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LEGUS and H_α-LEGUS Observations of Star Clusters in NGC 4449: Improved Ages and the Fraction of Light in Clusters as a Function of Age

Whitmore, Bradley C. and Chandar, Rupali and Lee, Janice and Ubeda, Leonardo and Adamo, Angela and Aloisi, Alessandra and Calzetti, Daniela and Cignoni, Michele and Cook, David and Dale, Daniel and Elmegreen, B. G. and Gouliermis, Dimitrios and Grebel, Eva K. and Grasha, Kathryn and Johnson, Kelsey E. and Kim, Hwihyun and Sacchi, Elena and Smith, Linda J. and Tosi, Monica and Wofford, Aida (2020) LEGUS and H_α-LEGUS Observations of Star Clusters in NGC 4449: Improved Ages and the Fraction of Light in Clusters as a Function of Age. Astrophysical Journal, 889 (2). Art. No. 154. ISSN 1538-4357. https://resolver.caltech.edu/CaltechAUTHORS:20200624-104211145

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Abstract

We present a new catalog and results for the cluster system of the starburst galaxy NGC 4449, based on multiband imaging observations taken as part of the LEGUS and H_α-LEGUS surveys. We improve the spectral energy fitting method used to estimate cluster ages, and find that the results, particularly for older clusters, are in better agreement with those from spectroscopy. The inclusion of H_α measurements, the role of stochasticity for low-mass clusters, the assumptions about reddening, and the choices of SSP model and metallicity all have important impacts on the age dating of clusters. A comparison with ages derived from stellar color–magnitude diagrams for partially resolved clusters shows reasonable agreement, but large scatter in some cases. The fraction of light found in clusters relative to the total light (i.e., T_L) in the U, B, and V filters in 25 different ≈ kiloparsec-size regions throughout NGC 4449 correlates with both the specific region luminosity, R_L , and the dominant age of the underlying stellar population in each region. The observed cluster age distribution is found to decline over time as dN/dτ ∝ τ^γ , with γ = −0.85 ± 0.15, independent of cluster mass, and is consistent with strong, early cluster disruption. The mass functions of the clusters can be described by a power law with dN/dM ∝ M^β and β = −1.86 ± 0.2, independent of cluster age. The mass and age distributions are quite resilient to differences in age-dating methods. There is tentative evidence for a factor of 2–3 enhancement in both the star and cluster formation rate ≈100–300 Myr ago, indicating that cluster formation tracks star formation generally. The enhancement is probably associated with an earlier interaction event.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.3847/1538-4357/ab59e5DOIArticle
https://arxiv.org/abs/2005.02840arXivDiscussion Paper
ORCID:
AuthorORCID
Whitmore, Bradley C.0000-0002-3784-7032
Chandar, Rupali0000-0003-0085-4623
Lee, Janice0000-0002-2278-9407
Adamo, Angela0000-0002-8192-8091
Aloisi, Alessandra0000-0003-4137-882X
Calzetti, Daniela0000-0002-5189-8004
Cignoni, Michele0000-0001-6291-6813
Cook, David0000-0002-6877-7655
Dale, Daniel0000-0002-5782-9093
Elmegreen, B. G.0000-0002-1723-6330
Gouliermis, Dimitrios0000-0002-2763-0075
Grebel, Eva K.0000-0002-1891-3794
Grasha, Kathryn0000-0002-3247-5321
Johnson, Kelsey E.0000-0001-8348-2671
Kim, Hwihyun0000-0003-4770-688X
Sacchi, Elena0000-0001-5618-0109
Smith, Linda J.0000-0002-0806-168X
Tosi, Monica0000-0002-0986-4759
Wofford, Aida0000-0001-8289-3428
Alternate Title:LEGUS and Hα-LEGUS Observations of Star Clusters in NGC 4449: Improved Ages and the Fraction of Light in Clusters as a Function of Age, LEGUS and Halpha-LEGUS Observations of Star Clusters in NGC 4449: Improved Ages and the Fraction of Light in Clusters as a Function of Ageb
Additional Information:© 2020. The American Astronomical Society. Received 2019 June 10; revised 2019 October 14; accepted 2019 October 24; published 2020 February 3. This work is based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 526555. These observations are associated with program #13364. R.C. acknowledges support from NSF grant 1517819. M.C. and M.T. acknowledge support from the INAF PRIN-SKA 2017 program 1.05.01.88.04. We thank Francesca Annibali for useful discussions. We also thank the referee for many useful comments that improved the paper. Facility: HST. -
Group:Infrared Processing and Analysis Center (IPAC)
Funders:
Funding AgencyGrant Number
NASANAS-526555
NSF1517819
Istituto Nazionale di Astrofisica (INAF)1.05.01.88.04
Subject Keywords:Galaxy stellar content ; Dwarf irregular galaxies ; Star clusters ; Young massive clusters ; Globular star clusters
Issue or Number:2
Classification Code:Unified Astronomy Thesaurus concepts: Galaxy stellar content (621); Dwarf irregular galaxies (417); Star clusters (1567); Young massive clusters (2049); Globular star clusters (656)
Record Number:CaltechAUTHORS:20200624-104211145
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20200624-104211145
Official Citation:Bradley C. Whitmore et al 2020 ApJ 889 154
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:103994
Collection:CaltechAUTHORS
Deposited By: George Porter
Deposited On:24 Jun 2020 21:52
Last Modified:24 Jun 2020 21:52

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