THE BRIGHTEST STAR IN THE MAGELLANIC IRREGULAR GALAXY DDO 155

Transcription

1 Publications of the Astronomical Society of the Pacific 98: , December 1986 THE BRIGHTEST STAR IN THE MAGELLANIC IRREGULAR GALAXY DDO 155 C. MOSS* Vatican Observatory, Castel Gandolfo, Italy AND G. DE VAUCOULEURS Department of Astronomy and McDonald Observatory, University of Texas, Austin, Texas Received 1986 August 9 ABSTRACT A spectrum has been obtained of the brightest star in the dwarf Magellanic irregular galaxy DDO 155. The heliocentric velocity of the star is V 0 = 134 ± 18 km s -1 which is 353 km s _1 less than the velocity of the galaxy, showing that it is a foreground star in the galaxy. The available evidence suggests that it is an early G-type metal-weak subdwarf, at a distance of 2.1 ± 0.4 kpc in the galactic halo. The value of the distance modulus of DDO 155 based on three separate distance indicators is x 0 = 25.1 corresponding to a distance A = 1.05 ± 0.1 Mpc, which is essentially unchanged from the previously adopted value. Key words: galaxy-stellar content-distance scale I. Introduction The dwarf Magellanic irregular galaxy DDO 155 = A = GR8, originally discovered by Reaves (1956), was first studied by Hodge (1967, 1974). More recently de Vaucouleurs and Moss (1983) have provided detailed surface photometry and B magnitudes for the 18 brightest stars, and have given an estimate of the distance 1.12 (1 ± 0.2) Mpc based on five separate distance indicators, to improve upon the rather uncertain estimate of the distance given by Hodge. Other recent studies are CCD photometry of 84 resolved stellar images by Hoes sel and Danielson (1983) using the photometric GR system of Thuan and Gunn (1976), and photographic photometry from image-tube plates of 123 stars in UBV by Ruotsalainen (1982). One of the principal uncertainties in any study of this galaxy is its distance, estimates of which are based on a variety of more-or-less certain distance indicators. In a continuing effort to improve on estimates of the distance, we have obtained a spectrum of the brightest star in the galaxy in order to determine whether or not it is a field star. Its prominent appearance in photographs of the galaxy led to the suspicion that it might be a foreground star in the galaxy; and indeed Hoessel and Danielson have shown that it has a color, (B V) = 0T67, in the yellow supergiant region of the C-M diagram, which is often heavily polluted by foreground stars. We will describe the observation (Section II. A), and give the radial velocities of the star and the galaxy showing * Present address: Institute of Astronomy, Madingley Road, Cambridge CB3 OHA, U.K. that the star is indeed a foreground star in the galaxy (Section II.B). The spectral type and distance to the star will be discussed briefly (Section II. C). Finally, we give a rediscussion of the distance of DDO 155 (Section III). II. Brightest Star in DDO 155 A. Observation The brightest star in DDO 155 (VM No. 1-de Vaucouleurs and Moss 1983) is at the position (1950) R.A. 12 h 56 m 10?95, Declination T3"9. A spectrum of this star was obtained on 1983 March 23 (UT) using the 2.3-m telescope of Steward Observatory at the University of Arizona on Kitt Peak. The telescope was equipped with a Boiler and Chivens spectrograph and an intensified photon-counting system which employs a Reticon dual diode array. The slit assembly of the spectrograph consisted of two circular apertures, separated by 24 arc sec in R.A. Each aperture was 2.5 arc sec in diameter, which was slightly larger than the diameter of the stellar image. Light from the two apertures is separately projected onto the two diode arrays allowing simultaneous star plus sky, and sky observations. A 600 line mm -1 grating was used for the observation which gave a wavelength coverage of 3400 Â-5600 Â with a FWHM of an unresolved line of approximately 8.2 A. An observation cycle consisted of four chops, each of duration 240 s. During the second and third chops the star was centered in the second aperture. Eight such cycles were made, giving a combined integration time on the object of 128 min. Exposures of a He-Ar lamp to provide a wavelength scale were made both before and at the end of 1282

2 GALAXY DDO the observation, and also at intervals of every two cycles during the observation. Finally, an extended exposure of a quartz lamp was made to provide calibration to remove the fixed pattern noise in the spectrum. Because the angular extent of DDO 155 is comparable to the separation of the apertures, the sky observations include some light from the galaxy. The sky position for the east aperture lies well outside the isophote, x B = mag arc sec -2, while for the west aperture it lies just outside the isophote, x B = mag arc sec -2 (see de Vaucouleurs and Moss 1983). For both sky observations the observed luminosity from the galaxy is thus a small fraction of the observed luminosity from the sky, and has been neglected. An upper limit to that component of the total observed luminosity in the sky-subtracted spectrum of the object which is contributed by the galaxy may be estimated by assuming that the surface brightness of the underlying galaxy equals the peak central surface brightness, x B = mag arc sec -2, as determined from an extrapolation of the exponential fit to the outer isophotes of the galaxy. The magnitude of the brightest star is B (see Section III). This gives an upper limit of only 14% for the fraction of the total observed luminosity which is from the galaxy. However, the surface-brightness profile of the underlying disk has been shown to have a luminosity deficiency in the central region of the galaxy ( x B < 23.0 mag arc sec -2 ) as compared to the extrapolated exponen- tial component. Thus the fraction of the total observed luminosity which is from the galaxy is expected to be even less than this upper limit. In the discussion which follows, it will be assumed that the luminosity in the continuum of the spectrum is mainly from the star, and the component of the luminosity which is from the galaxy will be neglected. B. Radial Velocities of Star and Galaxy Data for the two arrays were reduced separately. The He-Ar lamp observations were summed, and a fourth-order polynomial fit was made to 29 lines evenly spaced between 3492 A and 5187 A to convert the data to a wavelength scale. The standard deviations for the fits were 0.3 Â and 0.4 A, respectively, for the arrays. The sky-subtracted and summed data from the two arrays resulted in the spectrum shown in Figure 1. As was explained above, it is expected that the luminosity in the continuum is mainly from the brightest star. The emission lines are from an HII region which is coincident in position with the three brightest stars of the galaxy (Hodge 1974; de Vaucouleurs and Moss 1983). Identifications of the emission lines of the H II region and the absorption lines of the stellar spectrum are given in Figure 1. The measured wavelengths and corresponding radial velocities for these lines are listed in Table I. The columns of the table list the identified spectral line, the adopted rest wavelength, and the measured wavelength and corresponding radial velocity for the brightest star Fig. 1-The spectrum of the brightest star (VM No. 1) in DDO 155. Emission lines from the underlying H n region in the galaxy are superposed on the spectrum. The principal stellar absorption lines and emission lines from the H n region are identified. The original record has been smoothed using a Gaussian function with FWHM = 11.6 Â.

3 1284 MOSS AND DE VAUCOULEURS Line TABLE I Measured wavelengths of spectral lines X {l) Star DDO 155 o X (A) cz_ 1 X (A) cz 1 (km s ) (km s" ) [Oil] H$ Hh HÇ K H H< G Hy Hß [OUI] [OUI] NOTES: X, rest wavelength ; X, measured wavelength ; cz, radial velocity. and DDO 155, respectively. It is immediately evident that the radial velocity of the star is several hundred km s 1 less than the radial velocity of DDO 155, indicating that it is a foreground star in the galaxy. The mean heliocentric velocity of DDO 155 as measured from the emission lines of the H II region, with the exception of the Hy line, is V 0 = 230 ± 16 km s -1. The Hy line has been neglected since there is expected to be significant weakening of the short wavelength edge of this emission line due to the underlying stellar absorption line. This might be expected to result in a measured radial velocity which is too high, which in fact is what is observed. This value for the heliocentric velocity of DDO 155 is in good agreement with previous measurements by Hodge (1974) and Fisher and Tully (1975). Hodge measured the radial velocity, V r = 257 ± 30 km s -1, from an optical spectrum of the H II region located at the southwest end of the galaxy. Fisher and Tully give a 21-cm measurement of the heliocentric velocity of V 0 = 216 ±6.3 km s _1. Combining the above data, we will adopt a weighted mean value for the heliocentric velocity of V 0 = 219 ± 6 km s 1. Using the standard IAU correction for solar motion as given in RC2 (de Vaucouleurs, de Vaucouleurs, and Corwin 1976), the corrected velocity is V 0 = 168 km s" 1. For the determination of the radial velocity of the brightest star, the H8 line has been neglected since significant contamination of the long wavelength edge of this line by emission from the galaxy is expected to result in an underestimate of the radial velocity, which is what is observed. The radial velocity from the H0 line is anomalous and has been omitted. For the remaining lines, the weighted mean heliocentric velocity is V 0 = 134 ± 18 km s -1. The difference in the radial velocities of DDO 155 and the star is thus hv r = 353 km s -1. C. Spectral Type of the Star An approximate estimate of the spectral type of the star may be made using the identified absorption lines in the spectrum in Figure 1. The spectral type is estimated to be in the range F4-F7, with an adopted value of F5. This estimate of the spectral type of the star can be compared with its broad-band color. Hoes sel and Danielson (1983) give a color for this star, (G R) = With the transformation to the B V system given by Hoessel and Melnick (1980), we obtain (B V) = Using the procedure of RC2, and assuming a galactic extinction, A B = 0.19, the unreddened color is (B V) 0 = ± 0.06, where the error includes the estimated uncertainty in the color transformation. This color is too red for a main-sequence F5 star. Furthermore, if the galactic extinction value given by Sandage (1975) (A B = 0.00), or that given by Burstein and Heiles (1978) (A ß = 0.05) is adopted, this makes the intrinsic color of the star even redder by 0.5 or 0.4 mag, respectively. The most likely explanation is that the star is a metal-poor subdwarf with weak metal lines of early-g spectral type. This type of star is very common at fainter magnitudes. It is also to be noted that the measured heliocentric velocity is well within the range expected for G-type metal-poor subdwarfs in the galactic halo, which have a velocity dispersion, v v ~ 90 km s -1. The star has a magnitude B = (see Section III) which, combined with the (B V) color, gives a magnitude V = From the measured color we derive the absolute magnitude of the star (Chiu 1980) as M v = +5.8 ± 0.4, which gives a distance to the star of 2.1 ± 0.4 kpc. III. Distance Modulus of DDO 155 The distance modulus of DDO 155 was previously estimated from a set of five separate distance indicators which included the magnitudes of the brightest blue and red stars in the galaxy (de Vaucouleurs and Moss 1983). Using the additional photometry for the brightest stars which is now available, we will give revised estimates for the distance modulus based on these latter indicators. In Table II the B magnitudes and (B V) colors of the five brightest stars in DDO 155 are listed. The star num- TABLE II Brightest stars in DDO 155 Star B B-V no a NOTE: Star nos. are from de Vaucouleurs and Moss (1983). Star no.1 is a foreground star in the Galaxy.

4 GALAXY DDO bers are the same as given in de Vaucouleurs and Moss (1983). The B magnitudes have been derived from three sets of photometry by Hoessel and Danielson (1983), Ruotsalainen (1982), and de Vaucouleurs and Moss (1983), respectively. An intercomparison of these three sets of photometry shows that over the range 18.0 < B < 21.0, the mean error of the photographic B magnitudes given by Ruotsalainen is a B = For isolated stars within the galaxy, the photographic photometry of de Vaucouleurs and Moss has comparable accuracy with a B = However, for stars in more crowded fields, for which the local background is less well determined, their accuracy is much less with a B = Finally, as is expected, the mean error of the CCD photometry of Hoessel and Danielson is much better than either set of photographic photometry with a B = There are also significant zero-point shifts between the sets of photometry. ((B H d ~ Bvm) = +0.36; (B R B V m) 0.32.) The B magnitudes listed in Table II are weighted mean values from the three sets of photometry together with an adjustment to the zero point of de Vaucouleurs and Moss, which is intermediate between the two others. The (B V) colors in Table II are taken from Hoessel and Danielson (1983). As previously, we compare DDO 155 to the dwarf Magellanic irregular galaxy in the Sculptor group A We use the adopted values for the B magnitude of the galaxy and of the brightest blue supergiants ((B V) ^ 0.4): A0057: B t , B x , B x * , <B*> 3 = 20.97, from which ôirq* = 4.1, bm 3 = 4.47 (de Vaucouleurs and Moss 1983). Using the data in Table II, the revised corresponding values for DDO 155 are: A1256: B t = 14.7, B T , B * = 18.63, <B*> 3 = 18.80, from which bnii = 3.93, bm 3 * = We assume that A0057 is at the mean distance of the Sculptor group, x 0 = 27.0 ± 0.2 (de Vaucouleurs 1978, Table 5), and thus has an absolute magnitude M x = The absolute magnitude of DDO 155 is M x = 10.75, if we use the previously determined distance modulus, x 0 = (de Vaucouleurs and Moss 1983). Since the absolute magnitudes of the two galaxies are almost exactly equal, we may assume that the brightest blue supergiants in the two galaxies also have the same absolute magnitudes, and thus derive the following estimates of the differential modulus 8 x 0 and the modulus x 0 of DDO 155, 8 x 0 (B *) = 1.97, b x 0 (B 3 ) = 2.17, and p, 0 = The brightest red star (VM No. 14) in DDO 155 was previously taken to be at V X R = 19.3 with (B V) ~ +1 from approximate photometry given by Hodge (1967). This star was compared with the brightest red star in A (Lequeux and West 1981) to give a distance modulus for DDO 155 of x 0 = However, according TABLE III Distance moduli of DDD 155 Method y Total magnitude of galaxy compared to A Brightest blue stars compared to A Largest HII ring compared to IC Mean : to Hoessel and Danielson (1983), this staf is a much fainter blue star with V = 20.01, (B V) = A comparison of V magnitudes and (B V) colors from Hodge with corresponding values from Hoessel and Danielson, and from Ruotsalainen (1982) for stars brighter than V = 21, shows that the magnitude and color of this star given by Hodge are discrepant. We could use the C-M diagram of Hoessel and Danielson to give a new estimate of the magnitude of the brightest red star in DDO 155. However, this C-M diagram is quite dissimilar to the approximate C-M diagram of A (Lequeux and West 1981), which suggests that use of the magnitude of the brightest red star as a relative distance indicator between the two galaxies is likely to be unreliable. The above estimate of the distance modulus of DDO 155, together with two others taken from de Vaucouleurs and Moss (1983), are collected in Table III. The relatively very uncertain estimate based on the 21-cm linewidth and an extrapolation of the Tully-Fisher relation for DDO dwarf galaxies (de Vaucouleurs, de Vaucouleurs, and Buta 1983) has been neglected. The remaining estimates have a mean value x 0 = 25.1 ± 0.2:. This corresponds to a distance A = 1.05 ± 0.1 Mpc, which is essentially unchanged from the previously adopted distance, A 1.12 Mpc. It is a pleasure to thank Steward Observatory, the University of Arizona, for allocation of observing facilities. We are grateful to Dr. J. Liebert who assisted with the observations. This work has benefited from useful discussions with Dr. G. Gilmore. REFERENCES Burstein, D., and Heiles, C. 1978, Ap. J., 225, 40. Chiu, L.-T. G. 1980, Ap.]. Suppl, 44, 31. de Vaucouleurs, G. 1978, Ap. J., 224, 710. de Vaucouleurs, G., and Moss, C. 1983, Ap. J., 271, 123. de Vaucouleurs, G., de Vaucouleurs, A., and Buta, R. 1983, A.J., 88, 764. de Vaucouleurs, G., de Vaucouleurs, A., and Corwin, H. G. 1976, Second Reference Catalogue of Bright Galaxies (Austin: University of Texas Press) (RC2). Fisher, J. R., andtully, R. B. 1975, Astr. Ap., 44, 151. Gunn, J. E., and Westphal, J. A. 1981, Proc. SPIE, 290, 16. Hodge, P. W. 1967, Ap.J., 148, , Pub. A.S.P., 86, 645.

5 1286 MOSS AND DE VAUCOULEURS Hoessel, J. G., and Danielson, G. E. 1983, Ap. J., 271, 65. Hoessel, J. G., and Melnick, J. 1980, Astr. Ap., 84, 317. Lequeux, J., and West, R. 1981, Astr. Ap., 103, 319. Reaves, G. 1956, A.J., 61, 69. Ruotsalainen, R. W. 1982, Ph.D. thesis, University of Hawaii. Sandage, A. 1975, Ap. J., 202, 563. Thuan, T., and Gunn, J. E. 1976, Pub. A.S.P., 88, 543.