On the existence of bright IR galaxies at z>2: tension between Herschel and SCUBA-2 results?

Recent derivations of the galaxy star formation rate density (SFRD) obtained from sub-millimetre (sub-mm) surveys (e.g., SCUBA-2) show a tension with previous works based on Herschel and multi-wavelength data. Some of these works claim that the SFRD derived by pushing the Herschel surveys beyond z~2 are incorrect. However, the current sub-mm surveys obtained from SCUBA-2 data and the methods used to construct the total infrared (IR) luminosity function (LF) and the SFRD could be affected by some limitations. Here we show how these limitations (i.e., selection bias and incompleteness effects) might affect the total IR LF, making the resulting dusty galaxy evolution of difficult interpretation. In particular, we find that the assumed spectral energy distribution (SED) plays a crucial role in the total IR LF derivation, moreover, we confirm that the long-wavelength (e.g., 850-micron) surveys can be incomplete against "warm" SED galaxies, and that the use of a wide spectral coverage of IR wavelengths is crucial to limit the uncertainties and biases.Comment: 8 pages, 6 figures, accepted for publication in MNRA

HST imaging survey of sub-mJy star-forming galaxies I: morphologies at z ~ 0.2

We present the first results of our HST WFPC2 F814W snapshot imaging survey, targeting virtually all sub-mJy decimetric radio-selected star-forming galaxies. The radio selection at ~1 GHz is free from extinction effects and the radio luminosities are largely unaffected by AGN contamination, making these galaxies ideal tracers of the cosmic star formation history. A sub-sample of 4 targets is presented here, selected at 1.4 GHz from the spectroscopically homogenous and complete samples of Benn et al. (1993) and Hopkins et al. (1999). The redshifts are confined to a narrow range around z~0.2, to avoid differential evolution, with a radio luminosity close to L* where the galaxies dominate the comoving volume-averaged star formation rate. We find clearly disturbed morphologies resembling those of ultraluminous infrared galaxies, indicating that galaxy interactions may be the dominant mechanism for triggering star formation at these epochs. The morphologies are also clearly different from coeval quasars and radiogalaxies, as found in star-forming galaxies selected at other wavelengths. This may prove challenging for models which propose direct causal links between AGN evolution and the cosmic star formation history at these epochs. The asymmetries are typically much larger than seen in the CFRS at similar redshifts, optical luminosities and Halpha-derived star formation rates, indicating the possible existence of an obscuration-related morphological bias in such samples.Comment: MNRAS letters in press; 5 page

Unveiling the Evolution of Type I AGNs in the IR (15μm) — As Seen by ISO in the ELAIS-S1 Region

AbstractWe present the first estimate of the evolution of type 1 AGNs in the IR (15 μm) obtained from the ELAIS survey in the S1 region. We find that the luminosity function (LF) of Type 1 AGNs at 15μm is fairly well represented by a double power-law function with a bright slope of 2.9 and a faint slope of 1.1. There is evidence for significant cosmological evolution according to a pure luminosity evolution model L15(z)α(l+z)k, with in a (Ωm,ΩΛ)=(1.0,0.0) cosmology. This evolution is similar to what is observed at other wavebands. From the luminosity function and its evolution, we estimate a contribution of ~ 2% from Type 1 AGN to the total Cosmic Infrared Background (CIRB) at 15 μm

The molecular gas in the central region of NGC 7213

We present a multi-wavelength study (from X-ray to millimetre) of the nearby low-luminosity active galactic nucleus (LLAGN) NGC 7213. We combine the information from the different bands to characterize the source in terms of contribution from the AGN and the host-galaxy interstellar medium (ISM). This approach allows us to provide a coherent picture of the role of the AGN and its impact, if any, on the star formation and molecular gas properties of the host galaxy. We focused our study on archival ALMA Cycle 1 observations, where the CO(2-1) emission line has been used as a tracer of the molecular gas. Using the 3DBarolo code on ALMA data, we performed the modelling of the molecular gas kinematics traced by the CO(2-1) emission, finding a rotationally dominated pattern. The host-galaxy molecular gas mass was estimated from the integrated CO(2-1) emission line obtained with APEX data, assuming an $\alpha_{CO}$ conversion factor. By using the ALMA data, we would have underestimated the gas masses by a factor $\sim$3, given the filtering out of the large scale emission in interferometric observations. We also performed a complete X-ray spectral analysis on archival observations, revealing a relatively faint and unobscured AGN. The AGN results to be too faint to significantly affect the properties of the host-galaxy, such as star formation activity and molecular gas kinematics and distribution.Comment: 11 pages, 8 figures, to be published in A&