SCUBA Local Universe Galaxy Survey Research Articles

The CO(3–2)/CO(1–0) Luminosity Line Ratio in Nearby Star-forming Galaxies and Active Galactic Nuclei from xCOLD GASS, BASS, and SLUGS

Abstract We study the luminosity line ratio in a sample of nearby (z < 0.05) galaxies: 25 star-forming galaxies (SFGs) from the xCOLD GASS survey, 36 hard X-ray-selected active galactic nucleus (AGN) host galaxies from the BAT AGN Spectroscopic Survey, and 37 infrared-luminous galaxies from the SCUBA Local Universe Galaxy Survey. We find a trend for r 31 to increase with star formation efficiency (SFE). We model r 31 using the UCL-PDR code and find that the gas density is the main parameter responsible for the variation of r 31, while the interstellar radiation field and cosmic-ray ionization rate play only a minor role. We interpret these results to indicate a relation between SFE and gas density. We do not find a difference in the r 31 value of SFGs and AGN host galaxies, when the galaxies are matched in SSFR (〈r 31〉 = 0.52 ± 0.04 for SFGs and 〈r 31〉 = 0.53 ± 0.06 for AGN hosts). According to the results of the UCL-PDR models, the X-rays can contribute to the enhancement of the CO line ratio, but only for strong X-ray fluxes and for high gas density (n H > 104 cm−3). We find a mild tightening of the Kennicutt–Schmidt relation when we use the molecular gas mass surface density traced by CO(3–2) (Pearson correlation coefficient R = 0.83), instead of the molecular gas mass surface density traced by CO(1–0) (R = 0.78), but the increase in correlation is not statistically significant (p-value = 0.06). This suggests that the CO(3–2) line can be reliably used to study the relation between SFR and molecular gas for normal SFGs at high redshift and to compare it with studies of low-redshift galaxies, as is common practice.

SPITZEROBSERVATIONS OF COLD DUST GALAXIES

We combine new Spitzer Space Telescope observations in the mid-infrared and far-infrared (FIR) with SCUBA 850 μm observations to improve the measurement of dust temperatures, masses, and luminosities for 11 galaxies of the SCUBA Local Universe Galaxy Survey. By fitting dust models we measure typical dust masses of 107.9M☉ and dust luminosities of ∼1010L☉, for galaxies with modest star formation rates. The data presented in this paper combined with previous observations show that cold dust is present in all types of spiral galaxies and is a major contributor to their total luminosity. Because of the lower dust temperature of the SCUBA sources measured in this paper, they have flatter FIR νFν(160 μm)/νFν(850 μm) slopes than the larger Spitzer Infrared Nearby Galaxies Survey (SINGS), the sample that provides the best measurements of the dust properties of galaxies in the nearby universe. The new data presented here added to SINGS extend the parameter space that is well covered by local galaxies, providing a comprehensive set of templates that can be used to interpret the observations of nearby and distant galaxies.

The far-infrared-radio relationship at high and low redshift

We use the results of the SCUBA Local Universe Galaxy Survey, a submillimetre survey of galaxies in the nearby Universe, to investigate the relationship between the far-infrared--submillimetre and radio emission of galaxies at both low and high redshift. At low redshift we show that the correlation between radio and far-infrared emission is much stronger than the correlation between radio and submillimetre emission, which is evidence that massive stars are the source of both the far-infrared and radio emission. At high redshift we show that the submillimetre sources detected by SCUBA are brighter sources of radio emission than are predicted from the properties of galaxies in the local Universe. We discuss possible reasons for the cosmic evolution of the relationship between radio and far-infrared emission.

The SCUBA Local Universe Galaxy Survey — III. Dust along the Hubble sequence

We present new results from the SCUBA Local Universe Galaxy Survey (SLUGS), the first large systematic submillimetre survey of the local Universe. Since our initial survey of a sample of 104 IRAS-selected galaxies we have now completed a survey of a sample of 81 optically-selected galaxies, observed with the SCUBA camera on the James Clerk Maxwell Telescope. Since SCUBA is sensitive to the 90% of dust too cold to radiate significantly in the IRAS bands our new sample represents the first unbiased survey of dust in galaxies along the whole length of the Hubble sequence. We find little change in the properties of dust in galaxies along the Hubble sequence and detected 6 out of 11 elliptical galaxies. As in our earlier work on IRAS galaxies we find that the IRAS and submm fluxes are well-fitted by a two-component dust model with dust emissivity index beta=2. The major difference from our earlier work is that we find the ratio of the mass of cold dust to the mass of warm dust is much higher for our optically-selected galaxies and can reach values of ~1000. Comparison of the results for the IRAS- and optically-selected samples shows that there is a population of galaxies containing a large proportion of cold dust that is unrepresented in the IRAS sample. We derive local submm luminosity and dust mass functions, both directly from our optically-selected SLUGS sample, and by extrapolation from the IRAS PSCz survey using the method of Serjeant & Harrison, and find excellent agreement between the two. We find them to be well-fitted by Schechter functions except at the highest luminosities. We find that as a consequence of the omission of cold galaxies from the IRAS sample the luminosity function presented in our earlier work is too low by a factor of 2.

Dust masses and star formation in bright IRAS galaxies

We address the problem of modeling the far-infrared (FIR) spectrum and deriving the star-formation rate (SFR) and the dust mass of spiral galaxies. We use the realistic physical model of Popescu et al. ([CITE]) to describe the overall ultra-violet (UV), optical and FIR spectral energy distribution (SED) of a spiral galaxy. The model takes into account the 3-dimensional old and young stellar distributions in the bulge and the disk of a galaxy, together with the dust geometry. The geometrical characteristics of the galaxy and the intrinsic optical and near-infrared spectra are determined by the galaxy's observed K-band photometry. The UV part of the spectrum is assumed to be proportional to the SFR through the use of population synthesis models. By solving the radiative transfer equation, we are able to determine the absorbed energy, the dust temperature and the resulting FIR spectrum. The model has only three free parameters: SFR, dust mass, and the fraction of the UV radiation which is absorbed locally by dense dust in the HII regions. Using this model, we are able to fit well the FIR spectra of 62 bright IRAS galaxies from the “SCUBA Local Universe Galaxy Survey" of Dunne et al. ([CITE]). As a result, we are able to determine, among others, their SFR and dust mass. We find that, on average, the SFR (in absolute units), the star-formation efficiency, the SFR surface density and the ratio of FIR luminosity over the total intrinsic luminosity, are larger than the respective values of typical spiral galaxies of the same morphological type. We also find that the mean gas-to-dust mass ratio is close to the Galactic value, while the average central face-on optical depth of these galaxies in the V band is 2.3. Finally, we find a strong correlation between SFR or dust mass and observed FIR quantities like total FIR luminosity or FIR luminosity at 100 and 850 . These correlations yield well-defined relations, which can be used to determine a spiral galaxy's SFR and dust-mass content from FIR observations.

The distribution of atomic gas and dust in nearby galaxies - II. Further matched-resolution Very Large Array H i and SCUBA 850-μm images

We present Very Large Array (VLA) C-array 21-cm H i images of galaxies from the SCUBA Local Universe Galaxy Survey which have been observed at 850 μm with the James Clerk Maxwell Telescope. Matched-resolution (∼ 25 arcsec) H i images of 17 galaxies are presented and compared with 850-μm images. H i or 850-μm images of an additional six galaxies which were detected at only one wavelength are presented. Additionally, lower resolution H i observations of nine galaxies are presented. The observations of these galaxies, along with results previously presented, do not show any obvious trends in the H i/dust or H2/dust mass ratios with morphological type.

CO Molecular Gas in Infrared‐luminous Galaxies

We present the first statistical survey of the properties of the 12CO(1-0) and 12CO(3-2) line emission from the nuclei of a nearly complete subsample of 60 infrared (IR) luminous galaxies selected from SCUBA Local Universe Galaxy Survey (SLUGS). This subsample is flux limited at S60 ?m ? 5.24 Jy with far-IR (FIR) luminosities mostly at LFIR > 1010 L?. We compare the emission line strengths of 12CO(1-0) and (3-2) transitions at a common resolution of ~15''. The measured 12CO(3-2) to (1-0) line intensity ratios r31 vary from 0.22 to 1.72, with a mean value of 0.66 for the sources observed, indicating a large spread of the degree of excitation of CO in the sample. These CO data, together with a wide range of data at different wavelengths obtained from the literature, allow us to study the relationship between the CO excitation conditions and the physical properties of gas/dust and star formation in the central regions of galaxies. Our analysis shows that there is a nonlinear relation between CO and FIR luminosities, such that their ratio LCO/LFIR decreases linearly with increasing LFIR. This behavior was found to be consistent with the Schmidt law relating star formation rate to molecular gas content, with an index N = 1.4 ? 0.3. We also find a possible dependence of the degree of CO gas excitation on the efficiency of star-forming activity. Using the large velocity gradient (LVG) approximation to model the observed data, we investigate the CO-to-H2 conversion factor X for the SLUGS sample. The results show that the mean value of X for the SLUGS sample is lower by a factor of 10 compared to the conventional value derived for the Galaxy, if we assume the abundance of CO relative to H2, ZCO = 10-4. For a subset of 12 galaxies with H I maps, we derive a mean total face-on surface density of H2 + H of about 42 M? pc-2 within about 2 kpc of the nucleus. This value is intermediate between that in galaxies like our own and those with strong star formation.

Erratum: “CO Molecular Gas in Infrared Luminous Galaxies” (ApJ, 588, 771 [2003

We present the first statistical survey of the properties of the 12CO(1-0) and 12CO(3-2) line emission from the nuclei of a nearly complete subsample of 60 infrared (IR) luminous galaxies selected from SCUBA Local Universe Galaxy Survey (SLUGS). This subsample is flux limited at S60 mincron > 5.24 Jy with far-IR (FIR) luminosities mostly at LFIR > 10^10 LSUN. We compare the emission line strengths of 12CO(1-0) and (3-2) transitions at a common resolution of 15". The measured 12CO(3-2) to (1-0) line intensity ratios r31 vary from 0.22 to 1.72 with a mean value of 0.66 for the sources observed, indicating a large spread of the degree of excitation of CO in the sample. These CO data, together with a wide range of data at different wavelengths obtained from the literature, allow us to study the relationship between the CO excitation conditions and the physical properties of gas/dust and star formation in the central regions of galaxies. Our analysis shows that there is a non-linear relation between CO and FIR luminosities, such that their ratio LCO/LFIR decreases linearly with increasing LFIR. This behavior was found to be consistent with the Schmidt Law relating star formation rate to molecular gas content, with an index N = 1.4 +- 0.3. We also find a possible dependence of the degree of CO gas excitation on the efficiency of star forming activity. Using the large velocity gradient (LVG) approximation to model the observed data, we investigate the CO-to-H2 conversion factor X for the SLUGS sample. The results show that the mean value of X for the SLUGS sample is lower by a factor of 10 compared to the conventional value derived for the Galaxy, if we assume the abundance of CO relative to H2, ZCO = 10^-4.

Constraining the radio-submillimetre redshift indicator using data from the SCUBA Local Universe Galaxy Survey

Many of the faint submm sources uncovered by deep SCUBA surveys still remain unidentified at most, if not all, other wavelengths. The most successful method so far of obtaining accurate positions and hence allowing more secure identifications has been to use centimetre-wavelength imaging with the VLA. This has led to a tempting approach for obtaining redshift estimates for this population by Carilli & Yun, which relies on the tight FIR-radio relationship and takes advantage of the steep spectral slope in the submm. In this paper we use the submm data from the SCUBA Local Universe Galaxy Survey (SLUGS) to estimate the usefulness of, and the uncertainties in, the radio-submm redshift estimator. If the submm-radio spectral index were correlated with either dust temperature or 850-μm luminosity, this method could produce biased redshift estimates for 850-μm selected galaxies. We find, however, that within SLUGS, these correlations are not significant. The ratio of 850-μm to 1.4-GHz flux is found to decrease with increasing radio and FIR luminosity, and we propose that this is due to a component of the dust not associated with recent star formation, but which is instead heated to 15–20 K by the general interstellar radiation field.

The SCUBA Local Universe Galaxy Survey - II. 450- m data: evidence for cold dust in bright IRAS galaxies

This is the second in a series of papers presenting results from the SCUBA Local Universe Galaxy Survey. In our first paper we provided 850-μm flux densities for 104 galaxies selected from the IRAS Bright Galaxy Sample and we found that the 60-, 100-μm (IRAS) and 850-μm (SCUBA) fluxes could be adequately fitted by emission from dust at a single temperature. In this paper we present 450-μm data for the galaxies. With the new data, the spectral energy distributions of the galaxies can no longer be fitted with an isothermal dust model – two temperature components are now required. Using our 450-μm data and fluxes from the literature, we find that the 450/850-μm flux ratio for the galaxies is remarkably constant, and this holds from objects in which the star formation rate is similar to our own Galaxy, to ultraluminous infrared galaxies (ULIRGs) such as Arp 220. The only possible explanation for this is if the dust emissivity index for all of the galaxies is ∼2 and the cold dust component has a similar temperature in all galaxies . The 60-μm luminosities of the galaxies were found to depend on both the dust mass and the relative amount of energy in the warm component, with a tendency for the temperature effects to dominate at the highest L60. The dust masses estimated using the new temperatures are higher by a factor of ∼2 than those determined previously using a single temperature. This brings the gas-to-dust ratios of the IRAS galaxies into agreement with those of the Milky Way and other spiral galaxies which have been intensively studied in the submm.

The SCUBA Local Universe Galaxy Survey -- I. First measurements of the submillimetre luminosity and dust mass functions

This is the first of a series of papers presenting results from the SCUBA Local Universe Galaxy Survey (SLUGS), the first statistical survey of the submillimetre properties of the local Universe. As the initial part of this survey, we have used the SCUBA camera on the James Clerk Maxwell Telescope to observe 104 galaxies from the IRAS Bright Galaxy Sample. We present here the 850-μm flux measurements. The 60-, 100-, and 850-μm flux densities are well fitted by single-temperature dust spectral energy distributions, with the sample mean and standard deviation for the best-fitting temperature being Td=35.6±4.9 K and for the dust emissivity index β=1.3±0.2. The dust temperature was found to correlate with 60-μm luminosity. The low value of β may simply mean that these galaxies contain a significant amount of dust that is colder than these temperatures. We have estimated dust masses from the 850-μm fluxes and from the fitted temperature, although if a colder component at around 20 K is present (assuming a β of 2), then the estimated dust masses are a factor of 1.5–3 too low. We have made the first direct measurements of the submillimetre luminosity function (LF) and of the dust mass function. Unlike the IRAS 60-μm LF, these are well fitted by Schechter functions. The slope of the 850-μm LF at low luminosities is steeper than −2, implying that the LF must flatten at luminosities lower than we probe here. We show that extrapolating the 60-μm LF to 850 μm using a single temperature and β does not reproduce the measured submillimetre LF. A population of ‘cold’ galaxies (Td<25 K) emitting strongly at submillimetre wavelengths would have been excluded from the 60-μm-selected sample. If such galaxies do exist, then this estimate of the 850-μm flux is biased (it is underestimated). Whether such a population does exist is unknown at present. We correlate many of the global galaxy properties with the FIR/submillimetre properties. We find that there is a tendency for less luminous galaxies to contain hotter dust and to have a greater star formation efficiency (cf. Young). The average gas-to-dust ratio for the sample is 581±43 (using both the atomic and molecular hydrogen), which is significantly higher than the Galactic value of 160. We believe that this discrepancy is probably due to a ‘cold dust’ component at Td≤20 K in our galaxies. There is a surprisingly tight correlation between dust mass and the mass of molecular hydrogen, estimated from CO measurements, with an intrinsic scatter of ≃50 per cent.