María Luisa García Vargas

Violent star formation in NGC 2363

This is an electronic version of an article published in The Astrophysical Journal. Gonzalez-Delgado, R.M. et al. Violent star formation in NGC 2363. The Astrophysical Journal 437 (1994): 239-261

Nuclear Star-forming Structures and the Starburst-Active Galactic Nucleus Connection in Barred Spirals: NGC 3351 and NGC 4303

A high-resolution Hubble Space Telescope WFPC2 F218W UV image of the barred spiral NGC 4303 (classified as a LINER-type active galactic nucleus [AGN]) reveals for the first time the existence of a nuclear spiral structure of massive star-forming regions all the way down to the UV-bright unresolved core (size ≤ 8 pc) of an active galaxy. The spiral structure, as traced by the UV-bright star-forming regions, has an outer radius of 225 pc and widens as the distance from the core increases. The UV luminosity of NGC 4303 is dominated by the massive star-forming regions, and the unresolved LINER-type core contributes only 16% of the integrated UV luminosity. The nature of the UV-bright LINER-type core—stellar cluster or pure AGN—is still unknown. In contrast to NGC 4303, the UV F218W image of the non-AGN barred galaxy NGC 3351 shows a nuclear star-forming ring of 315 pc (semimajor axis) with a faint core. In the ring, the star formation is arranged in clumps of about 60-85 pc in diameter. Each clump consists of a few compact UV-bright clusters embedded in a more diffuse component. The integrated IUE spectrum of NGC 3351 shows the presence of Si IV 1400 A and C IV 1550 A absorption lines, typical features of young, 4-5 Myr old, massive star clusters. The presence of ring and spiral star-forming structures in the nuclear regions of these two barred spirals supports the bar-induced gas-fueling scenario by which bars accumulate gas in the nuclear regions of galaxies, produce nuclear star-forming rings (NGC 3351), and might eventually generate or feed an AGN (NGC 4303).

Evolutionary models for high-metallicity giant extragalactic H II regions

This is an electronic version of an article published in The Astrophysical Journal Supplement Series. Garcia Vargas, M.L. and A.I. Diaz. Evolutionary models for high-metallicity giant extragalactic H II regions. The Astrophysical Journal Supplement Series 91 (1994): 553-581

On the Origin of the Ultraviolet Continuum in Seyfert 2 Galaxies

High-resolution Hubble Space Telescope ultraviolet images of the Seyfert 2 galaxies NGC 1667, NGC 3982, NGC 5427, and NGC 5953, previously known to have circumnuclear star-forming rings, are presented. The UV images of all four Seyfert 2 galaxies show the presence of several compact UV-bright knots at distances of ~0.7-2 kpc from the nucleus and associated to the circumnuclear star-forming ring. The optically bright Seyfert 2 nucleus is barely detected in the UV. The UV images provide the first direct empirical evidence that the UV flux emitted by galaxies having a Seyfert 2-type nucleus surrounded by a circumnuclear star-forming ring is dominated by radiation coming from clusters of young hot stars distributed along the star-forming ring. The contribution of the Seyfert 2 nucleus amounts to ~1%-10% of the observed UV flux in these four galaxies. If circumnuclear star-forming rings of 2-3 kpc in diameter, or smaller, are a common feature of Seyfert 2 galaxies, we conclude that (a) the observed UV flux in Seyfert 2 galaxies is not dominated by the radiation from the nuclear ionizing source and (b) the observed UV flux is dominated by emission from clusters of young hot stars located in circumnuclear star-forming regions. Whatever the nature of the nuclear ionizing source in these Seyfert 2 galaxies, this must be obscured. The UV and Hα luminosities associated to the Seyfert 2 nucleus in these galaxies are consistent with the scenario of the nuclear ionizing source being an obscured power-law ionizing source or a young (2-4 Myr) obscured high-metallicity (≥Z☉) nuclear starburst.

Observations of Lyα Emission in Young Galaxies

We report IUE observations of two extremely low metallicity HII galaxies. Lyman alpha emission was detected in both galaxies.