Star-Forming Cloud Complexes in the Central Molecular Zone of NGC 253

Abstract

We report 350 and 230 GHz observations of molecular gas and dust in the starburst nucleus of NGC 253 at 20-40 pc (1 -2 ) resolution. The data contain CO(3-2), HCN(4-3), CO(2-1), 13 CO(2-1), C 18 O(2-1), and continuum at 0.87 mm and 1.3 mm toward the central kiloparsec. The CO(2-1) size of the galaxy's central molecular zone (CMZ) is measured to be about 300 pc × 100 pc at the half-maximum of intensity. Five clumps of dense and warm gas stand out in the CMZ at arcsecond resolution, and they are associated with compact radio sources due to recent massive star formation. They contribute one-third of the CO emission in the central 300 pc and have 12 CO peak brightness temperatures around 50 K, molecular gas column densities on the order of 10 4 M pc −2 , gas masses on the order of 10 7 M in the size scale of 20 pc, volume-averaged gas densities of n H 2 ∼ 4000 cm −3 , and high HCN-to-CO ratios suggestive of higher fractions of dense gas than in the surrounding environment. It is suggested that these are natal molecular cloud complexes of massive star formation. The CMZ of NGC 253 is also compared with that of our Galaxy in CO(2-1) at the same 20 pc resolution. Their overall gas distributions are strikingly similar. The five molecular cloud complexes appear to be akin to such molecular complexes as Sgr A, Sgr B2, Sgr C, and the l = 1.• 3 cloud in the Galactic center. On the other hand, the starburst CMZ in NGC 253 has higher temperatures and higher surface (and presumably volume) densities than its non-starburst cousin.

Diverse Nuclear Star-forming Activities in the Heart of NGC 253 Resolved with 10-pc-scale ALMA Images

“…All these clumps are approximately 10 pc in size. Sakamoto et al (2011) identified five peaks in their 1.3 mm continuum map with 20 pc (1 .1) resolution. One of the peaks, peak 3, is resolved into four clumps in our continuum map (Clumps 2-5; see Figure 1 (a)), and it turns out that those have totally diverse spectral features (see Section 3.2).…”

“…Rieke et al 1980); active star-formation in its central region with the star-formation rate of a few M yr −1 (e.g. Radovich et al 2001) is sustained by a large amount of interstellar medium (e.g., Sakamoto et al 2011).…”

The excitation of near-infrared H2emission in NGC 253

“…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. A high resolution (1 −2 ) study of the molecular gas in NGC 253 was performed by Sakamoto et al (2011). They found 5 warm, dense clumps of molecular gas, which are all coincident with radio sources.…”

Lessons from comparisons between the nuclear region of the Milky Way and those in nearby spirals

“…We adopt D = 3.9 Mpc. The galaxy is highly inclined and the starburst CMZ also shows an inclined, disky structure (Sakamoto et al 2011 …”