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The Astrophysical Journal, 633:871 ­ 893, 2005 November 10
# 2005. The American Astronomical Society. All rights reserved. Printed in U.S.A.

A

STAR FORMATION IN NGC 5194 ( M51a): THE PANCHROMATIC VIEW FROM GALEX TO SPITZER
D. Calzetti,2 R. C. Kennicutt, Jr.,3 L. Bianchi,4 D. A. Thilker,4 D. A. Dale,5 C. W. Engelbracht,3 C. Leitherer, M. J. Meyer,2 M. L. Sosey,2 M. Mutchler,2 M. W. Regan,2 M. D. Thornley,6 L. Armus,7 G. J. Bendo,3 S. Boissier,8 A. Boselli,9 B. T. Draine ,10 K. D. Gordon,3 G. Helou,7 D. J. Hollenbach,11 L. Kewley,12 B. F. Madore,8 D. C. Martin,13 E. J. Murphy,14 G. H. Rieke,3 M. J. Rieke,3 H. Roussel,7 K. Sheth,7 J. D. Smith,3 F. Walter,15 B. A. White,3 S. Yi,16 N. Z. Scoville,13 M. Polletta,13 and D. Lindler17
Received 2005 April 4; accepted 2005 June 15
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ABSTRACT Far-ultraviolet to far-infrared images of the nearby galaxy NGC 5194 ( M51a), from a combination of space-based (Spitzer, GALEX, and Hubble Space Telescope) and ground-based data, are used to investigate local and global star formation and the impact of dust extinction. The Spitzer data provide unprecedented spatial detail in the infrared, down to sizes $500 pc at the distance of NGC 5194. The multiwavelength set is used to trace the relatively young stellar populations, the ionized gas, and the dust absorption and emission in H ii ­ emitting knots, over 3 orders of magnitude in wavelength range. As is common in spiral galaxies, dust extinction is high in the center of the galaxy (AV $ 3:5 mag), but its mean value decreases steadily as a function of galactocentric distance, as derived from both gas emission and stellar continuum properties. In the IR / UV-UV color plane, the NGC 5194 H ii knots show the same trend observed for normal star-forming galaxies, having a much larger dispersion ($1 dex peak to peak) than starburst galaxies. We identify the dispersion as due to the UV emission predominantly tracing the evolved, nonionizing stellar population, up to ages $50 ­100 Myr. While in starbursts the UV light traces the current star formation rate (SFR), in NGC 5194 it traces a combination of current and recent past SFRs. Possibly, mechanical feedback from supernovae is less effective at removing dust and gas from the star formation volume in normal star-forming galaxies than in starbursts because of the typically lower SFR densities in the former. The application of the starburst opacity curve for recovering the intrinsic UV emission (and deriving SFRs) in local and distant galaxies appears therefore appropriate only for SFR densities k1 M yrþ1 kpcþ2. Unlike the UV emission, the monochromatic 24 m luminosity is an accurate local SFR tracer for the H ii knots in NGC 5194, with a peak-to-peak dispersion of less than a factor of 3 relative to hydrogen emission line tracers; this suggests that the 24 m emission carriers are mainly heated by the young, ionizing stars. However, preliminary results show that the ratio of the 24 m emission to the SFR varies by a factor of a few from galaxy to galaxy; this variation needs to be understood and carefully quantified before the 24 m luminosity can be used as an SFR tracer for galaxy populations. While also correlated with star formation, the 8 m emission is not directly proportional to the number of ionizing photons; it is overluminous, by up to a factor of $2, relative to the galaxy's average in weakly ionized regions and is underluminous, by up to a factor of $3, in strongly ionized regions. This confirms earlier suggestions that the carriers of the 8 m emission are heated by more than one mechanism. Subject headings: galaxies: interactions -- galaxies: ISM -- galaxies: starburst -- ISM: structure g Online material: color figures, machine-readable table

1. INTRODUCTION Over the past decade, discoveries of galaxy populations at earlier and earlier cosmic times have rekindled interest in star formation rate (SFR) indicators, estimated from a variety of monochromatic and nonmonochromatic emission measurements across the full spectrum. Of particular interest for cosmological studies
1 Based on observations obtained with the Spitzer Space Telescope and with GALEX. 2 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218; calzetti@stsci.edu. 3 Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721. 4 Department of Physics and Astronomy, Johns Hopkins University, Charles and 34th Street, Baltimore, MD 21218. 5 Department of Physics, University of Wyoming, Laramie, WY 82071. 6 Department of Physics, Bucknell University, Lewisburg , PA 17837. 7 Spitzer Science Center, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125. 8 Carnegie Observatories, Carnegie Institution of Washington, 813 Santa Barbara Street, Pasadena, CA 91101. 9 Laboratoire d'Astrophysique de Marseille, BP8, Traverse du Siphon, F-13376 Marseille, France.

are indicators exploiting measurements at rest-frame ultraviolet ( UV ), optical, and mid /far-infrared ( MIR / FIR) wavelengths; the interest has, however, accompanied a renewed awareness that potential limitations are not fully quantified yet. Presence of even small amounts of dust extinction in the early galaxies hampers significantly SFR measurements from the rest-frame UV emission of the high-redshift galaxies (e.g., the Lyman break galaxies; Steidel et al. 1999; Erb et al. 2003; Giavalisco et al. 2004; Reddy & Steidel 2004). At the other end of the spectrum, our still
Princeton University Observatory, Peyton Hall, Princeton, NJ 08544. NASA Ames Research Center, Moffett Field, CA 94035. Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822. 13 Astronomy Option, California Institute of Technology, MS 105-24, Pasadena, CA 91125. 14 Department of Astronomy, Yale University, P.O. Box 208101, New Haven, CT 06520. 15 Max-Planck-Institut f ur Astronomie, Koenigstuhl 17, D-69117 Heidelberg, ¨ Germany. 16 Physics Department, Oxford University, Clarendon Laboratory, Parks Road, Oxford OX13PU, UK. 17 Sigma Research and Engineering, Lanham, MD 20706.
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limited understanding of the infrared spectral energy distribution (SED) of galaxies may decrease the SFR prediction power of the submillimeter emission of the IR-bright SCUBA sources (e.g., Barger et al. 2000; Smail et al. 2000; Chapman et al. 2003, 2004). Even in the local universe, the widely different angular scales that have characterized until recently UV, optical, and infrared observations of galaxies, ranging from arcsecond /subarcsecond resolution for UV/optical to multiple arcseconds /arcminutes for FIR (IRAS, Infrared Space Observatory [ISO]), have so far limited our ability to understand in detail the applicability of each indicator within the realm of physically complex systems ( Kennicutt 1998b; Kewley et al. 2002; Rosa-Gonzalez et al. 2002). This in turn has inhibited attempts at cross-correlating calibrations of SFR indicators. The issue lays in how well tracers at each wavelength can measure the actual SFR. The main problem afflicting UV and optical SFR indicators is dust obscuration. There are two aspects to this problem. One is that regions with moderate amounts of dust will be dimmed in a way that depends not only on the amount of dust but also on the distribution of the emitters relative to the absorbers. This problem is exacerbated by the fact that populations of different ages suffer different amounts of dust extinction (Calzetti et al. 1994; Charlot & Fall 2000; Zaritsky et al. 2004). Recently it has been shown that quiescently star-forming galaxies follow a different dust opacity-reddening relation than starburst galaxies ( Buat et al. 2002; Bell 2002; Gordon et al. 2004; Buat et al. 2005; Seibert et al. 2005; Laird et al. 2005). In particular, their IR / UV ratio, a measure of dust opacity, is on average lower than that of starbursts for the same UV color, a measure of dust reddening, and shows a larger spread; differences in the ``b parameter '' (the ratio of current to lifetime SFR) between starforming and starburst galaxies have been invoked as an explanation for the observed difference ( Kong et al. 2004). A second problem is the unknown fraction of star formation that is completely obscured by dust at UV and optical wavelengths. The UV and FIR may, indeed, probe different regions /stages of star formation. Heavy obscuration is generally tied to the first temporal phases of star formation, roughly a few million years; as the stars age, they tend to drift off the parental cloud and diffuse in regions of lower gas /dust density or to disperse the natal gas /dust cloud ( Leisawitz & Hauser 1988). Estimates indicate that the fraction of completely obscured star formation is relatively small in the local universe, $20% ­ 30% (Calzetti et al. 1995; Heckman 1999; Calzetti 2001), but uncertainties are large and their impact on the calibration of SFR indicators mostly unprobed. A comprehensive attack to these problems is a core goal of the Spitzer Infrared Nearby Galaxies Survey project (SINGS; Kennicutt et al. 2003). This paper presents the first case study based on the well-known grand-design spiral galaxy NGC 5194 ( M51a, Whirlpool galaxy). We use a multiwavelength data set of the galaxy by combining UV images from GALEX, groundbased optical images, infrared emission line images from HST NICMOS (Scoville et al. 2001), and Spitzer 3.5 ­ 160 m images. These data provide a panchromatic view of the star formation in this galaxy, both locally (on the scales of star formation complexes) and globally. Spitzer and GALEX observations of nearby galaxies (closer than $10 Mpc) are uniquely suited for investigating issues of dust obscuration and star formation, thanks to a combination of comparatively high angular resolution (a few arcseconds) and the large fields of view (many arcminutes). We use the multiwavelength data to investigate the opacity-reddening properties of this quiescently star-forming galaxy on a detailed spatial scale. The UV, MIR, and FIR emissions are then compared with the optical (nebular lines) emission, both locally and

globally, to test the viability of each as an SFR indicator. For instance, the 8 m emission is a potentially attractive SFR indicator at high redshifts, as the rest-frame $8 m polycyclic aromatic hydrocarbon ( PAH ) bands are redshifted to k k 24 m for z k 2, thus still within the regime probed by, e.g., Spitzer.In addition, unlike tracers that probe directly the stellar light, MIR / FIR SFR tracers are little affected by dust extinction. There are a number of reasons for why NGC 5194 is an optimal target for this study. At a distance of about 8.2 Mpc ( from the systemic velocity of Tully [1988] and H0 ¼ 70 km sþ1 Mpcþ1), the typical angular resolution of our mid-infrared data, 500 ­1300 , corresponds to $200 ­ 520 pc, or the size of a large star formation complex. The relatively high level of spatial detail enables us to investigate the nature of the difference in the opacity-reddening properties between starbursts ( Meurer et al. 1999) and quiescent star-forming galaxies ( Kong et al. 2004). The total SFR, $3.4 M yrþ1, and the SFR /area, $0.015 M yrþ1 kpcþ2, of this galaxy place it among the ``quiescently'' star-forming systems, despite its interaction with the early-type galaxy NGC 5195 ( M51b), the latter located about 4A4 (10.5 kpc) to the north. NGC 5194 is a nearly face-on (i $ 20 ), grand-design spiral (SAbc), with intense star formation in the center and along the spiral arms. Its OB association population, the gas they ionize, and the diffuse ionized medium have been extensively investigated at optical and infrared wavelengths ( Kennicutt et al. 1989; Scoville et al. 2001; Thilker et al. 2002; Hoopes & Walterbos 2003). The UV emission shows a strong color gradient as a function of distance from the nucleus, becoming bluer at larger galactocentric distances, based on the GALEX images ( Bianchi et al. 2005). This is similar to what was previously found by Hill et al. (1997) from UV þ U radial color trends, and a comparison with IRAS images suggests that this color gradient is induced by a gradient in the dust extinction ( Boissier et al. 2004). NGC 5194 is a metal-rich galaxy [12 × log (O/H) $ 8:7 8:9; Bresolin et al. 2004], with a weak metallicity gradient as a function of distance from the nucleus out to at least 10 kpc radius, or $75% the B25 radius ( Zaritsky et al. 1994). When comparing properties of H ii knots within the galaxy, the shallow metallicity trend enables us to investigate stellar population aging effects unencumbered by the metallicity variations that affect galaxy-to-galaxy comparisons. The present paper is organized as follows: x 2 presents the observations, relevant data reduction considerations, and the main characteristics of the data set; x 3 is a short overview of the galaxy's morphology at different wavelengths; x 4 presents the measurements of H ii ­ emitting regions performed on the images; x 5 describes the observed properties of these H ii ­ emitting regions; x 6 presents dust extinction properties; x 7 analyzes the properties of popular SFR indicators, while the results are discussed in x 8. A summary is given in x 9. 2. OBSERVATIONS AND THE DATA SET 2.1. Spitzer Images The Spitzer images of M51 ( NGC 5194/ NGC 5195) were obtained with both IRAC (3.6, 4.5, 5.8, and 8.0 m) and MIPS (24, 70, and 160 m), as part of the SINGS Legacy project. A description of this project and the observing strategy can be found in Kennicutt et al. (2003). Each of the four IRAC images is a combination of two mosaics, each resulting from a 6 ; 9 grid covering a 18A5 ; 25 0 field. Observations of each mosaic were obtained on 2004 May 18 and 22, allowing a separation of a few days between the two to enable recognition and exclusion of asteroids and detector artifacts. Total exposure times in each filter are 240 s in the center of


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the field and 120 s at the edges (outer $2A5). The SINGS IRAC pipeline was used to create the final mosaics, which exploits the subpixel dithering to better sample the emission and resamples each mosaic into 0B76 pixels ( Regan et al. 2004). The measured 8 m point-spread function ( PSF ) FWHM is 2B1, and the 1 sensitivity limit in the central portion of the 8 m mosaic is 1:2 ; 10þ6 Jy arcsecþ2. A ``dust emission'' image at 8 m is obtained by subtracting the stellar contribution using the recipe of Pahre et al. (2004). The stellar emission ­ dominated 3.6 and 4.5 m images are combined assuming colors appropriate for an M0 III star (½3:6 ½4:5 ¼ þ0:15 in Vega mag; Pahre et al. 2004) and then rescaled under the same assumption to create a stellar-only image at 8 m (½3:6 ½8:0¼ 0:0 in Vega mag). A few percent adjustment of the rescaled ``stellar '' image is used to optimize the subtraction from the 8 m image. Potentially, the 3.6 and 4.5 m images can contain, in addition to photospheric emission from stars, also a component of hot dust emission. The impact of this component relative to the stellar contribution is different in the two images, with flux ratios ½ f (dust)/f (star)3:6 $ (0:3 0:7)½ f (dust)/f (star)4:5 (depending on the adopted stellar population), for dust with temperature T P 1000 K. To test whether a hot dust contribution may affect the derivation of the 8 m dust emission image, we have produced a second stellar continuum ­ subtracted 8 m image, using only the rescaled 3.6 m image as ``stellar continuum.'' The two dust images differ from each other by less than 3% across the entire region analyzed, suggesting that hot dust is not significantly impacting the stellar continuum subtraction process. MIPS observations of the galaxy were obtained on 2004 June 22 and 23. The reduction steps for MIPS mosaics are described in Gordon et al. (2005). The final mosaics have size 27 0 ; 60 0 , fully covering M51 and the surrounding background. At 24, 70, and 160 m, the PSF FWHM is $5B7, $1600 , and $3800 , respectively. The 1 detection limits are 1:1 ; 10þ6 , 8:7 ; 10þ6 , and 2:6 ; 10þ5 Jy arcsecþ2, respectively, for the 24, 70, and 160 m images. The three MIPS images are considered ``dust'' images for all purposes, as contributions from the photospheric emission of stars are negligible at these wavelengths. Consistency between the MIPS and IRAS flux scales has been checked by comparing the MIPS24 with the IRAS25 fluxes and, to a lesser extent, the MIPS70 with the IRAS60 fluxes. We get f (24) $ 12:3 Jy for NGC 5194, or about 20% lower than the IRAS25 value of 14.8 Jy (as measured from IRAS HiRes images); for the MIPS70 channel, we get f (70) $ 105 Jy, in better agreement with the IRAS60 value of 110.3 Jy, despite the slight offset between the two wave bands. The total FIR luminosity of NGC 5194, L( IR) ¼ L(3 1100 m), as derived from the MIPS fluxes (eq. [4] of Dale & Helou 2002), is log ½L( IR)/(ergs sþ1 ) ¼ 44:1, about 7% lower than the same quantity obtained from the IRAS fluxes (and using eq. [5] of Dale & Helou 2002). The nominal MIPS calibration uncertainties, $10% at 24 m and $20% in the longer wavelength bands, account for most of the discrepancies between the MIPS and IRAS fluxes and luminosities, with the possible exception of the 24 m band. However, removal / editing of the companion NGC 5195 is nontrivial in the lowresolution IRAS images, and this may account for some of the discrepancy. Indeed, the MIPS24 flux of the whole M51 pair ( NGC 5194+NGC 5195), f (24) $ 13:5 Jy, is in agreement with the 25 m flux, $13 Jy, obtained by COBE DIRBE ( from the DIRBE Point-Source Photometry Browser18).
18

GALEX ( Martin et al. 2005) imaging observations are centered at 1529 8 for the far-ultraviolet ( FUV, 1350 ­ 1750 8) and at 2312 8 for the near-ultraviolet ( NUV, 1750 ­ 2750 8) bands. Data for M51 were obtained on 2003 June 19 ­ 20 as part of the Nearby Galaxies Survey ( NGS; described by Bianchi et al. 2003a, 2003b). The exposure time of 1414 s yields an NUV ( FUV ) 1 sensitivity limit of 1:4 ; 10þ19 (3:6 ; 10þ19 ) ergs sþ1 cmþ2 8þ1 arcsecþ2 at the PSF scale ( FWHM ¼ 4B6). More details on the GALEX data, as well as a comparison with previous UIT data, are given by Bianchi et al. (2005). The latest photometric calibrations ( IR1 release, 2004 November) were applied to the two GALEX images of M51. The measured FWHM of the GALEX PSF is only slightly smaller than the MIPS 24 m PSF, making the comparison between the two sets of images straightforward. Distortions present in the FUV image were corrected by application of nonlinear geometric transformations to the image, using the optical images as reference. Residual distortions amount to P1B2, negligible for the purpose of this analysis (which employs $10 times larger apertures to perform photometry; see x 4). 2.3. HST NICMOS Images Observations with HST NICMOS are available for the central region of NGC 5194 in the Pa emission line (1.8756 m, F187N narrowband filter) and the adjacent continuum ( F190N narrowband filter). The image is a 3 ; 3 NIC3 mosaic (GO-7237; PI: Scoville) that spans the central 14400 , or the inner $6 kpc of the galaxy. Details of the observations, data reduction, and mosaicking are given in Scoville et al. (2001). Because of the proximity in wavelength of the two narrowband filters, the line-only image is obtained by subtracting the continuum-only image, previously rescaled by the ratio of the filters' efficiencies, from the line+continuum image. The NIC3 0B2 pixels undersample the NICMOS PSF, although this is not a concern for the diffuse ionized gas emission of interest here. The continuum-subtracted Pa image shows a diagonal tilt in the background, which is removed by fitting an inclined linear surface to the image (using the task IMSURFIT in IRAF ). The resulting image shows a relatively flat background. The sensitivity is variable, being lower at the seams of the nine images that form the mosaic. The average 1 sensitivity limit of the continuumsubtracted image is 1:8 ; 10þ16 ergs sþ1 cmþ2 arcsecþ2. The region of the galaxy imaged in Pa offers a unique opportunity, in conjunction with the H image (next section), to directly probe the impact of dust obscuration on the ionized gas and to measure star formation using an indicator ( Pa) weakly affected by dust. An extinction of 1 mag at V produces an extinction of 0.15 mag at Pa, i.e., a small, $14% change in the line intensity. We adopt an intrinsic ratio H /Pa ¼ 8:734 (Osterbrock 1989) and differential value k (H ) þ k (Pa ) ¼ 2:08 for the extinction curve. The central $6 kpc of NGC 5194 are characterized by observed H/Pa ratios that are smaller than the theoretical unreddened ratio, implying attenuations at V in the range AV $ 1 3:4. In what follows, the central region imaged in Pa will be referred to as the inner region, while areas external to this will be globally referred to as the outer region. 2.4. Ground-based Optical Images H-centered narrowband, B-band, and R-band images were obtained on 2001 March 28 with the Direct Camera at the 2.1 m Kitt Peak National Observatory ( KPNO) telescope, as part of the SINGS ancillary data program ( Kennicutt et al. 2003). Exposure times were 1800, 720, and 360 s for H, B, and R, respectively.

Available at http:// lambda.gsfc.nasa.gov/product /cobe/ browser.cfm.


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Standard reduction procedures were applied to the images. Both images are mosaics of two frames, displaced along the north-south direction to include both NGC 5194 and NGC 5195. Because of vignetting along one edge of the camera, correction procedures were applied; the photometric integrity along the seam of the mosaic was verified from comparing measurements of stars along the vignetted side of the mosaic with the same stars on the nonvignetted side. Standard-star observations were obtained during the observing run to derive photometric calibrations. A U-band image of the galaxy obtained on 2004 June 20 with the Steward 90 inch (2.29 m) Prime Focus Camera ( Williams et al. 2004) is also used in this analysis to construct the stellar population age ­ sensitive color U þ B. The final combined U image is the result of two dithered images, with a total exposure time of 1200 s. Photometric calibrations were also obtained for these observations (C. W. Engelbracht et al. 2005, in preparation). A comparison of the calibrated U-band image of NGC 5194 with the analogous image from the Sloan Digital Sky Survey (SDSS) indicates a disagreement between the two calibration scales of 28%, with our image being bluer than the SDSS one. We adopt our own calibration for this work, discussing the impact of the different photometric calibration from SDSS in x 6.1. The R-band image is rescaled and subtracted from the H image, which is then corrected for the contribution of the two [N ii] kk6548, 6584 emission lines. We adopt a fixed ratio ½ N ii k6584/ H ¼ 0:5, although it should be noted that this ratio covers a wide range in NGC 5194, from $0.3 in individual H ii regions up to $1.9 in the diffuse H component ( Hoopes & Walterbos 2003). The ratio ½ N ii k6584/ H ¼ 0:5 is typical of the spatially integrated line emission from a metal-rich galaxy (e.g., compare with M83 in McQuade et al. 1995). For a ratio ½ N ii k6548/½ N ii k6584 ¼ 0:335, the observed line emission is 1:617 ; H . The shallow metallicity trend as a function of galaxy radius ( Zaritsky et al. 1994) justifies the use of a single [ N ii]/ H ratio for all H ii knots in M51. The absolute photometry of the ground-based H image is checked against archival HST WFPC2 H images of M51. The WFPC2 images cover approximately the same region as the Pa mosaic, i.e., just the inner galaxy region; more details are given in Scoville et al. (2001). The HST images are used to check for three effects: (1) absolute photometry, since our ground-based H frames were obtained in marginally photometric conditions; (2) [ N ii] contamination, since the ground-based images require a large correction, while the HST WFPC2 H filter ( F656N ) is narrow enough that only a few percent of the total flux is due to [N ii] (Scoville et al. 2001); and (3) potential oversubtractions in regions with large H equivalent widths from using the R-band image (which includes H within its bandpass) as underlying continuum, as the HST continuum images exclude H. Points 1 and 2 are not independent, and flux comparisons between 12 common, isolated H ii regions, with H fluxes between 5:5 ; 10þ15 and 1:8 ; 10þ13 and equivalent widths between 25 and 500 8, indicate a mean systematic offset of about 20% between the ground-based and the HST images, the former having lower mean flux than the latter; we correct the ground-based image for this offset. The dispersion around this value is about 20% ­ 25% and can originate from intrinsic variations of the [ N ii]/ H ratio in the H ii regions, as observed by Hoopes & Walterbos (2003). We do not correct our individual data points for this caseby-case dependent offset but carry the uncertainties accordingly. The 1 sensitivity limit of our final H image is 1:8 ; 10þ17 ergs sþ1 cmþ2 arcsecþ2. The measured PSF in the optical images is 1B9, smaller than both the GALEX and Spitzer MIPS data and comparable to the PSF of the Spitzer IRAC data.

The high-resolution ( by infrared standards) maps obtained for this study allow for the first time a comparison of the spatial location of the emission at each wavelength, from the ultraviolet, through the optical, to the infrared for this galaxy. A three-color composite of M51 using three widely used SFR indicators ( Fig. 1, left panel ) shows that the FUV, H, and 24 m emissions do not always arise from the same regions. In particular, the FUV radiation emerges predominantly along the outer edges of the spiral arms, indicating relatively low dust extinction in these regions, while the FIR dominates the inner edges. The H emission appears to preferentially follow the infrared emission, down to a very detailed level, in both knots and areas of diffuse or filamentary emission. Presence of filamentary dust emission in the interarm regions is better appreciated in the higher angular resolution image, which combines continuum-subtracted H,3.6 m continuum emission from the aged, diffuse stellar population, and 8 m dust emission ( Fig. 1, right panel ). The complex structure of the dust emission contrasts the relatively smooth stellar emission from the 3.6 m IRAC image, while, quite expectedly, the H emission clusters along the spiral arms as do the brightest knots of dust emission. Along the outermost regions of the spiral arms of NGC 5194, H appears relatively unextincted, while dust emission (and extinction) increases steadily toward the center. The 8 and 24 m images also differ in the level of contrast between the luminosities of the arms and interarm regions: the contrast is lower, by a factor of 3 ­ 4, for the 8 m dust emission than for the 24 m emission within the central 15 kpc of NGC 5194. How can diagnostics, like the FUV and the FIR, derived from light emerging at seemingly different locations effectively provide a good calibration of the star formation? 4. MULTIWAVELENGTH PHOTOMETRY OF STAR-FORMING REGIONS 4.1. Aperture Photometry For the multiwavelength comparisons that are the main goal of this work, all images have been registered to the same coordinate system and pixel scale, using the H image as reference. The MIPS 24 m ( MIPS24) images have the lowest resolution, with a PSF FWHM $ 6 00,19 and thus will be driving the minimum spatial scale that can be investigated. We choose apertures of 1300 diameter, which correspond to about 520 pc at the distance of the galaxy. We perform photometry of 166 circular, 1300 diameter regions in the FUV, NUV, U, B,H, dust-only 8 m, and 24 m images, across NGC 5194 ( Figs. 2 and 3). The regions are selected primarily as being emission peaks in the MIPS24 image, although a second pass is made through the UV images to ensure that bright regions in these are also included. The apertures are selected to be as much as possible nonoverlapping, but for a few of them some overlap is unavoidable. In these cases, checks were performed to verify that the flux contained in the overlap region did not exceed 5% of the total flux in either of the two apertures. Of the 166 apertures, 54 are located within the inner region probed by the HST Pa image ( Fig. 3), and for these, the Pa flux is also measured. In about half of the regions the infrared emission peak and the UV emission peak are visibly displaced relative to each other
19 Images of observed ( IRAC) or simulated ( MIPS) PSFs were downloaded from the SSC Instruments' pages at http://ssc.spitzer.caltech.edu /obs; FWHMs and aperture corrections (see next section) are calculated from these images.


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Fig. 1.--Two three-color composites of M51. Left:FUV (blue), continuum-subtracted H ( green), and 24 mdust(red ) emission of the galaxy pair. The FUV and FIR images, from GALEX and Spitzer, respectively, have closely matched resolution (%600 ), while the resolution of the ground-based H image has been degraded to match that of the two space-borne images. Right: Continuum-subtracted H (blue), 3.6 m stellar continuum ( green), and 8 mdust (red ) emission of the galaxy pair. This second image exploits the higher angular resolution of the IRAC images (about 200 FWHM) to provide higher level of detail. The stellar continuum emission traces evolved (old) stellar populations. In this figure, a foreground star appears pure green. North is up, and east is to the left. The size of the pictures is $8A6 ; 11A8.

( Fig. 4). The displacement of peaks is of the order of a few arcseconds, larger than any displacement expected from misregistration of the images or from the residual distortions in the GALEX FUV image (%100 ; see x 2.2). The large apertures still allow us to encompass both UV and infrared emission, but the measured fluxes are clearly emerging from slightly different regions. Conversely, there is a high degree of coincidence, within the accuracy afforded by the images' resolution, between the infrared, both 8 and 24 m, and H emission peaks. Because of crowding, background annuli are generally difficult to define around each aperture, without including a neighboring region. We thus adopt a different approach for background removal from the measured fluxes. The 166 apertures are divided into 12 ``areas'' (one of these being the region probed by the HST Pa image), where the local background at each wavelength is fitted and globally removed from each area.20 Checks performed on the few isolated apertures that can be identified in the images indicate that this process of background removal is robust for the relatively small galactic areas selected (e.g., the inner region corresponds to about 20%