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Click here to close this overlay, or press the “Escape” key on your keyboard. Its membership of about 7, individuals also includes physicists, mathematicians, geologists, engineers, and others whose research and educational interests lie within the broad spectrum of subjects comprising contemporary astronomy.
The Institute of Physics IOP is a leading scientific society promoting physics and bringing physicists together for the benefit of all. It has a 22276 membership iraam around 50 comprising physicists from all sectors, as well as those with an interest in physics. It works to advance physics research, application and education; and engages with policy makers and the public to develop awareness and understanding of physics. Its publishing company, IOP Publishing, is a world leader in professional scientific communications.
The American Astronomical Society. You need an eReader or compatible software to experience the benefits of the ePub3 file format. What is article data? Cited by 17 articles. Get permission to re-use this article. Select your desired journals and corridors below. You will need to select a minimum of one iraj. The sample is mass-selected in the redshift interval from the Sloan Digital Sky Survey SDSS and therefore representative of the local galaxy population with. The CO 1—0 luminosity function is constructed and best fit with a Schechter function with parameters, and.
With the sample now complete down to stellar masses of 10 9we are able to irsm our study of gas scaling relations and confirm that both molecular gas fractions and depletion timescale vary with specific star formation rate or offset from the star formation main sequence much more strongly than they depend on stellar mass. Comparing the xCOLD GASS results with outputs from hydrodynamic and semianalytic models, we highlight the constraining power of cold gas scaling relations on models of galaxy formation.
Much of galaxy evolution is regulated by the availability of gas and the efficiency of the star formation process out of this material. For example, the shape, tightness, and redshift evolution of the main sequence of star-forming galaxies in the star formation rate—stellar mass SFR— plane can be explained by the availability of cold gas through inflows, the efficiency of the star formation process, and the balancing power of feedback e.
The cold atomic and molecular gas in the interstellar medium ISM of galaxies is not only intimately linked to star formation but also an excellent probe of the larger environment and of evolutionary mechanisms.
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While iraj mostly restricted to particularly luminous or nearby galaxies e. Improvements in instrument sensitivities and bandwidth have also made possible the investigation of the molecular gas contents of galaxies far beyond the local universe Daddi et al.
Unlike all irxm studies mentioned above, the sample was selected purely by redshift and stellar mass rather than targeting specific classes of galaxies and, with galaxies observed as part of a cohesive observing campaign, is homogeneous and large enough to statistically characterize scaling relations and their scatter. Recognizing the need to understand the link between gas, star formation, and global galaxy properties in lower-mass galaxies, we launched a second IRAM iraj m large program to extend the sample down to stellar masses of 10 9.
The sample was selected randomly out of the complete parent sample of SDSS galaxies within the ALFALFA footprint matching these criteria, making it unbiased and representative of the local galaxy population. A thorough description of the sample selection, survey strategy, and scientific motivation is given in Saintonge et al. Since the predicted CO luminosities of these galaxies are lower, the redshift range was lowered to for ease of detection.
In this redshift range, the SDSS spectroscopic sample is complete for galaxies withand the angular lram of these lower-mass galaxies are small enough that most of the CO flux can be recovered with a single pointing of the IRAM 30 m telescope and a small aperture correction.
The distributions of these objects on the sky and in redshift space are shown in Ifam 1 and 2. The redshift range of each survey is such that all sources could be observed with a single frequency tuning. Distribution of the “Spring” component of the sample in the redshift—right ascension plane top and as projected on the sky bottom.
Distribution of the “Fall” component of the sample in the redshift—right ascension plane top and as projected on the sky bottom. Irxm the sample selection and the observing strategy make xCOLD GASS the ideal sample to build scaling relations and serve as the benchmark for galaxy evolution studies.
Such key features are 1 the representative nature of the sample, being purely mass selected with no additional cuts 22776 quantities such as SFR, morphology, or infrared luminosity; 2 the size of the sample, which allows us to define both mean scaling relations and any scatter or third-parameter dependencies; 3 itam homogeneity of the CO measurements and the strict upper limits set in the case of nondetections; and 4 the large dynamic range of the various physical properties under consideration e.
For each survey, the sample was 2276 selected out of the SDSS parent sample to have a flat distribution to ensure an even sampling of the stellar mass parameter space. However, since the underlying stellar mass distribution of the full sample from SDSS is very well characterized, we can easily correct for this “mass bias” Catinella et al. As a starting point, we construct the expected mass distribution of 2726 purely volume-limited sample of galaxies based on the Baldry et al.
We assign as a statistical weight to each galaxy within bins of 0. To illustrate the impact of this weighting on other key parameters, Figure 3 shows the difference between the observed irak of stellar mass surface density, near-UV NUV color, and metallicity for the xCOLD GASS sample ira and after jram weights are taken into consideration filled gray and black solid line histograms, respectively.
Galaxies with CO 1—0 ieam and nondetection are shown separately as the blue and red histograms, respectively. The solid black line shows the distribution of the xCOLD GASS sample after weights are applied to correct for the flat distribution in the observed samples; this matches the stellar mass function as shown by the orange dashed line in panel a. We also extract information from the SDSS DR7 database to provide us with information about the structural properties and chemical composition of the galaxies.
A detailed description of this table’s contents lram given in Section 2. In order to increase survey efficiency, galaxies with good detections from the ALFALFA survey were not reobserved; hence, the observed sample lacked H i -rich objects. In the 3 mm band Ewe can make use of two sidebands each with a bandwidth of 8 GHz per linear polarization.
For each survey, the E band was tuned to a specific frequency that allowed us to detect the redshifted CO 1—0 line for all itam within the available bandwidth. All details regarding the setup of the instruments during both surveys are presented in Table 2. All observations were done in wobbler-switching mode. This allowed for simultaneous coverage of 4 GHz of bandwidth in each linear irma and for each band. The spectral resolution of the FTS is also a factor of 10 higher.
The strategy was to observe the galaxies predicted to be the most CO luminous under poorer weather conditions, as these require a typical rms sensitivity of 1.
When the precipitable water level was particularly low, we favored the redder galaxies predicted to be CO faint in order to achieve rms sensitivities of 0. A spectral window is kram for each 276 line to match the observed line width. The integrated line flux,is measured by adding the signal within this spectral window, and the standard deviation of the noise per 20 channel,is measured outside of it.
Properties of the spectral lines, such as central velocity and width, are measured using a custom-made IDL interactive program following the technique described in, e. The solid red line is the expected center of the line based 22776 the SDSS spectroscopic redshift.
When the CO line is detected, the dashed blue line indicates the central velocity of the line and the dotted lines represent the FWHM of the line based on the fitting technique described in Section 2.
The gray shaded area represents the region of the spectrum over which we integrated to calculate the total line flux.
The complete figure set 76 itam is available in the online journal. The complete figure set 76 images is available. Given the angular size of the galaxies, most of their flux can be recovered by irak single pointing of the IRAM 30 m telescope. However, to account for the larger angular size of some of the galaxies, we apply an aperture correction to all of the measured CO 1—0 line fluxes. The method presented in Saintonge et al. The model is given the inclination of the real galaxy and then convolved with a Gaussian matching the properties of the IRAM beam.
The aperture correction is the ratio between the flux of the model before and after this convolution. We performed tests to ensure that the scaling relations presented later in this paper are not caused by inadequate aperture corrections by confirming that key quantities that should not depend on distance within our sample such as molecular gas fraction and depletion timescale are indeed uncorrelated.
As presented in Table 3the catalog includes the following quantities:. The CO 1—0 emission line is only one of the many tracers available to measure the mass of molecular gas in galaxies. At higher redshifts, it is more common to observe higher- J transitions of this same molecule, as, for example, lines such as CO 2—1 and CO fall into the 3 mm atmospheric window at 2267respectively.
This is important, as a value of r 21 has to be assumed to convert an observed CO 2—1 flux into. With a beam size of 27” at this frequency, these observations are a good match for the CO 1—0 fluxes measured with the IRAM 22” beam.
Observations were performed through the allocation of a total of 78 hr via both the ESO proposal B and Max-Planck channels.
Integration times were set based on average observing conditions, rms noise requirements predicted from the measured IRAM CO 1—0a standard value ofand a Galactic conversion factor.
Shown are the 28 galaxies from Table 4 with detections of the CO 2—1 line. In each panel, the red solid line is the igam line center based on the SDSS optical redshift, while the dashed and dotted blue vertical lines are the measured CO line center and width, respectively. The gray shaded areas represent the region of the spectra over which we integrated to calculate the total line flux. A detailed description of this table’s ifam, as well as the additional columns available online, is given in Section 2.
Commonly used calibrations of the CO LF were derived either using samples biased toward more extreme starburst galaxies Keres et al. While the sample was selected to have a flat stellar mass distribution see Figure 3because it is extracted from a volume-limited parent sample, we only have to apply the statistical weights described in Section 2. We note that we do not attempt here to correct for the contribution of galaxies withand therefore we produce an LF that accounts for the contributions of galaxies more massive that 10 9.
However, given that there is a positive correlation between andand given that the most CO-bright galaxies with haveit is highly unlikely that oram galaxies contribute uram if at all to the CO LF at. Because the sample is gas fraction limited i. At the highest-mass end of the sample, where the observations are shallowest in terms of CO luminosity, the gas jram limit of the observations corresponds to.
The CO luminosity function for the xCOLD GASS sample is presented in Figure 6with the estimated completeness limits due to stellar mass cut of the sample and the gas fraction integration limit shown irram vertical lines at andrespectively. To address the completeness issue caused by itam fixed gas fraction integration limit, we produce two LFs by treating the upper limits differently: As expected, these two LFs are identical above the completeness limit of.
The error on each point in the luminosity function was calculated using a bootstrap method.
xCOLD GASS: The Complete IRAM 30 m Legacy Survey of Molecular Gas for Galaxy Evolution Studies
These are the points and errors shown in Figure 6for both the methods of treating the nondetections. The LF is shown if detections and the upper limits from nondetections are included red triangles and if only detections are used blue hexagons.
The true CO luminosity of many of these undetected galaxies is likely to be much lower than the upper limits used here, and we therefore expect the “true” CO LF to lie somewhere between the functions determined with and without nondetections.
The bootstrap errors on the individual points of the LFs are considered, and the covariance matrix is used to randomly sample the best-fit function. The best-fit Schechter functions to both LFs are shown in 276 6with the range of possible fits within the uncertainties illustrated by the red shaded area for the case where both CO detections and nondetections are considered. The parameters of the best-fit Schechter function for both cases are given in Table 5. When measurements are made within the same aperture, there are very few galaxy-to-galaxy variations in the integrated r 21 ratio.
These corrections, based on the technique described in Section 2. In the top panel, the CO 1—0 luminosities have been corrected by a small aperture correction to account for the beam size difference, and the solid line shows riam best-fitting value for r In the bottom panel, the dotted line is the 1: Galaxies are color-coded by their optical diameter, and the magenta stars represent merging iraam.