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Register a new userStar Formation Efficiencies at Giant Molecular Cloud Scales in the Molecular Disk of the Elliptical Galaxy NGC 5128 (Centaurus A)
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We present ALMA CO(1-0) observations toward the dust lane of the nearest elliptical and radio galaxy, NGC 5128 (Centaurus A), with high angular resolution ($sim$ 1 arcsec, or 18 pc), including information from large to small spatial scales and total flux. We find a total molecular gas mass of 1.6$times$10$^9$ $M_odot$ and we reveal the presence of filamentary components more extended than previously seen, up to a radius of 4 kpc. We find that the global star formation rate is $sim$1 Msol yr$^{-1}$, which yields a star formation efficiency (SFE) of 0.6 Gyr$^{-1}$ (depletion time $tau =$1.5 Gyr), similar to those in disk galaxies. We show the most detailed view to date (40,pc resolution) of the relation between molecular gas and star formation within the stellar component of an elliptical galaxy, from several kpc scale to the circumnuclear region close to the powerful radio jet. Although on average the SFEs are similar to those of spiral galaxies, the circumnuclear disk (CND) presents SFEs of 0.3 Gyr$^{-1}$, lower by a factor of 4 than the outer disk. The low SFE in the CND is in contrast to the high SFEs found in the literature for the circumnuclear regions of some nearby disk galaxies with nuclear activity, probably as a result of larger shear motions and longer AGN feedback. The higher SFEs in the outer disk suggests that only central molecular gas or filaments with sufficient density and strong shear motions will remain in $sim$1 Gyr, which will later result in the compact molecular distributions and low SFEs usually seen in other giant ellipticals with cold gas.
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We present the first census of giant molecular clouds (GMCs) complete down to 10$^6 M_{odot}$ and within the inner 4 kpc of the nearest giant elliptical and powerful radio galaxy, Centaurus A. We identified 689 GMCs using CO(1--0) data with 1 spatial resolution ($sim 20$ pc) and 2 km/s velocity resolution obtained with the Atacama Large Millimeter/submillimeter Array (ALMA). The $I$(CO)-$N$(H$_2$) conversion factor based on the virial method is $X_{rm CO}$ = $(2 pm 1 )times10^{20}$ cm$^{-2}$(K km/s)$^{-1}$ for the entire molecular disk, consistent with that of the disks of spiral galaxies including the Milky Way, and $X_{rm CO}$ = $(5 pm 2)times10^{20}$ cm$^{-2}$(K km/s)$^{-1}$ for the circumnuclear disk (CND, within a galactocentric radius of 200 pc). We obtained the GMC mass spectrum distribution and find that the best-truncated power-law fit for the whole molecular disk, with index $gamma simeq -2.41 pm 0.02$ and upper cutoff mass $sim 1.3 times 10^{7} M_{odot}$, is also in agreement with that of nearby disk galaxies. A trend is found in the mass spectrum index from steep to shallow as we move to inner radii. Although the GMCs are in an elliptical galaxy, the general GMC properties in the molecular disk are as in spiral galaxies. However, in the CND, large offsets in the line-width-size scaling relations ($sim$ 0.3 dex higher than those in the GMCs in the molecular disk), a different $X_{rm CO}$ factor, and the shallowest GMC mass distribution shape ($gamma = -1.1 pm 0.2$) all suggest that there the GMCs are most strongly affected by the presence of the AGN and/or shear motions.
Cloud-scale surveys of molecular gas reveal the link between molecular clouds properties and star formation (SF) across a range of galactic environments. Cloud populations in galaxy disks are considered to be representative of the `normal SF. At high resolution, however, clouds with exceptional gas properties and SF activity may also be observed in normal disk environments. In this paper, we study the brightest cloud traced in CO emission in the disk of NGC628. The cloud is spatially coincident with an extremely bright HII region. We characterize its molecular gas properties and investigate how feedback and large-scale processes influence the properties of the molecular gas. High resolution CO ALMA observations are used to characterize its mass and dynamical state, which are compared to other clouds in NGC628. A LVG analysis is used to constrain the beam-diluted density and temperature of the molecular gas. We analyze the MUSE spectrum using Starburst99 to characterize the young stellar population associated with the HII region. The cloud is massive ($1-2times10^7$M$_{odot}$), with a beam-diluted density of $n_{rm H_2}=5times10^4$ cm$^{-3}$. It has a low virial parameter, suggesting that its CO emission may be overluminous due to heating by the HII region. A young ($2-4$ Myr), massive $3times10^{5}$ M$_{odot}$ stellar population is associated. We argue that the cloud is currently being destroyed by feedback from young massive stars. Due to the clouds large mass, this phase of the clouds evolution is long enough for the impact of feedback on the excitation of the gas to be observed. Its high mass may be related to its location at a spiral co-rotation radius, where gas experiences reduced galactic shear compared to other regions of the disk, and receives a sustained inflow of gas that can promote the clouds mass growth.
Observations find a median star formation efficiency per free-fall time in Milky Way Giant Molecular Clouds (GMCs) on the order of $epsilon_{rm ff}sim 1%$ with dispersions of $sim0.5,{rm dex}$. The origin of this scatter in $epsilon_{rm ff}$ is still debated and difficult to reproduce with analytical models. We track the formation, evolution and destruction of GMCs in a hydrodynamical simulation of a Milky Way-like galaxy and by deriving cloud properties in an observationally motivated way, measure the distribution of star formation efficiencies which are in excellent agreement with observations. We find no significant link between $epsilon_{rm ff}$ and any measured global property of GMCs (e.g. gas mass, velocity dispersion). Instead, a wide range of efficiencies exist in the entire parameter space. From the cloud evolutionary tracks, we find that each cloud follow a emph{unique} evolutionary path which gives rise to wide diversity in all properties. We argue that it is this diversity in cloud properties, above all else, that results in the dispersion of $epsilon_{rm ff}$.
We report on a multi parameter analysis of giant molecular clouds (GMCs) in the nearby spiral galaxy M33. A catalog of GMCs identifed in 12CO(J=3-2) was used to compile associated 12CO(J=1-0), dust, stellar mass and star formation rate. Each of the 58 GMCs are categorized by their evolutionary stage. Applying the principal component analysis on these parameters, we construct two principal components PC1 and PC2 which retain 75% of the information in the original dataset. PC1 is interpreted as expressing the total interstellar matter content, and PC2 as the total activity of star formation. Young (<10Myr) GMCs occupy a distinct region in the PC1-PC2 plane, with lower ISM content and star formation activity compared to intermediate age and older clouds. Comparison of average cloud properties in different evolutionary stages imply that GMCs may be heated or grow denser and more massive via aggregation of diffuse material in their first ~10 Myr. The PCA also objectively identified a set of tight relations between ISM and star formation. The ratio of the two CO lines is nearly constant, but weakly modulated by massive star formation. Dust is more strongly correlated with the star formation rate than the CO lines, supporting recent findings that dust may trace molecular gas better than CO. Stellar mass contributes weakly to the star formation rate, reminiscent of an extended form of the Schmidt Kennicutt relation with the molecular gas term substituted by dust.
The distance to NGC 5128, the central galaxy of the Centaurus group and the nearest giant elliptical to us, has been determined using two independent distance indicators: the Mira period-luminosity (PL) relation and the luminosity of the tip of the red giant branch (RGB). The data were taken at two different locations in the halo of NGC 5128 with the ISAAC near-IR array on ESO VLT. From more than 20 hours of observations with ISAAC a very deep Ks-band luminosity function was constructed. The tip of the RGB is detected at Ks=21.24 pm 0.05 mag. Using an empirical calibration of the K-band RGB tip magnitude, and assuming a mean metallicity of [M/H]=-0.4 dex and reddening of E(B-V)=0.11, a distance modulus of NGC 5128 of (m-M)_0=27.87 pm 0.16 was derived. The comparison of the H-band RGB tip magnitude in NGC 5128 and the Galactic Bulge implies a distance modulus of NGC 5128 of (m-M)_0=27.9 pm 0.2 in good agreement with the K-band RGB tip measurement. The population of stars above the tip of the RGB amounts to 2176 stars in the outer halo field and 6072 stars in the inner halo field. The large majority of these sources belong to the asymptotic giant branch (AGB) population in NGC 5128 with numerous long period variables. Mira variables were used to determine the distance of NGC 5128 from a period-luminosity relation calibrated using the Hipparcos parallaxes and LMC Mira period-luminosity relation in the K-band. This is the first Mira period-luminosity relation outside the Local Group. A distance modulus of 27.96 pm 0.11 was derived, adopting the LMC distance modulus of 18.50 pm 0.04. The mean of the two methods yields a distance modulus to NGC 5128 of 27.92 pm 0.19 corresponding to D=3.84 pm 0.35 Mpc.
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