC 4372 between 1.4 and 2.3 M /L , representative of an old, purely stellar population.

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Section: Photometrymentioning

confidence: 99%

A study of rotating globular clusters

Kacharov

Bianchini

Koch

et al. 2014

A&A

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“…However considering that the first one covers a small area of the sky (5.6 × 5.7 arcmin) and that the second one is based on WIFI@ESO2.2 data which is known to suffer of significant inhomogeneity (Koch et al 2004;Calamida et al 2008) we didn't consider useful to perform such comparisons. In conclusion, of the two studies we compared with, one (Stetson) supports our calibration while the other (LS00) presents trends both in position and in magnitude which are impossible to disentangle at the moment.…”

Section: Comparison With Previous Photometrymentioning

confidence: 99%

A wide angle view of the Sagittarius dwarf spheroidal galaxy

Giuffrida

Sbordone

Zaggia

et al. 2010

A&A

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Context. The Sagittarius dwarf Spheroidal Galaxy (Sgr dSph) provides us with a unique possibility of studying a dwarf galaxy merging event while still in progress. Moving along a short-period, quasi-polar orbit in the Milky Way Halo, Sgr dSph is being tidally dispersed along a huge stellar stream. Due to its low distance (25 kpc), the main body of Sgr dSph covers a vast area in the sky (roughly 15 × 7 degrees). Available photometric and spectroscopic studies have concentrated either on the central part of the galaxy or on the stellar stream, but the overwhelming majority of the galaxy body has never been probed. Aims. The aim of the present study is twofold. On the one hand, to produce color magnitude diagrams across the extension of Sgr dSph to study its stellar populations, searching for age and/or composition gradients (or lack thereof). On the other hand, to derive spectroscopic low-resolution radial velocities for a subsample of stars to determine membership to Sgr dSph for the purpose of high resolution spectroscopic follow-up. Methods. We used VIMOS@VLT to produce V and I photometry on 7 fields across the Sgr dSph minor and major axis, plus 3 more centered on the associated globular clusters Terzan 7, Terzan 8 and Arp 2. A last field has been centered on M 54, lying in the center of Sgr dSph. VIMOS high resolution spectroscopic mode has then been used to derive radial velocities for a subsample of the observed stars, concentrating on objects having colors and magnitudes compatible with the Sgr dSph red giant branch. Results. We present photometry for 320 000 stars across the main body of Sgr dSph, one of the richest, and safely the most wide-angle sampling ever produced for this fundamental object. We also provide robust memberships for more than one hundred stars, whose high resolution spectroscopic analysis will be the object of forthcoming papers. Sgr dSph appears remarkably uniform among the observed fields. We confirm the presence of a main Sgr dSph population characterized roughly by the same metallicity of 47 Tuc, but we also found the presence of multiple populations on the peripheral fields of the galaxy, with a metallicity spanning from [Fe/H] = -2.3 to a nearly solar value.

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“…This effect is well known to exist at the WFI to the level of a few hundredths of a magnitude throughout most of the field, reaching slightly above 0.1 mag near the edges of the field. Since no calibrations suitable for a quantification of this effect in our images were obtained, we applied instead the WFI illumination correction map in the R band derived by Koch et al (2003) and assumed it to be approximately applicable to the I and z WFI filters as well. Judging from the residual differences between the illumination correction maps computed by Koch et al in different filters, we expect the systematic error introduced in this way not to exceed 0.03 mag anywhere in the field.…”

Section: Observations and Data Reductionmentioning

confidence: 99%

A new population of cool stars and brown dwarfs in the Lupus clouds

Comerón¹,

Spezzi

Martí

2009

A&A

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Context. Most studies of the stellar and substellar populations of star-forming regions rely on using the signatures of accretion, outflows, disks, or activity characterizing the early stages of stellar evolution. However, these signatures rapidly decay with time.Aims. We present the results of a wide-area study of the stellar population of clouds in the Lupus star-forming region. When combined with 2MASS photometry, our data allow us to fit the spectral energy distributions of over 150 000 sources and identify possible new members based on their photospheric fluxes, independent of any display of the signposts of youth. Methods. We used the Wide Field Imager (WFI) at the La Silla 2.2 m telescope to image an area of more than 6 square degrees in the Lupus 1, 3 and 4 clouds in the R C , I C , and z WFI bands, selected so as to overlap with the areas observed in the Spitzer Legacy Program "From molecular cores to planet-forming disks". We complement our data with 2MASS photometry to sample the spectral energy distribution from 0.6 μm to 2.2 μm. We validate our method on the census of known members of the Lupus clouds, for which spectroscopic classification is available. The temperatures derived for cool objects are generally accurate, with most of the exceptions attributed to veiling, strong emission lines at short wavelengths, near-infrared excess, variability, or the presence of close companions. Results. Considering that the dereddened fluxes of most cool (T eff < 3500 K) young stellar objects at the distance of Lupus occupy a gap between those typical both of field cool dwarfs and of background giants, we identify a new population of cool members of Lupus 1 and 3. The approximately 130 new members are only moderately concentrated toward the densest clouds, they appear to have ages in the same range as the known members, and very few show the infrared excess caused by warm disks. This population is absent in Lupus 4. Conclusions. This new population of Lupus members seems to be composed of stars and brown dwarfs that have lost their inner disks on a timescale of a few Myr or less. Almost all these objects are in low extinction regions. We speculate that dissipation of unshielded disks caused by nearby O stars or fast collapse of the pre-(sub)stellar cores triggered by the passage of old supernova shocks may have led to disk properties and evolutionary paths very different from those resulting from the more quiescent environment provided by dense molecular clouds.

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A calibration map for Wide Field Imager photometry

Cited by 32 publications

References 23 publications

A study of rotating globular clusters

A study of rotating globular clusters

A wide angle view of the Sagittarius dwarf spheroidal galaxy

A new population of cool stars and brown dwarfs in the Lupus clouds

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