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Star Formation Histories of the LEGUS Dwarf Galaxies. I. Recent History of NGC 1705, NGC 4449, and Holmberg II

Cignoni, M. and Sacchi, E. and Aloisi, A. and Tosi, M. and Calzetti, D. and Lee, J. C. and Sabbi, E. and Adamo, A. and Cook, D. O. and Dale, D. A. and Elmegreen, B. G. and Gallagher, J. S., III and Gouliermis, D. A. and Grasha, K. and Grebel, E. K. and Hunter, D. A. and Johnson, K. E. and Messa, M. and Smith, L. J. and Thilker, D. A. and Ubeda, L. and Whitmore, B. C. (2018) Star Formation Histories of the LEGUS Dwarf Galaxies. I. Recent History of NGC 1705, NGC 4449, and Holmberg II. Astrophysical Journal, 856 (1). Art. No. 62. ISSN 1538-4357. doi:10.3847/1538-4357/aab041.

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We use Hubble Space Telescope observations from the Legacy Extragalactic UV Survey to reconstruct the recent star formation histories (SFHs) of three actively star-forming dwarf galaxies, NGC 4449, Holmberg II, and NGC 1705, from their UV color–magnitude diagrams (CMDs). We apply a CMD fitting technique using two independent sets of stellar isochrones, PARSEC-COLIBRI and MIST, to assess the uncertainties related to stellar evolution modeling. Irrespective of the adopted stellar models, all three dwarfs are found to have had almost constant star formation rates (SFRs) in the last 100–200 Myr, with modest enhancements (a factor of ~2) above the 100 Myr averaged SFR. Significant differences among the three dwarfs are found in terms of the overall SFR, the timing of the most recent peak, and the SFR/area. The initial mass function of NGC 1705 and Holmberg II is consistent with a Salpeter slope down to ≈5 M ⊙, whereas it is slightly flatter, s = −2.0, in NGC 4449. The SFHs derived with the two different sets of stellar models are consistent with each other, except for some quantitative details, attributable to their input assumptions. They also share the drawback that all synthetic diagrams predict a clear separation in color between the upper main-sequence and helium-burning stars, which is not apparent in the data. Since neither differential reddening, which is significant in NGC 4449, nor unresolved binaries appear to be sufficient to fill the gap, we suggest this calls for a revision of both sets of stellar evolutionary tracks.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Cignoni, M.0000-0001-6291-6813
Sacchi, E.0000-0001-5618-0109
Aloisi, A.0000-0003-4137-882X
Tosi, M.0000-0002-0986-4759
Calzetti, D.0000-0002-5189-8004
Lee, J. C.0000-0002-2278-9407
Sabbi, E.0000-0003-2954-7643
Adamo, A.0000-0002-8192-8091
Cook, D. O.0000-0002-6877-7655
Dale, D. A.0000-0002-5782-9093
Elmegreen, B. G.0000-0002-1723-6330
Gallagher, J. S., III0000-0001-8608-0408
Gouliermis, D. A.0000-0002-2763-0075
Grasha, K.0000-0002-3247-5321
Grebel, E. K.0000-0002-1891-3794
Hunter, D. A.0000-0002-3322-9798
Johnson, K. E.0000-0001-8348-2671
Messa, M.0000-0003-1427-2456
Smith, L. J.0000-0002-0806-168X
Thilker, D. A.0000-0002-8528-7340
Alternate Title:Star Formation Histories of the LEGUS Dwarf Galaxies (I): recent History of NGC1705, NGC4449 and Holmberg II
Additional Information:© 2018. The American Astronomical Society. Received 2017 October 2; revised 2018 February 1; accepted 2018 February 14; published 2018 March 26. We warmly thank Kristy McQuinn for helpful discussions and suggestions. M.C. and M.T. acknowledge funding from the INAF PRIN-SKA 2017 program D.A.G. kindly acknowledges financial support by the German Research Foundation (DFG) through programme GO 1659/3-2. B.G.E. acknowledges support from NASA grant HST-GO-13364.14-A.
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Istituto Nazionale di Astrofisica (INAF)
Deutsche Forschungsgemeinschaft (DFG)GO 1659/3-2
NASANAS 5-26555
Subject Keywords:galaxies: dwarf; galaxies: formation; galaxies: starburst; galaxies: stellar content; Hertzsprung–Russell and C–M diagrams ; stars: massive
Issue or Number:1
Record Number:CaltechAUTHORS:20180411-150615795
Persistent URL:
Official Citation:M. Cignoni et al 2018 ApJ 856 62
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:85769
Deposited By: George Porter
Deposited On:11 Apr 2018 23:05
Last Modified:15 Nov 2021 20:32

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