Stellar Populations in the Outer Disk of M101

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1 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: 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 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.

2 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 V = 29.2 M101-E-SPUR 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

3 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 insideout, 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 wellstudied 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 in M101 s outer disk. With a limiting surface brightness of µ B 29.5 mag/arcsec 2, the imaging reveals the stellar structure of M101 s disk out to nearly 25 (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 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

4 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). A variety of stellar evolutionary stages are represented in the color-magnitude diagram (CMD): bright young main sequence stars, red and blue Helium burning stars, and the AGB and RGB from both young and old stellar populations. The relative mix of stars along these different features in the CMD can be used in a variety of ways to constrain age and metallicity in the stellar populations. Young stellar populations should have both an upper main sequence and significant numbers of high mass blue loop helium burning stars crossing the color magnitude diagram 2

5 (Fig 4); at the distance of M101 these stars will have m I 27 for 200 Myr ages, and m I 28.5 (the limit of our data) for 500 Myr ages. Detection of these young stars would confirm the young stellar populations implied by the broadband surface photometry and GALEX imaging. We can also search for a well-populated red giant branch (RGB) indicating the presence of old stellar populations; the relative mix of RGB versus MS and HeB stars thus constrains the star formation history over much longer timescales than those probed by the GALEX or broadband surface photometry alone. For older populations, metallicity information comes from the shape of the RGB (Figs 3 & 4); metal-rich populations have a fainter and redder RGB. For young populations the situation is more complex, but some metallicity constraints come from the BHeB sequence, which is largely absent at solar metallicities (Fig 4; McQuinn et al. 2011). Of particular interest would be a comparison between the stellar and gas-phase metallicity (e.g., Kennicutt et al. 2003) of the outer disk, as this would tell us whether the outer disk stars we see are formed in-situ from gas in M101 s outskirts, or were formed in M101 s inner disk and subsequently moved outwards due to tidal stripping or radial migration processes. We plan four pointings (two primary, two parallel) in the outerskirts of M101, shown in Fig 2. One primary ACS pointing will focus on the NE Plume, while the other will image populations in the E Spur. Each primary pointing will be paired with a nearby parallel blank WFC3 pointing located off the optical disk. When coupled with background corrections measured from the Hubble Ultra Deep and Frontier Fields, these parallel fields will allow us to measure any M101 halo population and correct the primary disk pointings for contamination from this halo population. The study of M101 halo populations from the parallel fields will be an interesting secondary science goal for the project. Our team has extensive experience both in discrete stellar populations (Durrell, Harding, Feldmeier) and deep surface photometry (Mihos, Harding, Feldmeier). Our data reduction and analysis strategy will follow previous efforts by members of our group studying extragalactic stellar populations (e.g., Durrell et al. 2007, 2010). After extraction of the CMDs for each field, we will compare the data both to theoretical isochrones and to full-fledged CMD modeling using the StarFISH code (Harris & Zaritsky 2001), comparing to artificial CMDs created under a variety of star formation history models. Particular scientific goals include: measuring the recent and extended star formation history of the NE Plume and E Spur, searching for and characterizing the metallicity of the stellar populations in the outer disk, examining differences in the stellar populations in the NE Plume and E Spur, and measuring M101 s halo populations in the parallel fields. In addition to the specific study of M101 s outer disk, a broader goal of our project will be a comparison of stellar population constraints based on deep surface photometry to those obtained via direct HST imaging of individual stars. This will be particularly helpful in understanding systematic uncertainties in studies of distant galaxies; while exquisite information is coming from HST direct imaging studies of nearby galaxies, most galaxies are far too distant for this kind of analysis. Connecting lessons learned from individual galaxies in the local universe to those gleaned from large samples of more distant galaxies studied in integrated light depends critically on understanding how the two techniques compare. 3

6 Bigiel F., et al. 2008, AJ, 136, 2846 Bush S. J., et al. 2010, ApJ, 713, 780 Dalcanton J. J., et al. 2012, ApJS, 200, 18 Dalcanton J. J., et al. 2009, ApJS, 183, 67 de Mello D. F., et al. 2008, AJ, 135, 548 Debattista V. P., et al. 2006, ApJ, 645, 209 Durrell P. R., et al. 2010, ApJ, 718, 1118 Durrell P. R., et al. 2007, ApJ, 656, 746 Gallart C. 1998, ApJ, 495, L43 Gil de Paz A., et al. 2005, ApJ, 627, L29 Girardi L., et al. 2010, ApJ, 724, 1030 Harris J., Zaritsky D. 2001, ApJS, 136, 25 Hoopes C. G., et al. 2001, ApJ, 559, 878 Huchtmeier W. K., Richter O.-G. 1988, A&A, 203, 237 Kennicutt R. C., Jr. 1989, ApJ, 344, 685 Marigo P., et al. 2008, A&A, 482, 883 Martin C. L., Kennicutt R. C., Jr. 2001, ApJ, 555, 301 Description of the Observations REFERENCES: Matheson T., et al. 2012, ApJ, 754, 19 Mihos J. C., et al. 2013, ApJ, 762, 82 Mihos J. C., et al. 2012, ApJ, 761, 186 Radburn-Smith D. J., et al. 2011, ApJS, 195, 18 Radburn-Smith D. J., et al. 2012, ApJ, 753, 138 Rejkuba M., et al. 2005, ApJ, 631, 262 Roškar R., et al. 2008, ApJ, 684, L79 Roškar R., et al. 2008, ApJ, 675, L65 Sancisi R., et al. 2008, A&ARv, 15, 189 Sellwood J. A., Binney J. J. 2002, MNRAS, 336, 785 Stewart K. R., et al. 2011, ApJ, 738, 39 Thilker D. A., et al. 2007, ApJS, 173, 538 van der Hulst T., Sancisi R. 1988, AJ, 95, 1354 Walter F., et al. 2008, AJ, 136, 2563 Williams B. F., et al. 2011, ApJ, 734, L22 Williams B. F., et al. 2009, AJ, 137, 419 Our program seeks to obtain deep photometry of the various stellar populations found in the outskirts of M101. We plan two primary ACS pointings: one on the NE Plume and the other on the E Spur, which will each be coupled with parallel WFC3 blank fields. These blank parallels will beused to quantify any M101 halo population and allow for proper extraction of the stellar populations in the primary outer disk fields. Exposure times: In order to properly sample the wide range of ages that are possible in these fields, we need to be able to perform photomery of M101 stars on the old (> 1Gyr) RGB, as well as the younger main sequence (MS) and He-burning stars blue loop stars with ages up to at least 400 Myr. We will need to reach at least three magnitudes down the RGB to (a) sample the blue MS stars up to ages of 400Myr, (b) obtain adequately strong photometry of both the red and blue helium-burning stars in order to place limits on both the age and metallicity of any young stars present in the fields and (c) search for old RGB stars and provide reasonable (within 0.2 dex) metallicity estimates of the RGB population. A further check on the age of the older RGB stars would be provided by detection of the AGB bump (e.g., Gallart 1998, Rejkuba et al. 2005, Williams et al. 2009), which would lie at I at the distance of M101. Thus we require photometry of stars down to F 814W I = 28.7 to adequately investigate all of these stellar populations. We have estimated our exposure times based on the ACS ETC (assuming the brightest possible sky background at µ V 22.9), as well as our own experiences with deep photometry of the M81 halo (Durrell et al. 2010) to similar depths as we are planning. To reach S/N 6 (roughly the 50% completeness level) for 4

7 N5422 N5473 N5485 NE Plume N5477 E Spur M N5474 Figure 1: Wide-field deep B image of M101 from Mihos et al. (2013), spanning 2.5 degrees (300 kpc) across, with a limiting surface brightness of µ B =29.5 mag/arcsec 2. North is up and east is to the left. In this image, regions of high surface brightness (µ B < 25.5) are shown at native resolution (1.45 /pixel) and rescaled in intensity to show the bright inner regions of galaxies. At lower surface brightnesses, the image has been masked of bright sources and median binned into 9x9 pixel (13 x13 ) bins to show fainter detail. Bright galaxies in the field are labeled; NGC 5477 and NGC 5474 are physically associated with M101, while NGC 5422, NGC 5473, and NGC 5485 are all background galaxies. The NE Plume and E Spur, the two features we plan to target with HST, are also marked. 5

8 NE Plume Fields E Spur Fields Op2cal Deep Op2cal B- V Color HI intensity Far UV Hα intensity Figure 2: Multiwavelength view of M101, with our planned HST fields overlaid. Top left: Our B-band image. Top middle: B image at a deeper stretch. Top right: Our B-V color map, scaled from B V<0.2 (magenta) to B V>0.7) (red). Bottom left: THINGS neutral hydrogen map (Walter et al. 2008). Bottom middle: GALEX near ultraviolet map (Gil de Paz et al. (2007). Bottom right: Hα map (Hoopes et al. 2001). All images have been registered to the same orientation and spatial scale as the optical image, with north up and east to the left. Figure 3: HST imaging of discrete stellar populations in the outer disk of the spiral galaxy NGC 2403, taken from Radburn-Smith et al Our data would go similarly far down the luminosity function of stellar populations in M101 s disk. A variety of evolutionary phases can be seen (as sketched in the right hand panel), including main sequence (MS) stars, blue and red Helium burning stars (BHeB/RHeB) and the red giant branch (RGB) and asymptotic giant branch (AGB) populations. The effect of metallicity on the RGB can be seen in the left hand panel. 6

9 Figure 4: Stellar isochrones from Mariago et al. (2008) using TP-AGB models of Girardi et al. (2010). The isochrones have been shifted to the distance of M101 (6.9 Mpc), and our detection limits are shown by the dotted line. Our data should be able to detect blue loop HeB stars down to an age of 400 Myr, probing the star formation history of the outer disk over the past several hundred Myr. The two panels show the effects of metallicity on the old RGB population and extent of the blue Helium burning loop for younger stars. I = 28.7 requires a total of 26800s, or 10 orbits. To measure the full color extent of the RGB at these levels, we need to reach V 29.9 (F 606W 29.7); to reach this level with the ACS requires a total exposure time of 30700s, or 11 orbits. Thus we require 21 orbits per field, and a total request of 42 orbits to complete our science goals. We will obtain a single exposure per orbit, thus a total visibility of 47min 60s=2820s per orbit; adopting this strategy will minimize the effects of CTE degredation on the photometry of our faintest sources (see below). WFC3 Parallel Observations: Our NE Spur and E Plume fields will contain a mixture of M101 halo stars and stars from the outer disk of M101. To properly extract the stellar populations for each of these regions, we need comparison fields of similar photometric depth located very close to the primary fields (to correctly predict the star counts from the halo stars). To accomplish this, we are also requesting co-ordinated parallel observations of M101 s outer disk and halo populations with WFC3. In F606W, the efficiency of WFC3 is similar to that of the ACS; in F814W, it is slightly lower. To balance the WFC3 exposures with those of the ACS, our parallel observations will consist of 11 orbits in F814W, and 10 orbits in F606W per filter, per field. Use of the WFC3 ETC shows that in 29,400 s (11 orbits, 1 image per orbit, 47 minute assumed visibility), we can reach F 814W I = 28.3 and V = 29.7 with S/N 6 for point sources. This is similar to the depth obtainable with the ACS, once the higher CTE effects in the latter are accounted for. CTE corrections: As noted in ISR ACS , ISR WFC and the CTE White Paper, high backgrounds of 15e (WFC3) and 100e (ACS) are needed per image 7

10 in order to mitigate severe losses of the signal of faint sources due to CTE degradation in both instruments. By observing a single image per orbit, the predicted image background levels will be comfortably above these limits adopting µ V = 22.9 (as above) for the average sky brightness yields expected levels of 250e for the ACS observations, and 50e for the WFC3. There will be at least 10 exposures per filter per field, and that will be enough to facilitate cosmic-ray removal. Crowding: Our fields are located far from M101, with surface brightnesses µ V 28 based on our deep imaging (Fig. 1). ACS/WFC observations of resolved RGB stars in Virgo s intracluster space (Williams et al. 2007) yielded 5300 RGB stars from the top 1.5 magnitudes of the RGB at a similar surface brightness. If we scale those numbers to include the additional 2 magnitudes of the RGB we will detect in M101 (a factor of 6.5 for an old, intermediate-metallicity population), and account for M101 lying at half the distance of the Virgo cluster, we predict a total of 8000 M101 stars in our planned ACS fields ( 5000 per WFC3). These stellar densities correspond to 1 star per pixel region. Stellar crowding will be insignificant. Background Fields: While the large fraction of the point sources in our fields will be M101 disk stars (along with some halo stars), we also expect a small contribution from faint foreground stars and unresolved background galaxies. To quantify this, we will compare our data to point-source photometry from other deep HST images, including (a) a subset of the Hubble Ultra Deep Field F606W and F775W exposures to match the signal-to-noise characteristics of our program fields, and (b) the deep F606W and F814W images that will be taken as part of the Hubble Space Telescope Frontiers program. Special Requirements Coordinated Observations Justify Duplications While M101 has been the focus of many HST studies, none have probed the outer disk to sufficient depth to study its resolved stellar populations. WFPC2 imaging (GO-11289) exists for a few fields near our NE Plume pointing, but these images are only 400s long. There is no HST imaging available for fields in the E Spur. Past HST Usage The PI has not led an HST program in recent cycles. 8