The Vast Population of Wolf-Rayet and Red Supergiant Stars in M101: I. Motivation and First Results
Michael M. Shara, Joanne L. Bibby, David Zurek, Paul A. Crowther, Anthony F.J. Moffat, Laurent Drissen
aa r X i v : . [ a s t r o - ph . S R ] F e b Submitted to AJ
The Vast Population of Wolf-Rayet and Red Supergiant Stars inM101: I. Motivation and First Results
Michael M. Shara , Joanne L. Bibby , , David Zurek , Paul A. Crowther , Anthony F.J.Moffat , and Laurent Drissen ABSTRACT
M101 is an ideal target in which to test predictions of massive star birth andevolution. The large abundance gradient across M101 (a factor of 20) suggeststhat many more WR stars must be found in the inner parts of this galaxy thanin the outer regions. Many H ii regions and massive star-forming complexes havebeen identified in M101; they should be rich in WR stars, and surrounded byRSG stars. Finally, the Wolf-Rayet stars in M101 may be abundant enough forone to explode as a Type Ib or Ic supernova and/or GRB within a generation.The clear identification of the progenitor of a Type Ib or Ic supernova as a WRstar would be a major confirmation of current stellar evolution theory.Motivated by these considerations, we have used the Hubble Space Telescopeto carry out a deep, He ii optical narrowband imaging survey of the massive starpopulations in the ScI spiral galaxy M101. Combined with archival broadbandimages, we were able to image almost the entire galaxy with the unprecedenteddepth and resolution that only HST affords.We describe the extent of the survey and our images, as well as our datareduction procedures. A detailed study of a field east of the center of M101,containing the giant star-forming region NGC 5462, demonstrates how we find Department of Astrophysics, American Museum of Natural History, Central Park West and 79th Street,New York, NY 10024-5192 Jeremiah Horrocks Institute for Maths, Physics & Astronomy, University of Central Lancashire, Preston,PR1 2HE, United Kingdom Department of Physics and Astronomy, University of Sheffield, Hounsfield Road, Sheffield S3 7RH,United Kingdom D´epartement de Physique, Universit´e de Montr´eal, CP 6128 Succ. C-V, Montr´eal, QC, H3C 3J7, Canada D´epartement de Physique, Universit´e Laval, Pavillon Vachon, Quebec City, QC, G1K 7P4 Canada
Subject headings: galaxies: individual (M101) — galaxies: stellar content —stars: Wolf-Rayet —stars: Supergiants
1. Introduction and Motivation1.1. Starburst Regions
The study of individual luminous stars and stellar populations in nearby giant H ii regions is a prerequisite to understanding the starburst phenomenon and interpreting theobservations of distant starburst galaxies and those containing starburst regions, for whichonly integral properties can be observed (Leitherer 1997). In this context, the Hubble SpaceTelescope (HST) has been crucial in providing us with high-resolution images of nearby, butvery dense and massive, stellar clusters which are ionizing giant H ii regions (Hunter et al.(1995), Hunter et al. (1996) and Malumuth et al. (1996)).Our HST-based investigation of the stellar populations of the most luminous star-forming complexes in the nearby late-type (ScIII) spiral galaxy NGC 2403 was very fruitful(Drissen et al. 1999), and underpinned our request for HST time to observe M101. A mem-ber of the M81 group, at a distance of 3.2 ± ii regions (Sivan et al. 1990). Its abundance level and O/H radialgradient have been well established by Martin & Belley (1996); they are similar to those ofM33 (Henry & Howard 1995). In contrast with M33, which contains relatively modest giantH II regions, four of NGC 2403’s H ii regions are exceptionally bright, with H α luminositiesL( Hα ) ∼ . − . × erg/s, comparable to the most massive starburst region in theLocal Group, the 30 Doradus complex. We also found direct evidence for the presence ofWolf-Rayet (WR) stars in five of the six giant H ii regions investigated; 25 - 40 WR stars arepresent in the NGC 2403-I giant H ii region alone. HST has also provided optical and UVspectra of individual massive stars in NGC 1569 (Maoz et al. 2001), NGC 5398 (Sidoli et al.2006) and NGC 925 (Adamo et al. 2011). Ground-based imagery and spectroscopy has re-vealed rich WR populations in M83 (Crowther et al. 2004) and NGC 5253 (Crowther et al.1999). 3 –M101 (also known as NGC 5457 and the Pinwheel Galaxy) is the logical galaxy in whichto extend this work. As the nearest giant grand design ScI spiral galaxy, it is brimmingwith H ii regions, massive stars and at least 3,000 luminous star clusters (Barmby et al.2006). About 6500 WR stars are estimated to exist in the Milky Way (Shara et al. 1999);simplistically scaling up to the size and luminosity of M101 suggests a population of 10-20,000 WR stars and even more Red Supergiants (RSGs). The star formation rate (SFR)in M101 (Lee et al. 2009) is probably a few times that of the Milky Way, and M101 is 50%or more larger than our Galaxy. This, again, suggests a very substantial population ofM101 WR stars. In addition, a rich treasury of (mostly) continuum imagery with HST wasalready in hand: over 135 ksec of HST exposures. As described below, we used this databaseextensively to perform the image subtractions needed to isolate the strong emission-line WRstars and very red Red Supergiants from the other stellar populations. Wolf-Rayet (Crowther 2007) and Red Supergiant stars (Levesque (2010); Meynet et al.(2011)) are the massive stars that are easiest to identify in imaging surveys of galaxies becauseof their strong emission lines and extreme colors, respectively. They provide importantconstraints on the age of a starburst (Gazak et al. 2012) and on the mode of star formation(Crowther 2013). Single stars with initial masses (M i > ⊙ ) are predicted to advance to theWR phase at approximately solar metallicity. WR stars possess strong stellar winds whichproduce a unique, emission–line spectrum displaying broad He ii λ iii λ iv λ classical cousins.Since the hydrogen–rich envelope has been removed from classical WN stars it followsthat they are probably the progenitors of at least a subset of H–poor Type Ib SN. Similarly,the removal of both the hydrogen and helium envelopes from WC stars should correspondto the absence of both these elements in the spectra of Type Ic SN. However, the WR-TypeIbc SN question remains unresolved as, to date, no direct detection of a Type Ib or Ic SNprogenitor has been obtained. (Eldridge et al. 2013) have recently claimed that 12 SNIbcprogenitors are invisible to as faint as absolute B, V and R magnitudes of -4 to -5.In contrast, RSG are predicted to arise from the evolution of less massive (8 – 20 M ⊙ )stars and are therefore expected to appear later in the life of a starburst. Evolutionarymodels predict that single massive stars with M i ∼ ⊙ end their lives during the Red 4 –Supergiant (RSG) phase as H–rich Type II core–collapse supernovae (ccSNe). HST broad–band pre-SN imaging has been able to confirm the RSG–Type II SN connection, particularlyfor SN 2003gd (Smartt et al. 2009). However, the highest mass RSG progenitor to date isonly ∼ ⊙ (Smartt 2009), making these limits uncertain.Models indicate that in instantaneous starbursts of low metallicity, these two popula-tions are well separated in time, since only the most extreme stars (M i ≥
50 M ⊙ ) can shedenough mass to reach the WR stage. In regions of high metallicity, however, the simul-taneous presence of WR and RSG stars can be expected for a short period of time, sincelower mass stars ( ∼
25 M ⊙ ) can also become WR after having spent some time as RSG(Maeder & Meynet 1994). WR and RSG are observed to coexist in the massive Galacticcluster Westerlund 1 (Clark et al. 2005). One of our key goals is to directly test this predic-tion in a single galaxy: M101. Kennicutt et al. (2003) have shown that in M101, over thegalactocentric range 6-41 kpc, oxygen abundances are well fitted by an exponential distribu-tion from approximately 1.3 (O/H) solar in the center to 1/15 (O/H) solar in the outermostregions. Equivalently, log O/H +12 = 8.8 in the center of M101, and 7.5 in that galaxy’souter regions. Observing across the entire range of galactocentric distances in M101 (from0 to 50 kpc) to measure how the absolute numbers and WR/RSG ratio changes across thegalaxy is an equally important goal of our study. An early HST–based investigation by Drissen et al. (1993) targeted the WR popula-tion of M33 using narrow–band λ ii region in the nearby star-forming galaxy M33 which lies at a distance of only d ∼ ∼ ′′ revealed that NGC 604 hasa moderate WR population (Drissen et al. 1991). However this was significantly increasedvia high spatial resolution HST narrow–band imaging (Drissen et al. 1993), identifying thefainter WR population which corresponds to the lowest mass WR stars. At the core of eachof the two most luminous giant H II regions in NGC 2403 lies a luminous, compact object(Drissen et al. 1999).The discovery that very dense, massive star clusters form at the cores of all typesof starbursts led to the suggestion that globular clusters were once located at the cores ofmassive starbursts (Meurer 1995; Whitmore & Schweizer 1995; Ho & Filippenko 1996a). Thecases of HD 97950 (the ionizing core of NGC 3603 (Drissen et al. 1995)), R136 (Moffat et al.1994) and (Crowther et al. 2010), NGC 2363 (Drissen et al. 2000), NGC 2403-I and NGC2403-II (Drissen et al. 1999), M31 (Neugent et al. 2012) and M33 (Neugent & Massey 2011) 5 –show that massive compact stellar clusters also form in more normal galaxies as well. Howmuch more common are they in a very massive, actively star-forming galaxy like M101?A striking feature in the NGC 2403 clusters is that RSG stars are mainly present overa more extended halo, while the young blue stars and most WR stars are in or close to acompact core. Stars more massive than ∼
25 M ⊙ are not expected to go through a RSGphase before becoming WR stars. For M i ≤
20 M ⊙ , stars evolve to RSG and explode as TypeII supernovae without entering the WR phase. The timescale of these evolutionary paths islonger as one considers lower masses, hence the absence of RSG and presence of WR stars inthe cores indicate that the population is dominated by very young and very massive stars.The presence of RSG in the halos signifies that we have an older mix of stars of various masseswith M ' ⊙ . The relative age spread and the spatial exclusion between RSG and WRstars are most obvious for the largest H ii regions. Although of different ages, the proximitythere of the WR and RSG stars suggests a triggering link between the two populations,which is a key part of this research program. WR stars and RSG are observed to co-existin the Milky Way’s most massive compact cluster Westerlund 1 (Clark et al. 2005). Theluminosity, inferred mass and compact nature of Westerlund 1 are comparable with those ofSuper Star Clusters - previously identified only in external galaxies (see e.g. Bastian et al.(2012), Larsen et al. (2011),Whitmore et al. (2005) and Whitmore et al. (2011)). Another key goal of this and related studies is obtaining narrow–band imaging of sev-eral nearby galaxies to produce a catalogue of ∼ WR stars. When a Type Ibc SNand/or gamma ray burst (Georgy et al. 2012) eventually occurs in one of these galaxies,our catalog should reveal the WR progenitor, confirming one of the strongest predictions ofstellar evolutionary theory. We have obtained ground-based narrow-band imaging of sev-eral nearby star-forming galaxies (Bibby & Crowther (2012), Bibby & Crowther (2010) andHadfield & Crowther (2007)) and confirmed a subset of the WR candidates with multi-objectspectroscopy. Given the average lifetime of a WR star of ∼
2. Techniques and Observations
The need for targeted surveys that uniquely pick out WR stars is highlighted bySmartt et al. (2009) and by Eldridge et al. (2013), who discussed Type Ibc SN for whichbroad–band pre-SN imaging exists. Not one progenitor has been identified from these dozenSNe. This is because short exposure times did not allow the images to go deep enoughto detect the continuum of a WR star. Had proper narrow–band surveys been available wewould almost certainly, by now, have been able to provide strong evidence for the WR-SNIbcconnection.A powerful technique to detect individual Wolf-Rayet stars in crowded fields, such as theones in M101, consists of subtracting a continuum image, normalized in both PSF and inten-sity, from an image obtained with a narrow-band filter centered on the He ii λ ii λ ii regions in galaxies such as M33 (Drissen et al. 1993) and NGC 2403 (Drissen et al.1999), detecting both isolated, individual WR stars and WR stars in unresolved clusters thatinclude only a very small fraction of WR stars.M101 lies at a distance of 6.4 Mpc (Shappee & Stanek 2011), almost twice as far asNGC 2403 (Freedman & Madore 1988), so that ∼ × longer exposures are needed to reachsimilar magnitudes. In Cycle 17 we obtained HST/Wide Field Camera 3 (WFC3) pointingsof 2 orbits per M101 field, under program ID 11635 (PI. Shara), with a total exposure time of6106 seconds per field. This permitted us to image to a similar depth in M101 as we achievedin NGC 2403. We note that the systemic redshift of M101 (+372 km/s) shifts the center ofthe He ii λ λ
3. Data Reductions
Each WFC3 pointing was treated individually to ensure the best alignment with theACS/WFC data and to make the size of each dataset more manageable. The correspondingACS frames were drizzled with the WFC3 F469N pointing using the multidrizzle taskwithin iraf to produce a coordinate system that was consistent between each differentinstrument and filter. Often small shifts and rotations were required to achieve the bestalignment; these were calculated using geomap . To achieve the best match, the WFC3 data IRAF is distributed by the National Optical Astronomy Observatory, which is operated by the Asso-ciation of Universities for Research in Astronomy (AURA) under cooperative agreement with the NationalScience Foundation. and each WFC3 field highlighted is 2.7 × ′′ , corresponding to 4.65 pc at the 6.4 Mpc distanceof M101. While slightly degraded from the 0.1 ′′ optimum sampling offered by HST, this wasnecessary to allow us to produce the best continuum subtractions possible.In the following we describe the methods applied to all 18 pointings, and present theresults of one of those pointings (M101-I). Later papers in this series, describing the remaining17 fields, use exactly the same methodology. Once the broad– and narrow–band images had been aligned, photometry was performedon each filter separately using the standalone code, daophot (Stetson 1987). A point-spreadfunction (PSF), based on isolated, point-like stars within the field, was built and applied toall the other stars detected. Individual zero-points from the HST literature were applied foreach filter to transform the observed magnitudes into ST magnitudes and Vega magnitudes.For this study we find the Vega magnitude system to be more useful, since (under the Vegasystem) the F435W, F555W, and F814W filters correspond to the Johnson B, V and Ifilters. This enabled us to use literature color cuts. Henceforth, any magnitudes listed areVega magnitudes unless otherwise stated.Typical photometric errors for the narrow-band F469N images were ± F N = 21mag) and ± F N = 25.5 mag);the distribution of the photometric errors relative to the source brightness for the F469Nimages is shown in Figure 2. For the ACS broad-band images the errors were slightlylower for sources of similar magnitudes, with ± F W = 21mag) and ± F W = 25.5mag). For the faintest sources in the ACS image with m F W = 28 mag thephotometric error is typically ± In order to assess the completeness of our M101 survey we must determine to what depthour images probe. Following the method of Bibby & Crowther (2010) we fit a polynomial tothe distribution, where the 100% detection limit is defined by the point at which the powerlaw deviates from the observed data. Figure 4 shows the distribution of sources detectedin our WFC3/F469N data, indicating that our 100% detection limit is m F N = 24.3 mag.If we adopt the extinction from Lee et al. (2009) of A(H α ) = 1.06 mag, corresponding to 10 –Fig. 2.— Photometric errors as a function of zero-point corrected, apparent (Vega) magni-tude for all sources found in the WFC3/F469N image of M101-I . 11 –Fig. 3.— Photometric errors as a function of zero-point corrected, apparent (Vega) magni-tude for all sources found in the ACS/WFC F435W image of M101-I . 12 –A(F469N) = 1.53 mag following the extinction law from Cardelli et al. (1989), and adopt theCepheid distance of 6.4 Mpc (Shappee & Stanek 2011), our 100% completeness detectionlimit corresponds to M F N = –6.26 mag.The magnitude distribution of sources in the ACS/F435W data is shown in Figure 5,which shows that we sample ∼ F W = 26.6 mag. Again, we adopt the extinction law of Cardelli et al. (1989)so A(F435W) = 1.74 mag determines a 100% completeness detection limit of M F W = –4.17 mag.
4. Source Selection
The F469N filter, centered at λ ii λ iii λ iii λ λ F N –m F W was determined, where m F N –m F W ≤ λ ≥ σ wereconsidered to be WR candidates. In addition, sources that were only identified in the F469Nimage, and not in the F435W or F555W images, were also flagged as WR candidates, sinceit is likely that these stars are faint WR stars with little or no detectable continuum. Figure6 shows the photometric properties of each WR candidate, with the 100% detection limit ofm F W = 26.6 mag adopted as the continum magnitude for those candidates only detectedin the F469N image.An efficient way of identifying bonafide WR candidates is via the “blinking” method(Moffat & Shara 1983), which compares the F469N, F435W and continuum subtracted im-ages in sequence. However, the broad–band F435W filter bandpass is almost 30 × the widthof the narrow-band F469N filter, hence the F435W image was scaled to create a narrow-bandcontinuum. Since most stars should have m F N –m F W =0 we determined the scale factorwhich allowed most stars to disappear from the continuum subtracted image. We note thatthis could affect the detection of WR stars with a low He ii excess. 13 –Fig. 4.— The magnitude distribution of photometric sources identified in the WFC3/F469Nimage of M101-I using 0.2 magnitude bins. A 100% detection limit of m F N = 24.3 mag isderived from this plot using the solid line which represents a 3rd degree polynomial fit tothe brightest sources. 14 –Fig. 5.— The magnitude distribution of photometric sources identified in the ACS/WFCF435W image of M101-I using 0.2 magnitude bins. A 100% detection limit ofm F W = 26.6 mag is derived from this plot using the solid line which represents a 3rd degreepolynomial fit to the brightest sources. 15 –The “blinking” technique was applied to all of the 3 σ photometric WR candidatesto confirm the He ii λ ∼ scaled filters.
5. Contamination by Variable Stars
Since the narrow-band images were obtained in 2010, and the continuum images in 2002,it is possible that stellar variability over the 8 year baseline leads to contamination of ourWR candidates with Cepheid and other variable stars.Shappee & Stanek (2011) identified Cepheids in 2 fields to determine the distance toM101. They showed that Cepheids have a (V–I) color ≥ ≥ σ excess in m F N –m F W , 15 are provisionallyeliminated ( ∼ > ≥ < -1 mag; this is unexpected. However, inspection of the image reveals that these two sourceslie in very crowded regions and hence their photometry is likely to be more unreliable thanfor the other candidates. The (V–I) colors for the 18 candidates which have a ≥ σ excessand -1 < (V–I) < F W – m F N , versus F469N magnitude for WR candidates in M101-I.Open squares indicate WR candidates with a 3 σ detection in both F469N and F435W images,which also have an excess of > σ . Open triangles indicate sources for which there was no de-tection in the F435W image, so excesses represent lower limits assuming m F W = 26.6 mag– our 100% detection limit. 17 – (a) (b) Fig. 7.— Postage stamp images produced during blinking showing the F435W continuum(bottom), F469N narrow-band (middle) and continuum subtracted image (top) for each WRcandidate. Here we show an example of a WR candidate a) detected in the continuum(source ∼
6. Identifying Red Supergiants
The archival ACS images also allow us to identify RSG candidates from their (B–V) and(V–I) colors. Based on the colors of RSGs in the LMC we apply color cuts of (B–V) ≥ ≥ ≥ . L ⊙ , equivalent to an apparent magnitude of m F W ≤
22 mag. Intotal we identify 164 RSG candidates in our single M101 field; their photometry is presentedin Table 2.In their study of RSGs in M31, Massey et al. (2009) suggest a more stringent B–V cut of 19 –Fig. 8.— Here we present the I (F814W) versus (V-I) of the WR candidates in M101which have a detection limit in both the F469N and F435W filter images of > σ . FollowingShappee & Stanek (2011) we use a color cut of V-I > ≥ ∼
25% of RSGs already spectroscopically confirmed by Massey (1998) fromtheir candidate list (their Figure 3). If B–V ≥ ∼
30% of candidates would be missed, which is consistent with the 35% reduction inRSG candidates we find if we apply the B–V ≥ ∼ F W ≥ F W -m F W ≥ δ m F W = 3 mag and δ m F W -m F W = 2 mag range is consistent with that found for theconfirmed RSGs in the LMC and as such we do not remove the RSGs from the candidatelist. In Figure 10 we focus on the largest star-forming complex NGC 5462 in the field M101-I,which includes multiple bright H ii regions from Hodge et al. (1990), H1159, 1169, 1170 and1176. This region has also been studied by Chen et al. (2005) using HST images and forreference we show an archival HST/WFPC2 H α image of this region, marking the clustersidentified by these authors.RSG and WR candidates are plotted as red squares and red triangles, respectively. Thebrightest 1% of all pixels in the ACS field are colored blue, while the next brightest 4% of allpixels are colored black. 7 WR and 36 RSG candidates are ”isolated”, i.e. not surrounded byblack pixels. 6 WR and 30 RSG are largely or entirely surrounded by black pixels. Finally, 12WR and 5 RSG are surrounded by blue pixels. The corresponding ratios of numbers of WR toRSG candidates are 0.194, 0.2 and 2.4 . The former two are statistically indistinguishable,but the strong clustering of WR stars in the core of the star-forming complex M101-I issuggested by the latter ratio. This suggestion can be made indisputable only with a muchlarger sample of stars. In later papers in this series we will greatly strengthen these smallnumber statistics with thousands of M101 WR and RSG stars.
7. Summary and Conclusions
We describe the motivation for, and data collected by HST to search for the progenitorsof type Ib/c supernovae in the nearby giant spiral galaxy M101. The analysis methodologyand early results of a search for WR and RSG stars in one HST WFC3 pointing of M101are reported. 75 WR and 164 RSG candidates are identified. There is a suggestion of 21 – (a)(b)
Fig. 9.— Photometry of Red Supergiant candidates in a single field of M101 showing a) theV–I versus B–V colors and b) V–I versus I-band magnitude for all candidates. The solidsquares show sources which are identified as RSGs using a (B–V) ≥ ≥ (a)(b) Fig. 10.— a) Broad band image of the main region of M101-I containing multiple clusters.The contours plotted related to 99% (blue) and 95% (black) cuts for the brightness. Wolf-Rayet stars are shown as red triangles and Red Supergiants as red squares - assuming a cutof B-V ≥ α image of the sameregion with clusters identified from Chen et al. (2005) plotted as red circles. 23 –Table 1: Photometry of M101 WR candidates. The RA and DEC of each candidate is takenfrom the calibration of the F469N/WFC3 image. Narrow- and broad-band magnitudes arelisted for each candidate, unless the object was not detected in that filter. Errors listed arethe 1 σ errors determined by the daophot routine. RA DEC F469N err F435W err F435W-F469N err F555W err F814W err14:03:51.802 +54:21:39.55 22.11 0.07 23.22 0.01 1.11 0.07 23.61 0.03 23.91 0.0514:03:51.378 +54:21:35.21 22.53 0.12 23.49 0.03 0.96 0.12 23.10 0.04 22.57 0.0414:03:51.722 +54:21:50.05 23.21 0.13 24.60 0.04 1.39 0.13 24.71 0.04 24.54 0.0514:03:52.347 +54:21:54.57 22.25 0.13 24.37 0.04 2.12 0.14 24.44 0.06 25.04 0.0814:03:52.755 +54:21:55.90 23.37 0.09 25.27 0.05 1.90 0.10 25.48 0.08 25.84 0.1114:03:54.324 +54:21:55.99 23.34 0.12 25.27 0.05 1.93 0.10 25.48 0.08 25.84 0.1114:03:53.820 +54:22:03.57 23.94 0.14 25.32 0.05 1.38 0.15 25.07 0.06 24.68 0.0714:03:53.690 +54:22:08.78 22.43 0.10 23.65 0.02 1.22 0.10 23.84 0.03 23.86 0.0414:03:53.863 +54:22:09.11 21.58 0.12 23.91 0.04 2.33 0.13 24.18 0.06 24.74 0.0714:03:54.030 +54:22:09.25 23.39 0.12 25.97 0.17 2.58 0.21 25.91 0.16 26.38 0.5214:03:53.344 +54:21:50.08 23.73 0.11 25.43 0.05 1.70 0.12 25.96 0.08 – –14:03:53.172 +54:21:50.63 23.19 0.12 25.43 0.05 2.24 0.12 25.96 0.08 – –14:03:52.987 +54:22:00.44 21.67 0.10 23.15 0.09 1.48 0.13 23.35 0.10 23.60 0.1114:03:53.072 +54:22:01.25 22.15 0.14 24.35 0.05 2.20 0.14 24.47 0.04 25.19 0.0814:03:53.676 +54:22:02.08 22.23 0.14 24.20 0.03 1.97 0.14 24.10 0.03 25.28 0.0714:03:53.294 +54:21:55.20 22.97 0.13 23.95 0.04 0.98 0.13 24.17 0.04 24.19 0.0414:03:54.942 +54:22:03.39 23.51 0.17 24.42 0.04 0.91 0.17 24.53 0.05 24.70 0.0714:03:53.180 +54:22:04.81 22.49 0.17 24.21 0.06 1.72 0.18 24.29 0.08 25.49 0.2014:03:54.810 +54:22:08.67 22.95 0.10 24.21 0.06 1.26 0.18 24.29 0.08 24.73 0.1314:03:54.551 +54:22:13.94 22.97 0.15 25.00 0.05 2.03 0.16 25.03 0.07 25.81 0.1014:03:36.863 +54:23:00.19 22.96 0.14 24.47 0.04 1.51 0.15 24.79 0.05 24.36 0.0414:03:53.453 +54:21:37.31 23.31 0.17 24.17 0.03 0.86 0.17 24.48 0.02 24.66 0.0314:03:52.091 +54:21:27.92 24.08 0.17 – – – – – – – –14:03:44.772 +54:21:38.30 23.93 0.17 – – – – – – – –14:03:47.409 +54:21:40.92 23.99 0.18 – – – – – – – –14:03:53.946 +54:21:46.79 24.21 0.14 – – – – – – – –14:03:43.399 +54:22:10.12 24.41 0.20 – – – – – – – –14:03:46.325 +54:22:24.73 23.88 0.11 – – – – – – – –14:03:49.713 +54:23:06.55 23.75 0.12 – – – – – – – –14:03:45.544 +54:23:10.90 23.78 0.15 – – – – – – – –14:03:48.384 +54:23:21.04 23.94 0.10 – – – – – – – –14:03:45.830 +54:23:23.92 24.05 0.12 – – – – – – – –
24 –Table 1: (continued)
RA DEC F469N err F435W err F435W-F469N err F555W err F814W err14:03:43.572 +54:20:43.93 24.15 0.17 – – – – – – – –14:03:42.956 +54:20:56.05 23.94 0.17 – – – – – – – –14:03:43.450 +54:21:03.90 24.33 0.12 – – – – – – – –14:03:42.025 +54:21:38.77 23.38 0.13 – – – – – – – –14:03:52.247 +54:21:50.81 23.89 0.13 – – – – – – – –14:03:53.618 +54:21:51.54 23.68 0.11 – – – – – – – –14:03:47.278 +54:21:52.04 23.67 0.18 – – – – – – – –14:03:51.110 +54:22:05.84 24.00 0.12 – – – – – – – –14:03:53.577 +54:22:09.21 23.93 0.18 – – – – – – – –14:03:43.080 +54:22:17.27 23.99 0.11 – – – – – – – –14:03:46.935 +54:22:18.13 24.33 0.16 – – – – – – – –14:03:48.954 +54:22:18.41 23.85 0.13 – – – – – – – –14:03:52.630 +54:22:19.51 23.73 0.15 – – – – – – – –14:03:50.926 +54:22:31.61 23.86 0.15 – – – – – – – –14:03:46.177 +54:22:33.89 23.34 0.11 – – – – – – – –14:03:36.468 +54:22:40.98 24.46 0.19 – – – – – – – –14:03:51.363 +54:22:42.89 24.27 0.23 – – – – – – – –14:03:39.703 +54:23:07.76 23.89 0.14 – – – – – – – –14:03:43.267 +54:23:10.18 23.64 0.11 – – – – – – – –14:03:48.790 +54:23:30.75 23.90 0.10 – – – – – – – –14:03:46.573 +54:20:58.59 24.09 0.19 – – – – – – – –14:03:40.353 +54:21:10.67 24.31 0.30 – – – – – – – –14:03:51.245 +54:21:17.95 23.77 0.14 – – – – – – – –14:03:47.905 +54:21:19.20 24.00 0.18 – – – – – – – –14:03:45.569 +54:22:03.20 24.02 0.16 – – – – – – – –14:03:53.393 +54:22:22.78 23.48 0.17 – – – – – – – –14:03:35.495 +54:22:50.76 23.89 0.18 – – – – – – – –14:03:43.473 +54:23:19.11 24.89 0.31 – – – – – – – –14:03:40.107 +54:21:14.66 23.85 0.26 – – – – – – – –14:03:44.015 +54:21:48.57 24.30 0.20 – – – – – – – –14:03:40.020 +54:21:49.87 24.62 0.48 – – – – – – – –14:03:49.533 +54:21:57.22 24.18 0.35 – – – – – – – –14:03:46.586 +54:22:44.52 23.78 0.13 – – – – – – – –14:03:43.833 +54:23:10.16 24.00 0.15 – – – – – – – –14:03:48.099 +54:23:39.66 24.67 0.26 – – – – – – – –14:03:45.029 +54:21:01.14 23.57 0.20 – – – – – – – –14:03:52.031 +54:21:51.80 23.04 0.12 – – – – – – – –14:03:52.886 +54:21:55.63 23.44 0.14 – – – – – – – –14:03:50.001 +54:23:19.95 24.07 0.30 – – – – – – – –14:03:40.677 +54:22:19.23 24.65 0.20 – – – – – – – –14:03:43.142 +54:22:34.54 24.39 0.19 – – – – – – – –14:03:43.161 +54:22:33.92 24.35 0.18 – – – – – – – –14:03:38.669 +54:22:18.74 24.12 0.16 – – – – – – – –
25 –Table 2: HST/ACS photometry of Red Supergiant candidates in one pointing of M101. Intotal we identify 164 RSG candidates using color and magnitude cuts provided by B. Davies(priv. communication). Note that all magnitudes presented use the Vega magnitude system,for which zero points were provided by Josh Sokol from the ACS instrument team. Errorslisted represent 1 σ errors which are calculated in daophot . RA DEC F435W err F555W err F814W err14:03:53.731 +54:21:53.65 25.40 0.05 24.02 0.03 21.62 0.0514:03:51.361 +54:21:13.24 25.40 0.05 23.95 0.04 21.61 0.0514:03:52.891 +54:21:45.49 24.94 0.04 23.48 0.02 21.62 0.0314:03:51.615 +54:21:35.75 24.93 0.08 23.58 0.03 21.60 0.0314:03:52.696 +54:21:38.56 25.63 0.08 24.32 0.04 21.73 0.0514:03:52.978 +54:21:45.75 25.35 0.04 23.89 0.03 21.74 0.0414:03:55.206 +54:21:54.40 25.65 0.04 24.12 0.03 21.74 0.0414:03:36.509 +54:22:49.55 25.30 0.05 23.91 0.04 21.75 0.0514:03:51.397 +54:22:46.20 25.33 0.05 23.98 0.03 21.78 0.0614:03:53.121 +54:21:27.84 25.14 0.07 23.84 0.03 21.79 0.0414:03:35.420 +54:21:11.50 26.33 0.08 25.08 0.07 21.79 0.0814:03:53.275 +54:21:21.52 26.43 0.07 24.91 0.05 21.79 0.0614:03:52.172 +54:21:47.34 25.19 0.07 23.68 0.04 21.81 0.0614:03:36.375 +54:20:48.33 25.23 0.04 23.96 0.03 21.82 0.0414:03:54.280 +54:21:32.75 25.32 0.07 23.89 0.04 21.82 0.0514:03:53.141 +54:21:47.40 25.25 0.07 23.84 0.03 21.85 0.0414:03:53.387 +54:21:27.42 25.03 0.10 23.73 0.05 21.86 0.0614:03:56.414 +54:22:32.07 25.43 0.08 24.03 0.05 21.88 0.0614:03:51.467 +54:21:14.10 25.38 0.05 23.96 0.05 21.89 0.0614:03:42.430 +54:23:36.30 25.15 0.03 23.61 0.02 21.57 0.0414:03:52.154 +54:21:39.26 25.25 0.07 23.67 0.04 21.52 0.0514:03:51.011 +54:21:08.24 25.94 0.08 24.46 0.04 21.70 0.0514:03:54.974 +54:21:53.05 25.53 0.07 24.14 0.05 21.49 0.0514:03:53.114 +54:21:47.13 25.06 0.04 23.46 0.03 21.42 0.0314:03:52.144 +54:21:38.98 25.07 0.07 23.49 0.03 21.42 0.0414:03:34.066 +54:22:47.59 25.25 0.03 23.97 0.03 21.41 0.0414:03:52.364 +54:21:34.34 24.93 0.07 23.45 0.02 21.39 0.0314:03:52.121 +54:21:41.35 24.65 0.04 23.37 0.03 21.40 0.0414:03:41.571 +54:23:52.62 24.89 0.04 23.37 0.03 21.40 0.0514:03:36.366 +54:23:07.19 25.37 0.07 24.15 0.06 21.38 0.0914:03:51.947 +54:21:47.81 25.02 0.12 23.60 0.05 21.37 0.0614:03:51.641 +54:21:37.61 24.62 0.10 23.17 0.02 21.08 0.0414:03:54.354 +54:21:33.96 24.45 0.06 23.23 0.03 21.30 0.0414:03:54.335 +54:21:33.92 25.35 0.07 23.98 0.04 21.30 0.0514:03:45.593 +54:22:35.95 25.09 0.11 23.63 0.11 21.27 0.1614:03:52.214 +54:21:28.58 25.27 0.04 23.96 0.06 21.28 0.0714:03:50.700 +54:21:17.50 25.13 0.06 23.63 0.06 21.26 0.0714:03:51.731 +54:21:37.81 25.49 0.04 23.94 0.02 21.15 0.02
26 –Table 2: (continued)
RA DEC F435W err F555W err F814W err14:03:51.696 +54:21:18.03 24.66 0.05 23.15 0.06 21.13 0.0914:03:52.707 +54:21:48.57 24.40 0.06 22.87 0.05 21.03 0.0614:03:51.869 +54:21:33.31 24.61 0.04 23.34 0.12 21.05 0.1314:03:54.266 +54:22:09.33 24.16 0.06 22.91 0.04 21.06 0.0614:03:52.993 +54:21:54.36 24.06 0.10 22.85 0.10 20.99 0.1014:03:36.424 +54:21:20.63 24.25 0.03 22.87 0.03 20.99 0.0414:03:53.214 +54:21:58.09 24.32 0.07 22.84 0.05 20.70 0.0614:03:53.119 +54:21:40.82 24.67 0.03 23.13 0.02 20.72 0.0314:03:51.525 +54:21:21.18 24.27 0.02 22.80 0.02 20.68 0.0314:03:51.344 +54:21:00.17 24.25 0.09 22.85 0.06 20.51 0.0714:03:52.993 +54:21:54.36 24.06 0.10 22.85 0.10 20.44 0.1014:03:35.731 +54:22:49.13 23.59 0.03 22.20 0.03 19.60 0.0414:03:53.537 +54:21:59.31 23.27 0.08 21.91 0.03 20.01 0.0414:03:53.403 +54:21:59.38 23.48 0.05 22.12 0.03 20.23 0.0514:03:54.224 +54:21:51.88 23.61 0.06 22.23 0.03 20.24 0.0414:03:52.916 +54:21:50.23 23.63 0.03 22.28 0.03 20.29 0.0514:03:56.477 +54:22:11.98 23.84 0.03 22.02 0.04 19.41 0.0514:03:56.795 +54:21:54.97 23.90 0.03 22.02 0.03 20.01 0.0414:03:41.464 +54:23:35.21 24.46 0.05 22.75 0.03 20.37 0.0414:03:52.410 +54:21:49.64 24.34 0.09 22.60 0.03 20.43 0.0414:03:44.033 +54:23:05.94 25.49 0.05 23.84 0.03 20.45 0.0414:03:52.476 +54:21:34.12 24.68 0.03 22.80 0.02 20.50 0.0314:03:52.475 +54:21:33.35 24.57 0.04 22.90 0.04 20.61 0.0414:03:54.025 +54:22:01.54 25.00 0.06 23.09 0.04 20.62 0.0514:03:52.267 +54:21:50.93 24.64 0.07 22.94 0.03 20.69 0.0514:03:53.067 +54:22:03.94 24.36 0.04 22.53 0.03 20.72 0.0414:03:55.499 +54:21:35.99 24.88 0.04 23.07 0.03 20.78 0.0414:03:53.845 +54:21:54.61 24.53 0.05 22.84 0.03 20.79 0.0414:03:54.357 +54:21:34.98 24.48 0.05 22.76 0.04 20.79 0.0614:03:51.656 +54:22:05.41 25.01 0.05 23.33 0.03 20.81 0.0514:03:53.157 +54:21:54.75 25.15 0.10 23.31 0.09 20.85 0.1314:03:53.882 +54:21:50.27 24.90 0.03 23.06 0.02 20.87 0.0314:03:52.740 +54:21:43.79 24.57 0.03 22.83 0.03 20.90 0.0314:03:52.598 +54:22:08.88 24.68 0.04 22.94 0.03 20.91 0.0314:03:51.297 +54:21:19.64 24.76 0.05 22.96 0.04 20.92 0.0514:03:36.903 +54:22:09.95 25.33 0.04 23.49 0.04 20.93 0.0514:03:50.700 +54:21:17.50 25.13 0.06 23.37 0.04 20.96 0.0514:03:51.813 +54:21:34.43 24.72 0.04 22.95 0.03 20.98 0.0414:03:52.838 +54:21:36.65 24.76 0.05 23.02 0.03 20.99 0.0414:03:53.105 +54:21:36.27 24.93 0.03 23.08 0.02 21.00 0.0214:03:54.367 +54:21:46.68 25.23 0.04 23.42 0.02 21.02 0.0314:03:50.964 +54:21:07.60 25.37 0.06 23.52 0.03 21.02 0.04
27 –Table 2: (continued)
RA DEC F435W err F555W err F814W err14:03:56.166 +54:21:59.42 24.90 0.04 23.17 0.04 21.03 0.0514:03:51.522 +54:21:36.49 25.64 0.05 23.81 0.04 21.08 0.0414:03:54.866 +54:21:54.63 25.00 0.05 23.16 0.03 21.09 0.0414:03:52.377 +54:21:29.64 25.22 0.03 23.37 0.04 21.11 0.0514:03:53.444 +54:21:14.05 24.97 0.04 23.17 0.03 21.12 0.0514:03:51.733 +54:21:35.50 25.18 0.07 23.35 0.09 21.13 0.1214:03:52.200 +54:21:36.24 25.33 0.07 23.07 0.03 21.18 0.0414:03:52.195 +54:21:36.42 24.74 0.03 23.07 0.03 21.18 0.0414:03:52.744 +54:21:20.43 25.61 0.05 23.84 0.04 21.18 0.0514:03:51.808 +54:21:35.15 25.24 0.03 23.42 0.02 21.21 0.0314:03:51.182 +54:21:57.21 26.08 0.07 24.27 0.03 21.23 0.0514:03:54.198 +54:22:00.43 24.92 0.03 23.29 0.02 21.24 0.0314:03:53.621 +54:21:47.58 25.09 0.04 23.36 0.03 21.24 0.0414:03:54.963 +54:21:55.03 25.34 0.04 23.60 0.03 21.26 0.0414:03:54.378 +54:22:13.81 25.54 0.05 23.61 0.03 21.26 0.0414:03:50.700 +54:21:17.50 25.13 0.06 23.37 0.04 21.26 0.0614:03:54.288 +54:21:52.26 25.00 0.06 23.32 0.02 21.27 0.0314:03:51.808 +54:21:43.68 25.08 0.06 23.31 0.02 21.29 0.0214:03:42.360 +54:23:48.05 25.04 0.08 23.26 0.06 21.30 0.0814:03:44.006 +54:23:36.89 25.37 0.10 23.62 0.12 21.32 0.1814:03:55.025 +54:22:27.55 25.15 0.06 23.46 0.03 21.33 0.0414:03:51.405 +54:21:41.58 24.98 0.03 23.28 0.01 21.34 0.0214:03:52.443 +54:21:23.97 25.73 0.06 23.77 0.04 21.34 0.0514:03:54.388 +54:21:34.39 25.89 0.07 23.30 0.03 21.35 0.0414:03:52.154 +54:21:37.76 25.54 0.05 23.78 0.03 21.36 0.0414:03:38.802 +54:21:47.99 25.63 0.06 23.88 0.03 21.37 0.0414:03:56.427 +54:22:04.82 25.48 0.05 23.60 0.03 21.41 0.0514:03:54.025 +54:21:42.82 25.19 0.04 23.59 0.03 21.43 0.0414:03:50.921 +54:21:22.55 25.37 0.05 23.53 0.03 21.44 0.0414:03:52.789 +54:21:44.94 25.20 0.03 23.44 0.01 21.45 0.0214:03:53.838 +54:21:38.62 26.29 0.08 24.27 0.03 21.46 0.0314:03:52.447 +54:21:49.52 24.99 0.06 23.31 0.05 21.46 0.0614:03:54.564 +54:21:19.97 25.25 0.05 23.50 0.02 21.48 0.0714:03:53.208 +54:21:50.69 25.27 0.05 23.46 0.03 21.50 0.0514:03:54.040 +54:21:57.44 25.67 0.06 23.78 0.03 21.53 0.0414:03:53.151 +54:21:33.80 25.82 0.05 24.16 0.05 21.53 0.0614:03:54.772 +54:22:01.28 25.45 0.05 23.76 0.04 21.54 0.0514:03:53.027 +54:21:34.49 25.34 0.05 23.56 0.03 21.55 0.0414:03:53.238 +54:21:51.47 25.13 0.07 23.50 0.03 21.59 0.0414:03:51.968 +54:21:33.02 25.84 0.07 24.06 0.03 21.60 0.0414:03:52.200 +54:21:36.24 25.33 0.07 23.58 0.07 21.60 0.0914:03:35.520 +54:22:17.92 25.22 0.07 23.61 0.02 21.62 0.0414:03:53.763 +54:21:53.60 25.77 0.07 24.15 0.04 21.62 0.06
28 –Table 2: (continued)
X Y F435W err F555W err F814W err14:03:54.566 +54:21:45.77 25.26 0.04 23.57 0.02 21.63 0.0314:03:56.658 +54:22:22.04 25.63 0.05 23.82 0.03 21.64 0.0414:03:52.822 +54:21:44.51 25.45 0.04 23.84 0.03 21.64 0.0414:03:54.379 +54:21:48.41 25.84 0.06 24.04 0.02 21.65 0.0314:03:49.694 +54:21:13.17 25.89 0.07 24.15 0.04 21.65 0.0514:03:54.057 +54:22:02.87 25.09 0.05 23.48 0.03 21.66 0.0414:03:54.617 +54:22:56.56 25.32 0.04 23.69 0.02 21.66 0.0314:03:37.847 +54:22:02.26 25.47 0.05 23.81 0.03 21.66 0.0414:03:43.282 +54:22:26.49 25.72 0.05 24.01 0.02 21.67 0.0314:03:51.967 +54:22:53.10 25.58 0.07 23.98 0.05 21.67 0.0614:03:52.185 +54:21:18.89 25.65 0.07 24.04 0.05 21.67 0.0614:03:53.076 +54:21:35.38 25.73 0.06 23.86 0.04 21.68 0.0414:03:53.446 +54:22:02.87 25.87 0.16 23.79 0.04 21.68 0.0514:03:52.282 +54:21:58.84 26.00 0.08 24.05 0.04 21.68 0.0514:03:53.326 +54:21:52.79 25.32 0.08 23.65 0.06 21.69 0.0814:03:51.768 +54:21:23.80 25.75 0.07 23.90 0.04 21.70 0.0514:03:51.756 +54:21:23.63 25.76 0.10 24.01 0.06 21.70 0.0614:03:56.264 +54:21:53.28 25.64 0.11 24.04 0.04 21.70 0.0514:03:50.450 +54:21:54.07 25.53 0.06 23.75 0.03 21.70 0.0514:03:54.868 +54:21:59.07 25.89 0.09 24.07 0.03 21.72 0.0414:03:53.418 +54:21:51.24 25.52 0.06 23.77 0.03 21.73 0.0414:03:55.619 +54:21:56.82 25.72 0.06 23.94 0.03 21.75 0.0414:03:51.115 +54:21:27.39 25.83 0.06 24.12 0.03 21.76 0.0414:03:53.779 +54:21:59.47 25.50 0.05 23.63 0.04 21.76 0.0514:03:39.781 +54:21:07.18 25.76 0.07 23.91 0.03 21.76 0.0514:03:54.274 +54:21:40.79 25.83 0.05 24.21 0.04 21.80 0.0514:03:51.490 +54:21:21.41 25.64 0.05 24.02 0.03 21.82 0.0414:03:43.061 +54:22:28.83 25.60 0.06 23.94 0.04 21.82 0.0514:03:52.248 +54:21:32.04 25.50 0.05 23.87 0.02 21.83 0.0314:03:37.785 +54:22:13.83 25.93 0.04 24.15 0.03 21.83 0.0414:03:40.408 +54:23:25.01 25.58 0.06 23.91 0.03 21.83 0.0514:03:52.086 +54:21:37.68 25.66 0.06 23.92 0.02 21.84 0.0314:03:52.948 +54:21:42.57 25.51 0.06 23.88 0.03 21.84 0.0414:03:51.309 +54:21:20.38 25.57 0.07 23.95 0.06 21.84 0.0714:03:42.129 +54:23:49.29 25.39 0.08 23.75 0.05 21.86 0.0614:03:53.636 +54:21:38.60 25.82 0.04 24.17 0.02 21.87 0.0314:03:53.212 +54:21:59.41 25.44 0.06 23.82 0.04 21.87 0.0514:03:54.313 +54:22:04.51 25.85 0.15 24.05 0.04 21.88 0.0514:03:53.181 +54:21:45.97 25.76 0.05 24.08 0.04 21.89 0.05
29 –clustering of WR candidates in the central core of the largest star-forming complex in thefield. Thousands of WR and RSG candidates, and hundreds of spectrographically confirmedWR stars will be reported in future papers in this series.This research is based on NASA/ESA Hubble Space Telescope observations obtained atthe Space Telescope Science Institute, which is operated by the Association of Universities forResearch in Astronomy Inc. under NASA contract NAS5-26555. JLB and MMS acknowledgethe interest and generous support of Hilary and Ethel Lipsitz. AFJM and LD are gratefulto NSERC (Canada) and FQRNT (Quebec) for financial assistance. We thank Or Graur forsuggestions on displaying the contours shown in Figure 10.
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