Compact Steep Spectrum Objects Research Articles

ATCA detections of massive molecular gas reservoirs in dusty, high-zradio galaxies

Observations using the 7 mm receiver system on the Australia Telescope Compact Array have revealed large reservoirs of molecular gas in two high-redshift radio galaxies: HATLAS J090426.9+015448 (z = 2.37) and HATLAS J140930.4+003803 (z = 2.04). Optically the targets are very faint, and spectroscopy classifies them as narrow-line radio galaxies. In addition to harbouring an active galactic nucleus the targets share many characteristics of sub-mm galaxies. Far-infrared data from Herschel-ATLAS suggest high levels of dust (>10^9 M_solar) and a correspondingly large amount of obscured star formation (~1000 M_solar / yr). The molecular gas is traced via the J = 1-0 transition of 12CO, its luminosity implying total H_2 masses of (1.7 +/- 0.3) x 10^11 and (9.5 +/- 2.4) x 10^10 (alpha_CO/0.8) M_solar in HATLAS J090426.9+015448 and HATLAS J140930.4+003803 respectively. Both galaxies exhibit molecular line emission over a broad (~1000 km/s) velocity range, and feature double-peaked profiles. We interpret this as evidence of either a large rotating disk or an on-going merger. Gas depletion timescales are ~100 Myr. The 1.4 GHz radio luminosities of our targets place them close to the break in the luminosity function. As such they represent `typical' z > 2 radio sources, responsible for the bulk of the energy emitted at radio wavelengths from accretion-powered sources at high redshift, and yet they rank amongst the most massive systems in terms of molecular gas and dust content. We also detect 115 GHz rest-frame continuum emission, indicating a very steep high-radio-frequency spectrum, possibly classifying the targets as compact steep spectrum objects.

First X-ray observations of low-power compact steep spectrum sources

We report first X-ray Chandra observations of a sample of seven low luminosity compact (LLC) sources. They belong to a class of young compact steep spectrum (CSS) radio sources. Four of them have been detected, the other three have upper limit estimations for X-ray flux, one CSS galaxy is associated with an X-ray cluster. We have used the new observations together with the observational data for known strong CSS and gigahertz-peaked spectrum (GPS) objects and large scale FRIs and FRIIs to study the relation between morphology, X-ray properties and excitation modes in radio-loud AGNs. We found that: (1) The low power objects fit well to the already established X-ray - radio luminosity correlation for AGNs and occupy the space among, weaker in the X-rays, FRI objects. (2) The high excitation galaxies (HEG) and low excitation galaxies (LEG) occupy distinct locus in the radio/X-ray luminosity plane, notwithstanding their evolutionary stage. This is in agreement with the postulated different origin of the X-ray emission in these two group of objects. (3) We have tested the AGN evolution models by comparing the radio/X-ray luminosity ratio with the size of the sources, and indirectly, with their age. We conclude that the division for two different X-ray emission modes, namely originate in the base of the relativistic jet (FRIs) or in the accretion disk (FRIIs) is already present among the younger compact AGNs. (4) Finally, we found that the CSS sources are less obscured than the more compact GPSs in X-rays. However, the anti-correlation between X-ray column density and radio size does not hold for the whole sample of GPS and CSS objects.

H i absorption towards nearby compact radio sources

We present the results of HI absorption measurements towards a sample of nearby Compact Steep-Spectrum (CSS) and Giga-Hertz Peaked Spectrum (GPS) radio sources, the CORALZ sample, using the Giant Metrewave Radio Telescope (GMRT). We observed a sample of 18 sources and find 7 new detections. These sources are of lower luminosity than earlier studies of CSS and GPS objects and we investigate any dependence of HI absorption features on radio luminosity. Within the uncertainties, the detection rates and column densities are similar to the more luminous objects, with the GPS objects exhibiting a higher detection rate than for the CSS objects. The relative velocity of the blueshifted absorption features, which may be due to jet-cloud interactions, are within $\sim$$-$250 km s$^{-1}$ and do not appear to extend to values over 1000 km s$^{-1}$ seen for the more luminous objects. This could be due to the weaker jets in these objects, but requires confirmation from observations of a larger sample of sources. There appears to be no evidence of any dependence of HI column density on either luminosity or redshift, but these new detections are consistent with the inverse relation between HI column density and projected linear size.

A survey of low-luminosity compact sources and its implication for the evolution of radio-loud active galactic nuclei - II. Optical analysis

This is the second in a series of papers concerning a new sample of low-luminosity compact (LLC) objects. Here we discuss the optical properties of the sample based on images and spectra from the Sloan Digital Sky Survey (SDSS). We have generated different diagnostic diagrams and we have classified the sources as high and low excitation galaxies (HEGs and LEGs, respectively). We have studied the jet–host interactions, the relation between radio and optical line emission and the evolution of the radio source within a larger sample that also includes the published samples of compact steep spectrum (CSS) and GHz-peaked spectrum (GPS) sources and Fanaroff–Riley type 1 and type II (FR I and FR II, respectively) objects. The optical and radio properties of the LLC sample are, in general, consistent with brighter CSS and large-scale radio sources, although the LLC objects have lower values of [O iii] luminosity than the more powerful CSS sources (L1.4 GHz > 1025 W Hz−1). However, when LLC objects are added to the other samples, HEGs and LEGs seem to follow independent, parallel evolutionary tracks. Regarding ionization mechanisms, LLC and luminous CSS objects behave like FR II sources, while FR Is seem to belong to a different group of objects. Based on our results, we propose independent, parallel evolutionary tracks for HEGs and LEGs, evolving from GPS to CSS to FR.

AN OPTICAL STUDY OF COMPACT STEEP-SPECTRUM RADIO SOURCES

ABSTRACT We present results from an optical study of Compact Steep-Spectrum (CSS) radio galaxies and quasars. The distinguishing characteristic of the CSS class is that their powerful radio lobes do not extend outside the host galaxy. CSS sources resemble miniature versions of the classical double radio galaxies and quasars, but high resolution radio interferometry is required to identify the double-lobed or core-jet radio morphologies. One explanation for such intrinsically small radio structures is that CSS objects are youthful sources which have not had time to expand into classical double radio sources. Alternatively, CSS objects might be of a normal age yet still compact due to confinement through strong interaction with the interstellar medium. Extensive radio investigations have yielded insights into the nature of CSS sources, but this is the first comprehensive optical study. Using the KPNO 2.1-m and 4-m telescopes we obtained broad band images and both low and high dispersion spectra for a sample of 19 CSS sources. We find that the CSS sources in our sample have similar optical characteristics and conclude that the CSS classification is physically meaningful, rather than being a collection of objects which appear to be compact for various unrelated reasons. Our optical data (Gelderman & Whittle 1994, ApJS, 91, 491) support a picture in which the CSS radio sources are interacting with the optical line emitting gas; although we are unable to distinguish between the normal expansion of a young source and a source which is contained by an exceptionally dense NLR. The most dramatic aspect of the spectra is that as a class, the CSS sources have remarkably broad, structured [O III] profiles with kinematics which are distinct from all other classes of AGN. Such exceptional velocity structure is presumably due to acceleration of the ambient gas by the powerful jets and lobes. The CSS objects in our sample may be typified as extremely luminous, high excitation line emission sources. As a class they have greater [O III] flux and equivalent width than other classes of AGN. Unexpectedly, we find little relation between the enhanced line emission and the powerful subgalactic radio source. The data from our CSS sample fall along the established correlation between line luminosity and total radio power. If the high NLR luminosities are due to ionization related to the radio structure then one might expect CSS sources would have greater line luminosity at a given radio power than other radio galaxies or quasars. Furthermore, we are unable to identify emission line flux ratios which might indicate that the radio jet plays a direct role in the ionization. Instead, photoionization by the active nucleus and a high covering factor in the NLR can explain the luminous, high equivalent width, high excitation line emission. From our imaging program we conclude that galaxy-galaxy interactions, as indicated by disturbed hosts and/or nearby companions, are suggested for every member of our CSS sample. Such interactions are important for other classes of radio-loud AGN and it is not surprising that they should be an important aspect of the CSS phenomenon. However, we are unable to determine any link to the other notable properties of CSS sources which might allow us to favor either explanation as to why CSS have such powerful yet compact radio structure.