90E 4 . 69. THE ASTRONOMICAL JOURNAL VOLUME 69, NUMBER 7 SEPTEMBER 1964 Atomic Hydrogen in Galaxies* 1964AJ Eugene E. Epstein Harvard College Observatory, Cambridge 38, Massachusetts, and Radio Astronomy Program, Aerospace Corporation, Los Angeles 45, California (Received 14 April 1964) This paper deals with all currently available 21-cm H i emission line observations of galaxies: 40 galaxies investigated with the Harvard maser-equipped 60-ft antenna and 8 galaxies previously investigated else- where. H i emission was positively detected from 3 Sb, 13 Sc, and 13 Ir galaxies; for the galaxies from which H i emission has not been detected, upper limits have been established. The bluer or “later” galaxies defi- nitely tend to have higher values of 911# i/Lm, 911# i/91Z, and <r# i and smaller values of 911 than the less blue galaxies ; o-# i is the mean H i areal density when one projects the entire H i mass onto the fundamental plane of the galaxy. The average values of the pertinent parameters for the detected Sb, Sc, and Ir galaxies are : Morphological type Sb Sc Ir Number detected 3 13 13 H i mass 9E#i (solar masses) 3X109 4X109 1X109 Total mass 911 (solar masses) 3X1011 6X1010 1X1010 911# i/91Z 0.01 0.08 0.16 9E/Zpg 7 7 9 311* i/Zpg 0 .09 0 .41 1 .19 Mean H i areal density cr# i(10 3 g/cm2) 0.6 1.7 3.3 The values of 911# i/Zpg and <r# i are independent of the distance estimates. The paper also presents average values and upper limits for undetected galaxies. I. INTRODUCTION these results and the tests performed thereon for possible dependence on morphological type and other THE Harvard maser-equipped 60-ft antenna was parameters. Appendix A contains details about the used to study atomic hydrogen in galaxies. The instrument, the observation techniques, and the reduc- principal objectives of the study were : tion procedures. Appendix B presents a series of (1) To obtain the velocity distribution of the neutral, reliability tests, and Appendix C discusses the effects atomic hydrogen (H i) in each galaxy observed. We of continuum radiation on the H i radiation. can use this distribution to determine the H i content and systemic velocity of each galaxy. II. OBSERVATIONAL DATA (2) To test the H i radiation data for possible A. Observing Program correlation with morphological type and other charac- teristics of the galaxies. The observations were made with a five-channel (3) To study, as functions of assumed internal maser radiometer and the Harvard 60-ft antenna; the motions, orientation, and angular size, the variations total system-noise temperature was 100 °K, and the of the predicted H i emission from a series of galaxy observing bandwidth was 32 km/sec. Further details models; to use these models to (a) generate Hi line appear in Appendix A. profiles similar to the observed profiles and (b) estimate An ideal test of the dependence oî H i data on the errors that result from the fact that the observed morphological type would require the observation of galaxies do not appear as point sources to the Harvard equal numbers of galaxies of each type. However, this antenna. We have reported these investigations in a is not practical; in the Holmberg (1958) catalogue of separate paper (Epstein 1964). northern galaxies, one must include objects at distances of «11 Mpc to obtain as few as four Sa galaxies, Section II presents the observational program and whereas there are several times that number of Sc reduced results. Section HI discusses the determination galaxies within the same distance. Since H i emission of the H i masses and the total mass estimates ; the will in general be more difficult to detect from distant section concludes with a tabular presentation of the than from nearby galaxies, this difference in relative results of all currently available 21-cm studies of abundance precluded attempts to observe equal galaxies. Section IV presents statistical summaries of numbers of all morphological types. The observing program included the 32 galaxies * The Agassiz Station Radio Astronomy Project of Harvard College Observatory is supported by the U. S. National Science listed in Table I: 4 E’s, 2 SO^ 2 Sa’s, 6 Sb’s, 1 SBb, Foundation. 8 Sc’s and 9 Ir’s. These represent three categories of 490 © American Astronomical Society • Provided by the NASA Astrophysics Data System 90E 4 . 69. ATOMIC HYDROGEN IN GALAXIES 491 Table I. Galaxies used in the observing program. from 4438), NGC 205 (too near 224), NGC 3623l(too near 3627 and 3628), and NGC 4374 (too near 4486). 1964AJ NGC *IC Type NGC Type B. Observational Results 45 Sc 3368 Sa 55a Sc 4214* Ir Figure 1 displays the H i emission line profiles of 26 *10* Ir 4258b Sb of the galaxies listed in Table I. Figure 2 shows the 147 E 4382 SO observed rotation curves constructed for NGC 55, 300, 185 E 4449* Ir and 3109. These galaxies are of large angular size and 247* Sc 4486b E 253b Sc 4565 Sb have major axes approximately parallel to lines of 300* Sc 4594 Sa constant declination. The curve constructed for NGC *1613* Ir 4631*’b Sc 4214 has a total radial velocity extent of ^90 km/sec 1023 SO 4656* Ir and suggests an east-west extent of ~ 15' ; this curve Fornax system E 4725 Sb is not illustrated. We were unable to construct rotation 1097 SBb 4736 Sb Holmberg IIa Ir 4826 Sb curves for the remaining galaxies because it was 3109* Ir 5055 Sb impossible to assign positions with sufficient accuracy. Sextans A* Ir 5236*-b Sc (See Appendix A5 for data reduction procedures.) *2574* Ir 6946*-° Sc The position information for the rotation curve of NGC 55 indicates that the signals which contribute to a 17 i emission detected. b Continuum emission detected. the noncentral profile peak originate in the same region 0 Continuum emission provisionally detected ; because of the low Galactic («10' east of the center) as the asymmetry in the latitude, some confusion with the Galactic background exists. However, Heeschen (1961), using a higher resolution antenna, identifies this galaxy as a continuum source. Table II. Positions of rotation curve maxima. galaxies for which Humason, Mayall, and Sandage Rp Rb.o (1956) have given radial velocities: (1) The apparently NGC Type (min of arc) (min of arc) Rp/Rb.o largest previously unobserved objects in each major class in Holmberg’s catalogue, (2) several well-known 157 Sc 1* 3 0.3 224 Sb 40b 98 0.4 dwarf galaxies, and (3) five southern galaxies of large 598 Sc 16c 42 0.4 angular size not in Holmberg^ catalogue. 2903 Sc 1.6d 7 0.2 Fourteen of the fifteen Hi emission sources listed 5005 Sb Ie 4 0.2 f in Table I were first detected with the Harvard maser 5055 Sb 1.7 8 0.2 installation. Cooper et al. (1960) detected five of the fourteen during a preliminary maser investigation ; the b» Burbidge, Burbidge, and Prendergast (1961b), 0 Brandt (1960a). rest are reported here for the first time. d Mayall and Aller (1942). Burbidge, Burbidge, and Prendergast (1960b). Initial searches (only one observation at each velocity «Burbidge,f Burbidge, and Prendergast (1961a). setting within the search range) for the Sb galaxies Burbidge, Burbidge, and Prendergast (1960a). NGC 4565, 4725, 4826, and 5055 were unsuccessful. We therefore decided to make more extended obser- luminosity distribution (see de Vaucouleurs 1961b). vations of NGC 4258 and NGC 4736, the two ap- About 10% of both the total photographic luminosity parently largest and nearest Sb galaxies listed in and the total H i flux of NGC 55 originates in this Table I. Since HI emission was not positively detected region. from these two most likely-to-be-detected Sb sources, no further observations were made of the more remote III. DATA ANALYSIS and/or apparently smaller Sb galaxies. Only a limited A. The HI Masses search was made for NGC 45 because emission had already been detected from a large sample of Sc None of the signals observed during this program galaxies. was measurably broadened (see Appendix A2) ; conse- We removed an Sa galaxy, NGC 3368, from the quently, the apparent solid angles of the emitting observing program because of confusion with a con- regions (within the galaxies) which give rise to the tinum source (flux«4XlO~26 W m~2 cps-1) approxi- individual signals were treated as being small with mately one beamwidth west; this source coincides in respect to the equivalent solid angle of the antenna position with NGC 3351, a blue-nucleus Sb galaxy beam (3.2X103 sq min). The expression used by Roberts (de Vaucouleurs 1961a and Tifft 1963). (1962a) for the È i mass, Sfïl#i (in solar masses), of a Certain galaxies, though within the scope of the first source which is small with respect to the beam is category above, were not considered; these included: NGC 4438 (its small radial velocity allows possible = 2.36X RPC's2 / SydV, (1) confusion between H i emission from the Galaxy and 9Fo J-* © American Astronomical Society • Provided by the NASA Astrophysics Data System 90E 4 . 69. 492 EUGENE E. EPSTEIN KM/SEC 1964AJ I Í (o) KM/SEC 8.0 _ 4.0 Ä 2.0 co 0.0 (b) KM/SEC (O Fig. 1. The H i emission line profiles, arranged in order of increasing right ascension of the galaxies. Ordinate units of both antenna temperature and flux are used; the latter is more reliably calibrated.