VLA Observations of NGC 253: Supernova Remnants and HiiRegions at 1 Parsec Resolution

Abstract

The edge-on starburst galaxy NGC 253 has been imaged with the VLA at wavelengths ranging from 1.3 to 20 cm, with resolutions between 1 and 15 pc. These images reveal a large number of compact radio sources embedded within the di †use radio structure in the inner 200 pc of the galaxy. We have identiÐed approximately 64 individual compact radio sources in the galaxy. Of the strongest 17 sources, for which the Ñux densities are high enough to measure spectral indices with moderately low errors, about half have Ñat spectra and half have steep spectra ; this indicates that perhaps half of the individual radio sources are dominated by thermal radio emission from H II regions. Over an 8 yr timescale, few, if any, of the strongest sources have varied in Ñux density, with limits of 1%È2% yr~1 on the rate of decrease. At the highest resolution of 1 pc, a number of radio sources are beginning to be resolved, and at least 15 di †erent sources can be identiÐed within the innermost 20 pc of the galaxy.Individual radio sources have been explored in more detail. The strongest source, 5.79[39.0 (TH2), assumed to be at the nucleus of the galaxy, has a brightness temperature greater than 20,000 K at 22 GHz and greater than 40,000 K at 15 GHz. It is unresolved at the VLA and may be either an active galactic nucleus or a very compact (nonvariable) supernova remnant. A resolved Ñat-spectrum source, 5.72[40.1 (TH6), is located D20 pc to the southwest. It has an apparent size of 2.4 ] 1.2 pc and appears to be an H II region similar to the inner part of 30 Doradus, containing approximately 105 M _ in stars, as well as D600 of ionized gas. Source counts, the lack of variability, and the lack of new M _ sources imply that the radio supernova rate is no more than 0.3 yr~1, consistent with estimates made in other wave bands.

The excitation of near-infrared H2emission in NGC 253

“…Fernández-Ontiveros et al (2009) suggests that Region 2 is a starburst of 6 Myr, which is also consistent with our age calculations. Our integrated supernova rate in the inner 300 pc of the galaxy, is 0.2 yr −1 , which is well matched to previous supernova rate measurement of 0.1−0.3 (Rieke et al 1980(Rieke et al , 1988Ulvestad & Antonucci 1997). Unlike Brγ and [FeII], the mechanism which excites the H 2 gas is not as clear.…”

“…Many dense, luminous, dust enshrouded young stellar clusters have been found in the nucleus (Forbes et al 1993(Forbes et al , 2000. In addition to the stellar clusters, Alonso-Herrero et al (2003) and Ulvestad & Antonucci (1997) have observed many compact radio sources in [FeII] and radio respectively, representing young supernova remnants. Turner & Ho (1985) observed over 60 compact radio sources, the brightest Reduced datacubes and extracted images for each line are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/550/A12 of which is named TH2.…”

“…The second, 3.0 to the northeast is coincident to the radio peak TH2. A more recent study by Ulvestad & Antonucci (1997) suggests that TH2 is the true nucleus of NGC 253 and represents either a low-luminosity active galactic nucleus (AGN) or a compact supernova remnant.…”

A Search for Active Galactic Nuclei in Sc Galaxies with HiiSpectra

“…The three H ii galaxies with nuclear radio detections that are shown in Figure 1 have radio sizes of a few hundred parsecs, indicating that any intense star formation must be confined to the near-nuclear regions of the galaxies. The 4.9 GHz radio sizes of NGC 864, NGC 4123, and NGC 4535 are quite similar to the 500-600 pc nuclear starburst in the nearby Sc galaxy NGC 253 (Antonucci & Ulvestad 1988;Ulvestad & Antonucci 1997). NGC 253 has a total 5 GHz flux density of 2.4 Jy integrated over the entire galaxy (Griffith et al 1994).…”

“…Thus, it is entirely possible that the detected H ii galaxies harbor nuclear star formation somewhat weaker than that in NGC 253. Supporting this possibility, Figure 2 shows the 6 cm image of NGC 253 (Ulvestad & Antonucci 1997), as it would appear at a distance of 20 Mpc instead of at 2.5 Mpc, having the flux density reduced by a factor of 64, and angular resolution degraded by a factor of 8 in each dimension. The radio morphology of this degraded NGC 253 image looks similar to the H ii galaxies in Figure 1, except that the respective peak and total flux densities are somewhat higher, at 8.2 mJy beam À1 and 20 mJy.…”