Revisiting newly Large Magellanic Cloud age gap star clusters
DDraft version February 9, 2021
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Revisiting newly Large Magellanic Cloud age gap star clusters
Andr´es E. Piatti
1, 2 Instituto Interdisciplinario de Ciencias B´asicas (ICB), CONICET-UNCUYO, Padre J. Contreras 1300, M5502JMA, Mendoza,Argentina Consejo Nacional de Investigaciones Cient´ıficas y T´ecnicas (CONICET), Godoy Cruz 2290, C1425FQB, Buenos Aires, Argentina
ABSTRACTRecently, a noticeable number of new star clusters was identified in the outskirts of the LargeMagellanic Cloud (LMC) populating the so-called star cluster age gap, a space of time ( ∼ Keywords: methods:observational – technine:photometric – galaxies:individual:LMC – galaxies:starcluster:general INTRODUCTIONThe absence of star clusters with ages between ∼ ∼
12 Gyr, Piatti & Mackey 2018; Piatti et al. 2018b).The lower age limit, however, has been changed as moreintermediate-age stars clusters were studied in detail.
Corresponding author: Andr´es E. Piattie-mail: [email protected]
For instance, Sarajedini (1998) found that NGC 2121,2155 and SL 663 are ∼ ∼ ∼ a r X i v : . [ a s t r o - ph . GA ] F e b Andr´es E. Piatti tory and increase of the star cluster formation rates (Pi-atti 2011a,b; Piatti & Geisler 2013; Kallivayalil et al.2013; Lucchini et al. 2020).Recently, Gatto et al. (2020) performed a search forunidentified star clusters in LMC outermost regions anddetected 20 star cluster candidates with estimated ages (cid:38) THE DATAWe use the portal of the Astro Data Lab , whichis part of the Community Science and Data Center ofNSF’s National Optical Infrared Astronomy ResearchLaboratory, to retrieve R.A and Dec. coordinates, PSF g, i magnitudes and their respective errors, interstellarreddening E ( B − V ) and χ and sharpness parametersof stellar sources located inside a radius of 6 (cid:48) from thestar clusters’ centers listed by Gatto et al. (2020). Theretrieved data sets consist of sources with 0.2 ≤ sharp-ness ≤ χ < ≥ ≥ https://datalab.noao.edu/smash/smash.php SMASH DR2. Figure 1 illustrates a typical star clusterfield, where the variation of the interstellar reddening isshown with color-coded symbols.The radii of the studied star cluster candidates arerelatively small, from 0.2 (cid:48) up to 0.55 (cid:48) , with an averageof 0.35 (cid:48) (see Table B1 in Gatto et al. 2020). Because wedownloaded information for circular areas much largerthan the star cluster fields, we thoroughly monitored thecontamination of field stars in the star clusters’ CMDs.Indeed, we selected for each star cluster field, repre-sented by a circle centered on the star cluster with aradius 3 times that of the star cluster, 6 adjacent refer-ence star field regions of equal star cluster field area dis-tributed around the cluster region, as depicted in Fig. 1.We based our analysis on dereddened CMDs, so we firstcorrected by interstellar extinction the g and i magni-tudes using the E ( B − V ) values provided by SMASHand the A λ / E ( B − V ) ratios, for λ = g, i , given by Ab-bott et al. (2018). The retrieved SMASH E ( B − V )value for each star (see color bar in Fig. 1) correspondsto the median E ( B − V ) around it obtained by usingthe Schlegel et al. (1998)’s reddening map (see also Choiet al. 2018; Nidever et al. 2021).As for the data sets used by Gatto et al. (2020), theywere kindly provided by V. Ripepi. We note that the g, i bands of the STEP/YMCA surveys are not the sameused by SMASH (see figure 1 in Ripepi et al. 2014).Therefore, g − i color ranges are not straightforwardlycomparable, nor their CMDs. Both, STEP/YMCA andSMASH data sets are then independent sources. Nideveret al. (2017) presented in their Table 4 the SMASH av-erage photometric transformation equations. By addingin quadrature zero-point, extinction and color term er-rors, we computed an accuracy (cid:46) gi . Theyshowed that these calibration errors imply a SMASHphotometry precision of ∼ gi . Such a preci-sion implies in turn an uncertainty of ∼ gi , for a star at the main sequence turnoff of a ∼ g (cid:38) g and i , respectively, which is compa-rable to that of SMASH. We note that the zero point andcolor terms errors obtained by Ripepi et al. (2014, seetheir Table 8), on which Gatto et al. (2020)’s photome-try relies, are of the same order than those of SMASH. DATA ANALYSISThe contamination of field stars plays an importantrole when dealing with star cluster CMDs, because itis not straightforward to consider a star as a clustermember only on the basis of its position in that CMD.Sometimes, additional information like proper motions,
MC age gap clusters − − × cos(Dec.) ( ) − − ∆ ( D ec . )( ) . . . . . E ( B - V ) Figure 1.
Schematic chart centered on STEP-0029. Thesize of the symbols is proportional to the g brightness,while their color excesses are coded according to the colorbar. The radius of the superimposed circles is 3 times thatadopted as the cluster’s radius (Gatto et al. 2020). Six la-beled reference star fields distributed around the star clustercircle are also drawn. radial velocities, and/or chemical abundances of indi-vidual stars can help with disentangling between fieldand star cluster members. Unfortunately, in the caseof our star cluster sample, Gaia
DR2 proper motions(Gaia Collaboration et al. 2016, 2018) are unreliable atthe main sequence turnoff level ( g (cid:38) g , ∆( g − i ) ) Andr´es E. Piatti g g i ) g g i ) g i ) Figure 2.
Color-magnitude diagram of STEP-0029. Blackpoints represent all the measured stars in SMASH DR2 datasets located within a circle with a radius equal to 3 times thecluster radius. Large magenta points represent the stars thatremained unsubtracted after the CMD cleaning procedure.The reference star field used to decontaminate the star clus-ter CMD is indicated at the top-left margin (see also Fig. 1). = (2.0 mag,1.0 mag) centered on the ( g , ( g − i ) ) valuesof each reference field star.In practice, for each reference field star, we first ran-domly selected the position of a subregion inside the starcluster field where to subtract a star. These subregionswere devised as annular segments of 90 ◦ wide and ofconstant area. Their external radii are chosen randomly,while the internal ones are calculated so that the areasof the annular sectors are constant. Here we adopted anarea for the subregions equal to πr cls , where r cls is thestar cluster radius. We then looked for a star with ( g ,( g − i ) ) values within a box defined as described above.If no star is found in that annular sector, we randomlyselected another one and repeated the search, allowingthe procedure to iterate up to 1000 times. If no star inthe star cluster field with a magnitude and a color simi-lar to ( g , ( g − i ) ) is found after 1000 iterations, we donot subtract any star for that ( g , ( g − i ) ) values. Thesame procedure was applied for all the stars in the ref-erence star field. The photometric errors of the stars inthe star cluster field were also taken into account whilesearching for a star to be subtracted from the star clus-ter CMD. With that purpose, we iterated up to 1000times the search within each defined box, allowing thestars in the star cluster CMD to vary their magnitudesand colors within an interval of ± σ , where σ representsthe errors in their magnitude and color, respectively.Figure 2 illustrates the different results of the decon-tamination of field stars when the different 6 referencestar fields (see Fig. 1) are used, separately. As can be seen, the different resulting cleaned star cluster CMDs(magenta points) show distinct groups of stars, depend-ing on the reference star field used, which suggests thatdifferences in the astrophysical properties of the com-posite star field population do exist. If all the referencestar fields showed a uniform distribution of stars in mag-nitude and color, all the resulting cleaned CMDs shouldlook similar. The spatial distribution of the stars thatremained unsubtracted is shown in Fig. 3. From Figs. 2and 3 is readily visible that the stars that have survivedthe cleaning procedure are not spatially distributed in-side the cluster radius (black circle), nor they unques-tionably follow the expected sequences in the star clusterCMD either. This means that those stars could ratherrepresent fluctuations in the stellar density along theLOS of the composite stellar field population.We finally assigned a membership probability to eachstar that remained unsubtracted after the decontamina-tion of the star cluster CMD. Because the stars in thecleaned CMDs vary with respect to the reference starfield employed (see the distribution of magenta pointsin Figs. 2 and 3), we defined the probability P (%) =100 × N /6, where N represent the number of time a starwas not subtracted during the six different CMD clean-ing executions. With that information on hand, we builtFig. 4, which shows the spatial distribution and theCMD of all the measured stars located in the field ofSTEP-0029. Stars with different P values were plotted with different colors. We applied the above cleaningprocedure to the remaining 16 star cluster candidatesdiscovered by Gatto et al. (2020) with ages (cid:38) DISCUSSION AND CONCLUSIONSBy examining the spatial distributions of stars withassigned P values (color-coded symbols in Figs. 4, 5-10),none of the analyzed fields show groups of stars with P >
50% concentrated inside the star cluster radius.This means that the spatial overdensities discovered byGatto et al. (2020) do not highlight themselves in termsof stellar brightness and color distributions from thoseof the surrounding field neither in the STEP/YMCAnor SMASH data sets, when the field star decontami-nation procedure described in Sect. 3 is applied. Theycould rather reveal small stellar density fluctuations inthe studied LMC regions. Isolated stars spread through-out the cleaned areas spanning a wide range of P val-ues are seen in all the studied fields. We also detect MC age gap clusters ( D e c . ) ( ) ( D e c . ) ( ) ) Figure 3.
Chart of the stars in the field of STEP-0029.The size of the symbols is proportional to the g brightnessof the star. Open black circles represent all the measuredstars in SMASH DR2 data sets located within a circle witha radius equal to 3 times the cluster radius. Filled magentacircles represent the stars that remained unsubtracted afterthe CMD cleaning procedure. The reference star field usedto decontaminate the star cluster CMD is indicated at thetop-left margin (see also Fig. 1). The large centered circlerepresents that of the star cluster radius. some local concentrations of stars that distinguish fromthe surrounding field in the SMASH data, for example,toward the southern outskirts of STEP-0012, YMCA-0021, and YMCA-0023. Their positions in the cleanedCMDs, however, do not provide hints for any star clustersequence.Because of the LMC distance (49.9 kpc; de Grijs et al.2014), stars projected along a particular LOS could pro-duce CMDs with features similar to those seen in starcluster CMDs. For example, from the SMASH cleanedCMD of YMCA-0002 (Fig. 6), we could conclude on theexistence of a star cluster with some few red clump andmain sequence turnoff stars, whereas the spatial distri-bution of stars with P >
70% does not support such apossibility. The SMASH cleaned CMD of YMCA-0007shows a populous red clump of stars with P ∼
50% thatbelong to the field, as judged by their spatial distribu-tion. In summary, Figs. 4, 5-10 most likely reveal thecomposite stellar population of the studied LMC regionsand their local fluctuations.Piatti (2018b) arrived to a similar conclusion on thenew identified star clusters by Bitsakis et al. (2017), whofound that the population of LMC star clusters locatedat deprojected distances < ◦ was nearly double theknown size of the system. Piatti (2018b) based his find-ings on the remarkable large number of objects withassigned ages older than 2.5 Gyr, which contrasts with the existence of the LMC star cluster age gap; the factthat the assumption of a cluster formation rate similarto that of the LMC star field does not help to recon-cile the large amount of star clusters either; and nearly50% of them come from star cluster search methodsknown to produce more than 90% of false detections.Bitsakis et al. (2017) identified only ∼
35% of the pre-viously known cataloged LMC star clusters. The LMCstar cluster frequency, i.e., number of star clusters pertime unit, is a distribution function that basically doesnot change if low mass star clusters are not considered.The known LMC star cluster population is statisticallycomplete down to 5 × M (cid:12) and their star cluster fre-quency does not show clusters in the age-gap, and thisis a feature seen all throughout the LMC body (Piatti2014). The LMC fields analyzed here are located beyond4 ◦ from the LMC center; the number of new detectionsis not such high; and the recovery fraction of known cat-aloged LMC star clusters is much higher (see figure 7 inGatto et al. (2020)). Nevertheless, it is hardly possiblethat age gap star clusters have been formed only in theoutskirts of the LMC. Indeed, the 15 ancient LMC glob-ular clusters are distributed in the halo and in the diskof the galaxy (Piatti et al. 2019).The different outcomes obtained by Gatto et al. (2020)and in the present work reflect the different perfor-mances of the techniques employed for cleaning theCMDs of field star contamination. When comparing thepresent constructed CMDs (Figs. 4, 5-10) and those ofGatto et al. (2020, see their figure B1) we note that: i)those built from SMASH and STEP/YMCA data con-tain a similar number of stars and reach in general simi-lar limiting magnitudes per unit area. ii) By comparingthe observed and cleaned CMDs built by Gatto et al.(2020), it would seem that the number of field starssubtracted was relatively small. Likewise, the numberof stars considered as star cluster members in Gattoet al. (2020)’s CMDs would seem also to be small as toconclude on clear features of a relatively old star clus-ter. In the present work, we subtracted a larger num-ber of field stars per unit area using the SMASH andSTEP/YMCA data sets and no definitive signature ofstar clusters are observed in the cleaned CMDs. There-fore, we speculate with the possibility that Gatto et al.(2020)’s results and ours are based on a low numberstatistics. iii) As can be seen, the star distribution alongthe theoretical isochrones in CMDs built from SMASHand STEP/YMCA data are comparable. Here we stressthe issue that for most of the studied objects, those starswould not seem to belong to a physical aggregate, but tothe composite LMC field. Indeed, the observed main se-quence turnoffs are mostly populated by field stars. iv) Andr´es E. Piatti )0.750.500.250.000.250.500.75 ( D e c . ) ( ) g i ) g P (\%) Figure 4.
Left panel:
Chart of the stars in the field ofSTEP-0029. The size of the symbols is proportional to the g brightness of the star. Open black circles represent all themeasured stars in SMASH DR2 data sets located within acircle with a radius equals to 3 times the cluster radius. Thelarge centered circle represents that of the star cluster radius. Right panel:
Color-magnitude diagram of STEP-0029. Blackpoints represent all the measured stars in SMASH DR2 datasets located within the star cluster radius. The theoreticalisochrone plotted by Gatto et al. (2020, see their figure B1)is overplotted for comparison purposes. Filled circles in bothpanels represent the stars that remained unsubtracted afterthe CMD cleaning procedure, color-coded according to theassigned membership probabilities ( P ). The metallicities of the isochrones used by Gatto et al.(2020) (Z= 0.004, 0.006 and 0.008) are much metal-richthan the known metallicity of the only one confirmedLMC age gap cluster ESO 121-SC03 (Z ≈ Abbott, T. M. C., Abdalla, F. B., Allam, S., et al. 2018,ApJS, 239, 18, doi: 10.3847/1538-4365/aae9f0Bekki, K., Couch, W. J., Beasley, M. A., et al. 2004, ApJL,610, L93, doi: 10.1086/423372Bitsakis, T., Bonfini, P., Gonz´alez-L´opezlira, R. A., et al.2017, ApJ, 845, 56, doi: 10.3847/1538-4357/aa8090Choi, Y., Nidever, D. L., Olsen, K., et al. 2018, ApJ, 869,125, doi: 10.3847/1538-4357/aaed1fDa Costa, G. S. 1991, in IAU Symposium, Vol. 148, TheMagellanic Clouds, ed. R. Haynes & D. Milne, 183de Grijs, R., Wicker, J. E., & Bono, G. 2014, AJ, 147, 122,doi: 10.1088/0004-6256/147/5/122Gaia Collaboration, Prusti, T., de Bruijne, J. H. J., et al.2016, A&A, 595, A1, doi: 10.1051/0004-6361/201629272Gaia Collaboration, Brown, A. G. A., Vallenari, A., et al.2018, A&A, 616, A1, doi: 10.1051/0004-6361/201833051Gatto, M., Ripepi, V., Bellazzini, M., et al. 2020, MNRAS,499, 4114, doi: 10.1093/mnras/staa3003Geisler, D., Bica, E., Dottori, H., et al. 1997, AJ, 114, 1920,doi: 10.1086/118614 Kallivayalil, N., van der Marel, R. P., Besla, G., Anderson,J., & Alcock, C. 2013, ApJ, 764, 161,doi: 10.1088/0004-637X/764/2/161Lucchini, S., D’Onghia, E., Fox, A. J., et al. 2020, Nature,585, 203. https://arxiv.org/abs/2009.04368Maia, F. F. S., Dias, B., Santos, J. F. C., et al. 2019,MNRAS, 484, 5702, doi: 10.1093/mnras/stz369Mateo, M., Hodge, P., & Schommer, R. A. 1986, ApJ, 311,113, doi: 10.1086/164757Nidever, D. L., Olsen, K., Walker, A. R., et al. 2017, AJ,154, 199, doi: 10.3847/1538-3881/aa8d1cNidever, D. L., Olsen, K., Choi, Y., et al. 2021, AJ, 161, 74,doi: 10.3847/1538-3881/abceb7Olszewski, E. W., Schommer, R. A., Suntzeff, N. B., &Harris, H. C. 1991, AJ, 101, 515, doi: 10.1086/115701Piatti, A. E. 2011a, MNRAS, 418, L40,doi: 10.1111/j.1745-3933.2011.01139.x—. 2011b, MNRAS, 418, L69,doi: 10.1111/j.1745-3933.2011.01145.x—. 2014, MNRAS, 437, 1646, doi: 10.1093/mnras/stt1998
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101 (R.A.)×cos(Dec.) ( )1.00.50.00.51.0 ( D e c . ) ( ) g i ) g P (\%) 1011.00.50.00.51.0 ( D e c . ) ( ) g
101 (R.A.)×cos(Dec.) ( )1.00.50.00.51.0 ( D e c . ) ( ) g i ) g P (\%) 1011.00.50.00.51.0 ( D e c . ) ( ) g
101 (R.A.)×cos(Dec.) ( )1.00.50.00.51.0 ( D e c . ) ( ) g i ) g P (\%) Figure 5.
Same as Fig. 4 for STEP-0001, STEP-0005, STEP-0012, STEP-0024, STEP-0035, and YMCA-0001 from top bottom,and from left to right. ( D e c . ) ( ) g )0.50.00.5 ( D e c . ) ( ) g i ) g P (\%) 101101 ( D e c . ) ( ) g )101 ( D e c . ) ( ) g i ) g P (\%) 0.50.00.50.50.00.5 ( D e c . ) ( ) g )0.500.250.000.250.50 ( D e c . ) ( ) g i ) g P (\%) Figure 6.
Same as Fig. 4 for YMCA-0002, YMCA-0004, YMCA-0006, YMCA-0007, YMCA-0008, and YMCA-0012 from topbottom, and from left to right. ( D e c . ) ( ) g
101 (R.A.)×cos(Dec.) ( )1.00.50.00.51.0 ( D e c . ) ( ) g i ) g P (\%) 101101 ( D e c . ) ( ) g
101 (R.A.)×cos(Dec.) ( )101 ( D e c . ) ( ) g i ) g P (\%) Figure 7.
Same as Fig. 4 for YMCA-0013, YMCA-0017, YMCA-0021, and YMCA-0023 from top bottom, and from left toright.
APPENDIX A. CLEANED STAR CLUSTER CMDSFigures 5 to 7 show the SMASH cleaned star cluster CMDs and the respective spatial distribution of the starcandidates with ages (cid:38)
MC age gap clusters Figure 8.
Same as Fig. 4 for STEP-0001, STEP-0005, STEP-0012, STEP-0024, STEP-0035, and YMCA-0001 from top bottom,and from left to right. Data were kindly provided by V. Ripepi.
Figure 9.
Same as Fig. 4 for YMCA-0002,YMCA-0004, YMCA-0006, YMCA-0007, YMCA-0008, and STEP-0029 from topbottom, and from left to right. Data were kindly provided by V. Ripepi.