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1.
The unsurpassed sensitivity and resolution of the Square Kilometer Array (SKA) will make it possible for the first time to probe the continuum emission of normal star forming galaxies out to the edges of the universe. This opens the possibility for routinely using the radio continuum emission from galaxies for cosmological research as it offers an independent probe of the evolution of the star formation density in the universe. In addition it offers the possibility to detect the first star forming objects and massive black holes.In deep surveys SKA will be able to detect Hi in emission out to redshifts of z ≈ 2.5 and hence be able to trace the conversion of gas into stars over an era where considerable evolution is taking place. Such surveys will be able to uniquely determine the respective importance of merging and accreting gas flows for galaxy formation over this redshift range (i.e. out to when the universe was only one third its present age). It is obvious that only SKA will able to see literally where and how gas is turned into stars.These and other aspects of SKA imaging of galaxies will be discussed.  相似文献   

2.
The redshift dependence of spectral index in powerful radio galaxies   总被引：1，自引：0，他引：1  
We present and discuss in this paper the rest frame radio spectra (1–25 GHz) of a sample of fourteen radio galaxies atz >2 from the newly defined MRC/1Jy complete sample of 558 radio sources. These galaxies are among the most powerful radio sources known and range in luminosity from 1028-1028·8 watt Hz-1 at 1 GHz. We find that the median rest frame spectral index of this sample of galaxies atz >2 is significantly steeper than that of a matched luminosity sample of 3CRR galaxies which are at a much lower redshift (0.85 <z < 1.7). This indicates that spectral index correlates primarily with redshift, at least in the luminosity range considered here. The difference between the distributions of rest frame spectral curvatures for the two samples does not appear to be statistically significant. We suggest a new explanation for the steeper spectra of radio galaxies at high redshift involving steeper electron energy spectra at injection. Electron energy spectra are expected to steepen in a first-order Fermi acceleration process, at both non-relativistic and relativistic shock fronts, as the upstream fluid velocity decreases. This may well be the case at high redshifts: the hotter and denser circum-galactic medium at high redshifts could result in slower speeds for the hotspot and the jet material behind it. The smaller sizes of radio sources at higher redshifts provide support to this scenario. Since deceased.  相似文献   

3.
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4.
We present RIJHK imaging of seven radio galaxies from the 7C Redshift Survey (7CRS) which lack strong emission lines and we use these data to investigate their spectral energy distributions (SEDs) with models that constrain their redshifts. Six of these seven galaxies have extremely red colours ( R − K >5.5) and we find that almost all of them lie in the redshift range 1< z <2. We also present near-infrared spectroscopy of these galaxies which demonstrate that their SEDs are not dominated by emission lines, although tentative lines, consistent with H α at z =1.45 and z =1.61, are found in two objects. Although the red colours of the 7CRS galaxies can formally be explained by stellar populations that are either very old or young and heavily reddened, independent evidence favours the former hypothesis. At z ∼1.5 at least 1/4 of powerful radio jets are triggered in massive (> L *) galaxies, which formed the bulk of their stars several Gyr earlier, that is at epochs corresponding to redshifts z ≳5. If a similar fraction of all z ∼1.5 radio galaxies are old, then extrapolation of the radio luminosity function shows that, depending on the radio source lifetimes, between 10 and 100 per cent of the near-IR selected extremely red object (ERO) population undergo a radio outburst at epochs corresponding to 1< z <2. An ERO found serendipitously in the field of one of the 7CRS radio sources appears to be a radio-quiet analogue of the 7CRS EROs with an emission line likely to be [O  ii ] at z =1.20. The implication is that some of the most massive elliptical galaxies formed the bulk of their stars at z ≳5 and these objects probably undergo at least two periods of active galactic nucleus activity: one at high redshift during which the black hole forms and another one at an epoch corresponding to z ∼1.5.  相似文献   

5.
The redshifted spectral line radiation emitted from both atomic fine-structure and molecular rotational transitions in the interstellar medium (ISM) of high-redshift galaxies can be detected in the centimetre, millimetre and submillimetre wavebands. Here we predict the counts of galaxies detectable in an array of molecular and atomic lines. This calculation requires a reasonable knowledge of both the surface density of these galaxies on the sky, and the physical conditions in their ISM. The surface density is constrained using the results of submillimetre-wave continuum surveys. Follow-up OVRO Millimeter Array observations of two of the galaxies detected in the dust continuum have provided direct measurements of CO rotational line emission at redshifts of 2.56 and 2.81. Based on these direct high-redshift observations and on models of the ISM that are constrained by observations of low-redshift ultraluminous infrared galaxies, we predict the surface density of line-emitting galaxies as a function of line flux density and observing frequency. We incorporate the sensitivities and mapping speeds of existing and future millimetre/submillimetre-wave telescopes and spectrographs, and so assess the prospects for blank-field surveys to detect this line emission from gas-rich high-redshift galaxies.  相似文献   

6.
We analyze the properties of a sample of optically polarized radio galaxies with a wide range of redshifts. The galaxies were selected both from our survey and from the literature. The aim of this work is checking whether high linear polarization is a general property of high redshift radio galaxies and how it depends on redshift. This provides a critical test on the suggestion that a considerable fraction of rest frame UV light in high redshift radio galaxies is scattered nuclear radiation. Our results show that radio galaxies withz > 0.7 are strongly polarized and that there is a strong dependence of the degree of linear polarization on the redshift. We discuss the possible origin of this correlation. Our results provide support to the validity of the Radio Quasar and Radio Galaxies unifying schemes.  相似文献   

7.
We present detailed science cases that a large fraction of the Indian AGN community is interested in pursuing with the upcoming Square Kilometre Array (SKA). These interests range from understanding low luminosity active galactic nuclei in the nearby Universe to powerful radio galaxies at high redshifts. Important unresolved science questions in AGN physics are discussed. Ongoing low-frequency surveys with the SKA pathfinder telescope GMRT, are highlighted.  相似文献   

8.
In the standard galaxy formation scenario plasma clouds with a high thermal energy content must exist at high redshifts since the protogalactic gas is shock heated to the virial temperature, and extensive cooling, leading to efficient star formation, must await the collapse of massive haloes (as indicated by the massive body of evidence, referred to as downsizing ). Massive plasma clouds are potentially observable through the thermal and kinetic Sunyaev–Zel'dovich effects and their free–free emission. We find that the detection of substantial numbers of galaxy-scale thermal Sunyaev–Zel'dovich signals is achievable by blind surveys with next generation radio telescope arrays such as EVLA, ALMA and SKA. This population is even detectable with the 10 per cent SKA, and wide field of view options at high frequency on any of these arrays would greatly increase survey speed. An analysis of confusion effects and of the contamination by radio and dust emissions shows that the optimal frequencies are those in the range 10–35 GHz. Predictions for the redshift distributions of detected sources are also worked out.  相似文献   

9.
K.E. Johnson   《New Astronomy Reviews》2004,48(11-12):1337
The Square Kilometer Array (SKA) will enable studies of star formation in nearby galaxies with a level of detail never before possible outside of the Milky Way. Because the earliest stages of stellar evolution are often inaccessible at optical and near-infrared wavelengths, high spatial resolution radio observations are necessary to explore extragalactic star formation. The SKA will have the sensitivity to detect individual ultracompact HII regions out to the distance of nearly 50 Mpc, allowing us to study their spatial distributions, morphologies, and populations statistics in a wide range of environments. Radio observations of Wolf-Rayet stars outside of the Milky Way will also be possible for the first time, greatly expanding the range of conditions in which their mass loss rates can be determined from free-free emission. On a vastly larger scale, natal of super star clusters will be accessible to the SKA out to redshifts of nearly z 0.1. The unprecedented sensitivity of radio observations with the SKA will also place tight constraints on the star formation rates as low as 1M yr−1 in galaxies out to a redshift of z 1 by directly measuring the thermal radio flux density without assumptions about a galaxy’s magnetic field strength, cosmic ray production rate, or extinction.  相似文献   

10.
Maser action in the interstellar medium produces the brightest and most spectacular molecular lines that radio astronomers can study. Strong maser action was first detected in OH (at 1.6 GHz) and water (at 22 GHz) in star-forming regions, but with improvements in mm and submm-wave technology, and improved laboratory data, many new maser transitions are being identified. For methanol alone over 20 maser transitions have been identified in star-forming regions. This review summarizes recent observational developments. Masers provide the most readily detectable indicators of the formation of massive young stars, and offer our best prospect for making a complete census of starforming regions in the Galaxy. Using radio interferometers the structure of the regions can be probed on angular scales of 1 milliarcsecond. The use of masers as probes of the physical conditions in these regions is discussed.  相似文献   

11.
The results of a comprehensive analysis of continuous radio spectra of a sample of Gigahertz-Peaked Spectrum (GPS) sources are reported. The sources are selected from a flux-density-complete sample (S ν ≥ 200 mJy at 4.8 or 5 GHz) using multifrequency measurements of the RATAN-600 radio telescope and data from the CATS astrophysical catalogs support system. The analysis revealed a very small number (1–2%) of “classical” GPS objects, which is significantly less than the expected fraction of 10%. GPS galaxies are found to have narrower and steeper radio spectra than quasars. The low-frequency part of the spectrum is seen to become steeper with increasing redshift. Galaxies and quasars at the same z have comparable angular sizes, whereas their luminosities may differ by one order of magnitude. At large redshifts there is a deficit of objects with low (several GHZ) peak frequencies. The number of GPS galaxies decreases sharply with redshift, and most of them are found at z between 0.01 and 1.81. GPS quasars are found at large redshifts, from 0.11 to 3.99. A quarter of the sample consists of blazars whose spectra may temporarily have a convex shape when the object is in active state.  相似文献   

12.
The submillimeter (submm) extragalactic background light (EBL) traces the integrated star formation history throughout the cosmic time. Deep blank-field 850 μm and 1.4 GHz surveys and optical follow-up have been only able to determine the redshift of ∼20% of the submm EBL. The majority (80%) of the submm EBL is still below the confusion and sensitivity limits of current submm and radio instruments. We break through these limits with stacking analyses on our deep 850 μm image in the GOODS-N and find that the submm EBL mostly comes from galaxies at redshifts around 1.0. This redshift is much lower than the redshift of z=2–3 previously implied from radio identified submm sources. This result significantly decreases the number of high redshift galaxies that may be seen by ALMA.  相似文献   

13.
Radio galaxies are uniquely useful as probes of large-scale structure since their uniform identification with giant elliptical galaxies out to high redshift means that the evolution of their bias factor can be predicted. As the initial stage in a project to study large-scale structure with radio galaxies, we have performed a small redshift survey, selecting 29 radio galaxies in the range of 0.19相似文献   

14.
An analysis of the environments around a sample of 28 3CR radio galaxies with redshifts 0.6< z <1.8 is presented, based primarily upon K -band images down to K ∼20 taken using the UK Infrared Telescope (UKIRT). A net overdensity of K -band galaxies is found in the fields of the radio galaxies, with the mean excess counts being comparable to that expected for clusters of Abell Class 0 richness. A sharp peak is found in the angular cross-correlation amplitude centred on the radio galaxies that, for reasonable assumptions about the luminosity function of the galaxies, corresponds to a spatial cross-correlation amplitude between those determined for low-redshift Abell Class 0 and 1 clusters.
These data are complemented by J -band images also from UKIRT, and by optical images from the Hubble Space Telescope . The fields of the lower redshift ( z ≲0.9) radio galaxies in the sample generally show well-defined near-infrared colour–magnitude relations with little scatter, indicating a significant number of galaxies at the redshift of the radio galaxy; the relations involving colours at shorter wavelengths than the 4000 Å break show considerably greater scatter, suggesting that many of the cluster galaxies have low levels of recent or on-going star formation. At higher redshifts the colour–magnitude sequences are less prominent owing to the increased field galaxy contribution at faint magnitudes, but there is a statistical excess of galaxies with the very red infrared colours ( J − K ≳1.75) expected of old cluster galaxies at these redshifts.
Although these results are appropriate for the mean of all of the radio galaxy fields, there exist large field-to-field variations in the richness of the environments. Many, but certainly not all, powerful z ∼1 radio galaxies lie in (proto)cluster environments.  相似文献   

15.
We investigate the potential of the Square Kilometer Array Telescope (SKA) to constrain the sound speed of dark energy. The Integrated Sachs Wolfe (ISW) effect results in a significant power spectrum signal when Cosmic Microwave Background (CMB) temperature anisotropies are cross-correlated with galaxies detectable with the SKA in H  i . We consider using this measurement, the autocorrelation of H  i galaxies and the CMB temperature power spectrum to derive constraints on the sound speed. We study the contributions to the cross-correlation signal made by galaxies at different redshifts and use redshift tomography to improve the signal-to-noise. We use a  χ2  analysis to estimate the significance of detecting a sound speed different from that expected in quintessence models, finding that there is potential to distinguish very low sound speeds from the quintessence value.  相似文献   

16.
We investigate the dependence of the Fanaroff–Riley (FR) 1/2 dichotomy of radio galaxies on their luminosities and redshifts. Because of a very strong redshift-luminosity correlation (Malmquist bias) in a flux-limited sample, any redshift-dependent effect could appear as a luminosity-related effect and vice versa. A question could then arise—do all the morphological differences seen in the two classes (FR 1 and 2 types) of sources, usually attributed to the differences in their luminosities, could as well be primarily a redshift-dependent effect? A sharp break in luminosity, seen among the two classes, could after all reflect a sharp redshift dependence due to a rather critical ambient density value at some cosmic epoch. A doubt on these lines does not seem to have been raised in past and things have never been examined with this particular aspect in mind. We want to ascertain the customary prevalent view in the literature that the systematic differences in the two broad morphology types of FR 1 and 2 radio galaxies are indeed due to the differences in their luminosities, and not due to a change in redshift. Here we investigate the dependence of FR 1/2 dichotomy of radio galaxies on luminosity and redshift by using the 3CR sample, where the FR 1/2 dichotomy was first seen, supplemented by data from an additional sample (MRC), that goes about a factor of 5 or more deeper in flux-density than the original 3CR sample. This lets us compare sources with similar luminosities but at different redshifts as well as examine sources at similar redshifts but with different luminosities, thereby allowing us a successful separation of the otherwise two intricately entangled effects. We find that the morphology type is not directly related to redshift and the break between the two types of morphologies seems to depend only upon the radio luminosity.  相似文献   

17.
We present an analysis of the optical spectra of a volume-limited sample of 375 radio galaxies at redshift  0.4 < z < 0.7  from the 2dF-SDSS (Sloan Digital Sky Survey) Luminous Red Galaxy (LRG) and QSO (quasi-stellar object) (2SLAQ) redshift survey. We investigate the evolution of the stellar populations and emission-line properties of these galaxies. By constructing composite spectra and comparing with a matched sample of radio-quiet sources from the same survey, we also investigate the effect on the galaxy of the presence of an active nucleus.
The composite spectra, binned by redshift and radio luminosity, all require two components to describe them, which we interpret as an old and a younger population. We found no evolution with redshift of the age of the younger population in radio galaxies, nor were they different from the radio-quiet comparison sample. Similarly, there is no correlation with radio power, with the exception that the most powerful radio sources  ( P 1.4 > 1026  W Hz−1) have younger stars and stronger emission lines than the less powerful sources. This suggests that we have located the threshold in radio power where strong emission lines 'switch on', at radio powers of around 1026 W Hz−1. Except for the very powerful radio galaxies, the presence of a currently active radio active galactic nucleus (AGN) does not appear to be correlated with any change in the observed stellar population of a luminous red galaxy at   z ∼ 0.5  .  相似文献   

18.
The properties of OH megamaser galaxies in the radio continuum are discussed. Many radio sources in OH megamaser galaxies exhibit relatively flat (α ≥ −0.5) radio spectra between frequencies of 1.49 and 8.44 GHz along with high brightness temperatures (Tb ≥ 104 K). In these galaxies the line and radio continuum fluxes are not correlated. The continuum radio emission of OH megamasers is predominantly nonthermal and is associated either with an active nucleus or with compact star formation. The thermal component of the radio emission from these galaxies can be neglected. The observed flat radio spectra and high brightness temperatures imply the existence of an active galactic nucleus, although some megamasers may be associated with compact star formation.__________Translated from Astrofizika, Vol. 48, No. 2, pp. 281–290 (May 2005).  相似文献   

19.
MERLIN images of Mrk 348 at 22 GHz show H2O maser emission at 0.02–0.11 Jy , within ∼ 0.8 pc of the nucleus. This is the first direct confirmation that molecular material exists close to the Seyfert 2 nucleus. Mrk 348 was observed in 2000 May one month after Falcke et al. first identified the maser in single-dish spectra. The peak maser flux density has increased about threefold. The masing region is ≲ 0.6 pc in radius. The flux density of radio continuum emission from the core has been rising for about 2 yr. The maser–core separation is barely resolved, but at the 3 σ significance level maser and core are not coincident along the line of sight. The masers lie in the direction of the northern radio lobes and probably emanate from material shocked by a jet with velocity close to c . The correlation between the radio continuum increase and maser flare is explained as arising from high-level nuclear activity through a common excitation mechanism, although direct maser amplification of the core by masers tracing a Keplerian disc is not completely ruled out.  相似文献   

20.
In this paper, we investigate how the Square Kilometre Array (SKA) can aid in determining the evolutionary history of active galactic nuclei (AGN) from redshifts z = 0 → 6. Given the vast collecting area of the SKA, it will be sensitive to both ‘radio-loud’ AGN and the much more abundant ‘radio-quiet’ AGN, namely the radio-quiet quasars and their ‘Type-II’ counterparts, out to the highest redshifts. Not only will the SKA detect these sources but it will also often be able to measure their redshifts via the Hydrogen 21-cm line in emission and/or absorption. We construct a complete radio luminosity function (RLF) for AGN, combining the most recent determinations for powerful radio sources with an estimate of the RLF for radio-quiet objects using the hard X-ray luminosity function of [ApJ 598 (2003) 886], including both Type-I and Type-II AGN. We use this complete RLF to determine the optimal design of the SKA for investigating the accretion history of the Universe for which it is likely to be a uniquely powerful instrument.  相似文献   

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