Molecular and atomic gas in the Local Group galaxy M 33

Abstract

We present high-resolution large-scale observations of the molecular and atomic gas in the Local Group galaxy M 33. The observations were carried out using the HEterodyne Receiver Array (HERA) at the 30 m IRAM telescope in the CO(2-1) line, achieving a resolution of 12 × 2.6 km s −1 , enabling individual giant molecular clouds (GMCs) to be resolved. The observed region is 650 square arcminutes mainly along the major axis and out to a radius of 8.5 kpc, and covers entirely the 2 × 40 radial strip observed with the HIFI and PACS Spectrometers as part of the HERM33ES Herschel key program. The achieved sensitivity in main-beam temperature is 20-50 mK at 2.6 km s −1 velocity resolution. The CO(2-1) luminosity of the observed region is 1.7 ± 0.1 × 10 7 K km s −1 pc 2 and is estimated to be 2.8 ± 0.3 × 10 7 K km s −1 pc 2 for the entire galaxy, corresponding to H 2 masses of 1.9 × 10 8 M and 3.3 × 10 8 M respectively (including He), calculated with N(H 2 )/I CO(1−0) twice the Galactic value due to the half-solar metallicity of M 33. The H i 21 cm VLA archive observations were reduced, and the mosaic was imaged and cleaned using the multi-scale task in the CASA software package, yielding a series of datacubes with resolutions ranging from 5 to 25 . The H i mass within a radius of 8.5 kpc is estimated to be 1.4 × 10 9 M . The azimuthally averaged CO surface brightness decreases exponentially with a scale length of 1.9 ± 0.1 kpc whereas the atomic gas surface density is constant at Σ H i = 6± 2 M pc −2 deprojected to face-on. For an N(H 2 )/I CO(1−0) conversion factor twice that of the Milky Way, the central kiloparsec H 2 surface density is Σ H 2 = 8.5 ± 0.2 M pc −2 . The star formation rate per unit molecular gas (SF efficiency, the rate of transformation of molecular gas into stars), as traced by the ratio of CO to H α and FIR brightness, is constant with radius. The SFE, with a N(H 2 )/I CO(1−0) factor twice galactic, appears 2-4 times greater than for large spiral galaxies. A morphological comparison of molecular and atomic gas with tracers of star formation is presented showing good agreement between these maps both in terms of peaks and holes. A few exceptions are noted. Several spectra, including those of a molecular cloud situated more than 8 kpc from the galaxy center, are presented.

A Molecular Star Formation Law in the Atomic-Gas-Dominated Regime in Nearby Galaxies

“…There are two straightforward physical interpretations for this. The efficiency of star formation from H 2 gas may be genuinely higher in these systems, a view advocated by Gardan et al (2007) and Gratier et al (2010). Alternatively, CO emission may be depressed relative to the true amount of H 2 mass due to changes in the dust abundance.…”

The Impact of Interactions, Bars, Bulges, and Active Galactic Nuclei on Star Formation Efficiency in Local Massive Galaxies

“…However, toward the low-mass end of our sample, and even more so at M * < 10 10 M , the M H 2 /SFR ratio may very well be modulated simultaneously by variations in α CO and actual changes in star formation efficiency. Even after accounting for metallicity effects on α CO , high star formation efficiencies have indeed been reported for several low-mass, low-μ * local galaxies where high-resolution measurements of both gas and star formation are possible, for example, M33 (Gardan et al 2007;Gratier et al 2010), IC 10 (Leroy et al 2006, and NGC 6822 (Gratier et al 2010) (see, however, the counterargument in the case of the Small Magellanic Cloud (SMC) in Bolatto et al 2011). Disentangling the two effects will require both large statistical samples of low-mass galaxies akin to COLD GASS and resolved gas maps of galaxies spanning a broader range of properties.…”

Wind and nebula of the M 33 variable GR 290 (WR/LBV)

“…In order to check hypothesis about the density gradients in surrounding ISM we have searched for archival data on the neutral gas distribution in this part of the M 33 galaxy. Figure 8 demonstrates the relative distribution of H i 21 cm column density in the vicinity of the GR 290 from Gratier et al (2010). It is clearly seen that while the nearby OB associations are located in the extended dense cloud of H i, the GR 290 is observed toward the edge of the local shell in the H i distribution.…”