J/MNRAS/520/63 Cl* ages, masses and extinctions of 4 galaxies (Whitmore+, 2023) ================================================================================ Improving Star Cluster Age Estimates in PHANGS-HST Galaxies and the Impact on Cluster Demographics in NGC 628. Whitmore B.C., Chandar R., Lee J.C., Floyd M., Deger S., Lilly J., Minsley R., Thilker D.A., Boquien M., Dale D.A., Henny K., Scheuermann F., Barnes A.T., Bigiel F., Emsellem E., Glover S., Grasha K., Groves B., Hannon S., Klessen R.S., Kreckel K., Kruijssen J.M.D., Larson K.L., Leroy A., Mok A., Pan H.-A., Pinna F., Sanchez-Blazquez P., Schinnerer E., Sormani M.C., Watkins E., Williams T. =2023MNRAS.520...63W (SIMBAD/NED BibCode) ================================================================================ ADC_Keywords: Galaxies, nearby ; Clusters, globular ; Photometry ; Stars, ages ; Spectroscopy ; Molecular data ; Optical ; Positional data ; Photometry, millimetric/submm ; Photometry, hydrogen-line ; Stars, masses ; Extinction Keywords: galaxies: star clusters: general - galaxies: star formation Abstract: A long-standing problem when deriving the physical properties of stellar populations is the degeneracy between age, reddening, and metallicity. When a single metallicity is used for all the star clusters in a galaxy, this degeneracy can result in 'catastrophic' errors for old globular clusters. Typically, approximately 10-20 per cent of all clusters detected in spiral galaxies can have ages that are incorrect by a factor of 10 or more. In this paper, we present a pilot study for four galaxies (NGC 628, NGC 1433, NGC 1365, and NGC 3351) from the PHANGS-HST survey. We describe methods to correct the age-dating for old globular clusters, by first identifying candidates using their colours, and then reassigning ages and reddening based on a lower metallicity solution. We find that young 'Interlopers' can be identified from their H{alpha} flux. CO (2-1) intensity or the presence of dust can also be used, but our tests show that they do not work as well. Improvements in the success fraction are possible at the ~=15 per cent level (reducing the fraction of catastrophic age-estimates from between 13 and 21 per cent, to between 3 and 8 per cent). A large fraction of the incorrectly age-dated globular clusters are systematically given ages around 100 Myr, polluting the younger populations as well. Incorrectly age-dated globular clusters significantly impact the observed cluster age distribution in NGC 628, which affects the physical interpretation of cluster disruption in this galaxy. For NGC 1365, we also demonstrate how to fix a second major age-dating problem, where very dusty young clusters with E(B - V) > 1.5 mag are assigned old, globular-cluster like ages. Finally, we note the discovery of a dense population of ~=300 Myr clusters around the central region of NGC 1365 and discuss how this results naturally from the dynamics in a barred galaxy. Description: The current paper is a pilot project using four PHANGS-HST galaxies (NGC 628, NGC 1365, NGC 1433, and NGC 3351). The project is designed to identify and fix the most common problems in the determination of cluster ages, masses, and reddening as performed in the PHANGS-HST data-reduction pipeline. The development of more sophisticated methods to optimize age estimates for the full sample will be briefly discussed, and its incorporation into the PHANGS-HST pipeline will be described in a future paper. Our approach in this paper is to first identify a set of clusters with age estimates that are clearly incorrect. We then experiment with how to objectively identify them in a colour-colour diagram and establish rules that fix the ages for most of these clusters. The few remaining exceptions (e.g. primarily young clusters with significant reddening due to dust that gives them colours similar to OGCs) are then corrected using information from either H{alpha}, CO intensity, or dust morphology. We find that, perhaps not surprisingly, H{alpha} emission offers the most direct and powerful way to establish the presence of young stars, but in the absence of such data, tracers that indicate the presence of dust, such as CO or dust morphology, can also be used. We describe the procedure used to identify 'bad ages' and investigate where they are generally found in the logAge versus E(B-V) diagram and the U-B versus V-I diagram. We describe how we build the hybrid age-solutions for clusters in the four program galaxies. We use the primary data sets from the PHANGS-HST project of Lee et al. (2022ApJS..258...10L), program which obtained multiband imaging for 38 nearby spiral galaxies in the following five filters: F275W (NUV), F336W (U), F438W (B), F555W (V), and F814W (I), with the WFC3 camera. The PHANGS-HST catalogs of compact star clusters are one of the key data products from the project. Cluster candidates are selected using a Multi-Concentration Index. They are then classified into four classes based on visual examination, in Whitmore et al. (2021MNRAS.506.5294W). In this work, we use only the human classified, C1 and C2 clusters which correspond respectively to the number of star clusters for our four galaxies in tables ngc628.dat, ngc1433.dat, ngc3351.dat, ngc1365.dat. The age-dating results presented throughout this paper are based on SED fits between cluster photometry in the NUV, U, B, V, and I bands and the Bruzual & Charlot (2003MNRAS.344.1000B) stellar population models, through the cigale fitting software. The age-dating results presented throughout this paper are base. For the original baseline solution, we adopted a fixed solar metallicity (Z=0.02) model and fit for the best combination. More, for our work we use MUSE and ALMA maps (H{alpha} and CO) to correct ages (i.e identification and correction procedures are presented in section 4 and 5). Objects: ---------------------------------------------------------------------------- RA (2000) DE Designation(s) ---------------------------------------------------------------------------- 01 36 41.74 +15 47 01.1 NGC 628 = 2E 0134.0+1531 03 33 36.45 -36 08 26.3 NGC 1365 = 1E 0331.7-3618 03 42 01.48 -47 13 18.9 NGC 1433 = 2MASX J03420155-4713194 10 43 57.73 +11 42 13.3 NGC 3351 = 2MASX J10435773+1142129 ---------------------------------------------------------------------------- File Summary: -------------------------------------------------------------------------------- FileName Lrecl Records Explanations -------------------------------------------------------------------------------- ReadMe 80 . This file ngc628.dat 445 486 New hybrid SED age solutions ages, masses and reddenings for cluster class 1 and 2 in NGC 628 ngc1433.dat 445 191 New hybrid SED age solutions ages, masses and reddenings for cluster class 1 and 2 in NGC 1433 ngc3351.dat 445 317 New hybrid SED age solutions ages, masses and reddenings for cluster class 1 and 2 in NGC 3351 ngc1365.dat 355 635 New hybrid SED age solutions ages, masses and reddenings for cluster class 1 and 2 in NGC 1365 -------------------------------------------------------------------------------- See also: J/MNRAS/512/1294 : HST study of star cluster in LEGUS galaxies (Hannon+, 2022) J/MNRAS/510/32 : Star clusters study in PHANGS-HST galaxies (Deger+, 2022) J/MNRAS/509/180 : GC in 5 nearby spiral galaxies (Lomeli-Nunez+, 2022) J/MNRAS/498/4906 : Pressure components in the central HII regions (Barnes+, 2020) J/MNRAS/404/1639 : MILES base models & new line index system (Vazdekis+, 2010) J/ApJ/909/121 : Young star clusters in NGC1313 with HST obs. (Messa+, 2021) J/ApJ/889/154 : LEGUS & Ha-LEGUS obs. of NGC4449 star clusters (Whitmore+, 2020) J/ApJ/887/80 : Gas phase oxygen abundances for HII regions (Kreckel+, 2019) J/ApJ/667/L145 : HST Photometry of M82 region B (Smith+, 2007) J/A+A/658/A188 : PHANGS-MUSE sample HII regions catalog (Santoro+, 2022) J/A+A/622/A103 : Python Code Investigating GALaxy Emission (Boquien+, 2019) J/A+A/527/A109 : Dust and PAH emission SEDs of disk galaxies (Popescu+, 2011) J/A+A/416/537 : UBVI and HST photometry of young stellar clusters (Larsen, 2004) J/ApJS/257/43 : PHANGS-ALMA: arcsecond CO(2-1) imaging of galaxies (Leroy+, 2021) J/ApJS/184/1 : Molecular clouds in the LMC by NANTEN. II. (Kawamura+, 2009) J/AJ/163/16 : Multiwavelength study in M33 star clusters (Moeller+, 2022) J/AJ/140/75 : Antennae galaxies (NGC 4038/4039) revisited (Whitmore+,2010) J/AJ/124/1418 : Radio-optical identification of star clusters (Whitmore+, 2002) II/342 : Hubble Source Catalog (V1 and V2) (Whitmore+, 2016) Byte-by-byte Description of file: ngc628.dat ngc1433.dat ngc3351.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 4 I4 --- ID ID number from compact cluster catalog (ID_PHANGS_CLUSTERS_V0_9_1) 6-23 F18.13 pix Xpos X coordinate (PHANGS_X) 25-42 F18.13 pix Ypos Y coordinate (PHANGS_Y) 44-62 F19.15 deg RAdeg Right ascension from PHANGS-HST (J2000) (PHANGS_RA) 64-82 F19.15 deg DEdeg Declination from PHANGS-HST (J2000) (PHANGS_DEC) 84-101 F18.16 mag CI Concentration Index CI as difference magnitude between a vega magnitude in 1 pixel radius minus a vega magnitude in 3 pixel radius (PHANGS_CI) 103-121 F19.16 [yr] logAgeT The original version 1 of logarithm age estimate with as described in Turner et al. 2021MNRAS.502.1366T (log_PHANGS_AGE_MINCHISQ) 123-140 F18.16 [Msun] logM*o Original logarithm of mass estimated with Chi2 minimization (log_PHANGS_MASS_MINCHISQ) 142-159 F18.16 mag E(B-V) The original E(B-V) estimated with Chi2 minimization (PHANGS_EBV_MINCHISQ) 161-181 E21.17 --- Chi The reduced phangs chi-sqr value (PHANGS_REDUCED_MINCHISQ) 183-185 F3.1 --- Class The human cluster classification symmetric as 1 and asymmetric as 2 (PHANGS_CLUSTER_CLASS) 187-209 F23.19 mag U-B ? F336W - F438W vega system color magnitude where objects out of field of view, or with negative flux = EMPTY flux (u-b) 211-232 E22.19 mag B-V F438W - F555W vega system color magnitude (b-v) 234-255 E22.19 mag V-I F555W - F814W vega system color magnitude (v-i) 257-274 F18.15 mag F555Wmag The F555W total magnitude in vega system (PHANGS_F555W_VEGA_TOT) 276-297 F22.14 10-18mW/m2/pix FHa The Halpha flux estimate where objects out of the Field of View are set to 0 value (HA6562_FLUX) 299-319 E21.18 K.km/s ICO The molecular CO (2-1) flux intensity where objects out of the Field of View are set to 1.0E20 value (CO_intensity) 321-339 F19.16 [yr] logAgeHO The logarithm of the age estimate for Old Globular Cluster=OGC box solution (hybrid_log_age_ogc) 341-358 F18.16 [Msun] logM*HO The logarithm of the mass estimate for Old Globular Cluster = OGC box solution (hybrid_log_mass_ogc) 360-382 F23.21 mag E(B-V)HO The E(B-V) estimate Old Globular Cluster = OGC box solution (hybrid_ebv_ogc) 384-402 F19.16 [yr] logAgeHVI The logarithm of the age estimate for VI limit solution (hybrid_log_age_vi) 404-421 F18.16 [Msun] logMHVI The logarithm of the mass estimate for VI limit solution (hybrid_log_mass_vi) 423-445 F23.21 mag E(B-V)HVI The E(B-V) estimate for VI limit solution (hybrid_ebv_vi) -------------------------------------------------------------------------------- Byte-by-byte Description of file: ngc1365.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 4 I4 --- ID ID number from compact cluster catalog (ID_PHANGS_CLUSTERS_V1P2) 6- 23 F18.13 pix Xpos X coordinate (PHANGS_X) 25- 42 F18.13 pix Ypos Y coordinate (PHANGS_Y) 44- 61 F18.15 deg RAdeg Right ascension from PHANGS-HST (J2000) (PHANGS_RA) 63- 81 F19.15 deg DEdeg Declination from PHANGS-HST (J2000) (PHANGS_DEC) 83-100 F18.16 mag CI Concentration Index CI as difference magnitude between a vega magnitude in 1 pixel radius minus a vega magnitude in 3 pixel radius (PHANGS_CI) 102-120 F19.16 [yr] logAgeT The original version 1 of logarithm age estimate with as described in Turner et al. 2021MNRAS.502.1366T (log_PHANGS_AGE_MINCHISQ) 122-139 F18.16 [Msun] logM*o Original logarithm of mass estimated with Chi2 minimization (log_PHANGS_MASS_MINCHISQ) 141-159 F19.17 mag E(B-V) The original E(B-V) estimated with Chi2 minimization (PHANGS_EBV_MINCHISQ) 161-163 F3.1 --- Class The human cluster classification symmetric as 1 for 366 occurences and asymmetric as 2 for 269 occurences (PHANGS_CLUSTER_CLASS) 165-185 E21.18 mag U-B ? F336W - F438W vega system color magnitude where objects out of field of view, or with negative flux = EMPTY flux (u-b) 187-208 E22.19 mag B-V F438W - F555W vega system color magnitude (b-v) 210-230 F21.18 mag V-I F555W - F814W vega system color magnitude (v-i) 232-249 F18.15 mag F555Wmag The F555W total magnitude in vega system (PHANGS_F555W_VEGA_TOT) 251-275 F25.16 10-18mW/m2/pix FHa The Halpha flux estimate where objects out of the Field of View are set to 0 value (HA6562_FLUX) 277-296 E20.17 K.km/s ICO The molecular CO (2-1) flux intensity where objects out of the Field of View are set to 1.0E20 value (CO_intensity) 298-316 F19.16 [yr] logAgeHhi The logarithm of the age estimate for high reddening solution (hybrid_log_age_high_red) 318-335 F18.16 [Msun] logM*Hhi The logarithm of the mass estimate for high reddening solution (hybrid_log_mass_high_red) 337-355 F19.17 mag E(B-V)Hhi E(B-V) estimate for high reddening solution (hybrid_ebv_high_red) -------------------------------------------------------------------------------- History: From electronic version of the journal ================================================================================ (End) Luc Trabelsi [CDS] 26-Feb-2026