Surveys of Nearby Galaxies – What Next? HST-ERS Daniela Calzetti (UMass) and the LEGUS Team (Angela Adamo, Göran Östlin, Matteo Messa) Surveys for All, Lund Observatory (Sweden), 1-2 February 2016
The LEGUS Team Red for Senior Advisory Group Blue for Science, Data Processing, EPO Leads 56 investigators (so far) at 30+ Institutions (US+EU): D. Calzetti (PI, UMass), J. Lee (Deputy PI, STScI), J. Andrews (U Arizona), A. Aloisi, S.N. Bright, T. Brown, C. Christian, M. Cignoni, K. Levay, M. Regan, E. Sabbi, L. Ubeda, B. Whitmore (STScI) , A. Adamo, M. Messa, G. Östlin (Stockholm U), R. Chandar (Utoledo), G. Clayton (LSU), D. Cook, D. Dale (U Wyoming), R. da Silva, M. Krumholz (UCSC), S. de Mink (Amsterdam U), C. Dobbs (UExeter), B. Elmegreen (IBM), D. Elmegreen (Vassar), A. Evans, K. Johnson (UVa), M. Fumagalli (U Durham), J. Gallagher, J. Ryon (UWisc), D. Gouliermis (MPIA), K. Grasha (UMass), E. Grebel, F. Shabani (Heidelberg U), A. Herrero, S. Taibi (IAC, Canarias), D. Hunter (Lowell Obs), L. Kahre, R. Walterbos (NMSU), R. Kennicutt (IoA, Cambridge), H. Kim (UT-Austin), D. Lennon (ESA), C. Martin, S. van Dyk (Caltech), P. Nair (U Alabama), A. Nota, L. Smith (STScI/ ESA), A. Pellerin (SUNY-Geneseo), J. Prieto (UC de Chile), D. Schaerer (Geneva Obs), D. Schiminovich (Columbia U), D. Thilker (JHU), M. Tosi, E. Sacchi (INAF-Ubologna), A. Wofford (IAP)
Madau+1996 Across Time… Cosmic SFR Madau & Dickinson 2014, ARAA We are currently in a position to describe to some accuracy the evolution of SFR and mass assembly across cosmic times… Mass Assembly
Across `Space’? Image kindly provided by D. Thilker We can’t yet connect the two scales of NGC 628, ~10 Mpc, galaxy-wide SF and individual stars/star GALEX FUV+NUV clusters: § How do stars form? Always clustered? In a scale—free hierarchy? (Elmegreen et al. 2006) § Do we have one or two modes of star formation (clustered and diffuse)? (Meurer et al. 1995, Crocker et al. 2014) § How has the mode of SF evolved with time? (10 9 M o clumps at z~1) The über-questions: § How does SF power the ISM? 1. How does the Hubble sequence form? § On what timescale do stars disperse? 2. How is SF linked to the gas supply § What are the bound structures (star (Kennicutt-Schmidt Law)? clusters) tracing? How do they evolve? 3. What is the role of feedback in § Do we have a universal stellar IMF? shaping galaxies and regulating SF? § How are `local’ SFRs affected?
The QuesCons • The Physics of GalacCc-Scale Star FormaCon – Mode(s) of star formaCon – Cluster-SF links – FormaCon and Erasure of structures – Links to dynamical structures, gas structures • The Physical Underpinning of the IMF – Is the IMF Universal? – What are the driving parameters/mechanisms?
The Schmidt-KennicuS Law KennicuS & Evans, 2012, Whole Galaxies Σ SFR ~ ( Σ gas ) γ , γ =1, ..., 2 Σ SFR The value of γ is connected to the underlying physics of the scaling between star formaCon and gas (a.k.a. the SK Law). Σ gas However, the SK Law does not appear to be `scalable’ from whole galaxies to galaxies’ consCtuents (star forming regions, molecular clouds, etc.). Problems with SFR & gas measurements? (C+2012) Changing of the dominant physics? (Hopkins+2013) Heiderman et al. 2010, Milky Way Molecular Clouds
SFR Measures and Stellar Pops Diffusion A common characteristic of local spirals: M101, ~7.5 Mpc GALEX FUV-NUV color maps show that GALEX FUV+NUV interarm regions have redder UV colors than arm regions This cannot be an effect of differential attenuation. Interarm regions in M101 do not contain stars younger than ~40 Myr (or more massive than ~10-15 M o ) (Crocker et al. 2015, HST UV) In starburst galaxies, the intercluster light only Clusters shows evidence for B stars (no O stars, like clusters). IC light = 80% of all UV light. Dispersion of clusters (Tremonti et al. 2001, Intercluster Light Chandar et al. 2005) or two modes of SF (Meurer et al. 1995) ?
12 The RepresentaCve Local Volume Drozdovsky+2008 Ø The Local Universe is over-dense relaCve to the cosmic average, by a factor of a few, up to at least ~10 Mpc. Ø Galaxies within the local 4 Mpc depart from the cosmic SFR for the most recent ~4 Gyr (Williams+2011) Ø The RepresentaCve Local Volume does not occur before 10 Mpc; need to be up to 100 Mpc, if U/LIRGs are included
The LEGUS Project C.+2015a Ø To make progress: investigate the spatially resolved SFHs and cluster formation histories of nearby galaxies with HST. Ø Cycle 21 HST Treasury Program: - UV-to-I imaging of 50 nearby (<18 Mpc) galaxies, targeting star clusters and resolved stars. First public release of Data Products: October 12, 2015: hSps://archive.stsci.edu/prepds/legus/ legus.stsci.edu LEGUS footprint=magenta LEGUS parallels=blue Archival data=red
Star Clusters: Age+Masses via BB Photometry Clusters LFs, Γ , env. variations… 3.15 Mpc 5 th band breaks degeneracy in SED fitting. UV more stable (~4X) than H α for separating young, Q o -deficient from aging clusters, esp. at low cluster masses – add stochastic models Estimated numbers: ~ 30,000 (Krumholz+2015)
UV, Masses, and Ages of Natal Clusters C+2015, ApJ NGC5253 – central nebula Cluster 5, extremely young, and behind dust. A V ~ 1.9 mag. Cluster 11: extremely young; ~3X less massive than earlier results. Still M V =-12.8, at the high-end envelope of the M V -SFR relation for star clusters. Mixed with A V ~50 mag dust. NO NEED for truncated IMF! The two clusters account for ~50% of the ionization in NGC5253. (and ~10 VMSs would account for it all, Smith+subm)
Survival and DissoluCon of Clusters >1200 star clusters idenCfied, divided into: 1. Symmetrical (red), older 2. Elongated (green) 3. MulCple Peaks (blue), youngest Analyze with 2p correlaCon funcCon NGC628, ~10 Mpc – Cluster randomizaCon -0.8 Cmescale at ~ 40 Myr in NGC628 – Mostly due to dissoluCon non-clustered of star clusters K. Grasha+2015, ApJ, in press 50 pc
Bringing it all together (caveat emptor: this is only 3 galaxies) • M101: interarm populaCon consistent with stars being older than 40 Myr (mass <10-15 M o, Crocker+2015 ) • ★ NGC6503: randomizaCon of stars occurs around 60 Myr ( Gouliermis+ 2015 ) • ★ NGC628: star clusters become randomly distributed around 40 Myr ( Grasha+2015 ) Taken all together this would appear to suggest that at least a fracCon of the stars in the field originate from dissolving star clusters over ~40-60 Myr (we already knew of `infant mortality’ at < 10 Myr!)
8 The Realm of JWST KennicuS & Evans 2012 • Address the physical founda/on of the Schmidt-KennicuS Law (scaling between SFR and gas) via YSOs within 1 Mpc and dust- enshrouded HII regions within ~10 Mpc. • InvesCgate the low-end of the IMF via resolved counts out to ~0.5 Mpc (NGC6822). Current evidence gives 2x mass varia/ons for galaxies. • SF Histories of galaxies between 200 Myr and 10 Gyr, out to ~6-8 Mpc for `fossil record’ studies (~a dozen giant spirals, and at most 1 Geha+2013 giant ellipCcal). • Physics of the Bulge mass – BH mass relaCon • Physics of dust processing (e.g., PAHs) out to ~10 Mpc and of dust formaCon (e.g., CCSNe) out to larger distances
10 The Realm of WFIRST (but also GAIA, EUCLID,…) Fossil Galaxy Assembly: § Mergers of small satellites into larger galaxies is one of the main mechanisms of the hierarchical galaxy assembly in Λ CDM models. § The signs of these 10:1 mergers survive for mulCple Gyrs, in the form of streams, tails, bridges, etc. § The stellar populaCons that trace these features can be detected in external galaxies out to ~10-12 Mpc with WFIRST, similarly to what is done today from the ground for the MW/MCs/stream systems Milky Way SimulaCon (Rensselaer/ Benjamin A. WilleS/Heidi Newberg)
23 The Far-Future: Outer Disk Physics • Galaxies are far more extended than their bright disks, in stellar populaCons and gas content. M83; 4.5 Mpc; GALEX+HI Regions with extreme condiCons of • density, pressure, metal enrichment, dust content, response to feedback. • Outskirts are dynamically `quiet’: imprints of structures persist for many Gyrs – testbeds for: - Modes of star formaCon - Upper IMF universality LUVOIR FoV - Star cluster evoluCon and internal (~5x WFC3) processes WFC3 FoV • Key requirements: UV (SFH<200 Myr) +efficiency (FoV+sensiCv.) Thilker et al. 2007
Conclusions 1. A multi-pronged approach is required to understand the spatial (and not only temporal) evolution of stellar populations in galaxies. LEGUS is designed to address all of these, and will attempt to address: 1. Presence of multiple modes of SF 2. Cluster formation and evolution 3. Variations of the high-end of the IMF 4. Calibration of local SFR(UV); (e.g., resolved KS Law) 5. Dynamical evolution of galaxies – links of scales of SF 6. Models of formation of massive stars 2. Preliminary results include: 1. Clusters dissolve and contribute to the field population on timescales beyond the `infant mortality’ one (40-60 Myr vs <10 Myr) 2. `Natal’ clusters can be detected at short (optical and possibly UV) wavelengths. BUT… We need a future UVOIR space facility that will address all of those quesCons across the many environments that may have been present at the Dawn of the Universe
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