Citation for published item:g tinell D f r r nd intongeD em¡ elie nd t nowie kiD teven nd gorteseD vu nd h v¡ eD omeel nd vemoni sD tenn t nd gooperD endrew nd himinovi hD h vid nd rummelsD g meron f nd p elloD ilvi nd qer¡ e D u tink nd uil ornD irgini nd ngD ting @PHIVA 9xqe X tot l old g s s ling rel tions nd mole ul rEtoE tomi g s r tios of g l xies in the lo l niverseF9D wonthly noti es of the oy l estronomi l o ietyFD RUT @IAF ppF VUSEVWSF Further information on publisher's website: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details.

We study the relation between molecular gas and star formation in a volume‐limited sample of 222 galaxies from the COLD GASS survey, with measurements of the CO(1–0) line from the IRAM 30‐m telescope. The galaxies are at redshifts 0.025 < z < 0.05 and have stellar masses in the range 10.0 < log M★/M⊙ < 11.5. The IRAM measurements are complemented by deep Arecibo H i observations and homogeneous Sloan Digital Sky Survey and GALEX photometry. A reference sample that includes both ultraviolet (UV) and far‐infrared data is used to calibrate our estimates of star formation rates from the seven optical/UV bands. The mean molecular gas depletion time‐scale [] for all the galaxies in our sample is 1 Gyr; however,  increases by a factor of 6 from a value of ∼0.5 Gyr for galaxies with stellar masses of ∼1010 M⊙ to ∼3 Gyr for galaxies with masses of a few ×1011 M⊙. In contrast, the atomic gas depletion time‐scale remains constant at a value of around 3 Gyr. This implies that in high‐mass galaxies, molecular and atomic gas depletion time‐scales are comparable, but in low‐mass galaxies, the molecular gas is being consumed much more quickly than the atomic gas. The strongest dependences of  are on the stellar mass of the galaxy [parametrized as ], and on the specific star formation rate (sSFR). A single  versus sSFR relation is able to fit both ‘normal’ star‐forming galaxies in our COLD GASS sample and more extreme starburst galaxies (luminous infrared galaxies and ultraluminous infrared galaxies), which have  yr. Normal galaxies at z = 1–2 are displaced with respect to the local galaxy population in the  versus sSFR plane and have molecular gas depletion times that are a factor of 3–5 times longer at a given value of sSFR due to their significantly larger gas fractions.

We introduce the GALEX Arecibo SDSS Survey (GASS), an on‐going large programme that is gathering high quality H i‐line spectra using the Arecibo radio telescope for an unbiased sample of ∼1000 galaxies with stellar masses greater than 1010 M⊙ and redshifts 0.025 < z < 0.05, selected from the Sloan Digital Sky Survey (SDSS) spectroscopic and Galaxy Evolution Explorer (GALEX) imaging surveys. The galaxies are observed until detected or until a low gas mass fraction limit (1.5–5 per cent) is reached. This paper presents the first Data Release, consisting of ∼20 per cent of the final GASS sample. We use this data set to explore the main scaling relations of the H i gas fraction with galaxy structure and NUV−r colour. A large fraction (∼60 per cent) of the galaxies in our sample are detected in H i. Even at stellar masses above 1011 M⊙, the detected fraction does not fall below ∼40 per cent. We find that the atomic gas fraction MH i/M★ decreases strongly with stellar mass, stellar surface mass density and NUV−r colour, but is only weakly correlated with the galaxy bulge‐to‐disc ratio (as measured by the concentration index of the r‐band light). We also find that the fraction of galaxies with significant (more than a few per cent) H i decreases sharply above a characteristic stellar surface mass density of 108.5 M⊙ kpc−2. The fraction of gas‐rich galaxies decreases much more smoothly with stellar mass. One of the key goals of GASS is to identify and quantify the incidence of galaxies that are transitioning between the blue, star‐forming cloud and the red sequence of passively evolving galaxies. Likely transition candidates can be identified as outliers from the mean scaling relations between MH i/M★ and other galaxy properties. We have fitted a plane to the two‐dimensional relation between the H i mass fraction, stellar surface mass density and NUV−r colour. Interesting outliers from this plane include gas‐rich red sequence galaxies that may be in the process of regrowing their discs, as well as blue, but gas‐poor spirals.

We are conducting COLD GASS, a legacy survey for molecular gas in nearby galaxies. Using the IRAM 30‐m telescope, we measure the CO(1−0) line in a sample of ∼350 nearby ( Mpc), massive galaxies (log(M*/M⊙) > 10.0). The sample is selected purely according to stellar mass, and therefore provides an unbiased view of molecular gas in these systems. By combining the IRAM data with Sloan Digital Sky Survey (SDSS) photometry and spectroscopy, GALEX imaging and high‐quality Arecibo H i data, we investigate the partition of condensed baryons between stars, atomic gas and molecular gas in 0.1–10L* galaxies. In this paper, we present CO luminosities and molecular hydrogen masses for the first 222 galaxies. The 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 0.066 ± 0.039, and this fraction does not depend on stellar mass, but is a strong function of (NUV − r) colour. Through stacking, we set a firm upper limit of  for red galaxies with NUV − r > 5.0. The average molecular‐to‐atomic hydrogen ratio in present‐day galaxies is 0.3, with significant scatter from one galaxy to the next. The existence of strong detection thresholds in both the H i and CO lines suggests that ‘quenching’ processes have occurred in these systems. 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.

Clump clusters and chain galaxies in the Hubble Ultra Deep Field (UDF) are examined for bulges in Near-Infrared Camera Multi-Object Spectrometer images. Approximately 50% of the clump clusters and 30% of the chains have relatively red and massive clumps that could be young bulges. Magnitudes and colors are determined for these bulgelike objects and for the bulges in spiral galaxies, and for all of the prominent star formation clumps in these three galaxy types. The colors are fitted to population evolution models to determine the bulge and clump masses, ages, star formation rate decay times, and extinctions. The results indicate that bulgelike objects in clump clusters and chain galaxies have similar ages and two to five times larger masses compared to the star formation clumps, while the bulges in spirals have roughly six times larger ages and 20 to 30 times larger masses than the clumps. All systems appear to have an underlying red disk population. The masses of starforming clumps are typically in a range from 10 7 to 10 8 M ; their ages have a wide range around ∼ 10 2 Myr. Ages and extinctions both decrease with redshift. Star formation is probably the result of gravitational instabilities in the disk gas, in which case the large clump mass in the UDF is the result of a high gas velocity dispersion, 30 km s −1 or more, combined with a high gas mass column density, ∼ 100 M pc −2 . Because clump clusters and chains dominate disk galaxies beyond z ∼ 1, the observations suggest that these types represent an early phase in the formation of modern spiral galaxies, when the bulge and inner disk formed.