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

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 RASS-SDSS galaxy cluster survey.

Galaxy clusters are the largest gravitationally bound systems in the universe. Clusters consist of three components: galaxies, gas, and dark matter. The galaxies themselves contribute the least, at most a few percent, to the total mass. The remainder consists of diffuse, hot gas (the intracluster medium, or ICM) and an unseen component which is needed to explain the gravitational stability of clusters (the dark matter). The two most obvious means of studying clusters of galaxies are by observing the optical light emitted from the constituent galaxies or the X-ray emission from the ICM. Clusters of galaxies, bound ensambles of hundreds of galaxies, are an ideal environment to study galaxy evolution and to learn how this is affected by different physical processes: gravity, starbursts and star formation, interactions with the intergalactic medium and galaxy-galaxy encounters. Since the very early works of Hubble in the thirties, it has been recognized that galaxies in dense environments differ systematically from those in low-density regions in their morphological types, stellar populations and gaseous content. When during the history of the Universe and why such environmental differences were established is currently one of the subjects of most intensive investigation in the international astrophysical community. On the other hand, clusters can teach us a great deal about cosmology. The distributions of galaxies on the sky shows a net-like structure in which thin walls and filaments surround large voids. The galaxy clusters are the nodes of this network. Therefore, they trace out the Large-Scale Structure (LSS) of the universe and can be used to study the LSS formation. Moreover, if clusters provide a 'fair sample' of the universe, then the fraction of their mass in baryons should equal the universal baryon fraction, known as $\Omega_b/\Omega_m$. Moreover, the evolution of cluster number density with redshift can determine the mass density parameter, known as $\Omega_M$, and possibly determine the equation of state (and nature) of the dark energy believed to be causing the expansion of the universe to accelerate. Thus, galaxy clusters have a twofold importance: first as laboratories of galaxy formation and evolution, and second as cosmological tool. The aim of this project is to study galaxy clusters from these two perspectives. For this purpose we use the largest optical and X-ray surveys ever realized, the Sloan Digital Sky Survey (SDSS) and the Rosat All Sky Survey (RASS), respectively, to conduct a multiwavelenght study of the properties of galaxy clusters. The project is called RASS-SDSS Galaxy Cluster Survey reflecting the name of the two big surveys used for this work. All the analyses are performed on two cluster samples specially created for the survey: the X-ray selected RASS-SDSS galaxy cluster catalog and a subsample of optically selected, isolated and spectroscopically confirmed Abell clusters. The project consists of two parts. The aim of the first part is to understand which role play the gravitational processes, galaxy mergers and collisions and the interaction with ICM in the process of galaxy formation and evolution. For this purpose, we study the variations of several properties of the cluster galaxy population such as the luminosity and spatial distribution, the morphological type mix, the Star Formation Rate (SFR) and stellar mass as a function of the environmental conditions and the cluster global properties. Our detailed analysis of the cluster individual and composite luminosity functions reveals that the LF clearly shows a bimodal behavior with an upturn and a evident steepening in the faint magnitude range in any SDSS band. The LF is well fitted by the sum of two Schechter functions. The bright end of the LF is found to be universal in all the clusters. The faint end of the LF is much steeper and varies significantly from system to system, when calculated within a fixed metric aperture. The variations are not ramdom however. The more massive a cluster, the lower its fraction of dwarf galaxies. This effect disappears when the cluster LF is calculated within the physical size of the system, as the virial radius ($r_{200}$). This indicates that the previously observed variations are due to aperture effects caused by the observed increase of the fraction of dwarf galaxies with the clustercentric distance. Our conclusion is that the shape of the cluster LF is universal in all the magnitude ranges when the LF is calculated within the virial region. Moreover, the analysis of the composite cluster LF per morphological type, shows that the upturn and the steepening at the faint end of the LF is caused by dwarf early type galaxies. These systems are quite rare in low density regions and appear to be a typical cluster population. We provide evidence that the process responsible for creating the excess population of dwarf early type galaxies in clusters is a threshold process that occurs when the density exceeds $\sim 500$ times the critical density of the Universe. We interpret our results in the context of the 'harassment' scenario, where faint early-type cluster galaxies are predicted to be the descendants of tidally-stripped late-type galaxies. In the same context, we investigate whether the cluster total star formation rate ($\Sigma SFR$) depends on the cluster global properties for a sample of 90 very nearby clusters. The total cluster SFR is given by the sum of the SFR of all the cluster members within the virial region. It is found to be proportional to the number of cluster galaxies involved ($N_{gal}$). The best relation between the total SFR and the cluster mass reflects the $N_{gal}-M$ relation, which is a power law with exponent smaller than 1. As a consequence, the more massive a cluster, the lower its number of cluster galaxies and total SFR per unit mass. The mean SFR per cluster galaxy ($\Sigma SFR/N_{gal}$) is constant troughout our cluster sample and does not depend on the global properties of the system. Moreover, in order to account for projection effects, we study the galaxy surface number density profile in our cluster sample. We find that clusters of different mass exibit different profiles. In the low and intermediate mass systems the best fit is provided by a core King profile, with the core radius decreasing with cluster mass, until, at the highest cluster masses, the profile is better represented by a cuspy Navarro, Frenk \& White profile. All these different analysis converge to the conclusion that the global properties of the cluster galaxy population, such as the luminosity distribution, the galaxy type mix, the mean and total cluster SFR are only weakly dependent on the cluster mass and X-ray luminosity. This suggests that the gravitational processes and the interaction galaxy-ICM are not likely to affect those properties of the cluster galaxy population. Only the spatial distribution of the cluster galaxies depends on the cluster mass, probably reflecting the different relaxation status of systems of different masses. Instead, the variations of the LF and the galaxy type mix with the clustercentric distance reflect a link between the galaxy formation process and the galaxy-galaxy encounters, as suggested by the 'harassment' scenario. In the second part of the thesis, galaxy clusters are used as cosmological tool. The aim of this work is to elucidate which component, galaxies or ICM, traces better the cluster mass in order to understand whether different selection methods select the same cluster population. This will clarify which bias is introduced by the different selection methods in the results of the cosmological tests. This will clarify which bias is introduced by the different selection methods in the results of the cosmological tests. For this porpuse, we analyse as a first step the relation between the optical ($L_{op}$) and the X-ray ($L_X$) luminosity, respectively, to the cluster mass in the X-ray selected RASS-SDSS cluster sample. The main motivation in deriving these dependences is to evaluate $L_{op}$ and $L_X$, as predictors of the cluster mass and to compare the quality of the two quantities as predictors. Our analysis reveals that $L_{op}$ is a key measure of the cluster mass. In this respect, the optical luminosity performs even better than the X-ray luminosity, which suggests that the mass distribution of a cluster is better traced by cluster galaxies rather than by intracluster gas. On the other hand, our conclusion is at odds with the generally accepted view that a cluster main physical properties are more easily revealed in the X-ray than in the optical. Such a view was established at an epoch when the lack of optical wide field surveys precluded a reliable determination of the optical luminosities of a large sample of clusters. With the advent of the Sloan Digital Sky survey, this problem is now overcome. The application of the same analysis to an optically selected cluster sample (the Abell subsample) confirms the result. Neverthless, the Abell sample comprises a subpopulation of systems which scatter significantly in the $L_X-M$ relation and appear to be extremely X-ray underluminous (on average one order of magnitude) with regard to their mass. On the other hand, these systems do follow the general scaling relation between optical luminosity and virial mass. Therefore, we call them 'Abell X-ray Underluminous clusters' or AXU clusters for short. To understand the particular nature of these systems, we examine the properties of their galaxy population. The velocity distribution of the AXU clusters is Gaussian within the virial region but is leptokurtic (more centrally concentrated than a Gaussian) in the outskirts, as expected for the systems in accretion. In addition, the AXU clusters have a higher fraction of blue galaxies in the external region and show a marginally significant paucity of galaxies at the center. Our results seem to support the interpretation suggested by Bower et al. (1997) that the AXU clusters are systems in formation undergoing a phase of mass accretion. Their low X-ray luminosity should be due to the still accreting Intracluster gas or to an ongoing merging process. Our results give supports to the conclusion of Donahue et al. (2002) concerning the biases inherent in the selection of galaxy clusters in different wavebands. While the optical selection is prone to substantial projection effects, also the X-ray selection is not perfect or not simple to characterize. The existence of X-ray underluminous clusters, even with large masses, makes it difficult to reach the needed completeness in mass for cosmological studies. Clearly, a multi-waveband approach is needed for optimizing the completeness and reliability of clusters samples. The 'RASS-SDSS Galaxy Clusters Survey' series comprises 7 scientific papers which are inserted as part of the thesis. Four of the papers are accepted for pubblication on a scientific Journal ('Astronomy & Astrophysics') and three are submitted

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

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

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

Photometric Redshifts of Submillimeter Galaxies

We use the photometric redshift method of Chakrabarti & McKee (2008) to infer photometric redshifts of submillimeter galaxies with far-IR (FIR) $\it{Herschel}$ data obtained as part of the PACS Evolutionary Probe (PEP) program. For the sample with spectroscopic redshifts, we demonstrate the validity of this method over a large range of redshifts ( 4 \ga z \ga 0.3) and luminosities, finding an average accuracy in $(1+z_{\rm phot})/(1+z_{\rm spec})$ of 10%. Thus, this method is more accurate than other FIR photometric redshift methods. This method is different from typical FIR photometric methods in deriving redshifts from the light-to-gas mass ($L/M$) ratio of infrared-bright galaxies inferred from the FIR spectral energy distribution (SED), rather than dust temperatures. Once the redshift is derived, we can determine physical properties of infrared bright galaxies, including the temperature variation within the dust envelope, luminosity, mass, and surface density. We use data from the GOODS-S field to calculate the star formation rate density (SFRD) of sub-mm bright sources detected by AzTEC and PACS. The AzTEC-PACS sources, which have a threshold 850 \micron flux \ga 5 \rm mJy, contribute 15% of the SFRD from all ULIRGs (L_{\rm IR} \ga 10^{12} L_{\odot}), and 3% of the total SFRD at $z \sim 2$.Comment: 7 pages, 2 figures, submitted to Ap

GOODS-Herschel: Separating High Redshift active galactic Nuclei and star forming galaxies Using Infrared Color Diagnostics

We have compiled a large sample of 151 high redshift (z=0.5-4) galaxies selected at 24 microns (S24>100 uJy) in the GOODS-N and ECDFS fields for which we have deep Spitzer IRS spectroscopy, allowing us to decompose the mid-infrared spectrum into contributions from star formation and activity in the galactic nuclei. In addition, we have a wealth of photometric data from Spitzer IRAC/MIPS and Herschel PACS/SPIRE. We explore how effective different infrared color combinations are at separating our mid-IR spectroscopically determined active galactic nuclei from our star forming galaxies. We look in depth at existing IRAC color diagnostics, and we explore new color-color diagnostics combining mid-IR, far-IR, and near-IR photometry, since these combinations provide the most detail about the shape of a source's IR spectrum. An added benefit of using a color that combines far-IR and mid-IR photometry is that it is indicative of the power source driving the IR luminosity. For our data set, the optimal color selections are S250/S24 vs. S8.0/S3.6 and S100/S24 vs. S8.0/S3.6; both diagnostics have ~10% contamination rate in the regions occupied primarily by star forming galaxies and active galactic nuclei, respectively. Based on the low contamination rate, these two new IR color-color diagnostics are ideal for estimating both the mid-IR power source of a galaxy when spectroscopy is unavailable and the dominant power source contributing to the IR luminosity. In the absence of far-IR data, we present color diagnostics using the WISE mid-IR bands which can efficiently select out high z (z~2) star forming galaxies.Comment: Accepted for publication in ApJ. 13 pages, 8 figure

Evidence for a wide range of UV obscuration in z ~ 2 dusty galaxies from the GOODS-Herschel survey

Dusty galaxies at z ~ 2 span a wide range of relative brightness between rest-frame mid-infrared (8um) and ultraviolet wavelengths. We attempt to determine the physical mechanism responsible for this diversity. Dust-obscured galaxies (DOGs), which have rest-frame mid-IR to UV flux density ratios > 1000, might be abnormally bright in the mid-IR, perhaps due to prominent AGN and/or PAH emission, or abnormally faint in the UV. We use far-infrared data from the GOODS-Herschel survey to show that most DOGs with 10^12 L_Sun < L_IR < 10^13 L_Sun are not abnormally bright in the mid-IR when compared to other dusty galaxies with similar IR (8--1000um) luminosities. We observe a relation between the median IR to UV luminosity ratios and the median UV continuum power-law indices for these galaxies, and we find that only 24% have specific star formation rates which indicate the dominance of compact star-forming regions. This circumstantial evidence supports the idea that the UV- and IR-emitting regions in these galaxies are spatially coincident, which implies a connection between the abnormal UV faintness of DOGs and dust obscuration. We conclude that the range in rest-frame mid-IR to UV flux density ratios spanned by dusty galaxies at z ~ 2 is due to differing amounts of UV obscuration. Of galaxies with these IR luminosities, DOGs are the most obscured. We attribute differences in UV obscuration to either: 1) differences in the degree of alignment between the spatial distributions of dust and massive stars, or 2) differences in the total dust content.Comment: 9 pages, 9 figures. Accepted by Ap

The lack of star formation gradients in galaxy groups up to z~1.6

In the local Universe, galaxy properties show a strong dependence on environment. In cluster cores, early type galaxies dominate, whereas star-forming galaxies are more and more common in the outskirts. At higher redshifts and in somewhat less dense environments (e.g. galaxy groups), the situation is less clear. One open issue is that of whether and how the star formation rate (SFR) of galaxies in groups depends on the distance from the centre of mass. To shed light on this topic, we have built a sample of X-ray selected galaxy groups at 0<z<1.6 in various blank fields (ECDFS, COSMOS, GOODS). We use a sample of spectroscopically confirmed group members with stellar mass M >10^10.3 M_sun in order to have a high spectroscopic completeness. As we use only spectroscopic redshifts, our results are not affected by uncertainties due to projection effects. We use several SFR indicators to link the star formation (SF) activity to the galaxy environment. Taking advantage of the extremely deep mid-infrared Spitzer MIPS and far-infrared Herschel PACS observations, we have an accurate, broad-band measure of the SFR for the bulk of the star-forming galaxies. We use multi-wavelength SED fitting techniques to estimate the stellar masses of all objects and the SFR of the MIPS and PACS undetected galaxies. We analyse the dependence of the SF activity, stellar mass and specific SFR on the group-centric distance, up to z~1.6, for the first time. We do not find any correlation between the mean SFR and group-centric distance at any redshift. We do not observe any strong mass segregation either, in agreement with predictions from simulations. Our results suggest that either groups have a much smaller spread in accretion times with respect to the clusters and that the relaxation time is longer than the group crossing time.Comment: Accepted for publication in MNRA