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The sample is selected purely according to stellar mass, and therefore provides an unbiased view of molecular gas in these systems.

In this paper, we present CO luminosities and molecular hydrogen masses for the first galaxies. Kram overall CO detection rate is 54 per cent, but our survey also uncovers the existence of sharp thresholds in galaxy structural parameters such as stellar mass surface density and concentration index, below which all galaxies have a measurable cold gas component but above which the detection rate of the CO line drops suddenly.

The mean molecular gas fraction of the CO detections is 0. The average molecular-to-atomic hydrogen ratio in present-day galaxies 35509 0. Intriguingly, atomic gas strongly dominates in the minority of galaxies with significant cold gas that lie above these thresholds. This suggests that some re-accretion of gas may still be possible following the quenching event.

Perhaps the most fascinating aspect of nearby galaxies is the intricately interwoven system of correlations between their global properties. Galaxy scaling relations also provide the route to understanding the internal physics of igam, as well as their formation and evolutionary histories.

Both relations provide important constraints on how these systems have assembled. However, a few well-established scaling laws exist describing how the cold gas is correlated with the other global physical properties of galaxies. The only well-studied relation is the Schmidt—Kennicutt star formation law Kennicuttrelating the formation rate of new stars and the surface density of cold gas in discs. The reason why so few scaling laws involving cold gas and global galaxy properties, such as masses, sizes and bulge-to-disc ratios, exist in the literature, is the difficulty in acquiring suitable data.

There are four general requirements on the data if the derived scaling laws are to be reliable: As we will describe, existing data sets do not, in general, meet all of these conditions. Line emission from the CO molecule was first detected in the central parts and discs of nearby galaxies 35 years ago Rickard et al.

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Because of a strong correlation between infrared luminosity and CO luminosity e. These pioneering studies constrained molecular gas properties in nearby galaxies as a function of morphology e.

Highlights from these studies include the observations that molecular gas distributions decline monotonically with galaxy-centric radius unlike the atomic gas distributions, that IR-luminous galaxies are also CO-bright, with molecular gas concentrated within the inner kpc of these mostly interacting systems, and that the total gas mass fraction as well as the molecular-to-atomic ratio are functions of Hubble type. Nevertheless, most of the samples did not meet all of the criteria listed above that would allow for accurate scaling laws to be derived; some samples were biased towards a particular galaxy type e.

Recently, much effort has been put into obtaining homogeneous and relatively deep high spatial resolution molecular gas maps covering the optical discs of nearby galaxies Regan et al. These samples are excellent for studying star formation laws within galaxies e.

With reliable measurements of molecular gas for a large, irsm sample of galaxies, it is possible not only to quantify scaling relations, but also to construct an accurate molecular gas mass function. We can also investigate the molecular gas properties of galaxy samples for which dedicated surveys do exist, but where the number of objects studied has been very small, for example early-type galaxies e.

A large unbiased sample of galaxies which can serve as a reference for such particular objects would also be very valuable. With its new, large-bandwidth receivers, the IRAM m telescope is the instrument of choice to conduct a new large 3590 gas survey, allowing the community to move from dedicated studies of particular types of galaxies, to larger systematic efforts.

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COLD GASS will provide a definitive, unbiased census of the partition of condensed baryons in the local Universe into stars, atomic and molecular gas in galaxies covering over 2 orders of magnitude in luminosity. In Sections 2—4we uram an overview of the survey and of the sample selection, and describe the CO measurements and ancillary data sets.

In Sections 5 and 6 we present the first COLD GASS scaling relations, correlating molecular gas masses with global galaxy parameters including stellar mass and jram gas mass. The conditions listed in Section 1which are required to obtain reliable scaling laws, are routinely met by optically selected samples of galaxies at a low redshift.

At radio wavelengths, a series of large blind H i surveys have become possible thanks to a number of new multifeed arrays. Although Lram measurements are accurate, homogeneous and unbiased, the survey is shallow, with the lram that it does not probe a large dynamic range in H i -to-stellar mass ratio for all but the very nearest galaxies. For example, in the redshift range 0. GASS is a large programme currently under way at the Arecibo m telescope, and is producing some of the first unbiased atomic gas scaling relations in the nearby Universe Catinella et al.

Details of the GASS survey design, target selection and observing procedures are given in Catinella et al. In short, the galaxies idam as a part of GASS are selected at random out of a larger parent 35509 of galaxies that meet the following criteria. No iraj selection criteria on colour, morphology or spectral properties for example were applied. This sample therefore provides us with iramm complete picture of how the cold atomic gas relates to other properties such as stellar mass, luminosity, 359 surface mass density or colour.

GASS also aims at studying the galaxies that are transitioning between iraam blue, star-forming state and a red passive state and vice versa.

These are identified as outliers from the mean scaling relations.

The ultimate goal is to understand the physical processes that affect the gas content of these galaxies e. We will then be able to quantify the link between atomic gas, molecular gas and stars in these systems. Galaxies in our redshift range 0. For the remaining objects, riam recover the total flux by adding a single offset pointing see Section 4.

Upon completion of the survey, the sample size of at least galaxies will be large enough to determine accurately a set of scaling laws involving three parameters and iarm measure the scatter around these relations. As seen in Fig. The uniform mass distribution also has the effect of flattening the colour distribution and reducing the number of galaxies with low stellar mass surface densities. The process includes registering the images and smoothing them to a common point spread function PSF.

The SDSS r -band images are convolved to the resolution of the UV imaging before SE xtractor is used to calculate magnitudes in consistent apertures, therefore ensuring that measurements in different bands represent similar physical regions of the galaxies.

The stellar mass assigned to a galaxy is then the mean of this distribution, while the measurement error is estimated from its width. Table 1 is published in its entirety in the electronic version of the journal see Supporting Information. A portion is shown here as an example of its format and contents.

Details of the H i observations are described in Catinella et al. The survey builds upon existing H i data bases: H i data for about 20 per cent of the GASS sample the most gas-rich objectscan be found in either of these sources. For 33509 rest of the sample, observations are carried out at the Arecibo Observatory.

Observations are carried out using the L -band Wide receiver and the interim correlator, providing coverage of the full frequency interval of the GASS targets at a velocity resolution of 1. Data reduction includes Hanning smoothing, ira, subtraction, radio frequency interference RFI excision, flux calibration and weighted combination of individual spectra.

Total H i -line 33509, velocity widths and recessional velocities are then measured using linear fitting of the edges of the H i profiles e. Observations are carried out at the IRAM m telescope. With a single tuning of the receiver at a frequency of This single tuning procedure results in enormous time savings of 15 min per source, and in an improved relative calibration accuracy. The second band is tuned to a frequency of The wobbler-switching mode is used for all the observations with a frequency of 1 Hz and a throw of arcsec.

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We also simultaneously record the data with the 4-MHz Filterbank, as a backup. Observations for this first data release were conducted between December and October. Atmospheric conditions varied greatly, with an average of 6 mm of precipitable water vapour PWV.

We also fold into this catalogue 15 galaxies observed in June, as a part of a pilot programme designed to test the feasibility of the survey. Observations were carried out in fixed observing blocks and as poor-weather back-ups for higher-frequency programmes. We observe the bluer galaxies generally CO-luminous under poorer weather conditions. These galaxies require on average an rms sensitivity of 1. When the atmospheric water vapour level is low, we preferentially observe the redder galaxies, which have a very low detection rate Fig.

In order to set firm upper limits for these galaxies, we require low noise values to reach our integration limit of per cent. Under good observing conditions, sensitivity to this minimum gas fraction, or an absolute minimum rms of 1. The results are shown in equally populated bins, each containing 37 galaxies. The grey shaded region shows the overall detection rate of The downward error bars show the effect of excluding tentative detections in each individual bin.

The efficiency of the observations is also maximized by our single tuning approach see Section 4. The data are reduced with the class software. All scans are visually examined, and those with distorted baselines, increased noise due to poor atmospheric conditions, or anomalous features are discarded.

The individual scans irma a single galaxy are baseline-subtracted first-order fit and then combined. If the line is detected, the window is set by hand to match the observed line profile. For the non-detection, an upper limit for the flux of see equation 5 is set. The central velocity and total width of the detected CO lines are then irzm using a custom-made idl interactive script. The peaks of the signal are identified, and a linear fit is applied to each side iarm the profile between the 20 and 80 per cent peak flux level.

The recession velocity is taken as the mid-point of this line. This method is described in Springob et al. However, some of the galaxies have optical diameters in excess of this, and an aperture correction needs to be applied. The large red symbols in panel a indicate the aperture corrections estimated using this method for the 25 COLD GASS galaxies for which we performed offset pointings to date, and the dashed line is the size threshold 40 arcsec for a galaxy to require an offset pointing.

In the right-hand panel of Fig. We adopted the three-quarter beam offset as a compromise between the requirements for independent flux measures and a modest fraction of our total observing time going into off-centre pointings. So far, we have performed offset pointings for 25 galaxies, that met the requirements listed above see Fig. We used equation 3 and Irzm. The blue circle shows the central position of the arcsec beam, and the orange circle the offset position, located three quarters of a beam from the centre for G, G, G, G and G, the offset was taken one full beam away from the centre; see Section 4.

On the spectra, we overplot the optical redshift igam the galaxies red linesand the central position and widths of the line detected in the central pointings blue lines. The shaded region shows the region of the offset spectrum integrated to measure the line flux.

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The optical diameter from SDSS g -band imaging D 25 and the flux ratio between offset and central pointings f off are given in each case. Our choice of is roughly the mean of values estimated in the Milky Way and in nearby galaxies e. It may be that the conversion factor is instead a function of a parameter such as gas surface density or metallicity Tacconi et al.