19 68Apj. . .151. .105D the Astrophysical Journal, Vol. 151

.105D The Astrophysical Journal, Vol. 151, January 1968 .151. 68ApJ. 19 INTEGRATED MAGNITUDES AND COLOR INDICES OF THE FORNAX DWARF GALAXY* G. BE VaUCOTJLEURS ANB H. D. ABLEsf McDonald Observatory, University of Texas Received October 24,1966; revised May 31,1967 ABSTRACT Integrated magnitudes and colors of the Fornax dwarf spheroidal galaxy derived from photoelectric scans with the 36-inch reflector are BT — 9.04 {MT — —13.0 if m — AT = 22 0), {B — V) = +0 63, (U — B) = +0 08. The luminosity distribution shows a nearly uniform central core of (r) 6' and maximum brightness po(B) = 24.6 mag sec“2 in an envelope of gradient UCa) = d(logI)/da — —0.077 min_1J = —1.15 kpc“1. 2 The effective equivalent radius is (re*) = 9Í4 where fie(B) = 25.2 mag sec“ and the system may be detected out to an average diameter in excess of Io. The photometry is compared with star counts by Hodge and with new counts on a 40-inch reflector plate. I. INTROBUCTION The Fornax system (Shapley 1938, 1939; Baade and Hubble 1939) is, together with the Sculptor system, the type example of the low-luminosity dwarf spheroidal galaxies. The true abundance of this galaxy type in space may be much larger than one might assume from the relatively small sample of a dozen or so at present known in the Local Group (de Vaucouleurs 1967). Similar systems are apparently present in abundance in other nearby groups (van den Bergh 1959) and clusters (Reaves 1956, 1966). The valu- able studies by Hodge (1961a, 6), including star counts in Fornax, have renewed interest in these systems. The luminosity distribution, integrated magnitudes, and colors are still very poorly known because of the extreme faintness of even the brighter of these objects; typically the surface brightness in the central regions is only one-tenth (or less) of the superim- posed night-sky luminosity. Until the present study the only available information on to- tal magnitude was the early estimate by Shapley (1939) of wPg = 9.0, from small-scale photographs. We report here on our attempts to derive integrated magnitudes and colors in the UBV system from direct photoelectric scans across the system with the 36-inch reflector of McDonald Observatory. General information on the Fornax system is given in Table 1, mainly from the Reference Catalogue (G. and A. de Vaucouleurs 1964) and a recent re-evaluation of data on the Local Group (de Vaucouleurs 1967). A photograph of the 40' X 47' central region of the system with the 40-inch reflector of the Naval Observatory is shown in Figure 1 (Plate 5). II. INSTRUMENTAL ANB OBSERVATIONAL BATA The observations consist of photoelectric scans (96, 9v, 6u) at declinations — 34°41', —34°43', —34°44',—34°46' (1950) near the center of the system made during three nights, October 5 and 6, 1961, and December 27,1962, with the UBV photometer at the Casse- grain focus of the 36-inch reflector (scale 1 mm = 16'H9) of the McDonald Observatory. * Contributions from the McDonald Observatory of the University of Texas, No. 414. f Now at U.S. Naval Observatory, Flagstaff Station. % Except where units of time are explicitly stated, all angular dimensions in this paper are expressed in arc sec and arc min. 105 © American Astronomical Society • Provided by the NASA Astrophysics Data System PLATE 5 Fig. 1.—Fornax dwarf galaxy. 40-inch reflector, U.S. Naval Observatory, December 1, 1964; 2 h.; 103a-D + GG 11; north at top, east at left. Dark lines show declination limits of photoelectric scans. Area of figure is approximately 40' X 47'. de Vaucotjleurs and Ables (see page 105) © American Astronomical Society Provided by the NASA Astrophysics Data System .105D 106 G. DE VAUCOULEURS AND H. D. ABLES Vol. 151 .151. The image of the galaxy was allowed to drift across the field aperture at the diurnal rate while the Brown recorder was driven at the rate of 0.5 inch/min of time. This combina- 68ApJ. tion gave a scale of about 62 mm/degree on which the diameters of the scanning holes 19 range from about 2 to 3 mm. The photometer used for the observations has been described by Johnson (1962). Different field apertures were used for the 1961 and 1962 observations. For the 1961 observations the effective diameters of the scanning holes were determined by recording and timing star transits at high declinations; for the 1962 observations the geometric diameters of the holes were adopted (Nos. 6 and 10). The aperture constants are listed in Table 2A. The last column gives the additive constant 2.5 log A for converting ap- parent magnitude measured through the hole to the corresponding surface brightness in magnitudes per square second of arc. Four of the six to eight Johnson-Morgan standards observed each night were measured through air masses greater than that of the Fornax dwarf. The measures for each night were reduced separately to the l/BV system by a computer program developed by Mr. F. Lopez. The extinction coefficients in magnitudes are listed in Table 2B. TABLE 1 Elements of the Fornax System a (1950) .. 2h37™7 Mean diameter, core 15' -20' S (1950) . _34°44' Mean diameter, envelope 65' -95'* II, P .. 203?1, —64?5 Mean axis ratio 0 65* III, b11 . 237?3, —65?7 Distance modulus 22 0: L, B . 266?0, —30?2 Absorption correction 0 2 Type ... dE3 Distance (kpc) 0.23 V (km/sec) - 7in Magnitude (pg) of Í19.1-19 3* Vo (km/sec) -141t/ brightest star [19 6-19.7f ♦AfterHodge (19616) and Shapley (1939). t Mount Wilson value for globular cluster NGC 1049. Í In globular clusters, after Hodge (1961a). TABLE 2A Instrumental Constants Hole No Effective Diameter 2 5 log A 7.. 115?0±0?5 10 041+0 010 8 . 148 4+0 5 10.595 + 0 010 6.. 114 5 10 034 10.. 161.0 10 772 TABLE 2B Extinction Coefficients Date ¿1(6-*) kz(u—b) h(v) Oct. 5, 1961. 0 086 0 259 0 135 Oct. 6,1961. .084 272 .129 Dec. 27, 1962 0 062 0.255 0.168 © American Astronomical Society • Provided by the NASA Astrophysics Data System .105D No. 1, 1968 FORNAX DWARF GALAXY 107 .151. III. MEASUREMENTS AND REDUCTION 68ApJ. Each scan was accurately located on a print of the galaxy by means of the field stars 19 recorded on the tracing, and a continuous trace was drawn through the undisturbed regions and interpolated through the regions affected by field stars. This curve was taken to represent the true luminosity profile of the galaxy. A prominent star was selected on each scan as a provisional origin of the abscissa scale. The “average” center was determined by folding the best scans, assuming a symmetrical east-west luminosity distribution. The abscissa of each scan was adjusted to fit this center by means of the reference stars. Finally, each scan was folded about its center to give a mean profile(ô') =/(r), where <ô'> is the mean deflection above an arbitrary reference level at the distance r from the center of the galaxy. Within the accuracy of the measurements all the scans can be considered to pass through the center of the galaxy because (1) their separation in declination is small with respect to the size of the galaxy and (2) the scans show the galaxy to have a “flat” central region of almost uniform brightness with a radius of about 6'. This simplifies the reduction since all the scans of a particular color on a given night can be averaged after the sky level has been determined for each. All of the scans were within the declination limits shown on the photograph in Figure 1 (Plate 5). In most of the scans, particularly the v and u scans, fluctuations in the brightness of the night sky complicated the determination of the sky level. From a visual inspection of the three (ô*/) profiles for a given night, a mean sky level, <5S'), was chosen for each scan. A somewhat more objective determination of the sky level is possible from a plot of log <ô&'> versus \¡r for each scan. For an exponential luminosity distribution in the outer regions of the galaxy, this plot will define a straight line which gives upper and lower limits for the sky level. The last measured point sets the upper limit for the sky level, and the lower limit is read by extrapolation at 1/r = 0. In general, the more prob- able value would be somewhere between these limits; but, for the scans uniform enough for the application of this method, the upper limit for the sky level is in closer agreement with the value chosen from visual inspection of the mean curves. The sky level for each scan was adopted after a careful comparison of the results from each of these methods. The more uniform scans were used to interpolate the outer profiles and sky levels for the scans with large sky fluctuations. A mean blue net deflection (i.e., above sky) <5&> = <5&'> — <5/> was determined at each abscissa r for each night by averaging the three blue scans. The mean sky level for each of the v and u scans was then determined from the mean b profile for each night by assuming the (6 — u) and {u — b) colors to be constant in the more reliably measured central and intermediate regions. For each of the <£/> and <5M') profiles an approximate value of the mean sky level was chosen and the resulting net deflections <5V> and <5W> were plotted versus <($&>.

Load more