J/MNRAS/510/32 Star clusters study in PHANGS-HST galaxies (Deger+, 2022)
Bright, relatively isolated star clusters in PHANGS-HST galaxies.
Aperture corrections, quantitative morphologies, and comparison with synthetic
stellar population models.
Deger S., Lee J.C., Whitmore B.C., Thilker D.A., Boquien M., Chandar R.,
Dale D.A., Ubeda L., White R., Grasha K., Glover S.C.O., Schruba A.,
Barnes A.T., Klessen R., Kruijssen J.M.D., Rosolowsky E., Williams T.G.
<Mon. Not. R. Astron. Soc. 510, 32-53 (2022)>
=2022MNRAS.510...32D 2022MNRAS.510...32D (SIMBAD/NED BibCode)
ADC_Keywords: Galaxies, nearby ; Associations, stellar ; Clusters, globular ;
Clusters, open ; Star Forming Region ; Ultraviolet ; Optical ;
Photometry, UBVRI ; Photometry, classification ; Positional data ;
Stars, ages ; Stars, masses ; Colors ; Morphology
Keywords: galaxies: star clusters: general - galaxies: star formation
Abstract:
Using PHANGS-HST NUV-U-B-V-I imaging of 17 nearby spiral galaxies, we
study samples of star clusters and stellar associations, visually
selected to be bright and relatively isolated, for three purposes: to
compute aperture corrections for star cluster photometry, to explore
the utility of quantitative morphologies in the analysis of clusters
and associations, and to compare to synthetic stellar population
models. We provide a technical summary of our procedures to determine
aperture corrections, a standard step in the production of star
cluster candidate catalogues, and compare to prior work. We also use
this specialized sample to launch an analysis into the measurement of
star cluster light profiles. We focus on one measure, M20 (normalized
second-order moment of the brightest 20 per cent of pixels), applied
previously to study the morphologies of galaxies. We find that M20
in combination with UB-VI colours, yields a parameter space where
distinct loci are formed by single-peaked symmetric clusters,
single-peaked asymmetric clusters, and multipeaked associations. We
discuss the potential applications for using M20 to gain insight
into the formation and evolution of clusters and associations.
Finally, we compare the colour distributions of this sample with
various synthetic stellar population models. One finding is that the
standard procedure of using a single-metallicity SSP track to fit the
entire population of clusters in a given galaxy should be revisited,
as the oldest globular clusters will be more metal- poor compared to
clusters formed recently.
Description:
Understanding the star formation histories and properties of galaxies
has strong ties with understanding the evolution of star clusters.
The Hubble Space Telescope (HST) has enabled the systematic study of
star clusters throughout the nearby galaxy population. HST increased the
number of star clusters detected per galaxy from hundreds to
thousands, and unlocked observational access to the wealth of clusters
and associations in nearby galaxies. The high angular resolution of
HST (FWHM ∼0.08", pixel scale = 0.04" for WFC3/UVIS) is essential for
the study of star clusters at the parsec scale in galaxies at
distances larger than 1 Mpc, where the typical half-light radius of
compact clusters is a few parsecs (Ryon et al. 2017ApJ...841...92R 2017ApJ...841...92R,
Cat. J/ApJ/841/92) (see section 1 Introduction). In this paper, we
analyse data obtained by the PHANGS-HST treasury survey (Lee et al.
2021AAS...23711301L 2021AAS...23711301L). The PHANGS-HST Treasury survey has obtained
5-band (NUBVI, mainly HST WFC3) imaging of 38 nearby spiral galaxies.
Observations started in 2019 April, and the programme completed
observations in 2021 May. The programme will ultimately yield
catalogues of tens of thousands of star clusters and associations.
In this study is organized as follows. We provide a summary of the
PHANGS-HST survey, and overview of the 5-band HST imaging. We discuss
the details of how the bright, isolated cluster and association sample
is constructed. We also describe the ensemble observed and physical
properties of this specialized sample, and compare with the properties
of the full cluster population. We then describe how we use clusters
from this sample to derive the aperture corrections for the PHANGS-HST
data processing pipeline. We discuss the quality assurance process
applied to the clusters prior to their use in the aperture correction
computation. Next, we provide the details of quantitative morphology
analysis, (see section Introducton).
Specifically, data presented are based on HST UV-optical imaging for
17 galaxies for which samples of bright, isolated star clusters
were constructed during the first year of the survey. A complete
description of the PHANGS-HST Treasury programme (GO-15654) is given
in Lee et al. (2021AAS...23711301L 2021AAS...23711301L), we provide a brief summary of the
imaging observations in the section 2 PHANGS-HST Imaging. Then as
explained in the section 3, this specialized sample is manually
identified by human visual inspection of the HST imaging by coauthor
BCW, classification groups is then acheived. Hereafter, physical
properties such as stellar masses, ages and aperture photometry
corrections, morphology parameters like CI and M20 are derived in the
section 3, 4 and 5 respectively.
Finally, we expose these data results for star clusters and stars
associations in the tablea1.dat regrouping 513 objects from 17 NGC
host galaxies which contains roughly 30 objects each.
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
galaxy.dat 11 17 Lists of the 17 nearby galaxies in this study
tablea1.dat 157 513 The bright, isolated specialized star clusters
and associations sample
--------------------------------------------------------------------------------
See also:
J/ApJ/841/92 : Young massive star clusters in 2 LEGUS galaxies (Ryon+, 2017)
J/A+A/622/A103 : Python Code Investigating GALaxy Emission (Boquien+, 2019)
J/AJ/128/163 : Galaxy morphological classification (Lotz+, 2004)
J/MNRAS/458/963 : CANDELS galaxy structure classification (Peth+, 2016)
https://archive.stsci.edu/hst/search_retrieve.html : Mikulski Archive home page
https://archive.stsci.edu/hlsp/phangs/phangs-hst : PHANGS-HST home page
Byte-by-byte Description of file: galaxy.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 8 A8 --- Galaxy Name of the nearby galaxy (galaxy)
10- 11 I2 --- Number Number of associated stars in the tablea1 (N)
--------------------------------------------------------------------------------
Byte-by-byte Description of file: tablea1.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 8 A8 --- Galaxy Host galaxy name as in the list seen
in section 2 PHANGS-HST Imaging (Galaxy)
10- 19 F10.6 deg RAdeg Right ascension (J2000) (RA)
21- 30 F10.6 deg DEdeg Declination (J2000) (Dec)
32- 38 F7.2 pix X pixel position (X)
40- 47 F8.2 pix Y pixel position (Y)
49- 53 F5.2 mag NUVmag Near-UV apparent magnitude in Vega Systems
four pixel circular aperture photometry
HST/WFC3 UVIS-F275W UV wide filter
(m{NUV}) (1)
55- 58 F4.2 mag e_NUVmag Mean error of NUVmag (δm{NUV})
60- 64 F5.2 mag Umag U apparent magnitude in Vega Systems
four pixel circular aperture photometry
HST/WFC3 UVIS-F336W Stromgren u filter
(m{U}) (1)
66- 69 F4.2 mag e_Umag Mean error of e_Umag (δm{U})
71- 75 F5.2 mag Bmag B apparent magnitude in Vega Systems
four pixel circular aperture photometry
HST/WFC3 UVIS-F438W WFPC2 B filter
(m{B}) (1)
77- 80 F4.2 mag e_Bmag Mean error of Bmag (δm{B})
82- 86 F5.2 mag Vmag V apparent magnitude in Vega Systems
four pixel circular aperture photometry
HST/WFC3 UVIS-F555W WFPC2 V filter
(m{V}) (1)
88- 91 F4.2 mag e_Vmag Mean error of Vmag (δm{V})
93- 97 F5.2 mag Imag I apparent magnitude in Vega Systems
four pixel circular aperture photometry
HST/WFC3 UVIS-F814W WFPC2 Wide I filter
(m{I}) (1)
99-102 F4.2 mag e_Imag Mean error of Imag (δm{I})
104-110 E7.2 Myr Age The Χ2 minimization age (Age) (2)
112-118 E7.2 Myr e_Age Mean error of Age (δAge)
120-126 E7.2 Msun M* Χ2 minimization stellar mass (Mass) (2)
128-134 E7.2 Msun e_M* Mean error of M* (δMass)
136-139 F4.2 mag E(B-V) Color excess of (B-V) as discussed in the
section 3.2 sample properties (E(B-V))
141-144 F4.2 mag e_E(B-V) (δE(B-V)
146-150 F5.2 --- M20 Normalized second-order moment (M{20}) (3)
152-155 F4.2 --- CI The morphology concentration index (CI) (4)
157 I1 --- Class The visual morphology class (Class) (5)
--------------------------------------------------------------------------------
Note (1): Described in the section 4 Apperture correction, apperture photometry
corrections are obtained for photometry (V, NUV, U, B, I) of the
PHANGS-HST star clusters performing a circular aperture with a radius
of 4 pixels. The size of the aperture is chosen to roughly correspond
to the half-light radius, so the corrections are expected to be
∼0.75 mag.
Note (2): As explained in the section 3.2 Sample properties, we visualized the
sample on two diagrams commonly used to characterize the properties
of star clusters and associations, the colour-colour diagram and the
age-stellar mass-reddening diagram (figure 2 and 3 of this section).
Determined via Χ2 minimization, ages and masses are derived via
SED fitting of the 5-band HST photometry with CIGALE (Boquien et al.
2019A&A...622A.103B 2019A&A...622A.103B, Cat. J/A+A/622/A103) as described in
Turner et al. (2021MNRAS.502.1366T 2021MNRAS.502.1366T).
Note (3): In the section 5 Quantitative measures of morphology, the first
measure we analysed is a non-parametric measure of morphology from
Lotz et al. (2004AJ....128..163L 2004AJ....128..163L, Cat. J/AJ/128/163), the M20
parameter, which has been extensively applied to galaxies
(Lotz et al. 2008ApJ...672..177L 2008ApJ...672..177L; Peth et al. 2016MNRAS.458..963P 2016MNRAS.458..963P,
Cat. J/MNRAS/458/963). M20 is the normalized second-order moment of
the 20 per cent brightest pixels, is a parameter that measures how
spatially separated the brightest regions of the given object are.
When the spatial separation, or the spatial variance, of the pixels
ranking among the 20 per cent brightest of pixels are large,
the object has a large M20 measure. If the spatial separation of
the 20 per cent brightest pixels is low, then so is the M20 value
of the object, (see the equatiions 4 and 5 of this section).
Note (4): As explicited in the section 3.2 Sample properties, previous work
has widely utilized the concentration index (CI) as a key metric
to distinguish candidate clusters of stars from point sources
(Adamo et al. 2017ApJ...841..131A 2017ApJ...841..131A; Cook et al. 2019MNRAS.484.4897C 2019MNRAS.484.4897C).
This metric is a measure of the difference in photometry between
circular apertures of different radii (the measure can also be
represented as CIij, where i and j denote the radius in pixels of
circular apertures to be compared), and the choice of 1 pixel and
3 pixel radii circular apertures has been standard practice.
The optimal CI threshold that results with maximal distinguishing
power is then determined using sets of bright and isolated star
clusters and point sources, similar in construction to the current
sample.
Note (5): Referring to the section 3.1 Sample construction, the objects studied
here fall into the standard four classes used to characterize sources
in star cluster candidate catalogues as follows:
1 = Star cluster - single peak, circularly symmetric, but with radial
profile more extended than point source, 192 objects in our sample
2 = Star cluster - same as Class 1, but elongated or asymmetric, 163
objects in our sample
3 = Stellar association - asymmetric, multiple peaks, 158 objects
in our sample
4 = Not a star cluster or stellar association, a broad range of
sources are included in this class (e.g. image artefacts,
background galaxies, individual stars), but for the purposes of
the sample used in this analysis, only point sources stars are
identified, 157 stars are selected across the 17 galaxies studied
here (see table 1 in the section 2 PHANGS-HST Imaging)
--------------------------------------------------------------------------------
History:
From electronic version of the journal
(End) Luc Trabelsi [CDS] 16-Oct-2024