Baryonic processes in galaxy evolution include the infall of gas onto galaxies to form neutral atomic hydrogen, which is then converted to the molecular state (H
), and, finally, the conversion of H
...to stars. Understanding galaxy evolution thus requires an understanding of the evolution of stars and of neutral atomic and molecular hydrogen. For the stars, the cosmic star-formation rate density is known to peak at redshifts from 1 to 3, and to decline by an order of magnitude over approximately the subsequent 10 billion years
; the causes of this decline are not known. For the gas, the weakness of the hyperfine transition of H I at 21-centimetre wavelength-the main tracer of the H I content of galaxies-means that it has not hitherto been possible to measure the atomic gas mass of galaxies at redshifts higher than about 0.4; this is a critical gap in our understanding of galaxy evolution. Here we report a measurement of the average H I mass of star-forming galaxies at a redshift of about one, obtained by stacking
their individual H I 21-centimetre emission signals. We obtain an average H I mass similar to the average stellar mass of the sample. We also estimate the average star-formation rate of the same galaxies from the 1.4-gigahertz radio continuum, and find that the H I mass can fuel the observed star-formation rates for only 1 to 2 billion years in the absence of fresh gas infall. This suggests that gas accretion onto galaxies at redshifts of less than one may have been insufficient to sustain high star-formation rates in star-forming galaxies. This is likely to be the cause of the decline in the cosmic star-formation rate density at redshifts below one.
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FZAB, GEOZS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
ABSTRACT We report a Giant Metrewave Radio Telescope search for H i 21 cm emission from a large sample of massive star-forming galaxies at z 1.18-1.34, lying in sub-fields of the DEEP2 Redshift ...Survey. The search was carried out by co-adding ("stacking") the H i 21 cm emission spectra of 857 galaxies, after shifting each galaxy's H i 21 cm spectrum to its rest frame. We obtain the 3 upper limit SH i < 2.5 Jy on the average H i 21 cm flux density of the 857 galaxies, at a velocity resolution of 315 km s−1. This yields the 3 constraint on the average H i mass of the 857 stacked galaxies, the first direct constraint on the atomic gas mass of galaxies at z > 1. The implied limit on the average atomic gas mass fraction (relative to stars) is MGAS/M* < 0.5, comparable to the cold molecular gas mass fraction in similar star-forming galaxies at these redshifts. We find that the cosmological mass density of neutral atomic gas in massive star-forming galaxies at z 1.3 is GAS < 3.7 × 10−4, significantly lower than GAS estimates in both galaxies in the local universe and damped Ly absorbers at z ≥ 2.2. Massive blue star-forming galaxies thus do not appear to dominate the neutral atomic gas content of the universe at z 1.3.
ABSTRACT
The detection of redshifted hyperfine line of neutral hydrogen (H i) is the most promising probe of the epoch of reionization (EoR). We report an analysis of 55 h of Murchison Widefield ...Array (MWA) phase II drift scan EoR data. The data correspond to a central frequency $\nu _0 = 154.24 \, \rm MHz$ (z ≃ 8.2 for the redshifted H i hyperfine line) and bandwidth $B = 10.24 \, \rm MHz$. As one expects greater system stability in a drift scan, we test the system stability by comparing the extracted power spectra from data with noise simulations and show that the power spectra for the cleanest data behave as thermal noise. We compute the H i power spectrum as a function of time in one and two dimensions. The best upper limit on the 1D power spectrum are: $\Delta ^2(k) \simeq (1000~\rm mK)^2$ at k ≃ 0.2h Mpc−1 and at k ≃ 1h Mpc−1. The cleanest modes, which might be the most suited for obtaining the optimal signal to noise, correspond to k ≳ 1h Mpc−1. We also study the time-dependence of the foreground-dominated modes in a drift scan and compare with the expected behaviour.
Abstract
We report a ≈400 hr Giant Metrewave Radio Telescope (GMRT) search for H
i
21 cm emission from star-forming galaxies at
z
= 1.18–1.39 in seven fields of the DEEP2 Galaxy Survey. Including ...data from an earlier 60 hr GMRT observing run, we co-added the H
i
21 cm emission signals from 2841 blue star-forming galaxies that lie within the FWHM of the GMRT primary beam. This yielded a 5.0
σ
detection of the average H
i
21 cm signal from the 2841 galaxies at an average redshift 〈
z
〉 ≈ 1.3, only the second detection of H
i
21 cm emission at
z
≥ 1. We obtain an average H
i
mass of 〈M
HI
〉 = (3.09 ± 0.61) × 10
10
M
⊙
and an H
i
-to-stellar mass ratio of 2.6 ± 0.5, both significantly higher than values in galaxies with similar stellar masses in the local universe. We also stacked the 1.4 GHz continuum emission of the galaxies to obtain a median star formation rate (SFR) of 14.5 ± 1.1 M
⊙
yr
−1
. This implies an average H
i
depletion timescale of ≈2 Gyr for blue star-forming galaxies at
z
≈ 1.3, a factor of ≈3.5 lower than that of similar local galaxies. Our results suggest that the H
i
content of galaxies toward the end of the epoch of peak cosmic SFR density is insufficient to sustain their high SFR for more than ≈2 Gyr. Insufficient gas accretion to replenish the H
i
could then explain the observed decline in the cosmic SFR density at
z
< 1.
ABSTRACT
Double radio relics in galaxy clusters are rare phenomena that trace shocks in the outskirts of merging galaxy clusters. We have carried out a spectral and polarization study of the ...spectacular double relics in the galaxy cluster A3376 using the Giant Metrewave Radio Telescope at 150 and 325 MHz and the Very Large Array at 1400 MHz. The polarization study at 1400 MHz reveals a high degree of polarization (∼30 per cent) and aligned magnetic field vectors (not corrected for Faraday rotation) in the eastern relic. A highly polarized (>60 per cent) filamentary radio source of size ∼300 kpc near the eastern relic and north of the bent‐jet radio galaxy is detected for the first time. The western relic is less polarized and does not show aligned magnetic field vectors. The distribution of spectral indices between 325 and 1400 MHz over the radio relics shows steepening from the outer to the inner edges of the relics. The spectral indices of the eastern and the western relics imply Mach numbers in the range 2.2–3.3. Remarkable features such as the inward filament extending from the eastern relic, the highly polarized filament, the complex polarization properties of the western relic and the separation of the brightest cluster galaxy from the intracluster medium by a distance >900 kpc are noticed in the cluster. A comparison with simulated cluster mergers is required to understand the complex properties of the double relics in the context of the merger in A3376. An upper limit log(P1.4 GHz W Hz−1) < 23.0 on the strength of a Mpc size radio halo in A3376 is estimated.
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BFBNIB, FZAB, GIS, IJS, IZUM, KILJ, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBMB, UL, UM, UPUK
We present a method to derive the dynamical mass of face-on galaxy disks using their neutral hydrogen (H i) velocity dispersion ( H i). We have applied the method to nearby, gas-rich galaxies that ...have extended H i gas disks and have low inclinations. The galaxy sample includes four large disk galaxies, NGC 628, NGC 6496, NGC 3184, and NGC 4214, and three dwarf galaxies, DDO 46, DDO 63, and DDO 187. We have used archival H i data from The H i Nearby Galaxy Survey (THINGS) and the LITTLE THINGS survey to derive the H i gas distributions and Spitzer mid-infrared images to determine the stellar disk mass distributions. We examine the disk dynamical and baryonic mass ratios in the extreme outer disks where there is H i gas but no visible stellar disk. We find that for the large galaxies, the disk dynamical and H i gas mass surface densities are comparable in the outer disks. But in the smaller dwarf galaxies, for which the total H i gas mass dominates the baryonic mass, i.e., M(H i) ≥ M(stars), the disk dynamical mass is much larger than the baryonic mass. For these galaxies, there must either be a very low-luminosity stellar disk which provides the vertical support for the H i gas disk or there is halo dark matter associated with their disks, which is possible if the halo has an oblate shape so that the inner part of the dark matter halo is concentrated around the disk. Our results are important for explaining the equilibrium of H i disks in the absence of stellar disks and is especially important for gas-rich, dwarf galaxies that appear to have significant dark matter masses associated with their disks.
Using the Murchison Widefield Array (MWA), the low-frequency Square Kilometre Array precursor located in Western Australia, we have completed the GaLactic and Extragalactic All-sky MWA (GLEAM) ...survey, and present the resulting extragalactic catalogue, utilizing the first year of observations. The catalogue covers 24 831 square degrees, over declinations south of +30... and Galactic latitudes outside 10... of the Galactic plane, excluding some areas such as the Magellanic Clouds. It contains 307 455 radio sources with 20 separate flux density measurements across 72-231 MHz, selected from a time- and frequency-integrated image centred at 200 MHz, with a resolution of ...2 arcmin. Over the catalogued region, we estimate that the catalogue is 90 per cent complete at 170 mJy, and 50 per cent complete at 55 mJy, and large areas are complete at even lower flux density levels. Its reliability is 99.97 per cent above the detection threshold of 5..., which itself is typically 50 mJy. These observations constitute the widest fractional bandwidth and largest sky area survey at radio frequencies to date, and calibrate the low-frequency flux density scale of the southern sky to better than 10 per cent. This paper presents details of the flagging, imaging, mosaicking and source extraction/characterization, as well as estimates of the completeness and reliability. All source measurements and images are available online. This is the first in a series of publications describing the GLEAM survey results. (ProQuest: ... denotes formulae/symbols omitted.)
We present a sample of 1483 sources that display spectral peaks between 72 MHz and 1.4 GHz, selected from the GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey. The GLEAM ...survey is the widest fractional bandwidth all-sky survey to date, ideal for identifying peaked-spectrum sources at low radio frequencies. Our peaked-spectrum sources are the low-frequency analogs of gigahertz-peaked spectrum (GPS) and compact-steep spectrum (CSS) sources, which have been hypothesized to be the precursors to massive radio galaxies. Our sample more than doubles the number of known peaked-spectrum candidates, and 95% of our sample have a newly characterized spectral peak. We highlight that some GPS sources peaking above 5 GHz have had multiple epochs of nuclear activity, and we demonstrate the possibility of identifying high-redshift (z > 2) galaxies via steep optically thin spectral indices and low observed peak frequencies. The distribution of the optically thick spectral indices of our sample is consistent with past GPS/CSS samples but with a large dispersion, suggesting that the spectral peak is a product of an inhomogeneous environment that is individualistic. We find no dependence of observed peak frequency with redshift, consistent with the peaked-spectrum sample comprising both local CSS sources and high-redshift GPS sources. The 5 GHz luminosity distribution lacks the brightest GPS and CSS sources of previous samples, implying that a convolution of source evolution and redshift influences the type of peaked-spectrum sources identified below 1 GHz. Finally, we discuss sources with optically thick spectral indices that exceed the synchrotron self-absorption limit.
We present a multi-wavelength analysis of the merging rich cluster of galaxies, Abell 2256 (A2256). We have observed A2256 at 150 MHz using the Giant Metrewave Radio Telescope and successfully ...detected the diffuse radio halo and the relic emission over a {approx}1.2 Mpc{sup 2} extent. Using this 150 MHz image and the images made using archival observations from the Very Large Array (VLA; 1369 MHz) and the Westerbrok Synthesis Radio Telescope (WSRT; 330 MHz), we have produced spectral index images of the diffuse radio emission in A2256. These spectral index images show a distribution of flat spectral index (S {proportional_to} {nu}{sup {alpha}}, {alpha} in the range -0.7 to -0.9) plasma in the region NW of the cluster center. Regions showing steep spectral indices ({alpha} in the range -1.0 to -2.3) are toward the SE of the cluster center. These spectral indices indicate synchrotron lifetimes for the relativistic plasmas in the range 0.08-0.4 Gyr. We interpret this spectral behavior as resulting from a merger event along the direction SE to NW within the last 0.5 Gyr or so. A shock may be responsible for the NW relic in A2256 and the megaparsec scale radio halo toward the SE is likely to be generated by the turbulence injected by mergers. Furthermore, the diffuse radio emission shows spectral steepening toward lower frequencies. This low-frequency spectral steepening is consistent with a combination of spectra from two populations of relativistic electrons created at two epochs (two mergers) within the last {approx}0.5 Gyr. Earlier interpretations of the X-ray and the optical data also suggested that there were two mergers in Abell 2256 in the last 0.5 Gyr, consistent with the current findings. Also highlighted in this study is the futility of correlating the average temperatures of thermal gas and the average spectral indices of diffuse radio emission in the respective clusters.
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