University of Massachusetts Amherst ScholarWorks@UMass Amherst
Astronomy Department Faculty Publication Series Astronomy
2010 The evolution of stellar structures in dwarf galaxies N. Bastian
D. R. Weisz
E. D. Skillman
K. B.W. Mc Quinn
A E. Dolphin
See next page for additional authors
Follow this and additional works at: https://scholarworks.umass.edu/astro_faculty_pubs Part of the Astrophysics and Astronomy Commons
Recommended Citation Bastian, N.; Weisz, D. R.; Skillman, E. D.; Mc Quinn, K. B.W.; Dolphin, A E.; and Gutermuth, R. A., "The ve olution of stellar structures in dwarf galaxies" (2010). Astronomy Department Faculty Publication Series. 116. 10.1111/j.1365-2966.2010.17841.x
This Article is brought to you for free and open access by the Astronomy at ScholarWorks@UMass Amherst. It has been accepted for inclusion in Astronomy Department Faculty Publication Series by an authorized administrator of ScholarWorks@UMass Amherst. For more information, please contact [email protected]. Authors N. Bastian, D. R. Weisz, E. D. Skillman, K. B.W. Mc Quinn, A E. Dolphin, and R. A. Gutermuth
This article is available at ScholarWorks@UMass Amherst: https://scholarworks.umass.edu/astro_faculty_pubs/116 arXiv:1010.1837v1 [astro-ph.CO] 9 Oct 2010 o.Nt .Ato.Soc. Astron. R. Not. Mon. cetd eevd noiia form original in Received; Accepted. eqatfidadtereouintae safnto of function a as traced evolution ca structures their which and with developed quantified the techniques we statistical be in works here- of these distribution (2009; series In stellar respectively. a al. LMC, the et et and investigated Bastian Gieles SMC we and which In B09) over G08) disperse. after timescale hereafter galaxies the (2008; dwarf measure al. in to structures is hierar- stellar goa work clustered, the and present a galaxies, in the within born distribution fractal be or to chical observed are stars Young INTRODUCTION 1 .Bastian galaxies N. dwarf in structures stellar of evolution The ..Gutermuth R.A. .Walter F. c 8 7 6 5 4 3 2 1 a-lnkIsiu ¨rAtooi,Koisul1,D-6 K¨onigstuhl 17, f¨ur Astronomie, Max-Planck-Institut nttt fAtooy nvriyo abig,Madingle Cambridge, of University Astronomy, of Institute eateto srnm,Uiest fMsahsts Amh Massachusetts, of University Astronomy, of Department ieCleeAtooyDprmn,SihClee Northam College, Smith Department, Astronomy College Five eateto hsc srnm,Mclse olg,160 College, Macalester 85756 Astronomy, AZ & Tucson, Physics Rd, of Hermans Department E 1151 Company, Raytheon srnm eatet nvriyo inst,Minneapol Minnesota, of University Department, Astronomy colo hsc,Uiest fEee,SokrRa,Exet Road, Stocker Exeter, of University Physics, of School 09RAS 2009 8 1 ..Weisz D.R. , 000 5 , 6 ..Cannon J.M. , –2(09 rne 2Otbr21 M L (MN 2010 October 12 Printed (2009) 1–12 , place udeso y,wihw uniyuigtetopitcorrelation two-point the using quantify dispers we o Q structures which degree These Myr, and high scales. groups of parsec clusters, a hundreds of of o with distribution hundreds continuous agre galaxies tip to a in within parsec the of find, born up on We made are th is time. based stars which can with galaxies that we evolves studies, ranges the stars vious age these of different of subset in distribution a stars spatial selecting for By stars main-sequence. OB of population M ( t SMC long), this least and at for survives iulsaswt tla scrns diinly eslc eyyo very a select we Additionally, ma isochrones. absolute stellar the with comparing st by stars Burning age-dated Helium vidual unambiguously Blue select be we can CMDs, dia which obtained colour-magnitude deep the composite use Using galaxy. We create each and age. stars population individual the resolve of Surveys function for a Camera Telescope/Advanced as galaxies dwarf within e words: Key t between ABSTRACT G 8,NC26,NC4068) NGC 2366, NGC 784, NGC prmtrdvlpdb atrgt&Wiwrh(04.O galac On (2004). Whitworth & Cartwright by developed -parameter 2 epeetasuyo h aito fsailsrcueo stellar of structure spatial of variation the of study a present We ∼ ..Skillman E.D. , oe limits lower ∼ 5 y DO15.Ti ssmlrt htw aefudpeiul for previously found have we what to similar is This 165). (DDO Myr 350 0Mr n h M ( LMC the and Myr) 80 evo n h uioiyo h otgalaxy. host the of luminosity the and aais-daf aais-idvda DO15 Holmberg 165, (DDO individual - galaxies dwarf; - galaxies 4 .Ercolano B. , ntetm ttkst eoeteoiia tutr ie,structur (i.e., structure original the remove to takes it time the on 17Hiebr,Germany Heidelberg, 9117 of l od abig B H,UK 0HA, CB3 Cambridge Road, y rE44L UK 4QL, EX4 er n s N545 USA 55455, MN is, rt as 10 USA 01002 Mass. erst, tn as 16 USA 01063 Mass. pton, rn vne t al N55125 MN Paul, St. Avenue, Grand 0 2 ...McQuinn K.B.W. , akruddsrbto y7 y n 7 y,respec- tim crossing Myr, the 175 to (t comparable and are Myr timescales These 75 th tively. by from indistinguishable distribution substruc- being background stellar gradually, LMC erased and was SMC the ture for that found We time. nw sifn otlt,ie,cutrepnindet th to due (also expansion clusters cluster star i.e., popping mortality, infant as structure, as such of known effects removal local the galac- than for general rather responsible that conclusion were the dynamics to tic led which galaxy the of cross ∼ evo 7 y) ed o n n togcorrelations strong any find not do We Myr). 175 = hc aisbetween varies which , 1 .Gieles M. , R galaxy mgn fnab wr aaisi re to order in galaxies dwarf nearby of imaging A T E /σ tl l v2.2) file style X where 7 σ ..Kennicutt R.C. , 2 steselrvlct disperison) velocity stellar the is ..Dolphin A.E. , ntmsae ftn to tens of timescales on e ∼ 0 y NC2366) (NGC Myr 100 rm CD)for (CMDs) grams nsuyhwthe how study en soitosfrom associations i cls ecan we scales, tic ucinadthe and function n ( ung mn ihpre- with ement ntd findi- of gnitude substructure f ubeSpace Hubble r (BHeBs), ars h young the f populations II . C2574, IC , 0Myr) 10 3 7 , , the e e e e 2 Bastian et al. loss of residual gas left over from the star-formation process Schmeja & Klessen 2006; Koenig et al. 2008, Kraus & Hille- - Boily & Kroupa 2003; Bastian & Goodwin 2006). brand 2008) with many regions appearing centrally concen- One of the major limitations of the previous works was trated and relaxed while others are still largely fragmentary. in defining samples of nearly coeval stars (i.e. stars of simi- On intermediate spatial scales (1−100pc) extragalactic stud- lar age). Previously, we selected groups of stars from regions ies on objects like NGC 346 in the SMC have revealed that in the colour-magnitude diagram along the main-sequence star-forming regions are made up of many individual clus- (MS) and then carried out simulations to estimate the mean ters and also distributed star-formation (e.g., Sabbi et al. age and spread of stars within that region of space in the 2007; Schmeja, Gouliermis, & Klessen 2009). On galactic colour-magnitude diagram (CMD). This resulted in a nearly scales, it appears that there is not a characteristic size for coeval population only for the youngest ages, whereas re- star- forming regions, and that there exists a continuous hi- gions further down the CMD were made up of older stars erarchy of structure from pc to kpc scales (Elmegreen & as well as some contribution of younger stars. Our results Elmegreen 2001, Elmegreen et al. 2006; Bastian et al. 2005, were tested against the evolution of the distribution of stel- 2007, 2009; G08; Odekon 2008). The framework that is de- lar clusters with known ages, and overall we found good veloping is that stars form in a highly structured distribu- agreement. tion on all scales probed so far, and that these structures The post-MS evolutionary phase of young (.1 Gyr) are erased on approximately the crossing time, regardless of intermediate mass (& 3 M⊙) stars provides an alternative scale. method of age dating that avoids the age degeneracies of MS The number of nearby dwarf galaxies with high quality stars. Of particular importance are core blue helium burning optical imaging available has significantly increased in the stars (BHeBs) which have a one-to-one relationship between past decade, thanks to a number of Hubble Space Telescope age and luminosity (e.g., Bertelli et al. 1994), making it pos- survey programs (e.g., Tully et al. 2006, Weisz et al. 2008, sible to assign ages to individual BHeBs on an optical CMD. Dalcanton et al. 2009). The excellent spatial resolution af- The short lifetimes of BHeBs, ∼ 10% of their MS lifetimes forded by HST allows for precise photometry and age dat- (e.g., Chiosi, Bertelli, & Bressan 1992), allow for a clear iden- ing of individual BHeBs. In this paper, we have selected six tification of coeval populations with minimal confusion be- star forming dwarf galaxies in the Local Volume (Weisz et tween stars of different ages (Dohm-Palmer et al. 1997). For al. 2008, McQuinn et al. 2010) in which we will explore the a given age, BHeBs are ∼ 2 magnitudes brighter compared evolution of stellar structure using a statistically significant to MS stars, thus providing higher photometric precision and number of BHeBs. longer look back times (∼ 500 Myr; e.g., Dohm-Palmer et This paper is organised as follows. In § 2 we introduce al. 1997, Weisz et al. 2008). Although there are fewer BHeBs the observations used in the present study. The technique than MS stars, the precise age-dating, possible with BHeBs, of age-dating BHeB stars is discussed in § 3 and in § 4 we compensates for the reduction in numbers and provides a introduce the methods used to quantify the evolution of spa- more secure final sample of stars. In those cases where the tial structures along with their caveats. The results for each photometry is sufficiently deep, the star formation histories individual galaxy are given in § 5 and in § 6 we summarise derived from the BHeB stars are consistent with those de- and discuss our main findings. rived from the main sequence stars (e.g., Dohm-Palmer et al. 2002). Imaging of resolved stellar populations, specifically 2 OBSERVATIONS AND PHOTOMETRY BHeBs, in nearby star forming dwarf galaxies, i.e., dwarf irregular galaxies (dIrrs), could reveal fundamental pro- Observations of all galaxies were taken with the Advanced cesses driving the evolution of stellar structures. Unlike Camera for Surveys (ACS, Ford et al. 1998) aboard HST. more massive galaxies, dIrrs are close to solid body rotators DDO 165, Holmberg II, NGC 2366, and IC 2574 were (e.g., Skillman et al. 1988, Skillman 1996), implying that the observed in the F555W and F814W filters as part of a locations of stars are not significantly disturbed by external survey of the M81 Group dwarf galaxies (GO-10605 - mechanisms such as sheer or spiral density waves. Combined Weisz et al. 2008; also GO-9755, PI Walter, was used with precise ages of BHeBs, the dynamic simplicity of dIrrs for IC 2574), while NGC 4068 (GO-9771 - Karachentsev may provide new insight into the intrinsic evolution of stellar & Kashibadze 2006) and NGC 784 (GO-10210 - Tully et structures. al. 2006) were both observed in the F606W and F814W fil- Ideally, one would like to study not only the timescale ters. The data we use in this paper have been processed as over which stellar structures are erased, but also how this de- described in Weisz et al. (2008) and McQuinn et al. (2010). pends on spatial scale. Sub-cluster (< 1 pc) structures are Here, we briefly summarize the data reduction process. erased quickly (Goodwin & Cartwright 2004) while larger Images were all downloaded from the HST archive fol- structures take more time to disperse. The exact dependence lowing standard reduction and processing with the HST of the structural evolutionary timescale and the spatial scale pipeline. To ensure uniformity among our data, photome- probed will be essential in confirming the cause of the evo- try of all images was performed using DOLPHOT1, a ver- lution. sion of HSTPHOT optimized for the ACS (Dolphin 2000). There have been many recent studies on quantifying Cosmic rays, hot pixel residuals, and extended objects were the distribution of point-like (i.e., stellar) structures on a all rejected based on their brightness profiles. The remain- variety of physical scales. Near- and mid-IR studies of star- ing objects were subjected to further photometric cuts. forming regions on scales of 0.1−10 pc have revealed a wealth of structural information (Lada & Lada 2003; Cartwright & Whitworth 2004; Gutermuth et al. 2005, 2008, 2009; 1 http://purcell.as.arizona.edu/dolphot/