1. Nearby Galaxies: What Next? D. Calzetti (Univ. of Massachusetts) and the LEGUS Team https://legus.stsci.edu/ HUBBLE2020: Hubble’s 25 th Anniversary Symposium STScI, Baltimore, April 20-24, 2015 NGC3344 NGC1566

2. Nearby Galaxies: The Hubble Revolution (a biased view) Enabled resolution and accurate measurements of individual stars in galaxies beyond the MCs and in otherwise crowded environments (MW GCs, MC star forming regions, etc.) -> SFHs, our window into the past Enabled UV measurements of the young structures in external galaxies (stars, clusters, associations, etc.) -> SFRs, our window into the present Enabled detecting the ionized gas component and its complex interrelation with the young stars -> SF feedback and chemical enrichment, our window into the physics Tarantula Nebula in the LMC (Sabbi et al.2014) 28 25 Ultra-faint Dwarfs, Brown et al. 2014

3. Nearby Galaxies: Still On-going… A number of studies on: – How is star formation structured within galaxies, where is it located, and how is it connected to its fuel (gas) – How does recent SFH distributes relative to past SFH – How is star formation related to the dynamical structures, and how this informs galaxy evolution – Whether one or more `modes’ of star formation exist within and among galaxies – Whether the stellar IMF is universal or not. -0.8 K. Grasha et al. (LEGUS) – see poster NGC628 55 pc 550 pc non-clustered

4. Star Formation Structures: Stars Self-similar clustering over large scales (~2.5 kpc) -> ring Structure sizes ~130 pc (similar to MCs) ~40% of young stars are not part of structures Structures dispersion ~60 Myr (shorter than MCs) U UV-U NGC6503, ~5 Mpc Thilker et al. (LEGUS) NGC6503, deprojectedHierarchy of clustering Gouliermis et al. (LEGUS)

5. Star Formation Structures: Clusters Goddard et al.; Silva-Villa et al. 2011, Adamo et al., Mora et al.  = Cluster Formation Rate/SFR Cook et al. 2012 (37 dwarfs) Careful accounting of star clusters, esp. in dwarfs, is key to test model predictions for cluster formation, e.g., whether clusters form at the density peaks of the ISM NGC2366

6. Star Formation Structures: Clusters Goddard et al.; Silva-Villa et al. 2011, Adamo et al., Mora et al.  = Cluster Formation Rate/SFR Cook et al. 2012 (37 dwarfs) We still need to perform that careful accounting! (strong synergy with other data) NGC2366 ACS footprints H  image

7. Recent and Past SFHs UGC5139, 3.9 Mpc Cignoni et al. (LEGUS) SFH (10 -> 0.1 Gyr) SFR(UV) SFR(H  ) Dalcanton+2011 SFH (0.1 -> 0 Gyr)

8. SFHs and SFRs SFRs are based on assumption of constant SFH over the relevant luminosity timescale SFR(UV) over ~100 Myr SFR(H  ) over ~6-7 Myr UGC5139, 3.9 Mpc Current SFR in UGC5139 is consistent with ~one 10 4 Mo cluster (or several lower mass ones) having recently formed. Decrease in recent-past SFR implies: SFR(UV) ~ 2-3 SFR(H  )

9. Outskirts of Galaxies: The New Frontier Galaxies are far more extended than their bright disks, in stellar populations and gas content. Regions with extreme conditions in terms of density, pressure, metal enrichment, dust content, response to feedback. Outskirts are dynamically `quiet’: imprints of structures persist for many Gyrs. Need to train Hubble (and JWST) on these extreme environments E.g., they can test predictions of  CDM galaxy evolution models Very little HST data exist on the outskirts of nearby galaxies, combination of efficiency and `competition’ Thilker et al. 2007 M83; 4.5 Mpc; GALEX+HI WFC3 FoV

10. Outer Disk Star Formation Low-density conditions: – resolve high mass (UV-optical) stars and young clusters with HST and – low mass stars (optical-IR) with JWST out to ~ 10-15 Mpc Hess diagrams to investigate recent and past SFHs, age and mass gradients and structures. Dynamically quiet environments can test: – multiple modes of star formation; – effects of internal processes in star cluster evolution. Test-beds for the universality of the IMF, beyond integrated-light comparisons (e.g., Koda et al. 2012) NGC1512+1510 (GALEX); ~10 Mpc

11. Tests of Galaxy Growth  CDM models = galaxy growth through multiple minor mergers (Penarrubia et al. 2006). Imprints remain in the outskirts of galaxies (Bullock & Johnston 2005). Outer disk of M31 consistent with this picture (Brown et al. 2008; Martin-Delgado et al. 2008; Ibata et al. 2014). Inside-out star formation (Roskar et al. 2008) observed in: breaks in exponential profiles and blue outskirts of HI-rich disk galaxies (Wang et al. 2011). About 20% of local galaxies have XUV disks (Lemonias et al. 2011 ). Minor mergers and inside-out star formation predict different outskirts population mixes. M31 CMDs; Brown et al. 2008 NGC5055 Thilker et al.

12. Massive Stars IMF The upper end of the stellar IMF impacts: – SFRs at all cosmic distances – Energy input into the ISM/IGM (feedback/outflows) – Metal enrichment Lee et al. 2009 Dwarf Galaxies Massive Galaxies Problems: 1.SFH/IMF degeneracy 2.Loss of ionizing photons

13. Massive Stars IMF - 2 Andrews, DC, et al. 2013, 2014 Log [L(H  )/M cluster] Log M cluster Universal IMF Top-Light IMF The universal IMF is preferred at the 2-  level only. Need to get tighter constraints using a larger number of star clusters in dwarf galaxies. The data show average values for star clusters in the listed galaxies. Crucial range

14. Conclusions Hubble’s `work’ for nearby galaxies is not done yet. Superb angular resolution with UV capabilities can still yield answers to: The physics of star formation in extreme environments; The universality of the stellar IMF; Galaxy growth models. This will provide a foundation for future studies of both nearby and distant galaxies with JWST.