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Arno Allan Penzias, a visionary explorer of the Universe, made significant contributions to cosmology and radio spectroscopy. Alongside Robert Wilson, he detected the cosmic microwave radiation from the big bang, revolutionizing cosmology. Penzias and Wilson also made the first detection of millimeter-wave radiation from interstellar carbon monoxide, opening up a new field of research. Penzias had a successful career at Bell Labs and received numerous awards and recognitions, including the Nobel Prize in Physics in 1978. He will be remembered for his groundbreaking discoveries and contributions to the scientific community. [Extracted from the article]

This is the second of two papers describing the observations and source catalogues derived from sensitive 3-GHz images of the Lockman Hole North using the Karl G. Jansky Very Large Array (VLA). We describe the reduction and cataloguing process, which yielded an image with 8 arcsec resolution and instrumental noise of σn = 1.01 μJy beam-1 rms (before primary-beam corrections) and a catalogue of 558 sources detected above 5σn. We include details of how we estimate source spectral indices across the 2-GHz VLA bandwidth, finding a median index of -0.76 ± 0.04. Stacking of source spectra reveals a flattening of spectral index with decreasing flux density. We present a source count derived from the catalogue. We show a traditional count estimate compared with a completely independent estimate made via a P(D) confusion analysis, and find very good agreement. Cross-matches of the catalogue with X-ray, optical, infrared, radio, and redshift catalogues are also presented. The X-ray, optical and infrared data, as well as active galactic nuclei (AGN) selection criteria allow us to classify 10 per cent as radio-loud AGN, 28 per cent as radio-quiet AGN, and 58 per cent as star-forming galaxies, with only 4 per cent unclassified. [ABSTRACT FROM AUTHOR]

We study the Extended Chandra Deep Field-South Very Large Array sample, which reaches a flux density limit at 1.4 GHz of 32.5 μJy at the field centre and redshift ~4, and covers ~0.3deg². Number counts are presented for the whole sample while the evolutionary properties and luminosity functions are derived for active galactic nuclei (AGN). The faint radio sky contains two totally distinct AGN populations, characterized by very different evolutions, luminosity functions, and Eddington ratios: radio-quiet (RQ)/radiative-mode and radio-loud (RL)/jet-mode AGN. The radio power of RQ AGN evolves α (1 + z)2.5, similarly to star-forming galaxies, while the number density of RL ones has a peak at z ~ 0.5 and then declines at higher redshifts. The number density of radio-selected RQ AGN is consistent with that of X-ray selected AGN, which shows that we are sampling the same population. The unbiased fraction of radiative-mode RL AGN, derived from our own and previously published data, is a strong function of radio power, decreasing from ~ 0.5 at P 1.4 GHz ~ 1024 WHz-1 to ~0.04 at P1.4GHz ~ 1022 WHz-1.Thanks to our enlarged sample, which now includes ~700 radio sources, we also confirm and strengthen our previous results on the source population of the faint radio sky: star-forming galaxies start to dominate the radio sky only below ~0.1 mJy, which is also where RQ AGN overtake RL ones. [ABSTRACT FROM AUTHOR]

Author(s): K. I. Kellermann [1] The Physics of Extragalactic Radio Sources by David S. De Young University of Chicago Press: 2002. 450 pp. $45 After the Second World War, scientists, [...]

Although the extragalactic nature of 3C 48 and other quasi stellar radio sources was discussed as early as 1960 by John Bolton and others, it was rejected largely because of preconceived ideas about what appeared to be unrealistically high radio and optical luminosities. Not until the 1962 occultations of the strong radio source 3C 273 at Parkes, which led Maarten Schmidt to identify 3C 273 with an apparent stellar object at a redshift of 0.16, was the true nature understood. Successive radio and optical measurements quickly led to the identification of other quasars with increasingly large redshifts and the general, although for some decades not universal, acceptance of quasars as the very luminous nuclei of galaxies.Curiously, 3C 273, which is one of the strongest extragalactic sources in the sky, was first cataloged in 1959 and the magnitude 13 optical counterpart was observed at least as early as 1887. Since 1960, much fainter optical counterparts were being routinely identified using accurate radio interferometer positions which were measured primarily at the Caltech Owens Valley Radio Observatory. However, 3C 273 eluded identification until the series of lunar occultation observations led by Cyril Hazard. Although an accurate radio position had been obtained earlier with the OVRO interferometer, inexplicably 3C 273 was initially misidentified with a faint galaxy located about an arc minute away from the true quasar position. [ABSTRACT FROM PUBLISHER]

We present our very recent results on the sub-mJy radio source populations at 1.4 GHz based on the Extended Chandra Deep Field South VLA survey, which reaches ~ 30 μJy, with details on their number counts, evolution, and luminosity functions. The sub-mJy radio sky turns out to be a complex mix of star-forming galaxies and radio-quiet AGN evolving at a similar, strong rate and declining radio-loud AGN. While the well-known flattening of the radio number counts below 1 mJy is mostly due to star-forming galaxies, these sources and AGN make up an approximately equal fraction of the sub-mJy sky. Our results shed also light on a fifty-year-old issue, namely radio emission from radio-quiet AGN, and suggest that it is closely related to star formation, at least at z ~ 1.5 − 2. The implications of our findings for future, deeper radio surveys, including those with the Square Kilometre Array, are also discussed. One of the main messages, especially to non-radio astronomers, is that radio surveys are reaching such faint limits that, while previously they were mainly useful for radio quasars and radio galaxies, they are now detecting mostly star-forming galaxies and radio-quiet AGN, i.e., the bulk of the extragalactic sources studied in the infrared, optical, and X-ray bands. [ABSTRACT FROM PUBLISHER]

Although the extragalactic nature of quasars was discussed as early as 1960, it was rejected largely because of preconceived ideas about what appeared to be an unrealistically high radio and optical luminosity. Following the 1962 occultations of the strong radio source 3C 273 at Parkes, and the subsequent identification with an apparent stellar object, Maarten Schmidt recognized that the relatively simple hydrogen line Balmer series spectrum implied a redshift of 0.16. Successive radio and optical measurements quickly led to the identification of other quasars with increasingly large redshifts and the general, although for some decades not universal, acceptance of quasars as being by far the most distant and the most luminous objects in the Universe. Arguments for a more local population continued for at least several decades, fueled in part by a greater willingness to accept the unclear new physics needed to interpret the large observed redshifts rather than the extreme luminosities and energies implied by the cosmological interpretation of the redshifts. Curiously, 3C 273, which is one of the strongest extragalactic sources in the sky, was first catalogued in 1959 and the magnitude 13 optical counterpart was observed at least as early as 1887. Since 1960, much fainter optical counterparts were being routinely identified using accurate radio interferometer positions, measured primarily at the Caltech Owens Valley Radio Observatory. However, 3C 273 eluded identification until the series of lunar occultation observations led by Cyril Hazard. Subsequent attempts to classify quasars into numerous sub-categories based on their observed optical, radio, IR and high energy properties have perhaps led to more confusion than clarity. However, quasars and the broader class of AGN are now a fundamental part of astrophysics and cosmology. They were the basis for the recognition of supermassive black holes in galactic nuclei, which are intimately tied to the formation and evolution of stars and galaxies. [ABSTRACT FROM AUTHOR]

ABSTRACT In order to trace the instantaneous star formation rate (SFR) at high redshift, and thus help in understanding the relation between the different emission mechanisms related to star formation, we combine the recent 4-Ms Chandra X-ray data and the deep Very Large Array radio data in the Extended Chandra Deep Field-South region. We find 268 sources detected both in the X-ray and radio bands. The availability of redshifts for ∼95 per cent of the sources in our sample allows us to derive reliable luminosity estimates and the intrinsic properties from X-ray analysis for the majority of the objects. With the aim of selecting sources powered by star formation in both bands, we adopt classification criteria based on X-ray and radio data, exploiting the X-ray spectral features and time variability, taking advantage of observations scattered across more than 10 years. We identify 43 objects consistent with being powered by star formation. We also add another 111 and 70 star-forming candidates detected only in the radio and X-ray bands, respectively. We find a clear linear correlation between radio and X-ray luminosity in star-forming galaxies over three orders of magnitude and up to z∼ 1.5. We also measure a significant scatter of the order of 0.4 dex, higher than that observed at low redshift, implying an intrinsic scatter component. The correlation is consistent with that measured locally, and no evolution with redshift is observed. Using a locally calibrated relation between the SFR and the radio luminosity, we investigate the LX(2-10 keV)-SFR relation at high redshift. The comparison of the SFR measured in our sample with some theoretical models for the Milky Way and M31, two typical spiral galaxies, indicates that, with current data, we can trace typical spirals only at z≤ 0.2, and strong starburst galaxies with SFRs as high as ∼100 M⊙ yr−1, up to z∼ 1.5. [ABSTRACT FROM AUTHOR]

18 days of MERLIN data and 42 h of A-array VLA data at 1.4 GHz have been combined to image a 10-arcmin field centred on theHubble Deep Field(HDF). This area also includes theHubble Flanking Fields(HFF). A complete sample of 92 radio sources withwas detected using the VLA data alone and then imaged with the MERLIN+VLA combination. The combined images offer (i) higher angular resolution (synthesized beams of diameter 0.2–0.5 arcsec), (ii) improved astrometric accuracy, and (iii) improved sensitivity compared with VLA-only data. The images are amongst the most sensitive yet made at 1.4 GHz, with rms noise levels of 3.3μJy beam−1 in the 0.2-arcsec images. Virtually all the sources are resolved, with angular sizes in the range 0.2 to 3 arcsec. The central 3-arcmin square was imaged separately to search for sources down to 27μJy. No additional sources were detected, indicating that sources fainter than 40μJy are heavily resolved with MERLIN and must have typical angular sizes>0.5 arcsec. Radio sources associated with compact galaxies have been used to align the HDF, the HFF and a larger CFHT optical field to the radio-based International Celestial Reference Frame. TheHSToptical fields have been registered to<50 mas in the HDF itself, and toin the outer parts of the HFF. We find astatisticalassociation of very faintradio sources with optically bright HDF galaxies down to∼23 mag. Of the 92 radio sources above 40μJy,∼85 per cent are identified with galaxies brighter than; the remaining 15 per cent are associated with optically faint systems close to or beyond the HFF (or even the HDF) limit. The high astrometric accuracy and the ability of radio waves to penetrate obscuring dust have led to the correct identification of several very red, optically faint systems, including the the strongest submillimetre source in the HDF, HDF 850.1. On the basis of their radio structures and spectra, 72 per cent (66 sources) can be classified as starburst or active galactic nucleus-type systems; the remainder are unclassified. The proportion of starburst systems increases with decreasing flux density; belowof the sources are starburst-type systems associated with major disc galaxies in the redshift range 0.3–1.3.Chandradetections are associated with 55 of the 92 radio sources, but their X-ray flux densities do not appear to be correlated with the radio flux densities or morphologies. The most recent submillimetre results on the HDF and HFF do not provide any unambiguous identifications with these latest radio data, except for HDF 850.1, but suggest at least three strong candidates. [ABSTRACT FROM AUTHOR]

Since the first radio astronomy observations in the 1930s, the angular resolution of radio telescopes has improved from tens of degrees to better than one thousandth of a second of arc. This advancement has been the result of technological innovations such as stable, sensitive, short-wavelength radio receivers, digital correlators, atomic clocks, and high-speed tape recorders, as well as the development of sophisticated image processing algorithms implemented on inexpensive, fast, digital computers. [ABSTRACT FROM AUTHOR]

With the goal of investigating the link between black hole (BH) and star formation (SF) activity, we study a deep sample of radio selected star forming galaxies (SFGs) and active galactic nuclei (AGNs). Using a multi-wavelength approach we characterize their host galaxies properties (stellar masses, optical colors, and morphology). Moreover, comparing the star formation rate derived from the radio and far-infrared luminosity, we found evidences that the main contribution to the radio emission in the radio-quiet AGNs is star-formation activity in their host galaxy. [ABSTRACT FROM PUBLISHER]