Draft version February 21, 2018 Preprint typeset using LATEX style AASTeX6 v. 1.0 STAR FORMATION HISTORIES OF THE LEGUS DWARF GALAXIES (I): RECENT HISTORY OF NGC 1705, NGC 4449 AND HOLMBERG II M. Cignoni1;2;3, E. Sacchi3;4, A. Aloisi5, M. Tosi3, D. Calzetti6, J. C. Lee5;7, E. Sabbi5, A. Adamo8, D. O. Cook19, D. A. Dale9, B. G. Elmegreen10, J.S. Gallagher III18, D. A. Gouliermis11;12, K. Grasha6, E. K. Grebel13, D. A. Hunter14, K. E. Johnson17, M. Messa8, L. J. Smith15, D. A. Thilker16, L. Ubeda5 and B.C. Whitmore5 1Dipartimento di Fisica, Universit`adi Pisa, Largo Bruno Pontecorvo, 3, 56127 Pisa, Italy 2INFN, Sezione di Pisa, Largo Pontecorvo 3, 56127 Pisa, Italy 3INAF{Osservatorio di Astrofisica e Scienza dello Spazio, Via Gobetti 93/3, I-40129 Bologna, Italy 4Dipartimento di Fisica e Astronomia, Universit`adegli Studi di Bologna, Via Gobetti 93/2, I-40129 Bologna, Italy 5Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA 6Department of Astronomy, University of Massachusetts { Amherst, Amherst, MA 01003, USA 7Visiting Astronomer, Spitzer Science Center, Caltech, Pasadena, CA, USA 8Department of Astronomy, The Oskar Klein Centre, Stockholm University, Stockholm, Sweden 9Department of Physics and Astronomy, University of Wyoming, Laramie, WY 10IBM Research Division, T.J. Watson Research Center, Yorktown Hts., NY 11Zentrum f¨urAstronomie der Universit¨atHeidelberg, Institut f¨urTheoretische Astrophysik, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany 12Max Planck Institute for Astronomy, K¨onigstuhl17, 69117 Heidelberg, Germany 13Astronomisches Rechen-Institut, Zentrum f¨urAstronomie der Universit¨atHeidelberg, M¨onchhofstr. 12-14, 69120 Heidelberg, Germany 14Lowell Observatory, Flagstaff, AZ 15European Space Agency/Space Telescope Science Institute, Baltimore, MD 16Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD 17Department of Astronomy, University of Virginia, Charlottesville, VA 18 Dept. of Astronomy, University of Wisconsin{Madison, Madison, WI 19 California Institute of Technology, Pasadena, CA ABSTRACT We use HST 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 uncer- tainties related to stellar evolution modelling. Irrespective of the adopted stellar models, all the 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 the overall SFR, the timing of the most recent peak and the SFR=area. The Initial Mass Function (IMF) 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 upper main sequence and arXiv:1802.06792v1 [astro-ph.GA] 19 Feb 2018 helium burning stars, which is not apparent in the data. Since differential reddening, significant in NGC 4449, or unresolved binaries don't appear to be sufficient to fill the gap, we suggest this calls for a revision of both sets of stellar evolutionary tracks. Keywords: stellar evolution - star forming region, galaxies: stellar content 1. INTRODUCTION namics presumably plays a minor role in regulating their Dwarf galaxies are important. Given the ubiquity of SF. Moreover, whilst massive galaxies have formed most of their stars in the first 3 Gyr, low mass galaxies have low metallicity galaxies at high redshift, understanding ∼ how star formation (SF) proceeds in such regimes is cru- been forming stars over the whole of cosmic time. This cial for cosmological purposes. Dwarf galaxies lack spiral results in a variety of star formation histories (SFH, see density waves and differential rotation, hence, global dy- e.g. Tolstoy et al. 2009; Weisz et al. 2011; Gallart et 2 al. 2015; McQuinn et al. 2011) and specific star forma- found that starbursts occur in 20% of the TNT isolated tion rates (sSFR), ranging from nearly inactive, as the pairs, compared to only 6% of the matched isolated single transition-type dwarf Leo T, to extremely active, as the dwarfs, suggesting that dwarf-dwarf interactions play a blue compact dwarfs (BCDs). key role in triggering starbursts in dwarfs. Interestingly, Potential mechanisms for triggering SF in dwarfs can the final coalescence stage of the dwarf-dwarf merger is be broadly categorized as either internally-driven or not required to produce starbursts, but rather they are externally-driven events. From the internal point of view, triggered at earlier stages of the interaction. Finally, col- intrinsic processes such as stochastic self-propagating SF lisions between dwarfs and satellites predominantly star- (Gerola et al. 1980) have certainly a role, in particular free, or even dark, could explain the strong bursts of at the low mass end where single star forming events SF that in some dwarfs occur without apparent cause can change the galaxy luminosity significantly. However, (Starkenburg et al. 2016). Indeed, HI clouds with no BCDs appear to be different from non-starbursting dwarf evident optical counterpart have been found around sev- irregulars in terms of their HI and internal dynamics. eral BCDs (Ramya et al. 2009), suggesting that infall of The higher central rotation-velocity gradients of BCDs such gas clouds is not an unlikely event. suggests a relation between the intense activity and the In this framework, quantitative information on the gravitational potential (Lelli et al. 2012). Moreover, de- SFH in dwarf galaxies (see e.g. Weisz et al. 2011) is fun- spite the isolation, BCDs show star forming regions kine- damental in order to understand the connection between matically decoupled from the rest of the galaxy (Koleva the observed activity and processes such as merging, ac- et al. 2014). Elmegreen et al.(2012) suggested that gi- cretion, and interaction. The SFH likely keeps a record ant star forming clumps in dwarf irregular galaxies could of the past interactions. Detailed high resolution SFH in-spiral towards the center, feeding intense star forma- studies may also provide invaluable hints on how star- tion. This is because in such small galaxies, star forming forming regions evolve and what turns off SF, from the clumps could be sufficiently massive to exceed a few per- smallest to galactic-size scales, and how these processes cent of the galaxy mass enclosed inside their orbital radii, depend on dynamical effects. therefore producing dynamically significant torques on The purpose of this paper is to quantify the most recent dark matter halo particles, halo stars, and the surround- SFH of individual galaxies using UV Color-Magnitude ing disk to lose their angular momentum with a timescale Diagrams (CMDs) of their resolved stars. While all stud- of 1 Gyr. Other proposed internal mechanisms are the ies of SFH performed so far (e.g. Tolstoy et al. 2009 and effect of massive triaxial dark haloes, promoting the mi- references therein, Weisz et al. 2011) aim at covering the gration of a significant fraction of gas from the periphery longest possible look-back time and infer the SFH back to the center of the dwarf (Bekki & Freeman 2002), or to the earliest epochs, in this case we aim at resolving bars made of dark matter (Hunter & Elmegreen 2004). in higher detail the most recent epochs. Here we exploit External processes must be also important. If Cold the Legacy ExtraGalactic UV Survey (LEGUS; Calzetti Dark Matter (CDM) is scale-free as predicted, groups of et al. 2015) to study at high temporal resolution the most dwarf galaxies are a natural expectation on small mass recent past of three actively star-forming dwarf galaxies, scales and dwarf galaxies could be formed through the NGC 1705, NGC 4449 and Holmberg II (hereafter Ho II, same physical process as massive galaxies. While dwarfs also known as UGC 4305). All three dwarfs have their accreting dwarfs have been observed in at least one case, optical SFH already derived by others (see e.g., Annibali DDO 668 (Annibali et al. 2016; Sacchi et al. 2016), dwarf et al. 2003; Weisz et al. 2008; McQuinn et al. 2010), but groups have been recently observed by Stierwalt et al. our study is complementary to theirs since it deals only (2017) using the panchromatic TiNy Titans (TNT; Stier- with the last 100-200 Myr but with the higher temporal walt et al. 2015) survey, a systematic study of SF in in- resolution achieved with UV photometry. Moreover, we teracting dwarf galaxies. According to this study, the derive the SFHs adopting two different and independent interaction between dwarf galaxies could be quantita- sets of isochrones, to estimate the uncertainties result- tively different as compared to more massive counter- ing from different approaches and input physics (rota- parts. In fact, both paired dwarfs and paired massive tion, mixing length, amount of overshooting, etc.) of the galaxies show enhanced SF out to separations of 100 stellar models. We also take advantage of the higher ca- ∼ kpc (e.g. Patton et al. 2013; Stierwalt et al. 2015), but pability of separating main-sequence (MS) and post-MS the effect in dwarfs is stronger by a factor of 1.3 and in- stars in UV CMDs to evaluate how well the evolutionary volves a larger fraction of the virial radius. Dwarfs are models are able to reproduce the observed properties of therefore more globally affected by small companions.