NGC 5291: IMPLICATIONS FOR THE FORMATION OF DWARF GALAXIES

THE ASTRONOMICAL JOURNAL VOLUME 114, NUMBER 4 OCTOBER 1997NGC 5291: IMPLICATIONS FOR THE FORMATION OF DWARF GALAXIESBENJAMIN K. MALPHRUSDepartment of Physical Sciences, Morehead State University, Morehead. Kentucky 40351Electronic mail: b.malphr@morehead-st.edu2 0 7 1 2/4, CAROLINE E. SIMPSONDepartment of Physics. Florida International Universily, Miami, Florida 33199Electronic mail: simpsonc@fiu.eduS. T. GOTTESMANDepartment of Astronomy, University of Florida, Gainesville, Florida 32611Electronic mail: gott@astro.ufl.eduTIMOTHY G. HAWARDEN lJoint Astronomy Centre, 660 North A'ohoku Place, Hilo, Hawaii 96270Electronic mail: hawarden@jach.hawaii.eduReceived 1997 April 20; revised 1997 July 7ABSTRACTThe possible formation and evolution of dwarf irregular galaxies from material derived from perturbedevolved galaxies is addressed via an H 1 study of a likely example, the peculiar system NGC 5291. Thissystem, located in the western outskirts of the cluster Abell 3574, contains the lenticular galaxy NGC 5291which is in close proximity to a disturbed companion and is flanked by an extensive complex of numerousknots extending roughly 4' north and 4' south of the galaxy. In an initial optical and radio study, Longmoreet al. (1979, MNRAS, 188,285) showed that these knots have the spectra of vigorous star-forming regions,and suggested that some may in fact be young dwarf irregular galaxies. High resolution 21-cm lineobservations taken with the VLA are presented here and reveal that the H 1 distribution associated with thissystem encompasses not only the entire N-S complex of optical knots, but also forms an incomplete ring ortail that extends approximately 3' to the west. The H I associated with NGC 5291 itself shows a highvelocity range; the Seashell is not detected. The formation mechanism for this unusual system is unclear andtwo models--a large, low-luminosity ram-swept disk, and a ram-swept interaction--are discussed. The H lin the system contains numerous concentrations, mostly along the N-S arc of the star-forming complexes,which generally coincide with one or more optical knots; the larger H 1 features contain several × 109 M,Sof gas. Each of the knots is compared to a set of criteria designed to determine if these objects are boundagainst their own internal kinetic energy and are tidally stable relative to the host galaxy. An analysis of theproperties of the H I concentrations surrounding the optical star-forming complexes indicates that at leastthe largest of these is a bound system; it also possesses a stellar component. It is suggested that this objectis a genuinely young dwarf irregular galaxy that has evolved from the material associated with the systemand that this entire complex contains several proto- or young dwarf irregular galaxies in various stages ofdevelopment. We are therefore witnessing the early evolution of a number of genuinely young galaxies.Given the evident importance of the NGC 5291 system as a "nursery" for young galaxies, careful modelingis required if we are to understand this remarkable galaxy. © 1997 American Astronomical Society.[S0004-6256(97)02110-9]I. INTRODUCTIONNGC 5291 lies at c_=13h47m24)3, b'=--30°24'26 "(J2000), in the western parts of the cluster Abell 3574 2(Abell 1958) of which it is probably a member. We adopt avalue for H 0 of 75 km s-_ Mpc-_; the cluster recessionIRoyal Observatory, Blackford Hill, Edinburgh EH9 3H J. United Kingdom.2A.k.a. Shapley 1346-30 (Shapley 1936), 1C 4329 (de Vaucouleurs & deVaucouleurs 1964), and Klemola 29 (Klemola 1969).velocity of 4350 km s J (e.g., Richter 1984) then implies adistance of 58 Mpc. Pedersen etal. (1978) presented andbriefly discussed deep images of the system taken with theESO 3.6 m telescope, drawing attention to the immense com-plexes of optical knots which extend several galactic diam-eters to the north and south and hypothesizing that these areH II regions (or at least, sites of recent star formation) fromtheir blue colors. In the Second Reference Catalog of BrightGalaxies (de Vaucouleurs et al. 1976) NGC 5291 is classi-fied as a peculiar elliptical (Ep?) with a nearby companion.1427 Astron. J. 114 (4), Oclober 1997 0004-6256/97/114(4)/1427/20/$10.00 © 1997 Am. Astron. Soc. 1427