21 research outputs found

    The halo distribution of 2dF galaxies

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    We use the clustering results obtained by Madgwick et al. (2003) for a sample of 96 791 galaxies from the 2dF Galaxy Redshift Survey with redshift 0.01 < z < 0.15 to study the distribution of late-type and early-type galaxies within dark matter haloes of different mass. Within the framework of our models, galaxies of both classes are found to be as spatially concentrated as the dark matter within haloes, even though, while the distribution of star-forming galaxies can also allow for some steeper profiles, this is ruled out drastically in the case of early-type galaxies. We also find evidence for morphological segregation, as late-type galaxies appear to be distributed within haloes of mass-scales corresponding to groups and clusters up to about two virial radii, whereas passive objects show a preference to reside closer to the halo centre. If we assume a broken power law of the form 〈Ngal〉(m) = (m/m0) for mcut≤m < m0 and 〈Ngal〉(m) = (m/m0) at higher masses to describe the dependence of the average number of galaxies within haloes on the halo mass, fits to the data show that star-forming galaxies start appearing in haloes of masses mcut≃ 1011 M⊙, much smaller than what is obtained for early-type galaxies (mcut≃ 1012.6 M⊙). In the high-mass regime m≥m0, 〈Ngal〉 increases with halo mass more slowly (α2≃ 0.7) in the case of late-type galaxies than for passive objects which present α2≃ 1.1. The above results imply that late-type galaxies dominate the 2dF counts at all mass-scales. We stress that — at variance with previous statements — there is no degeneracy in the determination of the best functional forms for ρ(r) and 〈Ngal〉, as they affect the behaviour of the galaxy-galaxy correlation function on different scale

    Cosmic evolution of quasar clustering: implications for the host haloes

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    We present detailed clustering measurements from the 2dF Quasi-Stellar Object Redshift Survey (2QZ) in the redshift range 0.8 1012 M⊙ and that the characteristic mass of their host haloes is of the order of 1013 M⊙. The observed clustering is consistent with assuming that the locally observed correlation between black hole mass and host galaxy circular velocity is still valid at z > 1. From the fraction of haloes which contain active quasars, we infer that the characteristic quasar lifetime is tQ∼ a few × 107 yr at z∼ 1 and approaches 108 yr at higher redshift

    The PEP Survey: evidence for intense star-forming activity in the majority of radio-selected AGN at z>~1

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    In order to investigate the FIR properties of radio-active AGN, we have considered three different fields where both radio and FIR observations are the deepest to-date: GOODS-South, GOODS-North and the Lockman Hole. Out of a total of 92 radio-selected AGN, ~64% are found to have a counterpart in Herschel maps. The percentage is maximum in the GOODS-North (72%) and minimum (~50%) in the Lockman Hole, where FIR observations are shallower. Our study shows that in all cases FIR emission is associated to star-forming activity within the host galaxy. Such an activity can even be extremely intense, with star-forming rates as high as ~10^3-10^4 Msun/yr. AGN activity does not inhibit star formation in the host galaxy, just as on-site star-formation does not seem to affect AGN properties, at least those detected at radio wavelengths and for z>~1. Furthermore, physical properties such as the mass and age distributions of the galaxies hosting a radio-active AGN do not seem to be affected by the presence of an ongoing star-forming event. Given the very high rate of FIR detections, we stress that this refers to the majority of the sample: most radio-active AGN are associated with intense episodes of star-formation. However, the two processes proceed independently within the same galaxy, at all redshifts but in the local universe, where powerful enough radio activity reaches the necessary strength to switch off the on-site star formation. Our data also show that for z>~1 the hosts of radio-selected star-forming galaxies and AGN are indistinguishable from each other both in terms of mass and IR luminosity distributions. The two populations only differentiate in the very local universe, whereby the few AGN which are still FIR-active are found in galaxies with much higher masses and luminosities.Comment: 20 pages, 22 figures, to appear in MNRA

    The clustering properties of high-redshift passive galaxies

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    We investigate the clustering properties of 3<z<5 candidate passive galaxies from the Merlin et al. (2019) sample residing in the GOODS-North (35 sources) and GOODS-South (33 sources) fields. Within the large uncertainties due to the paucity of sources we do not detect clustering signal in GOODS-North, while this is present in GOODS-South, highlighting the importance of the effects of cosmic variance. The estimated correlation length in GOODS-South is r_0=12^+4_-5 Mpc, while the estimated minimum mass for a halo capable to host one of such high-redshift quenched galaxies is log10(M_min/M_sun) =13.0\pm 0.3, once also the constraints from their space density are taken into account. Both values are compatible with the results from GOODS-North. Putting the above findings in a cosmological context, these suggest no evolution of the dark matter content of the hosts of passive galaxies during the past 12.5 Gyr, i.e. during more than 90% of the age of the Universe. We discuss possible scenarios for the observed trend.Comment: 6 pages, 3 figures, letter to appear in MNRA

    Exploring AGN Activity over Cosmic Time with the SKA

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    In this Chapter we present the motivation for undertaking both a wide and deep survey with the SKA in the context of studying AGN activity across cosmic time. With an rms down to 1 μ\muJy/beam at 1 GHz over 1,000 - 5,000 deg2^2 in 1 year (wide tier band 1/2) and an rms down to 200 nJy/beam over 10 - 30 deg2^2 in 2000 hours (deep tier band 1/2), these surveys will directly detect faint radio-loud and radio-quiet AGN (down to a 1 GHz radio luminosity of about 2×10232\times10^{23} W/Hz at z=6z=6). For the first time, this will enable us to conduct detailed studies of the cosmic evolution of radio AGN activity to the cosmic dawn (z6z\gtrsim6), covering all environmental densities.Comment: 17 pages, 7 figures, to appear as part of 'Continuum Science' in Proceedings 'Advancing Astrophysics with the SKA (AASKA14)

    A census of radio-selected AGNs on the COSMOS field and of their FIR properties

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    We use the new catalogue by Laigle et al. to provide a full census of VLA-COSMOS radio sources. We identify 90 per cent of such sources and sub-divide them into active galactic nuclei (AGNs) and star-forming galaxies on the basis of their radio luminosity. The AGN sample is complete with respect to radio selection at all z ≲ 3.5. Out of 704 AGNs, 272 have a counterpart in the Herschel maps. By exploiting the better statistics of the new sample, we confirm the results of Magliocchetti et al.: the probability for a radio-selected AGN to be detected at far-infrared (FIR) wavelengths is both a function of radio luminosity and redshift, whereby powerful sources are more likely FIR emitters at earlier epochs. Such an emission is due to star-forming processes within the host galaxy. FIR emitters and non-FIR emitters only differentiate in the z ≲ 1 universe. At higher redshifts, they are indistinguishable from each other, as there is no difference between FIR-emitting AGNs and star-forming galaxies. Lastly, we focus on radio AGNs which show AGN emission at other wavelengths. We find that mid-infrared (MIR) emission is mainly associated with ongoing star formation and with sources which are smaller, younger and more radio luminous than the average parent population. X-ray emitters instead preferentially appear in more massive and older galaxies. We can therefore envisage an evolutionary track whereby the first phase of a radio-active AGN and of its host galaxy is associated with MIR emission, while at later stages the source becomes only active at radio wavelengths and possibly also in the X-ray

    Hosts and environments of radio-active AGN

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    Investigations of the population of radio-active AGN up to z=3.5 not only show that these sources are hosted by galaxies of very large, M*>1010.5 Msun, stellar masses, but also that at all redshifts they reside in very massive dark matter halos, comparable to those associated with groups-to-clusters of galaxies. This result is found both via clustering studies and by directly pinpointing such sources to the cosmological structures they belong to. We also show how intense star-forming activity is encountered in the overwhelming majority of z>1 (massive) galaxies hosts of radio-active AGN, and how this activity is only halted by nuclear feedbacks in the relatively local universe. What emerges from our work is a scenario whereby physical processes at sub-pc/pc (e.g. AGN emission) and kpc scales strongly influence the large-scale structure behavior of the AGN and its host