Washington Photometry Research Articles

A Multiwavelength Study of the Segue 3 Cluster

Abstract We present new SDSS and Washington photometry of the young outer-halo stellar system Segue 3. Combined with archival VI-observations, our most consistent results yield , , , and , with a high binary fraction of 0.39 ± 0.05 derived using the Padova models. We confirm that mass-segregation has occurred, supporting the hypothesis that this cluster is being tidally disrupted. A three-parameter King model yields a cluster radius of , a core radius of , and a tidal radius of . A comparison of Padova and Dartmouth model-grids indicates that the cluster is not significantly α-enhanced, with a mean dex, and a population age of only 2.6 ± 0.4 Gyr. We rule out a statistically significant age spread at the main-sequence turnoff because of a narrow subgiant branch, and discuss the role of stellar rotation and cluster age, using Dartmouth and Geneva models: approximately 70% of the Seg 3 stars at or below the main-sequence turnoff have enhanced rotation. Our results for Segue 3 indicate that it is younger and more metal-rich than all previous studies have reported to date. From colors involving Washington C and SDSS-u filters, we identify several giants and a possible blue straggler for future follow-up spectroscopic studies, and we produce spectral energy distributions of previously known members and potential Segue 3 sources with Washington (CT 1), Sloan (ugri), and VI-filters. Segue 3 shares the characteristics of unusual stellar systems that have likely been stripped from external dwarf galaxies as they are being accreted by the Milky Way, or that have been formed during such an event. Its youth, metallicity, and location are all inconsistent with Segue 3 being a cluster native to the Milky Way.

Observational hints of a real age spread in the young LMC star cluster NGC 1971

Abstract We report that the serendipitous young Large Magellanic Cloud cluster NGC 1971 exhibits an extended main-sequence turn-off (eMSTO) possibly originated mostly by a real age spread. We used CT1 Washington photometry to produce a colour–magnitude diagram (CMD) with the fiducial cluster features. From its eMSTO, we estimated an age spread of ∼170 Myr (observed age range 100–280 Myr), once observational errors, stellar binarity, overall metallicity variations and stellar rotation effects were subtracted in quadrature from the observed age width.

Multiple Populations in NGC 1851: Abundance Variations and UV Photometric Synthesis in the Washington and HST/WFC3 Systems

Abstract The analysis of multiple populations (MPs) in globular clusters (GCs), both spectroscopically and photometrically, is key in understanding their formation and evolution. The relatively narrow Johnson U, F336W, and Stromgren and Sloan u filters have been crucial in exhibiting these MPs photometrically, but in Paper I we showed that the broader Washington C filter can more efficiently detect MPs in the test case GC NGC 1851. Additionally, In Paper I we detected a double main sequence (MS) that has not been detected in previous observations of NGC 1851. We now match this photometry to NGC 1851's published RGB abundances and find that the two RGB branches observed in C generally exhibit different abundance characteristics in a variety of elements (e.g., Ba, Na, and O) and in CN band strengths, but no single element can define the two RGB branches. However, simultaneously considering [Ba/Fe] or CN strength with either [Na/Fe], [O/Fe], or CN strength can separate the two photometric RGB branches into two distinct abundance groups. Matches of NGC 1851's published SGB and HB abundances to the Washington photometry shows consistent characterizations of the MPs, which can be defined as an O-rich/N-normal population and an O-poor/N-rich population. Photometric synthesis for both the Washington C filter and the F336W filter finds that these abundance characteristics, with appropriate variations in He, can reproduce for both filters the photometric observations in both the RGB and the MS. This photometric synthesis also confirms the throughput advantages that the C filter has in detecting MPs.

DEEP WASHINGTON PHOTOMETRY OF INCONSPICUOUS STAR CLUSTER CANDIDATES IN THE LARGE MAGELLANIC CLOUD

We present deep Washington photometry of 45 poorly populated star cluster candidates in the Large Magellanic Cloud (LMC). We have performed a systematic study to estimate the parameters of the cluster candidates by matching theoretical isochrones to the cleaned and de-reddened cluster color-magnitude diagrams (CMDs). We were able to estimate the basic parameters for 33 clusters, out of which, 23 are identified as single clusters and 10 are found to be members of double clusters. Other 12 cluster candidates have been classified as possible clusters/asterisms. About 50% of the true clusters are in the 100-300 Myr age range, while some are older or younger. We have discussed the distribution of age, location, reddening with respect to field as well as size of true clusters. The sizes and masses of the studied sample are found to be similar to that of open clusters in the Milky Way. Our study adds to the lower end of cluster mass distribution in the LMC, suggesting that the LMC apart from hosting rich clusters also has formed small, less massive open clusters in the 100-300 Myr age range.

CCD Washington photometry of four poorly studied open clusters in the two inner quadrants of the galactic plane

Complementing our Washington photometric studies on Galactic open clusters (OCs), we now focus on four poorly studied OCs located in the first and fourth Galactic quadrants, namely BH84, NGC5381, BH211 and Czernik37. We have obtained CCD photometry in the Washington system C and T1 passbands down to T1∼8.5mag for these four clusters. Their positions and sizes were determined using the stellar density radial profiles. We derived reddening, distance, age and metallicity of the clusters from extracted (C-T1,T1) color-magnitude diagrams (CMDs), using theoretical isochrones computed for the Washington system. There are no previous photometric data in the optical band for BH84, NGC5381 and BH211. The CMDs of the observed clusters show relatively well defined main sequences, except for Czernik37, wherein significant differential reddening seems to be present. The red giant clump is clearly seen only in BH211. For this cluster, we estimated the age in (1000-200+260)Myr, assuming a metallicity of Z=0.019. BH84 was found to be much older than it was previously believed, while NGC5381 happened to be much younger than previously reported. The heliocentric distances to these clusters are found to range between 1.4 and 3.4kpc. BH84 appears to be located at the solar galactocentric distance, while NGC5381, BH211 and Czernik37 are situated inside the solar ring.

Disentangling the physical reality of star cluster candidates projected towards the inner disc of the Large Magellanic Cloud

We have used Washington photometry for 90 star cluster candidates of small angular size -typically ~ 11" in radius- distributed within nine selected regions in the inner disc of the Large Magellanic Cloud (LMC) to disentangle whether they are genuine physical system, and to estimate the ages for the confirmed clusters. In order to avoid a misleading interpretation of the cluster colour-magnitude diagrams (CMDs), we applied a subtraction procedure to statistically clean them from field star contamination. Out of the 90 candidate clusters studied, 61 of them resulted to be genuine physical systems, whereas the remaining ones were classified as possible non- clusters since either their CMDs and/or the distribution of stars in the respective fields do not resemble those of stellar aggregates. We statistically show that ~ (13 +- 6)% of the catalogued clusters in the inner disc could be possible non-clusters, independently of their deprojected distances. We derived the ages for the confirmed clusters from the fit of theoretical isochrones to the cleaned cluster CMDs. The derived ages resulted to be in the age range 7.8 < log(t) < 9.2. Finally, we built cluster frequencies for the different studied regions and found that there exists some spatial variation of the LMC CF throughout the inner disc. Particularly, the innermost field contains a handful of clusters older than ~ 2 Gyr, while the wider spread between different CFs has taken place during the most recent 50 Myr of the galaxy lifetime.

Photometric metallicities in Boötes I

We present new Stromgren and Washington data sets for the Bootes I dwarf galaxy, and combine them with the available SDSS photometry. The goal of this project is to refine a ground-based, practical, accurate method to determine age and metallicity for individual stars in Bootes I that can be selected in an unbiased imaging survey, without having to take spectra. We produce photometric metallicities from Stromgren and Washington photometry, for stellar systems with a range of $-1.0>[Fe/H]>-3.5$. To avoid the decrease in sensitivity of the Stromgren metallicity index on the lower red-giant branch, we replace the Stromgren v-filter with the broader Washington C-filter; we find that $CT_1by$ is the most successful filter combination, for individual stars with $[Fe/H]<-2.0$, to maintain ~0.2 dex $[Fe/H]$-resolution over the whole red-giant branch. We demonstrate that we can break the isochrones' age-metallicity degeneracy with these filters, using stars with log g=2.5-3.0, which have less than a 2% change in their $(C-T_1)$-colour due to age, over a range of 11-14 Gyr.

Mass distribution and structural parameters of Small Magellanic Cloud star clusters

In this work we estimate, for the first time, the total masses and mass function slopes of a sample of 29 young and intermediate-age SMC clusters from CCD Washington photometry. We also derive age, interstellar reddening and structural parameters for most of the studied clusters by employing a statistical method to remove the unavoidable field star contamination. Only these 29 clusters out of 68 originally analysed cluster candidates present stellar overdensities and coherent distribution in their colour-magnitude diagrams compatible with the existence of a genuine star cluster. We employed simple stellar population models to derive general equations for estimating the cluster mass based only on its age and integrated light in the B, V, I, C and T1 filter. These equations were tested against mass values computed from luminosity functions, showing an excellent agreement. The sample contains clusters with ages between 60 Myr and 3 Gyr and masses between 300 and 3000 Mo distributed between ~0.5 deg. and ~2 deg. from the SMC optical centre. We determined mass function slopes for 24 clusters, of which 19 have slopes compatible with that of Kroupa IMF (2.3 +/- 0.7), considering the uncertainties. The remaining clusters - H86-188, H86-190, K47, K63 and NGC242 - showed flatter MFs. Additionally, only clusters with masses lower than ~1000 Mo and flatter MF were found within ~0.6 deg. from the SMC rotational centre.

A sample of relatively unstudied star clusters in the Large Magellanic Cloud: fundamental parameters determined from Washington photometry

To enlarge our growing sample of well-studied star clusters in the Large Magellanic Cloud (LMC), we present CCD Washington CT1 photometry to T1 ~ 23 in the fields of twenty-three mostly unstudied clusters located in the inner disc and outer regions of the LMC. We estimated cluster radii from star counts. Using the cluster Washington (T1,C-T1) colour-magnitude diagrams, statistically cleaned from field star contamination, we derived cluster ages and metallicities from a comparison with theoretical isochrones of the Padova group. Whenever possible, we also derived ages using delta_T1 - the magnitude difference between the red giant clump and the main sequence turn off - and estimated metallicities from the standard giant branch procedure. We enlarged our sample by adding clusters with published ages and metallicities determined on a similar scale by applying the same methods. We examined relationships between their positions in the LMC, ages and metallicities. We find that the two methods for age and metallicity determination agree well with each other. Fourteen clusters are found to be intermediate-age clusters (1-2 Gyr), with [Fe/H] values ranging from -0.4 to -0.7. The remaining nine clusters turn out to be younger than 1 Gyr, with metallicities between 0.0 and -0.4. Our 23 clusters represent an increase of ~ 30% in the current total amount number of well-studied LMC clusters using Washington photometry. In agreement with previous studies, we find no evidence for a metallicity gradient. We also find that the younger clusters were formed closer to the LMC centre than the older ones.

The paucity of globular clusters around the field elliptical NGC 7507

Context: There is strong evidence that globular cluster systems (GCSs) of massive galaxies are largely assembled by infall/accretion processes. Therefore, we expect the GCSs of isolated elliptical galaxies to be poor. Alhough not completely isolated, NGC 7507 is a massive field elliptical galaxy with an apparently very low dark matter content. Aims: We determine the richness, the colour distribution, and the structural properties of the GCS of NGC 7507. Methods: We perform wide-field Washington photometry with data obtained with the MOSAIC II camera at the 4m-Blanco telescope, CTIO. Results: The GCS is very poor with S_N ~ 0.6. We identify three subpopulations with peaks at (C-T1) colours of 1.21, 1.42, and 1.72. The bluest population may represent the old, metal-poor component. This interpretation is supported by its shallow density profile. The red population is more concentrated, resembling the galaxy light. The intermediate-colour population is strongly peaked in colour and we interpret this population as the signature of a starburst, whose age depends on the metallicity, but should be quite old, since no signatures of a merger are identifiable. In addition, we find a main sequence in the stellar foreground population, which we attribute to the Sagittarius dwarf tidal stream. Conclusions: The extraordinarily poor GCS of NGC 7507, a massive elliptical galaxy, is an illustration of how important the environmental conditions are for producing rich GCSs.

Washington photometry of candidate star clusters in the Small Magellanic Cloud

We present for the first time Washington CT1 photometry for 11 unstudied or poorly studied candidate star clusters. The selected objects are of small angular size, contain a handful of stars, and are projected towards the innermost regions of the Small Magellanic Cloud (SMC). The respective Colour-Magnitude Diagrams (CMDs) were cleaned from the unavoidable star field contamination by taking advantage of a procedure which makes use of variable size Colour-Magnitude Diagram cells. This method has shown to be able to eliminate stochastic effects in the cluster CMDs caused by the presence of isolated bright stars, as well as, to make a finer cleaning in the most populous CMD regions. Our results suggest that nearly 1/3 of the studied candidate star clusters would appear to be genuine physical systems. In this sense, the ages previously derived for some of them mostly reflect those of the composite stellar populations of the SMC field. Finally, we used the spatial distribution in the SMC of possible non-clusters to statistically decontaminate that of the SMC cluster system. We found that there is no clear difference between both expected and observed cluster spatial distributions, although it would become more important at a 2\sigma\, level between a ~ 0.3deg and 1.2deg (the semi-major axis of a ellipse parallel to the SMC bar and with b/a = 1/2), if the asterisms were increased up to 20%.

Washington photometry of 26 moderately young small angular size clusters in the Large Magellanic Cloud

Aims. With the aim of enlarging the number of studied LMC clusters in the age range 8.0 < log(t) < 9.0, we focus here on a sample of mostly unstudied cluster candidates. Methods. We present for the first time CCD Washington CT1T2 photometry of stars in the field of 26 LMC clusters. Results. The studied clusters turned out to be small angular size objects with ages within the age range 8.0 < log(t) < 9.0, which are

Washington photometry of 14 intermediate-age to old star clusters in the Small Magellanic Cloud

We present CCD photometry in the Washington system C, T1 and T2 passbands down to T1∼ 23 in the fields of L3, L28, HW 66, L100, HW 79, IC 1708, L106, L108, L109, NGC 643, L112, HW 84, HW 85 and HW 86, 14 Small Magellanic Cloud (SMC) clusters, most of them poorly studied objects. We measured T1 magnitudes and C−T1 and T1−T2 colours for a total of 213 516 stars spread throughout cluster areas of 14.7 × 14.7 arcmin2 each. We carried out an in-depth analysis of the field star contamination of the colour–magnitude diagrams (CMDs) and statistically cleaned the cluster CMDs. Based on the best fits of isochrones computed by the Padova group to the (T1, C−T1) CMDs, as well as from the δ(T1) index and the standard giant branch procedure, we derived ages and metallicities for the cluster sample. With the exception of IC 1708, a relatively metal-poor Hyades-age cluster, the remaining 13 objects are between intermediate and old age (from 1.0 to 6.3 Gyr), their [Fe/H] values ranging from −1.4 to −0.7 dex. By combining these results with others available in the literature, we compiled a sample of 43 well-known SMC clusters older than 1 Gyr, with which we produced a revised age distribution. We found that the present clusters’ age distribution reveals two primary excesses of clusters at t∼ 2 and 5 Gyr, which engraves the SMC with clear signs of enhanced formation episodes at both ages. In addition, we found that from the birth of the SMC cluster system until approximately the first 4 Gyr of its lifetime, the cluster formation resembles that of a constant formation rate scenario.

THE GLOBULAR CLUSTER SYSTEM OF THE VIRGO GIANT ELLIPTICAL GALAXY NGC 4636. I. SUBARU/FAINT OBJECT CAMERA AND SPECTROGRAPH SPECTROSCOPY AND DATABASE

We present a spectroscopic study of the globular clusters (GCs) in the giant elliptical galaxy NGC 4636 in the Virgo cluster. We selected target GC candidates using the Washington photometry derived from the deep CCD images taken at the KPNO 4m. Then we obtained the spectra of 164 target objects in the field of NGC 4636 using the Multi-Object Spectroscopy (MOS) mode of Faint Object Camera and Spectrograph (FOCAS) on the SUBARU 8.2m Telescope. We have measured the velocities for 122 objects: 105 GCs in NGC 4636, the nucleus of NGC 4636, 11 foreground stars, 2 background galaxies, and 3 probable intracluster GCs in the Virgo cluster. The GCs in NGC 4636 are located in the projected galactocentric radius within 10arcmin (corresponding to 43 kpc). The measured velocities for the GCs range from 300km/s to 1600km/s, with a mean value of 932_{-22}^{+25} km/s, which is in good agreement with the velocity for the nucleus of NGC 4636, 928\pm 45 km/s. The velocity dispersion of the GCs in NGC 4636 is derived to be 231_{-17}^{+15} km/s and the velocity dispersion of the blue GCs is slightly larger than that of the red GCs. Combining our results with data in the literature, we produce a master catalog of radial velocities for 238 GCs in NGC 4636. The velocity dispersion of the GCs in the master catalog is found to be 225_{-9}^{+12} km/s for the entire sample, 251_{-12}^{+18} km/s for 108 blue GCs, and 205_{-13}^{+11} km/s for 130 red GCs.

Washington photometry of five star clusters in the Large Magellanic Cloud

AbstractWe present CCD photometry in the Washington-system C and T1 passbands down to T1 ~ 22.5 mag in the fields of NGC 1697, SL 133, NGC 1997, SL 663, and OHSC 28, five mostly unstudied star clusters in the LMC. Cluster radii were estimated from star counts in appropriately sized boxes distributed throughout the entire observed fields. We perform a detailed analysis of field-star contamination and derive cluster colour–magnitude diagrams (CMDs). Based on the best fits of isochrones computed by the Padova group to the (C − T1, T1) CMDs, the δ(T1) index and the ‘standard giant-branch’ procedure, we derive metallicities and ages for the five clusters. With the exception of NGC 1697 (age = 0.7 Gyr, [Fe/H] = 0.0 dex), the remaining four clusters are of intermediate age (from 2.2 to 3.0 Gyr) and relatively metal poor ([Fe/H] = −0.7 dex). We combine our sample with clusters with ages and metallicities on a similar scale and examine relationships between position in the LMC, age and metallicity. We confirm previous results that clusters younger than ~ 1 Gyr were formed during an outside-in process; this occurred after a burst of cluster formation that took place mainly in the outer disk and peaked ~ 2 Gyr ago. Finally, the cluster and field age–metallicity relations (AMRs) show evidence for a metallicity offset but do overlap, particularly on the upper-envelope side of the cluster AMR.

Washington photometry of five star clusters in the Large Magellanic Cloud

Aims. We present CCD photometry in the Washington system C and T 1 passbands down to T 1 ~ 22.5 in the fields of NGC 1697, SL 133, NGC 1997, SL 663, and OHSC 28, five mostly unstudied star clusters in the LMC.Methods. Cluster radii were estimated from star counts in appropriate-sized boxes distributed throughout the entire observed fields. We perform a detailed analysis of the field star contamination and derive cluster colour-magnitude diagrams (CMDs). Based on the best fits of isochrones computed by the Padova group to the (T 1 , ) CMDs, the index and the Standard Giant Branch procedure, we derive metallicities and ages for the five clusters. We combine our sample with clusters with ages and metallicities on a similar scale and examine relationships between position in the LMC, age and metallicity.Results. With the exception of NGC 1697 (age = 0.7 Gyr, [Fe/H] = 0.0 dex), the remaining four clusters are of intermediate-age (from 2.2 to 3.0 Gyr) and relatively metal-poor ([Fe/H] = –0.7 dex). The cluster and field age-metallicty realtions show evidence for a metallicity offset but do overlap, particularly on the upper envelope side of the cluster age-metallicity relation.Conclusions. We confirm previous results that clusters younger than ~1 Gyr were formed during an outside-in process; this occurred after a burst of cluster formation that took place mainly in the outer disk and peaked at ~2 Gyr ago.

THE AGE AND METALLICITY OF THE BOÖTES I SYSTEM

We present Washington photometry of a field central to the Bootes I dwarf spheroidal galaxy, which was discovered as a stellar overdensity in the Sloan Digital Sky Survey (DR5). We show that the Washington filters are much more effective than the Sloan filters in separating the metal-poor turn-off stars in the dwarf galaxy from the foreground stars. We detect 165 objects in the field, and statistically determine that just over 40% of the objects are non-members. Our statistical analysis mostly agrees with radial velocity measurements of the brighter stars. We find that that there is a distinct main-sequence turn-off and subgiant branch, where there is some evidence of a spread in chemical abundance. Any evidence of an age spread is limited to a few billion years. The brightest 7 Bootes I members give a (photometric-color derived) weighted mean iron-abundance of [Fe/H]=-2.1+/-0.4, and the best-fit isochrone is the 14.1 Gyr, Z=0.0002 model, with DM=19.11 and E(B-V)=0.02.

CCD Washington photometry of three highly field star contaminated open clusters in the third Galactic quadrant

We present CCD photometry in the Washington system C and T 1 passbands down to T 1 ∼ 19.5 magnitudes in the fields of Czernik 26, Czernik 30, and Haffner 11, three poorly studied open clusters located in the third Galactic quadrant. We measured T 1 magnitudes and C − T 1 colors for a total of 6472 stars distributed throughout cluster areas of 13.6′ × 13.6′ each. Cluster radii were estimated from star counts in appropriate-sized boxes distributed throughout the entire observed fields. Based on the best fits of isochrones computed by the Padova group to the ( C − T 1, T 1) color-magnitude diagrams (CMDs), we derived color excesses, heliocentric distances and ages for the three clusters. These are characterized by a relatively small angular size and by a high field star contamination. We performed a firm analysis of the field star contamination of the CMDs and examined different relationships between the position in the Galaxy of known open clusters located within 1 kpc around the three studied ones, their age and their interstellar visual absorption. We confirm previous results in the sense that the closer the cluster birthplace to the Galactic plane, the higher the interstellar visual absorption. We also found that the space velocity dispersion perpendicular to the Galactic plane diminishes as the clusters are younger. The positions, interstellar visual absorptions, ages, and metallicities of the three studied clusters favor the hypothesis that they were not born in the recently discovered Canis major (CMa) dwarf galaxy before it was accreted by the Milky Way.

The Globular Cluster System of M60 (NGC 4649). I. Canada‐France‐Hawaii Telescope MOS Spectroscopy and Database

We present radial velocity measurements for globular clusters in M60, a giant elliptical galaxy in the Virgo Cluster. Target globular cluster candidates were selected using Washington photometry based on deep 16 -->' × 16 -->' images taken at the KPNO 4 m telescope and using VI photometry derived from Hubble Space Telescope WFPC2 archival images. The spectra of the target objects were obtained with the Multi-Object Spectrograph at the Canada-France-Hawaii Telescope. We have measured the radial velocities of 111 objects in the field of M60: 93 globular clusters (72 blue globular clusters with 1.0 ≤ C– -->T1 T1 + 27−25 km s−1, which is in good agreement with the velocity of the nucleus of M60 ( -->vgal = 1056 km s−1). Combining our results with data from the literature, we present a master catalog of radial velocities for 121 globular clusters in M60. The velocity dispersion of the clusters in the master catalog is found to be 234 -->+ 13−14 km s−1 for the entire sample, 223 -->+ 13−16 km s−1 for 83 blue globular clusters, and 258 -->+ 21−31 km s−1 for 38 red globular clusters.

Sizes of Confirmed Globular Clusters in NGC 5128: A Wide-Field High-Resolution Study

Using Magellan/IMACS images covering a 1.2 × 1.2 deg2 field of view with seeing of 0.4''-0.6'', we have applied convolution techniques to analyze the light distribution of 364 confirmed globular clusters in the field of NGC 5128 and to obtain their structural parameters. Combining these parameters with existing Washington photometry from Harris et al., we are able to examine the size difference between metal-poor (blue) and metal-rich (red) globular clusters. For the first time, this can be addressed on a sample of confirmed clusters that extends to galactocentric distances about 8 times the effective radius, Reff, of the galaxy. Within 1Reff, red clusters are about 30% smaller on average than blue clusters, in agreement with the vast majority of extragalactic globular cluster systems studied. As the galactocentric distance increases, however, this difference becomes negligible. Thus, our results indicate that the differences between the effective radii, re, of the clusters could be explained purely by projection effects, with red clusters being more centrally concentrated than blue ones, and by an intrinsic re-Rgc dependence, like the one observed for the galaxy.