Gustavo J. Carranza

Open clusters or their remnants: B and V photometry of NGC 1901 and NGC 1252

Photometry in theB andV bands is presented for the southern stellar groups NGC 1901 and NGC 1252. NGC 1901 is often described as an open cluster while NGC 1252 consists of a concentration of about 20 stars centered20 0 north of the original New General Catalogue coordinates, and at the southwest edge of the large region previously assigned to this object in the literature. NGC 1901 has a clear main sequence and shares similarities with the Hyades. We derive a reddening value E(B V )=0 :04, a distance from the Sund =0 :45 kpc (Z = 0:23 kpc) and an age 0.6 0.1 Gyr. NGC 1901 is conclusively a physical system, dynamically comparable to or more evolved than the Hyades. The colour-magnitude diagram of NGC 1252 suggests a turno and main sequence, and a total of 12 probable members. We simulated the Galactic eld colour-magnitude diagram in the same direction and found it to be a poor match to NGC 1252, suggesting that NGC 1252 is not a eld fluctuation. Isochrone tting to the probable members is consistent with E(B V )=0 :02, d =0 :64 kpc (Z = 0:46 kpc) and an age 3 1 Gyr. NGC 1252 cannot be ruled out as a physical group with the available data. If so, evidence is found that it is not a classical open cluster, but rather an open cluster remnant.

Study of McLeish’s Interacting Object

We discuss the morphology, kinematics, and physical conditions of the emitting gas of the interacting system IRAS 20048-6621. We present as well numerical simulations of this interacting system, discovered by David McLeish in 1946. The main galaxy (McL A) is an edge-on spiral galaxy with highly distorted NW side. On this side is also located McL B, the perturber galaxy. We determined a distance of 151 Mpc (h = 0.75) and a diameter of 70 kpc for McL A. It presents a bright nucleus with broad red emission lines (FWHM ≈ 500 km s-1). McL A has far-IR color indexes closely comparable to NGC 3628, one the few nearby edge-on galaxies which is a bright infrared emitter. Nevertheless, McL A is more luminous (in these bands) than any of the edge-on galaxies in the sample of bright infrared galaxies of Young et al. (1988). The two sides of McL A rotation curve are remarkably different. The N-body model that best reproduces McL A kinematical and morphological data (Kuijken &Dubinski 1995) gives a total mass 7 × 1011 M⊙ for McL A. Numerical simulations with the TREESPH code closely reproduce the morphology and radial velocity observations. The best scenario for this system is that of a prograde encounter between McL A and B, with McL Bs orbit 35° tilted with respect to the spiral disk of McL A and a perigalactic distance of 17.6 kpc. The derived mass ratio is McL B/McL A ≈ 1/26. In the last 5 × 108 yr the perturber has crossed the main galaxy disk twice, in between it crossed the perigalacticon. According to our simulations, the emitting gas present in McL B has not been stripped out from the McL A disk, so that leads us to conclude that McL B is an irregular or small spiral galaxy.

Separating C Stars from Dust in the Central Region of the Seyfert 2 Galaxy NGC 1241

The Seyfert 2 galaxy NGC 1241 presents a 1.5 kpc large circumnuclear ring (CNR) of star formation embracing a small bar plus leading arms. Those structures are Paα emitters but barely seen in Hα. It also presents stellar trailing arms inside the CNR. Gemini and HST imagery allow the construction of high-resolution (V - H) and (J - Ks) color maps, as well as a (J - Ks) versus K color-magnitude diagram of this complex region. The CNR is heavily obscured in V, but a fairly transparent window appears in the direction of the nucleus. Nonetheless, the nucleus presents a (J - Ks) color that is redder than the CNR. The CNR is composed of extremely young H II regions still enshrouded in their dust cocoons. However, the nuclear (J - Ks) color cannot be explained in this manner. Therefore, we propose the contribution of C stars as the most feasible mechanism for explaining the colors. If the nuclear stellar population is comparable to that of the Large Magellanic Cloud bar, 500 C stars and 25,000 asymptotic giant branch O-rich stars inside 50 pc may reproduce the observed colors. C stars release enriched material to the nuclear environment, probably fueling the central engine of this Seyfert 2 galaxy during the lifetime of stars with masses in the range 2 M☉ < MC star < 6 M☉ (C-star phase). The ejected material that remains trapped in the central potential might also explain the systematically observed increased strength of the optical CN bands in Seyfert 2 galaxies and is consistent with the significant contribution of intermediate age stars to the optical continuum of low-luminosity active galactic nuclei.

Internal motions in NGC 55

The kinematics of the central zones of NGC 55 is discussed on the basis of interferometric observations. The emission regions have radial velocities well differentiated showing also a significant gradient and coinciding in general with those obtained in 21 cm. The exception consisting of an emission complex of high velocity is interpreted as moving towards the centre of the galaxy indicating flux of material along the bar.

The nitrogen abundance in NGC 55

The purpose of this paper is to propose an interpretation for the line spectrum of NGC 55, whose most remarkable feature is the absence of the generally noticeable λ6584[Nii]. From the physical conditions of the ionized gas responsible of the observed emission, we infer that the N(N)/N(O) abundance ratio is significantly smaller than usual, probably reflecting a nitrogen underabundance in a medium of temperatures ∼9100 K and densities ∼500 cm−3.

The ionized gas in NGC 4945

Physical conditions and relative abundance ratios of the ionized medium of NGC 4945 are discussed. The spectral characteristics indicate forTe=104 K a relatively low electron density (Ne≤1250 cm−3) decreasing outwards. N(N)/N(H) and N(S)/N(H) diminish with the distance from the nucleus while N(N)/N(S) is nearly constant. These abundance gradients, the lack of kinematical evidences of a bar and the observational difficulties to grasp its morphology, make it improbable that NGC 4945 could be classified as a barred galaxy.

Physical conditions in NGC 6164-5

From spectroscopic observations of the emission lines Hβ, λ4959 and λ5007[Oiii], Hα, λ6584[Nii], λ6717 and λ6731[Sii] and some interferometric data in Hα and [Nii] it was established that NGC 6164 and NGC 6165 have characteristics of normal Hii regions for Ne and Te. They are embedded in a very tenuous medium nearly coincident with the Strömgren sphere of the central star HD 148937, corresponding to the outer peripheral structure described by Westerlund. Variations of the relative abundances of N, O, and S are discussed.

Radial variations of line ratios across NGC 5236

The ionized gas in NGC 5236 was studied by spectroscopic means. The radial behaviour of the line ratios [Nii]/[Sii], Hα/[Sii] suggest a real nitrogen overabundance in the central regions of that galaxy.

Interferometric study of NGC 1313

Results of Hα interferometric observations of NGC 7793 are reported. This galaxy contains about 93 HII regions and a general emission background. Its radial velocity is 215 km s−1.The total mass is low, 2–3×109 solar masses, as well as the average density; the meanM/L is 0.4 suggesting a high proportion of young objects.