The H II Regions of M33. II. A Photometric Catalog of 1272 Newly IdentiÐed Emission Regions

Transcription

1 PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF THE PACIFIC, 111:685È690, 1999 June ( The Astronomical Society of the PaciÐc. All rights reserved. Printed in U.S.A. The H II Regions of M33. II. A Photometric Catalog of 1272 Newly IdentiÐed Emission Regions PAUL W. HODGE, JEFF BALSLEY,1 TED K. WYDER, AND BROOKE P. SKELTON Astronomy Department, Box , University of Washington, Seattle, WA ; hodge=astro.washington.edu, jmb6r=virginia.edu, wyder=astro.washington.edu, skelton=astro.washington.edu Received 1999 February 23; accepted 1999 March 1 ABSTRACT. We have identiðed a total of 1272 newly recognized emission regions in M33. Combined with the previously cataloged total of 1066 H II regions and supernova remnants, this brings M33Ïs total to 2338 emission regions. This paper provides photometry of the new objects in Ha, which is combined with data from previous catalogs to produce a global H II region luminosity function (corrected for incompleteness) that reaches a faint luminosity limit of 2 ] 1034 ergs s~1 and shows a broad maximum with a peak frequency at luminosities of 6 ] 1035 ergs s~1. We also plot the H II region size distribution and comment on unusual morphologies. 1. INTRODUCTION The Local Group galaxy M33 (NGC 598) is rich in H II regions. Early narrowband optical surveys revealed hundreds of them (Aller 1942; Courtès & Cruvellier 1965; Sabbadin, Rafanelli, & Bianchini 1980; Boulesteix et al. 1974; Courtès et al. 1987; and many others). A recent compilation revealed that a total of 1030 H II regions and 36 supernova remnants were identiðed prior to 1998 (Hodge, Skelton, & Ashizawa 1999). These objects, especially the brightest ones, have been the subject of a large number of papers, which have used them to study the composition, the abundance gradient, the star formation rate, the modes of star formation, and the velocity Ðeld of the galaxy; e.g., see the review published by Sharov (1988). In a previous paper we used this early sample to investigate the luminosity function and size distribution of M33Ïs H II regions (Wyder, Hodge, & Skelton 1997, hereafter Paper I). This paper reports the identiðcation of a large number of additional H II regions (and related emission regions) in M33. Most of the newly recognized objects are fainter and/or smaller than those in previous catalogs. For that reason we use them to explore the faint end of the H II region luminosity function, which is generally incomplete for most observations of galaxies. As explained in 4, the luminosity function is interesting because it is the result of the combination of a number of astrophysically important features of a galaxy. In order to determine a luminosity function correctly, it is necessary to evaluate quantitatively the completeness of a ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ 1 Current address: Astronomy Department, University of Virginia, Charlottesville, VA sample. This paper investigates that aspect of the new catalog and reviews the completeness of previous catalogs, using the techniques described in 5. The corrected total luminosity function is presented in 6. In the process of measuring the Ha luminosities of our sample, we have also determined their isophotal sizes. These are used to complete the size distribution curve for the M33 sample, which is discussed in relation to the function found by van den Bergh (1981) in 7. In addition to classical H II regions, the present survey has also identiðed a large number of more di use objects; most of these are probably hydrogen clouds that are excited by the general ultraviolet Ñux in the disk of M33, which is rich in O and B stars (Humphreys & Sandage 1980). The morphology of the various types of emission regions is discussed in THE DATA SOURCE The images used to identify objects in this catalog were generously provided to us by Shawn Gordon. They were obtained by W. Blair, C. Smith, R. Kirshner, and K. Long with the Kitt Peak National Observatory (KPNO) 4 m telescope, using a narrowband (34 A FWHM) Ha Ðlter (centered at 6571 A to allow for the blueshift of M33) and a wider (145 A ) Ðlter for the continuum, centered at 6105 A. The original purpose was to discover and study supernova remnants in M33, using a third set of images exposed behind an S II Ðlter (Gordon et al. 1998). The images were calibrated as described in Paper I. Figure 1 outlines the coverage of the four sets of images obtained with the KPNO 4 m telescope; the area covers most but not quite all of the main disk of M33. The giant 685

2 686 HODGE ET AL. NGC 604 NGC 595 NGC 592 NGC 588 The last column in the catalog gives some descriptive notes. Unresolved emission regions, which are probably mostly compact H II regions, but which may also include some bright planetary nebulae or emission-line stars, are identiðed by the letter u. ÏÏ Di use2 regions for which there is no obvious excitation source (and for which boundaries are often somewhat arbitrarily drawn) are indicated by the letter d. ÏÏ Other notes call attention to unusual morphologies or other notable circumstances. Charts that identify the emission regions are not provided, as the objects, together with all previously cataloged emission regions, are included in the identiðcation atlas of M33 published elsewhere (Hodge et al. 1999). FIG. 1.ÈThe area sampled, based on a mosaic of the four images used H II region NGC 588 lies on the west edge of the covered area and several fainter H II regions lie beyond the boundaries of our search. Thus our catalog is not complete, and a new faint survey of the outer parts of the galaxy should reveal additional H II regions. 3. THE CATALOG Table 1 is a catalog of the newly identiðed emission regions in M33. Names have been assigned to them according to the precepts of the IAU Commission 5, and these designations are registered with the Commission and the Strasbourg Data Archives. The objects are ordered in increasing right ascension (with a few exceptions where last minute corrections to the catalog were made), and each number is given the preðx HBW. ÏÏ The positions of the centroids of each object were obtained by reference to the Digitized Sky Survey Catalog and are accurate to a few seconds of arc. Centroids were chosen by eye rather than by some photometric criterion, which would be awkward to deðne in the cases of complex morphologies. The Ha Ñuxes for most of the emission regions were measured by integrating over the area deðned by an outer isophote level of 10~16 ergs s~1 cm~2 arcsec~2, as measured above the local background. Very faint or di use H II regions had no areas with a surface brightness this bright, and therefore no Ha Ñuxes are listed for those. The sizes listed in the catalog are based on the area contained within that isophote. 4. THE LUMINOSITY FUNCTION OF THE CATALOG In Paper I we explained the motivation behind the determination of the luminosity function of a galaxyïs H II regions. Observationally, the Ðrmest conclusion is that the bright end of the luminosity function is nearly independent of the galaxyïs characteristics, with some minor exceptions (Kennicutt, Edgar, & Hodge 1989). The slope and shape of the bright portion of the luminosity function is determined largely by the mass spectrum of star clusters undergoing formation. Except, of course, for the problem of small number statistics near the bright limit, this portion of the function is well established. The faint end, on the other hand, is greatly uncertain observationally because of brightness limitations and the incompleteness of surveys, as discussed in Paper I. This portion of the luminosity function involves single-star H II regions; its shape is a function of the bolometric stellar initial mass function and certain characteristics of the interstellar gas. Because our present list of newly recognized H II regions essentially doubles the number of faint H II regions, our data should be useful in more clearly deðning the faint end of the luminosity function, making it more amenable to astrophysical interpretation. Not all of the newly discovered emission regions in Table 1 should be included in a luminosity function of H II regions. We have omitted all objects that we describe as patches of di use emission as they are probably not normal single-star or single-association H II regions. Also, as explained above, many of them have such low surface brightness that their luminosities were not measured. Figure 2 shows the di erential luminosity function for the newly identiðed H II regions in Table 1. The data are plotted in logarithmic bins of luminosity. Clearly, although a few fairly luminous H II regions are present, the bulk of the ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ 2 Our objects called di use ÏÏ correspond approximately to emission regions called Ðeld ÏÏ by Patel & Wilson (1995).

3 H II REGIONS OF M33. II. 687 TABLE 1 NEW EMISSION REGIONS IN M33 R.A. Decl. Fluxa Areab ID (2000) (2000) (10~15 ergs cm~2 s~1) (arcsec2) Commentsc HBW HBW Half o frame HBW HBW HBW HBW u HBW HBW Linear HBW HBW HBW d HBW d HBW d, linear HBW u HBW d, linear HBW u HBW d HBW d, linear HBW HBW d HBW d, linear HBW u HBW u HBW HBW d, linear HBW u HBW u HBW HBW HBW u HBW HBW HBW u HBW u HBW HBW HBW u HBW d HBW HBW HBW HBW d HBW HBW HBW u HBW HBW HBW HBW HBW u HBW Table 1 is presented in its entirety in the electronic edition of the PASP. A portion is shown here for guidance regarding its form and content. NOTE.ÈUnits of right ascension are hours, minutes, and seconds, and units of declination are degrees, arcminutes, and arcseconds. a The Ñuxes were measured by integrating over the area within an outer isophote level of 10~16 ergs s~1 cm~2 arcsec~2, as measured above the local background. Some of the emission regions identiðed in this table had no areas which were brighter than this limit and therefore no Ñux was measured for these objects. b The area for each object is the area on the sky within an outer isophote level of 10~16 ergs s~1 cm~2 arcsec~2, as measured above the local background. c Unresolved emission regions are identiðed by the letter u. ÏÏ Di use regions for which there is no obvious excitation source are identiðed by the letter d. ÏÏ

4 688 HODGE ET AL. FIG. 2.ÈThe di erential luminosity function for the H II regions newly cataloged in this paper. This sample does not include unresolved or di use emission regions. objects have luminosities in the range where previous luminosity functions were highly incomplete. 5. COMPLETENESS CORRECTION As we did in our previous paper, we carried out a number of tests to determine the completeness of the present catalog at various Ñux levels. Incompleteness at the faint end is largely a question of the detection limit, while at brighter luminosities emission regions in crowded areas can be undercounted when a photometric boundary is used as an identiðcation criterion. Two overlapping H II regions could be counted as one, or small faint H II regions could be lost if superimposed upon the image of a large, bright region. Two of us (T. W. and P. H.) independently carried out completeness tests using the Ðeld combination method described in Paper I. We used 19 di erent pairs of Ðelds, superimposed on the computer screen to present simulated, more crowded Ðelds. The areas were chosen from throughout the galaxy, with a preponderance of outer Ðelds in order to have the average e ective crowding similar to the original sample. We then used the same criteria used in the initial survey to identify separate emission regions, produc- ing a catalog for each image pair. This was then compared with the lists of H II regions in previous catalogs, including that in this paper. The number of emission regions recovered was then recorded and all results combined to produce a measure of the completeness of our searches as a function of the brightness of the H II regions. Areas of di use emission were not included for reasons described above. Figure 3 shows our results. Clearly the new survey reaches to fainter limits than did earlier surveys (the Boulesteix et al and Courtès et al catalogs), and is Ðnding at least 25% of the H II regions at the very faint level of2x1034 ergs s~1, our measurement (but not detection) limit. 6. THE TOTAL LUMINOSITY FUNCTION With all data corrected for incompleteness, we have combined the three catalogs (Boulesteix et al. 1974; Courtès et al. 1987, and this one) to produce a complete luminosity function for H II regions in M33 (limited, of course, to the area of our survey, which excludes a few remote H II regions). This is plotted in Figure 4, which uses logarithmic bins of 0.2 dex. The distance used for M33 is 840 kpc (Freedman, Wilson, & Madore 1991). The shape of the luminosity function is similar to that derived in Paper I, extending to a limit that is an order of magnitude fainter than previously reached. The peak of the function remains broad and is remarkably symmetrical. The peak is at L \ ergs s~1 and the width (FWHM) is approximately 2.4 dex. Before this curve can be compared with theoretical models, we need to correct the derived luminosities for the e ects of extinction. Unfortunately, measurements of the extinction in all of the H II regions are not available. However, Viallefond & Goss (1986) have compiled reddening data for 16 of M33Ïs emission regions for which FIG. 3.ÈResults of completeness tests performed on our present survey FIG. 4.ÈThe luminosity function for all of the H II regions in M33 measured here and in Wyder et al. (1997), corrected for completeness (squares) compared with that for the previous surveys of Boulesteix et al. (1974) and Courte` s et al. (1987) (circles).

5 H II REGIONS OF M33. II. 689 they had high-resolution radio data and for which published values were available for optical line strengths. Their Table 2 quotes extinction values at Hb based on both optical data (Ha/Hb line strengths) and radio/hb data. They argue that the radio/optical ratios produce the more reliable extinction values, the optical-only data being subject to the e ects of internal dust. Concerned that their assembled results might be skewed by the fact that they necessarily were limited to including only bright H II regions, we used their data to look for a possible correlation with luminosity, but no signiðcant correlation was found. Therefore we propose adopting a mean of their tabulated extinction values as the best correction to be made globally to the luminosity function. The mean extinction derived from the radio/optical ratios is 1.22 ^ 0.50 mag at Hb (this can be compared with the mean Ha/Hb, taken from both core and global values, of 0.80 ^ 0.38). Adopting this value as representative for all of M33Ïs H II regions means that the curve plotted in Figure 4 should be moved to higher luminosities by 0.16 dex before it can be compared with models (unless, of course, the models include the e ects of dust, both internal and external). Note also, however, that we have not corrected the Ñuxes for contamination by [N II] emission. Such correction will decrease our values by 10%È20%. Because this paper is primarily a data paper, we do not here pursue the interesting questions that can be addressed by comparing Figure 4 with various models of star formation regions. We will do so, however, in a future paper in this series. There we will also compare our results with those for other galaxies where the data reached comparably faint limits. 7. SIZE DISTRIBUTION For all H II for which we measured Ñuxes, the areas within our speciðed isophote limit was also determined. These Ðgures have been converted to e ective diameters, deðned as the diameter of a circle of the same area. These data have been used to plot a size distribution (Fig. 5). For uniformity, we have excluded the emission regions in the southeast quadrant, which had a shorter e ective exposure because of thin clouds (see Paper I). Also, all unresolved and di use regions were excluded. Figure 5 is shown in order to provide the reader with an idea of the size characteristics of this sample of newly cataloged H II regions. To e ect a comparison with the size distributions of H II regions in other galaxies, we have combined our new data with the sizes determined for all previously cataloged H II regions in M33. Since van den BerghÏs (1981) Ðrst presentation of this sort of data it has been traditional to plot the size distribution cumulatively, as we have done in Figure 6. For comparison we also plot the curve derived in our previous paper. As found by van den Bergh (1981) and by many FIG. 5.ÈThe di erential size distribution for the H II regions measured in this study. The diameter plotted here for each H II region is the diameter of the circle with the same area as the H II region. subsequent investigators, the size distribution is usually exponential, of the form N \ N e~d@do where D is the 0 diameter, D is a characteristic scale peculiar to the galaxy, o N is a constant related to the total H II region population 0 of a galaxy, and N is the number of H II regions with diameters greater than D. Our more complete sample does not change the shape of the M33 curve very much, except to straighten it out somewhat at both the large and the small ends of the distribution. A least squares solution for a linear Ðt yields a value of D \ 34 ^ 1 pc, in good agreement with o our previously derived value. 8. MORPHOLOGIES Most of the newly cataloged emission regions listed in Table 1 are normal H II regions. However, there are also several objects whose morphologies indicate that they may not be examples of traditional H II regions. A total of 227 objects, 18% of the sample, are classiðed in Table 1 as FIG. 6.ÈThe cumulative size distribution for all of the H II regions in M33 measured here and in Wyder et al. (1997), plotted as circles and squares, respectively. The diameter plotted here for each H II region is the diameter of the circle with the same area as the H II region. No correction for completeness was attempted.

6 690 HODGE ET AL. di use emission regions. Most of these are large, with very low surface brightnesses and do not appear to be clearly associated with massive stars or star groups. We believe them to be interstellar gas that is excited by the general ultraviolet Ñux in the galaxy, derived from the high density of O and B stars that exist throughout the disk. Their surface brightnesses are generally below 10~16 ergs s~1 cm~2 arcsec~2. The adopted borders of these di use regions were chosen by eye and do not necessarily have quantitative signiðcance. The average dimension of the cataloged examples, which we quote with the warning that it may have little real signiðcance, is 2A.9^ 0A.5 (D12 pc). An additional 168, or 13%, of the newly cataloged objects are unresolved, with diameters less than D2A (D8 pc). Many of these are fairly luminous; the brightest have luminosities on the order of 1036 ergs s~1. There are probably three di erent types of objects among the cataloged unresolved regions. Some are probably compact H II regions, some are probably planetary nebulae, and some may be emission-line stars whose Ha emission is bright enough to swamp the continuum images. Spectra of each of these sources would be needed to settle the question of their nature. We note the presence of three other kinds of unusual morphologies. These are various kinds of linear features, especially among the di use regions and some of the luminous complexes. These are identiðed in Table 1 as either linear, ÏÏ indicating a straight-line morphology (of which there are 36) or arcs ÏÏ or Ðlaments, ÏÏ indicating a narrow, curved morphology (of which there are 48). The third category includes the bubbles ÏÏ and rings.ïï These kinds of objects are also found among the brighter, previously cataloged H II regions and have been the subject of extensive study (e.g., Drissen et al and Hunter 1994b). Our catalog includes 18 of these objects, most of which are probably stellar windblown bubbles like N70 in the Large Magellanic Cloud (Skelton et al. 1999) but intrinsically fainter. A spectroscopic study like that published by Hunter (1994a) and Oey & Massey (1994) for Magellanic Cloud bubbles would be of interest. We are greatly in the debt of S. Gordon, who provided us with the Ðles of the images of M33 and to his colleagues, W. Blair, R. Kirshner, K. Long, and C. Smith, who obtained the data at Kitt Peak. The NSF partially supported the initial work on this project under grant AST Aller, L. 1942, ApJ, 95, 32 Boulesteix, J., Courtès, G., Laval, A., Monnet, G., & Petit, H. 1974, A&A, 37, 33 Courtès, G., & Cruvellier, P. 1965, Ann. Astrophys., 28, 683 Courtès, G., Petit, H., Sivan, J.-P., Dodonov, S., & Petit, M. 1987, A&A, 174, 28 Drissen, L., Shara, M., & Mo at, A. 1991, AJ, 101, 1659 Freedman, W., Wilson, C., & Madore, B. 1991, ApJ, 372, 455 Gordon, S., Kirschner, R., Long, K., Blair, W., Duric, N., & Smith, R. 1998, ApJS, 117, 89 Hodge, P. W., Skelton, B. P., & Ashizawa, J. 1999, An Atlas of Local Group Galaxies (Dordrecht: Kluwer), in press Humphreys, R., & Sandage, A. R. 1980, ApJS, 44, 319 REFERENCES Hunter, D. 1994a, AJ, 107, 565 ÈÈÈ. 1994b, AJ, 108, 1658 Kennicutt, R. C., Edgar, B. K., & Hodge, P. W. 1989, ApJ, 337, 761 Oey, S., & Massey, P. 1994, ApJ, 425, 635 Patel, K., & Wilson, C. D. 1995, ApJ, 451, 607 Sabbadin, F., Rafanelli, P., & Bianchini, A. 1980, A&AS, 39, 97 Sharov, A. S. 1988, The Spiral Galaxy M33 (Moscow: Nauka) Skelton, B. P., Waller, W., Gelderman, R. F., Brown, L. W., Woodgate, B. E., Caulet, A., & Schommer, R. A. 1999, PASP, 111, 465 van den Bergh, S. 1981, AJ, 86, 1464 Viallefond, F., & Goss, W. M. 1986, A&A, 154, 357 Wyder, T. K., Hodge, P. W., & Skelton, B. P. 1997, PASP, 109, 927