arXiv subjects

Dario Fadda

Publications and source records attributed to Dario Fadda.

At least 37 records · Page 2Linked to original sources

arXiv2017

Galaxy Evolution in the Cluster Abell 85: New Insights from the Dwarf Population

We present the first results of a new spectroscopic survey of the cluster Abell 85 targeting 1466 candidate cluster members within the central $\sim$1 deg$^2$ of the cluster and having magnitudes $m_r < 20.5$ using VIMOS/VLT and HYDRA/WIYN. A total of 520 galaxies are confirmed as either relaxed cluster members or part of an infalling population. A significant fraction are low mass; the median stellar mass of the sample is $10^{9.6} M_{\odot} $, and 25% have stellar masses below $10^9 M_{\odot}$ (i.e. 133 dwarf galaxies). We also identify seven active galactic nuclei (AGN), four of which reside in dwarf host galaxies. We probe the evolution of star formation rates, based on H$α$ emission and continuum modeling, as a function of both mass and environment. We find that more star forming galaxies are observed at larger clustercentric distances, while infalling galaxies show evidence for recently enhanced star forming activity. Main sequence galaxies, defined by their continuum star formation rates, show different evolutionary behavior based on their mass. At the low mass end, the galaxies have had their star formation recently quenched, while more massive galaxies show no significant change. The timescales probed here favor fast quenching mechanisms, such as ram-pressure stripping. Galaxies within the green valley, defined similarly, do not show evidence of quenching. Instead, the low mass galaxies maintain their levels of star forming activity, while the more massive galaxies have experienced a recent burst.

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arXiv2016

Transient effects in Herschel/PACS spectroscopy

The Ge:Ga detectors used in the PACS spectrograph onboard the Herschel space telescope react to changes of the incident flux with a certain delay. This generates transient effects on the resulting signal which can be important and last for up to an hour. The paper presents a study of the effects of transients on the detected signal and proposes methods to mitigate them especially in the case of the "unchopped" mode. Since transients can arise from a variety of causes, we classified them in three main categories: transients caused by sudden variations of the continuum due to the observational mode used; transients caused by cosmic ray impacts on the detectors; transients caused by a continuous smooth variation of the continuum during a wavelength scan. We propose a method to disentangle these effects and treat them separately. In particular, we show that a linear combination of three exponential functions is needed to fit the response variation of the detectors during a transient. An algorithm to detect, fit, and correct transient effects is presented. The solution proposed to correct the signal for the effects of transients substantially improves the quality of the final reduction with respect to the standard methods used for archival reduction in the case where transient effects are most pronounced. The programs developed to implement the corrections are offered through two new interactive data reduction pipelines in the latest releases of the Herschel Interactive Processing Environment.

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arXiv2016

Star formation and black hole accretion activity in rich local clusters of galaxies

We present a study of the star formation and central black hole accretion activity of the galaxies hosted in the two nearby (z$\sim$0.2) rich galaxy clusters Abell 983 and 1731. Aims: We are able to quantify both the obscured and unobscured star formation rates, as well as the presence of active galactic nuclei (AGN) as a function of the environment in which the galaxy is located. Methods: We targeted the clusters with unprecedented deep infrared Spitzer observations (0.2 mJy @ 24 micron), near-IR Palomar imaging and optical WIYN spectroscopy. The extent of our observations ($\sim$ 3 virial radii) covers the vast range of possible environments, from the very dense cluster centre to the very rarefied cluster outskirts and accretion regions. Results: The star forming members of the two clusters present star formation rates comparable with those measured in coeval field galaxies. The analysis of the spatial arrangement of the spectroscopically confirmed members reveals an elongated distribution for A1731 with respect to the more uniform distribution of A983. The emerging picture is compatible with A983 being a fully evolved cluster, in contrast with the still actively accreting A1731. Conclusions: The analysis of the specific star formation rate reveals evidence of on-going galaxy pre-processing along A1731's filament-like structure. Furthermore, the decrease in the number of star forming galaxies and AGN towards the cluster cores suggests that the cluster environment is accelerating the ageing process of galaxies and blocking further accretion of the cold gas that fuels both star formation and black hole accretion activity.

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arXiv2015

Rule-based Cross-matching of Very Large Catalogs in NED

The NASA/IPAC Extragalactic Database (NED) has deployed a new rule-based cross-matching algorithm called Match Expert (MatchEx), capable of cross-matching very large catalogs (VLCs) with >10 million objects. MatchEx goes beyond traditional position-based cross-matching algorithms by using other available data together with expert logic to determine which candidate match is the best. Furthermore, the local background density of sources is used to determine and minimize the false-positive match rate and to estimate match completeness. The logical outcome and statistical probability of each match decision is stored in the database, and may be used to tune the algorithm and adjust match parameter thresholds. For our first production run, we cross-matched the GALEX All Sky Survey Catalog (GASC), containing nearly 40 million NUV-detected sources, against a directory of 180 million objects in NED. Candidate matches were identified for each GASC source within a 7.5 arcsecond radius. These candidates were filtered on position-based matching probability, and on other criteria including object type and object name. We estimate a match completeness of 97.6% and a match accuracy of 99.75%. MatchEx is being used to cross-match over 2 billion catalog sources to NED, including the Spitzer Source List, the 2MASS Point-Source Catalog, AllWISE, and SDSS DR 10. It will also speed up routine cross-matching of sources as part of the NED literature pipeline.

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arXiv2014

The hunt for red AGN: a new infrared diagnostic

We introduce a new infrared diagnostic to separate galaxies on the basis of their dominant infrared emission: stellar or nuclear. The main novelty with respect to existing diagnostics, is the usage of a broad band encompassing at the same time the 9.7micron Silicate absorption feature and one of the adjacent broad PAH (polycyclic aromatic hydrocarbon) features. This provides a robust estimate of the near- to mid-infrared continuum slope and enables a clear distinction among different classes of galaxies up to a redshift z=2.5. The diagnostic can be applied to a wealth of archival data from the ISO, Spitzer, and Akari surveys as well as future JWST surveys. Based on data in the GOODS, Lockman Hole, and North Ecliptic Pole (NEP) fields, we find out that approximately 70% active galactic nuclei detected with X-ray and optical spectroscopy dominate the total mid-infrared emission. Finally, we estimate that AGN contribute less than 30% of the mid-infrared extragalactic integrated emission.

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arXiv2013

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 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.

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arXiv2011

A multi-wavelength analysis of Spitzer selected Coma Cluster galaxies: star formation rates and masses

We present a thorough study of the specific star formation rates for MIPS 24um selected galaxies in the Coma cluster. We build galaxy spectral energy distributions using optical (u',g',r',i',z'), Near-infrared (J,H,Ks), and Mid to Far-infrared (IRAC and MIPS) photometry. New and archival spectra confirm 210 cluster members. Subsequently, the total infrared luminosity, galaxy stellar mass, and specific star formation rate for the members are determined by measuring best fit templates. Using an array of complementary diagnostics, we search for contaminating AGN, but find few. We compare obscured star formation rates to unobscured rates derived from extinction-corrected H-alpha emission line measurements. The agreement between these two values leads us to conclude that there is no evidence for an additionally obscured component. In our spectroscopic sample, complete to 80% for r' < 19.5, we find that all starbursts are blue and are dwarfs, having masses < 10^9 solar masses. Examining the location of these starbursts within the cluster, we confirm that there is a lower fraction in the cluster core.

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arXiv2010

The first bent double lobe radio source in a known cluster filament: Constraints on the intra-filament medium

We announce the first discovery of a bent double lobe radio source (DLRS) in a known cluster filament. The bent DLRS is found at a distance of 3.4 Mpc from the center of the rich galaxy cluster, Abell~1763. We derive a bend angle alpha=25deg, and infer that the source is most likely seen at a viewing angle of Phi=10deg. From measuring the flux in the jet between the core and further lobe and assuming a spectral index of 1, we calculate the minimum pressure in the jet, (8.0+-3.2)x10^-13 dyn/cm^2, and derive constraints on the intra-filament medium (IFM) assuming the bend of the jet is due to ram pressure. We constrain the IFM to be between (1-20)x10^-29 gm/cm^3. This is consistent with recent direct probes of the IFM and theoretical models. These observations justify future searches for bent double lobe radio sources located several Mpc from cluster cores, as they may be good markers of super cluster filaments.

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arXiv2010

Spitzer observations of Abell 1763 - I: infrared and optical photometry

We present a photometric analysis of the galaxy cluster Abell 1763 at visible and infrared wavelengths. Included are fully reduced images in r', J, H, and Ks obtained using the Palomar 200in telescope, as well as the IRAC and MIPS images from Spitzer. The cluster is covered out to approximately 3 virial radii with deep 24um imaging (a 5? depth of 0.2 mJy). This same field of 40' by 40' is covered in all four IRAC bands as well as the longer wavelength MIPS bands (70 and 160um). The r' imaging covers 0.8 deg2 down to 25.5 magnitudes, and overlaps with most of the MIPS field of view. The J, H, Ks images cover the cluster core and roughly half of the filament galaxies, which extend towards the neighboring cluster, Abell 1770. This first, in a series of papers on Abell 1763, discusses the data reduction methods and source extraction techniques used for each dataset. We present catalogs of infrared (IR) sources (with 24 and/or 70um emission) and their corresponding emission in the optical (u', g', r', i', z'), and Near- to Far-IR (J, H, Ks, IRAC, and MIPS 160um). We provide the catalogs and reduced images to the community through the NASA/IPAC Infrared Science Archive (IRSA).

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arXiv2010

Spitzer observations of Abell 1763 - II: Constraining the nature of activity in the cluster-feeding filament with VLA and XMM-Newton data

The Abell 1763 superstructure at z=0.23 contains the first galaxy filament to be directly detected using mid-infrared observations. Our previous work has shown that the frequency of starbursting galaxies, as characterized by 24μm emission is much higher within the filament than at either the center of the rich galaxy cluster, or the field surrounding the system. New VLA and XMM-Newton data are presented here. We use the radio and X-ray data to examine the fraction and location of active galaxies, both active galactic nuclei (AGN) and starbursts. The radio far-infrared correlation, X-ray point source location, IRAC colors, and quasar positions are all used to gain an understanding of the presence of dominant AGN. We find very few MIPS-selected galaxies that are clearly dominated by AGN activity. Most radio selected members within the filament are starbursts. Within the supercluster, 3 of 8 spectroscopic members detected both in the radio and in the mid-infrared are radio-bright AGN. They are found at or near the core of Abell 1763. The five starbursts are located further along the filament. We calculate the physical properties of the known wide angle tail (WAT) source which is the brightest cluster galaxy (BCG) of Abell 1763. A second double lobe source is found along the filament well outside of the virial radius of either cluster. The velocity offset of the WAT from the X-ray centroid, and the bend of the WAT in the intracluster medium (ICM) are both consistent with ram pressure stripping, indicative of streaming motions along the direction of the filament. We consider this as further evidence of the cluster-feeding nature of the galaxy filament.

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arXiv2010

Infrared Luminosities and Aromatic Features in the 24um Flux Limited Sample of 5MUSES

We study a 24\,$μ$m selected sample of 330 galaxies observed with the Infrared Spectrograph for the 5\,mJy Unbiased Spitzer Extragalactic Survey. We estimate accurate total infrared luminosities by combining mid-IR spectroscopy and mid-to-far infrared photometry, and by utilizing new empirical spectral templates from {\em Spitzer} data. The infrared luminosities of this sample range mostly from 10$^9$L$_\odot$ to $10^{13.5}$L$_\odot$, with 83% in the range 10$^{10}$L$_\odot$$<$L$_{\rm IR}$$<10^{12}$L$_\odot$. The redshifts range from 0.008 to 4.27, with a median of 0.144. The equivalent widths of the 6.2\,$μ$m aromatic feature have a bimodal distribution. We use the 6.2\,$μ$m PAH EW to classify our objects as SB-dominated (44%), SB-AGN composite (22%), and AGN-dominated (34%). The high EW objects (SB-dominated) tend to have steeper mid-IR to far-IR spectral slopes and lower L$_{\rm IR}$ and redshifts. The low EW objects (AGN-dominated) tend to have less steep spectral slopes and higher L$_{\rm IR}$ and redshifts. This dichotomy leads to a gross correlation between EW and slope, which does not hold within either group. AGN dominated sources tend to have lower log(L$_{\rm PAH 7.7μm}$/L$_{\rm PAH 11.3μm}$) ratios than star-forming galaxies, possibly due to preferential destruction of the smaller aromatics by the AGN. The log(L$_{\rm PAH 7.7μm}$/L$_{\rm PAH 11.3μm}$) ratios for star-forming galaxies are lower in our sample than the ratios measured from the nuclear spectra of nearby normal galaxies, most probably indicating a difference in the ionization state or grain size distribution between the nuclear regions and the entire galaxy. Finally, we provide a calibration relating the monochromatic 5.8, 8, 14 and 24um continuum or Aromatic Feature luminosity to L$_{\rm IR}$ for different types of objects.

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arXiv2010

Mid-infrared spectroscopy of Spitzer-selected ultra-luminous starbursts at z~2

We want to study the mid-infrared properties and the starburst and AGN contributions, of 24um sources at z~2, through analysis of mid-infrared spectra combined with millimeter, radio, and infrared photometry. Mid-infrared spectroscopy allows us to recover accurate redshifts. A complete sample of 16 Spitzer-selected sources (ULIRGs) believed to be starbursts at z~2 ("5.8um-peakers") was selected in the (0.5 sq.deg.) J1064+56 SWIRE Lockman Hole field. These sources have S(24um)>0.5mJy, a stellar emission peak redshifted to 5.8um, and r'(Vega)>23. The entire sample was observed with the low resolution units of the Spitzer/IRS infrared spectrograph. These sources have 1.2mm observations with IRAM 30m/MAMBO and very deep 20cm observations from the VLA. Nine of our sources also benefit from 350um observation and detection from CSO/SHARC-II. The entire sample shows good quality IRS spectra dominated by strong PAH features. The main PAH features at 6.2, 7.7, 8.6, and 11.3um have high S/N average luminosities of 2.90, 10.38, 3.62, and 2.29x10^{10}Lsun, respectively. We derived accurate redshifts spanning from 1.75 to 2.28. The average of these redshifts is 2.017. This result confirms that the selection criteria of "5.8um-peakers" associated with a strong detection at 24um are reliable to select sources at z~2. We have analyzed the different correlations between PAH emission and infrared, millimeter, and radio emission. Practically all our sources are strongly dominated by starburst emission. We have also defined two subsamples based on the equivalent width at 7.7um to investigate AGN contributions. Our sample contains strong starbursts and represents a particularly 24um-bright class of SMGs. The very good correlation between PAH and far-IR luminosities is now confirmed in high-z starburst ULIRGs. These sources show a small AGN contribution to the mid-IR, around ~20% in most cases.

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arXiv2009

The ~0.9 mJy sample: A mid-infrared spectroscopic catalog of 150 infrared-luminous, 24 micron selected galaxies at 0.3<z<3.5

We present a catalog of mid-infrared (MIR) spectra of 150 infrared (IR) luminous galaxies in the Spitzer extragalactic first look survey obtained with IRS on board Spitzer. The sample is selected to be brighter than ~0.9 mJy at 24 micron and it has a z distribution in the range [0.3,3.5] with a peak at z=1. It primarily comprises ultraluminous IR galaxies at z>1 and luminous IR galaxies at z<1, as estimated from their monochromatic 14 micron luminosities. The number of sources with spectra that are dominated by an active galactic nucleus (AGN) continuum is 49, while 39 sources have strong, star-formation related features. For this classification, we used the equivalent width (EW) of the 11.3 micron polycyclic aromatic hydrocarbon (PAH) feature. Several intermediate/high z starbursts have higher PAH EW than local ULIRGs. An increase in the AGN activity is observed with increasing z and luminosity, based on the decreasing EW of PAHs and the increasing [NeIII]/[NeII] ratio. Spectral stacking leads to the detection of the 3.3 micron PAH, the H2 0-0 S(1) and S(3) lines, and the [NeV] line. We observe differences in the flux ratios of PAHs in the stacked spectra of IR-luminous galaxies with z or luminosity, which are not due to extinction effects. When placing the observed galaxies on IR color-color diagrams, we find that the wedge defining AGN comprises most sources with continuum-dominated spectra, but also contains many starbursts. The comparison of the 11.3 micron PAH EW and the H-band effective radius, measured from HST data, indicates that sources with EW>2 micron, are typically more extended than ~3 kpc. However, there is no strong correlation between the MIR spectral type and the near-IR extent of the sources. [Abridged].

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arXiv2007

Starburst galaxies in cluster-feeding filaments unveiled by Spitzer

We report the first direct detection with Spitzer of galaxy filaments. Using Spitzer and ancillary optical data, we have discovered two filamentary structures in the outskirts of the cluster Abell 1763. Both filaments point toward Abell 1770 which lies at the same redshift as Abell 1763 (z=0.23), at a projected distance of ~13 Mpc. The X-ray cluster emission is elongated along the same direction. Most of the far-infrared emission is powered by star formation. According to the optical spectra, only one of the cluster members is classified as an active galactic nucleus. Star formation is clearly enhanced in galaxies along the filaments: the fraction of starburst galaxies in the filaments is more than twice than that in other cluster regions. We speculate that these filaments are feeding the cluster Abell 1763 by the infall of galaxies and galaxy groups. Evidence for one of these groups is provided by the analysis of galaxy kinematics in the central cluster region.

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arXiv2007

Deep u*- and g-band Imaging of the Spitzer Space Telescope First Look Survey Field : Observations and Source Catalogs

We present deep u*-, and g-band images taken with the MegaCam on the 3.6 m Canada-France-Hawaii Telescope (CFHT) to support the extragalactic component of the Spitzer First Look Survey (hereafter, FLS). In this paper we outline the observations, present source catalogs and characterize the completeness, reliability, astrometric accuracy and number counts of this dataset. In the central 1 deg2 region of the FLS, we reach depths of g~26.5 mag, and u*~26.2 mag (AB magnitude, 5$σ$ detection over a 3" aperture) with ~4 hours of exposure time for each filter. For the entire FLS region (~5 deg2 coverage), we obtained u*-band images to the shallower depth of u*=25.0--25.4 mag (5$σ$, 3" aperture). The average seeing of the observations is 0.85" for the central field, and ~1.00" for the other fields. Astrometric calibration of the fields yields an absolute astrometric accuracy of 0.15" when matched with the SDSS point sources between 18<g<22. Source catalogs have been created using SExtractor. The catalogs are 50% complete and greater than 99.3% reliable down to g~26.5 mag and u*~26.2 mag for the central 1 deg2 field. In the shallower u*-band images, the catalogs are 50% complete and 98.2% reliable down to 24.8--25.4 mag. These images and source catalogs will serve as a useful resource for studying the galaxy evolution using the FLS data.

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arXiv2007

Absolute Calibration and Characterization of the Multiband Imaging Photometer for Spitzer. II. 70 micron Imaging

The absolute calibration and characterization of the Multiband Imaging Photometer for Spitzer (MIPS) 70 micron coarse- and fine-scale imaging modes are presented based on over 2.5 years of observations. Accurate photometry (especially for faint sources) requires two simple processing steps beyond the standard data reduction to remove long-term detector transients. Point spread function (PSF) fitting photometry is found to give more accurate flux densities than aperture photometry. Based on the PSF fitting photometry, the calibration factor shows no strong trend with flux density, background, spectral type, exposure time, or time since anneals. The coarse-scale calibration sample includes observations of stars with flux densities from 22 mJy to 17 Jy, on backgrounds from 4 to 26 MJy sr^-1, and with spectral types from B to M. The coarse-scale calibration is 702 +/- 35 MJy sr^-1 MIPS70^-1 (5% uncertainty) and is based on measurements of 66 stars. The instrumental units of the MIPS 70 micron coarse- and fine-scale imaging modes are called MIPS70 and MIPS70F, respectively. The photometric repeatability is calculated to be 4.5% from two stars measured during every MIPS campaign and includes variations on all time scales probed. The preliminary fine-scale calibration factor is 2894 +/- 294 MJy sr^-1 MIPS70F^-1 (10% uncertainty) based on 10 stars. The uncertainty in the coarse- and fine-scale calibration factors are dominated by the 4.5% photometric repeatability and the small sample size, respectively. The 5-sigma, 500 s sensitivity of the coarse-scale observations is 6-8 mJy. This work shows that the MIPS 70 micron array produces accurate, well calibrated photometry and validates the MIPS 70 micron operating strategy, especially the use of frequent stimulator flashes to track the changing responsivities of the Ge:Ga detectors.

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arXiv2007

Spitzer Mid-Infrared Spectroscopy of Infrared Luminous Galaxies at z~2 II: Diagnostics

We present mid-IR spectral decomposition of a sample of 48 Spitzer-selected ULIRGs spanning z~1-3 and likely L_IR~10^12-10^13Lsun. Our study aims at quantifying the star-formation and AGN processes in these sources which recent results suggest have evolved strongly between the observed epoch and today. To do this, we study the mid-IR contribution of PAH emission, continuum, and extinction. About 3/4 of our sample are continuum- (i.e. AGN) dominated sources, but ~60% of these show PAH emission, suggesting the presence of star-formation activity. These sources have redder mid-IR colors than typical optically-selected quasars. About 25% of our sample have strong PAH emission, but none are likely to be pure starbursts as reflected in their relatively high 5um hot dust continua. However, their steep 30um-to-14um slopes suggest that star-formation might dominate the total infrared luminosity. Six of our z~2 sources have EW6.2>~0.3um and L_14um>~10^12Lsun (implying L_IR>~10^13Lsun). At these luminosities, such high EW6.2 ULIRGs do not exist in the local Universe. We find a median optical depth at 9.7um of =1.4. This is consistent with local IRAS-selected ULIRGs, but differs from early results on SCUBA-selected z~2 ULIRGs. Similar to local ULIRGs about 25% of our sample show extreme obscuration (tau_9.7>~3) suggesting buried nuclei. In general, we find that our sources are similar to local ULIRGs, but are an order of magnitude more luminous. It is not clear whether our z~2 ULIRGs are simply scaled-up versions of local ULIRGs, or subject to fundamentally different physical processes.

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arXiv2006

Spitzer Mid-Infrared Spectroscopy of Infrared Luminous Galaxies at z~2 I: the Spectra

We present the mid-infrared (MIR) spectra obtained with the Spitzer InfraRed Spectrograph (IRS) for a sample of 52 sources, selected as infrared luminous, z>1 candidates in the Extragalactic First Look Survey (XFLS). The sample selection criteria are f(24um) > 0.9mJy, nu fnu(24um)/nu fnu(8um) > 3.16 and nu fnu(24um)/nu fnu(0.7um) > 10. Of the 52 spectra, 47 (90%) produced measurable redshifts based solely on the mid-IR spectral features, with the majority (35/47=74%) at 1.5 1. Their $L_{1600Å}$ and $L_{\rm IR}$ suggest that our sample is among the most luminous and most dust enshrouded systems of its epoch . Our study has revealed a significant population of dust enshrouded galaxies at z~2, whose enormous energy output, comparable to that of quasars, is generated by AGN as well as starburst. This IR luminous population has very little overlap with sub-mm and UV-selected populations (Abridged).

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