A growing body of evidence indicates that the star formation rate per unit stellar mass (sSFR) decreases with increasing mass in normal main-sequence star-forming galaxies. Many processes have been advocated as being responsible for this trend (also known as mass quenching), e.g., feedback from active galactic nuclei (AGNs), and the formation of classical bulges. In order to improve our insight into the mechanisms regulating the star formation in normal star-forming galaxies across cosmic epochs, we determine a refined star formation versus stellar mass relation in the local Universe. To this end we use the Hα narrow-band imaging followup survey (Hα3) of field galaxies selected from the HI Arecibo Legacy Fast ALFA Survey (ALFALFA) in the Coma and Local superclusters. By complementing this local determination with high-redshift measurements from the literature, we reconstruct the star formation history of main-sequence galaxies as a function of stellar mass from the present epoch up to z = 3. In agreement with previous studies, our analysis shows that quenching mechanisms occur above a threshold stellar mass M knee that evolves with redshift as ∝(1 + z) 2 . Moreover, visual morphological classification of individual objects in our local sample reveals a sharp increase in the fraction of visually classified strong bars with mass, hinting that strong bars may contribute to the observed downturn in the sSFR above M knee . We test this hypothesis using a simple but physically motivated numerical model for bar formation, finding that strong bars can rapidly quench star formation in the central few kpc of field galaxies. We conclude that strong bars contribute significantly to the red colors observed in the inner parts of massive galaxies, although additional mechanisms are likely required to quench the star formation in the outer regions of massive spiral galaxies. Intriguingly, when we extrapolate our model to higher redshifts, we successfully recover the observed redshift evolution for M knee . Our study highlights how the formation of strong bars in massive galaxies is an important mechanism in regulating the redshift evolution of the sSFR for field main-sequence galaxies.

Context. Active galactic nuclei (AGN) emit radiation over a wide range of wavelengths, with a peak of emission in the far-UV region of the electromagnetic spectrum, a spectral region that is historically difficult to observe. Aims. Using optical, GALEX UV, and XMM-Newton data we derive the spectral energy distribution (SED) from the optical/UV to X-ray regime of a sizeable sample of AGN. The principal motivation is to investigate the relationship between the optical/UV emission and the X-ray emission and provide bolometric corrections to the hard X-ray (2-10 keV) energy range, k bol , the latter being a fundamental parameter in current physical cosmology. Methods. We construct and study the X-ray to optical SED of a sample of 195 X-ray selected Type 1 AGN belonging to the XMM-Newton bright serendipitous survey (XBS). The optical-UV luminosity was computed using data from the Sloan Digital Sky Survey (SDSS), from our own dedicated optical spectroscopy and the satellite GALaxy evolution EXplorer (GALEX), while the X-ray luminosity was computed using XMM-Newton data. Because it covers a wide range of redshift (0.03 < z < ∼ 2.2), X-ray luminosities (41.8 < log L [2−10] keV < 45.5 erg/s) and because it is composed of "bright objects", this sample is ideal for this kind of investigation. Results. We confirm a highly significant correlation between the accretion disc luminosity L disc and the hard X-ray luminosity, where β = 1.18 ± 0.05. We find a very shallow dependence of k bol on the X-ray luminosity with respect to the broad distribution of values of k bol . We find a correlation between k bol and the hard X-ray photon index Γ 2−10 keV and a tight correlation between the optical-to-X-ray spectral index α ox and k bol , so we conclude that both Γ 2−10 keV and α ox can be used as a proxy for k bol .

We study the link between the X-ray emission in radio-quiet active galactic nuclei (AGN) and the accretion rate on the central supermassive black hole using a statistically well-defined and representative sample of 71 type 1 AGN extracted from the XMM-Newton Bright Serendipitous Survey. We search and quantify the statistical correlations between some fundamental parameters that characterize the X-ray emission, i.e. the X-ray spectral slope, , and the X-ray 'loudness', and the accretion rate, both absolute ( Ṁ) and normalized to the Eddington luminosity (Eddington ratio, λ). We parametrize the X-ray loudness using three different quantities: the bolometric correction K bol , the two-point spectral index α OX and the disc/corona luminosity ratio. We find that the X-ray spectral index depends on the normalized accretion rate while the 'X-ray loudness' depends on both the normalized and the absolute accretion rate. The dependence on the Eddington ratio, in particular, is probably induced by the -λ correlation. The two proxies usually adopted in the literature to quantify the X-ray loudness of an AGN, i.e. K bol and α OX , behave differently, with K bol being more sensitive to the Eddington ratio and α OX having a stronger dependence with the absolute accretion. The explanation of this result is likely related to the different sensitivity of the two parameters to the X-ray spectral index.

We present SDSSJ143244.91+301435.3, a new case of radio-loud narrow line Seyfert 1 (RL NLS1) with a relatively high radio power (P 1.4GHz =2.1×10 25 W Hz −1 ) and large radioloudness parameter (R 1.4 =600±100). The radio source is compact with a linear size below ∼1.4 kpc but, contrary to most of the RL NLS1 discovered so far with such a high R 1.4 , its radio spectrum is very steep (α=0.93, S ν ∝ ν −α ) and not supporting a "blazar-like" nature. Both the small mass of the central super-massive black-hole and the high accretion rate relative to the Eddington limit estimated for this object (3.2×10 7 M ⊙ and 0.27, respectively, with a formal error of ∼0.4 dex on both quantities) are typical of the class of NLS1. Through a modelling of the spectral energy distribution of the source we have found that the galaxy hosting SDSSJ143244.91+301435.3 is undergoing a quite intense star-formation (SFR=50 M ⊙ y −1 ) which, however, is expected to contribute only marginally (∼1 per cent) to the observed radio emission. The radio properties of SDSSJ143244.91+301435.3 are remarkably similar to those of compact steep spectrum (CSS) radio sources, a class of AGN mostly composed by young radio galaxies. This may suggest a direct link between these two classes of AGN, with the CSS sources possibly representing the misaligned version (the so-called "parent population") of RL NLS1 showing blazar characteristics.