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Register a new userCO(J=3-2) On-the-fly Mapping of the Nearby Spiral Galaxies NGC 628 and NGC 7793: Spatially-resolved CO(J=3-2) Star-formation Law
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We present the results of CO(J=3-2) on-the-fly mappings of two nearby non-barred spiral galaxies NGC 628 and NGC 7793 with the Atacama Submillimeter Telescope Experiment at an effective angular resolution of 25. We successfully obtained global distributions of CO(J=3-2) emission over the entire disks at a sub-kpc resolution for both galaxies. We examined the spatially-resolved (sub-kpc) relationship between CO(J=3-2) luminosities (LCO(3-2)) and infrared (IR) luminosities (LIR) for NGC 628, NGC 7793, and M 83, and compared it with global luminosities of JCMT Nearby Galaxy Legacy Survey sample. We found a striking linear LCO(3-2)-LIR correlation over the 4 orders of magnitude, and the correlation is consistent even with that for ultraluminous infrared galaxies and submillimeter selected galaxies. In addition, we examined the spatially-resolved relationship between CO(J=3-2) intensities (ICO(3-2)) and extinction-corrected star formation rates (SFRs) for NGC 628, NGC 7793, and M 83, and compared it with that for GMCs in M 33 and 14 nearby galaxy centers. We found a linear ICO(3-2)-SFR correlation with 1 dex scatter. We conclude that the CO(J=3-2) star formation law (i.e., linear LCO(3-2)-LIR and ICO(3-2)-SFR correlations) is universally applicable to various types and spatial scales of galaxies, from spatially-resolved nearby galaxy disks to distant IR-luminous galaxies, within 1 dex scatter.
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We have made the first map of CO(J=3-2) emission covering the disk of the edge-on galaxy, NGC~4631, which is known for its spectacular gaseous halo. The strongest emission, which we model with a Gaussian ring,occurs within a radius of 5 kpc. Weaker disk emission is detected out to radii of 12 kpc, the most extensive molecular component yet seen in this galaxy. From comparisons with infrared data, we find that CO(J=3-2) emission more closely follows the hot dust component, rather than the cold dust,consistent with it being a good tracer of star formation. The first maps of $R_{3-2/1-0}$, H$_2$ mass surface density and SFE have been made for the inner 2.4 kpc radius region. Only 20% of the SF occurs in this region and excitation conditions are typical of galaxy disks, rather than of central starbursts. The SFE suggests long gas consumption timescales ($>$ $10^9$ yr). The velocity field is dominated by a steeply rising rotation curve in the region of the central molecular ring followed by a flatter curve in the disk. A very steep gradient in the rotation curve is observed at the nucleus, providing the first evidence for a central concentration of mass: M$_{dyn},=,5,times,10^7$ M$_odot$ within a radius of 282 pc. The velocity field shows anomalous features indicating the presence of molecular outflows; one of them is associated with a previously observed CO(J=1-0) expanding shell. Consistent with these outflows is the presence of a thick ($z$ up to $1.4$ kpc) CO(J=3-2) disk. We suggest that the interaction between NGC~4631 and its companion(s) has agitated the disk and also initiated star formation which was likely higher in the past than it is now. These may be necessary conditions for seeing prominent halos.
Context. Outflows provide indirect means to get an insight on diverse star formation associated phenomena. On scales of individual protostellar cores, outflows combined with intrinsic core properties can be used to study the mass accretion/ejection process of heavily embedded protostellar sources. Methods. An area comprising 460x230 of the Serpens cloud core has been mapped in 12 CO J = 3to 2 with the HARP-B heterodyne array at the James Clerk Maxwell Telescope; J = 3to 2 observations are more sensitive tracers of hot outflow gas than lower J CO transitions; combined with the high sensitivity of the HARP-B receptors outflows are sharply outlined, enabling their association with individual protostellar cores. Results. Most of ~20 observed outflows are found to be associated with known protostellar sources in bipolar or unipolar configurations. All but two outflow/core pairs in our sample tend to have a projected orientation spanning roughly NW-SE. The overall momentum driven by outflows in Serpens lies between 3.2 and 5.1 x 10^(-1) Modot km s^(-1), the kinetic energy from 4.3 to 6.7 x 10^(43) erg and momentum flux is between 2.8 and 4.4 x 10^(-4) Modot km s^(-1) yr^(-1). Bolometric luminosities of protostellar cores based on Spitzer photometry are found up to an order of magnitude lower than previous estimations derived with IRAS/ISO data. Conclusions. We confirm the validity of the existing correlations between the momentum flux and bolometric luminosity of Class I sources for the homogenous sample of Serpens, though we suggest that they should be revised by a shift to lower luminosities. All protostars classified as Class 0 sources stand well above the known Class I correlations, indicating a decline in momentum flux between the two classes.
Star formation induced by a spiral shock wave, which in turn is generated by a spiral density wave, produces an azimuthal age gradient across the spiral arm, which has opposite signs on either side of the corotational resonance. An analysis of the spatial separation between young star clusters and nearby HII regions made it possible to determine the position of the corotation radius in the studied galaxies. Fourier analysis of the gas velocity field in the same galaxies independently confirmed the corotation radius estimates obtained by the morphological method presented here.
We report $^{12}$CO($J=$3--2) observations of 15 nearby elliptical galaxies carried out with the ASTE telescope. Thirteen were selected without regard to the presence of other tracers of cold interstellar matter. CO emission was detected from three of the galaxies, two of which are undetected by IRAS at 100 microns, suggesting that cold ISM may be present in more ellipticals than previously thought. The molecular gas masses range from $2.2 times 10^6$ to $4.3 times 10^8$ $M_odot$. The ratio of the CO(3--2) and (1--0) lines, $R_{31}$, has a lower value for elliptical galaxies than for spiral galaxies except for NGC 855, for which the value is close to the mean for spirals. The molecular gas in NGC 855 has a mean density in the range 300 -- 1000 cm$^{-3}$ adopting a temperature range of 15 -- 100 K.
We present a detailed study of the flocculent spiral galaxy NGC 7793, part of the Sculptor group. By analyzing the resolved stellar populations of the galaxy, located at a distance of ~3.7 Mpc, we infer for the first time its radial star formation history (SFH) from Hubble Space Telescope photometry, thanks to both archival and new data from the Legacy ExtraGalactic UV Survey. We determine an average star formation rate (SFR) for the galaxy portion covered by our F555W and F814W data of 0.23 +- 0.02 Msun/yr over the whole Hubble time, corresponding to a total stellar mass of 3.09 +- 0.33 x 10^9 Msun in agreement with previous determinations. Thanks to the new data extending to the F336W band, we are able to analyze the youngest stellar populations with a higher time resolution. Most importantly, we recover the resolved SFH in different radial regions of the galaxy; this shows an indication of a growing trend of the present-to-past SFR ratio, increasing from internal to more external regions, supporting previous findings of the inside-out growth of the galaxy.
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