Hubble Space Telescope Cycle 21 GO Proposal <ID> Stellar Populations in the Outer Disk of M101 Principal Investigator: Dr. Christopher Mihos Institution: Case Western Reserve University Electronic Mail: [email protected] Scientific Category: RESOLVED STELLAR POPULATIONS Scientific Keywords: Galaxy Disks, Galaxy Formation And Evolution, Resolved Stellar Populations, Spiral Galaxies, Stellar Populations In External Galaxies Instruments: ACS, WFC3 Proprietary Period: 12 Proposal Size: Medium Orbit Request Prime Parallel Cycle 21 42 42 Abstract The outskirts of disk galaxies hold a remarkable range of information on processes driving galaxy evolution. The distribution of age and metallicity in their stellar populations constrains the star formation history of the outer disk, and can help differentiate between models of inside-out disk building via fresh accretion and in-situ star formation versus those where outer disk populations come from interactions or stellar migration moving stars outwards from the inner disk. The nearby spiral galaxy M101 (NGC 5457) provides an ideal opportunity to study the outer disk populations of a giant Sc galaxy in great detail. Our deep imaging of the galaxy has mapped its stellar disk out to nearly 50 kpc (10 disk scale lengths), where we see signatures of diverse stellar populations tracing recent star formation. However, the constraints provided by integrated colors are limited; direct imaging of the stellar populations with HST will provide much stronger constraints on the evolutionary history of the outer disk. We propose ACS imaging of two fields in the outskirts of M101 to study the stellar populations there. One field targets the very blue NE Plume while a second images the redder E Spur. Each primary has a parallel WFC3 pointing that images a nearby blank field, to allow for separation of M101 halo populations from those in the outer disk fields. We will construct resolved stellar CMDs for each field and use the relative distribution of stars in different evolutionary phases to constrain the ages and metallicities of the stellar populations. Using these data, we can recover the star formation history of the outer disk and compare it to models of disk galaxy evolution. Dr. Christopher Mihos : Stellar Populations in the Outer Disk of M101 Investigators: Investigator Institution Country PI& Dr. Christopher Mihos Case Western Reserve University USA/OH CoI Dr. Patrick R. Durrell Youngstown State University USA/OH CoI Dr. Paul Harding Case Western Reserve University USA/OH CoI Dr. John J. Feldmeier Youngstown State University USA/OH CoI Mr. Aaron Watkins Case Western Reserve University USA/OH Number of investigators: 5 & Phase I contacts: 1 Target Summary: Target RA Dec Magnitude M101-NE-PLUME 14 04 48.7430 +54 32 32.13 V = 29.2 M101-E-SPUR 14 05 3.4470 +54 18 30.35 V = 29.2 Observing Summary: Target Config Mode and Spectral Elements Flags Orbits M101-NE-PLUME ACS/WFC Imaging F606W 11 M101-NE-PLUME ACS/WFC Imaging F814W 10 M101-E-SPUR ACS/WFC Imaging F606W 11 M101-E-SPUR ACS/WFC Imaging F814W 10 M101-NE-PLUME WFC3/UVIS Imaging F606W CPAR 10 M101-NE-PLUME WFC3/UVIS Imaging F814W CPAR 11 M101-E-SPUR WFC3/UVIS Imaging F606W CPAR 10 M101-E-SPUR WFC3/UVIS Imaging F814W CPAR 11 Total prime orbits: 42 Total coordinated parallel orbits: 42 Scientific Justification Introduction The diffuse, low surface brightness outskirts of disk galaxies hold a remarkable range of information about processes driving galaxy evolution. As disks are thought to grow \inside- out," where the inner regions form first, and the outskirts later, the most recent signatures of galaxy assembly should lie in their faint outer reaches. Interactions and accretion can deposit fresh gas in the outer disk to fuel additional disk building (see, e.g., Sancisi et al. 2008), and the outer disks of galaxies are ideal laboratories for studying mechanisms for star formation at low gas density (Kennicutt et al. 1989; Martin & Kennicutt 2001; Bigiel et al. 2008). In addition, spiral structure can drive substantial radial migration of stars, whereby stars formed in the inner disk can move outwards and populate the disk outskirts (Sellwood & Binney 2002; Debattista et al. 2006; Roskar et al. 2008ab). All of these different processes leave signatures in the structure, stellar populations, and kinematics of the outer disk which can be used to develop a more complete picture of disk galaxy evolution. The nearby galaxy M101 (NGC 5457; d=6.9 Mpc, Matheson et al. 2012) presents a unique opportunity to study the outskirts of a giant Sc galaxy in detail. The galaxy is well- studied at many wavelengths, giving us a comprehensive view of the structure and kinematics of its baryonic components. Recent deep optical imaging (Mihos et al. 2013; Fig 1) traces M101's outer stellar disk to nearly 50 kpc, similar in size to the galaxy's extended HI disk (van der Hulst & Sancisi 1988; Walter et al. 2008; Fig 2). Deep HI mapping has revealed an even more extended (∼ 100 kpc) envelope of HI at very low column density (Huchtmeier & Witzel 1979; Mihos et al. 2012). GALEX imaging has shown M101 to be an \extended ultraviolet (XUV) disk" (Thilker et al. 2007), with far-ultraviolet emission tracing star formation well beyond the radius at which low gas densities are thought to inhibit active star formation (e.g., Kennicutt 1989). Whether this star formation has only recently been triggered, perhaps by an interaction, or reflects a long-term process of ongoing disk building requires a more detailed understanding of the underlying stellar populations in the outer disk. Our deep optical surface photometry provides new information on the stellar populations 2 in M101's outer disk. With a limiting surface brightness of µB∼ 29.5 mag/arcsec , the imaging reveals the stellar structure of M101's disk out to nearly 250 (50 kpc), or ∼ 10 disk scale lengths (Fig 1). At these radii, the well-known asymmetry of the inner disk slews 180 degrees, resulting in an asymmetric plume of light at large radius which follows the very extended HI disk to the northeast of M101. This \NE Plume" has very blue colors (B−V=0.2), suggesting it is dominated by young stars, likely the somewhat more evolved (few hundred Myr to ∼ 1 Gyr) counterpart of the young far-UV emitting population traced by GALEX imaging. Combining the optical and far-UV data, we find that constant star formation models significantly underpredict the plume's luminosity when constrained by the optical color and UV-derived star formation rate. In contrast, starburst models with an order-of-magnitude burst in the star formation rate that peaked ∼ 250{350 Myr ago provide a much better match to the observed properties of the plume. Our imaging also shows a second spur of extended light to the east of M101's disk (the 1 \E Spur"), which is significantly redder (B−V=0.45) than the NE Plume. The E Spur's red color makes it much harder to constrain its star formation history, particularly when factoring in possible complications due to reddening of the populations by dust. In practice, a wide variety of star formation histories | aging bursts, truncated star formation models, and constant star formation histories | can each be combined with a judicial amount of dust to produce the redder colors of the E Plume. These uncertainies highlight the problem with using integrated colors to study stellar populations in detail. While the properties of the NE Plume are consistent with a recent burst of star formation, such a burst population will dominate the plume's integrated light and make it extremely difficult to detect any underlying older population. The presence of an old stars would help discriminate between scenarios where this outer disk star formation is solely a recent event, versus those in which star formation and disk building has been occuring at a low level for many Gyrs. In the case of the E Spur, where the red colors make it difficult to place any strong constraints on the stellar populations, we are left uncertain whether the E Spur is an example of in-situ disk building by secular processes such as outwardly propogating spiral waves (e.g., Bush et al. 2010), or simply material pulled out from the inner disk by a recent interaction. While both the NE Plume and E Spur are morphologically reminiscent of tidal features produced during fly-by galaxy interactions, similar disturbed morphologies are also seen in models of gas accretion from the surrounding environment (e.g., Stewart et al. 2011). Without concrete information about the stellar populations in M101's outer disk, it remains unclear whether we are seeing ongoing secular disk-building processes at work, or simply transient events triggered by a recent interaction. The Need for HST Much firmer constraints on the stellar populations in M101's outer disk can come from direct imaging of its stellar populations using HST. The technique of imaging extragalactic stellar populations has been one of HST's greatest successes, yielding information on the star formation history and metallicity of galaxies across a range of distances and Hubble types, from the disks and halos of spiral galaxies (e.g., de Mello et al. 2008; Dalcanton et al. 2009, 2012; Durrell et al. 2010; Radburn-Smith et al. 2011, 2012) to nearby elliptical galaxies (Harris et al. 2007; Rejkuba et al. 2005, 2011) and galaxies and intracluster stars in the Virgo Cluster (e.g., Durrell et al. 2007; Williams et al. 2007). An example of discrete stellar population imaging is shown in Fig 3, taken from Radburn- Smith et al. (2011), for the outer disk of the nearby spiral NGC 2403 (the greater distance of M101 coupled with the deeper imaging proposed here means that our data for M101 would go comparably far down the luminosity function for M101's stellar populations).