arXiv subjects

P. G. Perez-Gonzalez

Publications and source records attributed to P. G. Perez-Gonzalez.

At least 37 records · Page 2Linked to original sources

arXiv2011

Uv-to-fir analysis of spitzer/irac sources in the extended groth strip i: Multi-wavelength photometry and spectral energy distributions

We present an IRAC 3.6+4.5 microns selected catalog in the Extended Groth Strip (EGS) containing photometry from the ultraviolet to the far-infrared and stellar parameters derived from the analysis of the multi-wavelength data. In this paper, we describe the method used to build coherent spectral energy distributions (SEDs) for all the sources. In a companion paper, we analyze those SEDs to obtain robust estimations of stellar parameters such as photometric redshifts, stellar masses, and star formation rates. The catalog comprises 76,936 sources with [3.6]<23.75 mag (85% completeness level of the IRAC survey in the EGS) over 0.48 square degrees. For approximately 16% of this sample, we are able to deconvolve the IRAC data to obtain robust fluxes for the multiple counterparts found in ground-based optical images. Typically, the SEDs of the IRAC sources in our catalog count with more than 15 photometric data points, spanning from the UV to the FIR. Approximately 95% and 90% of all IRAC sources are detected in the deepest optical and near-infrared bands. Only 10% of the sources have optical spectroscopy and redshift estimations. Almost 20% and 2% of the sources are detected by MIPS at 24 and 70 microns, respectively. We also cross-correlate our catalog with public X-ray and radio catalogs. Finally, we present the Rainbow Navigator public web-interface utility designed to browse all the data products resulting from this work, including images, spectra, photometry, and stellar parameters.

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arXiv2010

An X-ray study of the SNR G344.7-0.1 and the central object CXOU J170357.8-414302

Aims. We report results of an X-ray study of the supernova remnant (SNR) G344.7-0.1 and the point-like X-ray source located at the geometrical center of the SNR radio structure. Methods. The morphology and spectral properties of the remnant and the central X-ray point-like source were studied using data from the XMM-Newton and Chandra satellites. Archival radio data and infrared Spitzer observations at 8 and 24 $μ$m were used to compare and study its multi-band properties at different wavelengths. Results. The XMM-Newton and Chandra observations reveal that the overall X-ray emission of G344.7-0.1 is extended and correlates very well with regions of bright radio and infrared emission. The X-ray spectrum is dominated by prominent atomic emission lines. These characteristics suggest that the X-ray emission originated in a thin thermal plasma, whose radiation is represented well by a plane-parallel shock plasma model (PSHOCK). Our study favors the scenario in which G344.7-0.1 is a 6 x 10^3 year old SNR expanding in a medium with a high density gradient and is most likely encountering a molecular cloud on the western side. In addition, we report the discovery of a soft point-like X-ray source located at the geometrical center of the radio SNR structure. The object presents some characteristics of the so-called compact central objects (CCO). However, its neutral hydrogen absorption column (N_{H}) is inconsistent with that of the SNR. Coincident with the position of the source, we found infrared and optical objects with typical early-K star characteristics. The X-ray source may be a foreground star or the CCO associated with the SNR. If this latter possibility were confirmed, the point-like source would be the farthest CCO detected so far and the eighth member of the new population of isolated and weakly magnetized neutron stars.

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arXiv2010

The AGN, Star-Forming, and Morphological Properties of Luminous IR-Bright/Optically-Faint Galaxies

We present the AGN, star-forming, and morphological properties of a sample of 13 MIR-luminous (f(24) > 700uJy) IR-bright/optically-faint galaxies (IRBGs, f(24)/f(R) > 1000). While these z~2 sources were drawn from deep Chandra fields with >200 ks X-ray coverage, only 7 are formally detected in the X-ray and four lack X-ray emission at even the 2 sigma level. Spitzer IRS spectra, however, confirm that all of the sources are AGN-dominated in the mid-IR, although half have detectable PAH emission responsible for ~25% of their mid-infrared flux density. When combined with other samples, this indicates that at least 30-40% of luminous IRBGs have star-formation rates in the ULIRG range (~100-2000 Msun/yr). X-ray hardness ratios and MIR to X-ray luminosity ratios indicate that all members of the sample contain heavily X-ray obscured AGN, 80% of which are candidates to be Compton-thick. Furthermore, the mean X-ray luminosity of the sample, log L(2-10 keV)(ergs/s)=44.6, indicates that these IRBGs are Type 2 QSOs, at least from the X-ray perspective. While those sources most heavily obscured in the X-ray are also those most likely to display strong silicate absorption in the mid-IR, silicate absorption does not always accompany X-ray obscuration. Finally, ~70% of the IRBGs are merger candidates, a rate consistent with that of sub-mm galaxies (SMGs), although SMGs appear to be physically larger than IRBGs. These characteristics are consistent with the proposal that these objects represent a later, AGN-dominated, and more relaxed evolutionary stage following soon after the star-formation-dominated one represented by the SMGs.

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arXiv2010

Infrared Excess Sources: Compton Thick QSOs, low luminosity Seyferts or starbursts?

We explore the nature of Infrared Excess sources (IRX), which are proposed as candidates for luminous L_X(2-10keV)>1e43erg/s Compton Thick (N_H>2e24cm^{-2}$) QSOs at z~2. Lower redshift, z~1, analogues of the distant IRX population are identified by firstly redshifting to z=2 the SEDs of all sources with secure spectroscopic redshifts in the AEGIS (6488) and the GOODS-North (1784) surveys and then selecting those that qualify as IRX sources at that redshift. A total of 19 galaxies are selected. The mean redshift of the sample is $z\approx1$. We do not find strong evidence for Compton Thick QSOs in the sample. For 9 sources with X-ray counterparts, the X-ray spectra are consistent with Compton Thin AGN. Only 3 of them show tentative evidence for Compton Thick obscuration. The SEDs of the X-ray undetected population are consistent with starburst activity. There is no evidence for a hot dust component at the mid-infrared associated with AGN heated dust. If the X-ray undetected sources host AGN, an upper limit of L_X(2-10keV) =1e43erg/s is estimated for their intrinsic luminosity. We propose that a large fraction of the $z\approx2$ IRX population are not Compton Thick QSOs but low luminosity [L_X(2-10keV)<1e43erg/s], possibly Compton Thin, AGN or dusty starbursts. It is shown that the decomposition of the AGN and starburst contribution to the mid-IR is essential for interpreting the nature of this population, as star-formation may dominate this wavelength regime.

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arXiv2008

Determining Star Formation Rates for Infrared Galaxies

We show that measures of star formation rates (SFRs) for infrared galaxies using either single-band 24 um or extinction-corrected Paschen-alpha luminosities are consistent in the total infrared luminosity = L(TIR) ~ 10^10 L_sun range. MIPS 24 micron photometry can yield star formation rates accurately from this luminosity upward: SFR(M_sun/yr) = 7.8 x 10^-10 L(24 um, L_sun) from L(TIR) = 5 x 10^9 L_sun to 10^11 L_sun, and SFR = 7.8 x 10^-10 L(24 um, L_sun) x (7.76 x 10^-11 L(24))^0.048 for higher L(TIR). For galaxies with L(TIR) >= 10^10 L_sun, these new expressions should provide SFRs to within 0.2 dex. For L(TIR) >= 10^11 L_sun, we find that the SFR of infrared galaxies is significantly underestimated using extinction-corrected Pa-alpha (and presumably using any other optical or near infrared recombination lines). As a part of this work, we constructed spectral energy distribution (SED) templates for eleven luminous and ultraluminous purely star forming infrared galaxies (LIRGs and ULIRGs) and over the spectral range 0.4 microns to 30 cm. We use these templates and the SINGS data to construct average templates from 5 microns to 30 cm for infrared galaxies with L(TIR) = 5 x 10^9 to 10^13 L_sun. All of these templates are made available on line.

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arXiv2008

Low-resolution spectroscopy and spectral energy distributions of selected sources towards sigma Orionis

Aims: We investigated in detail nine sources in the direction of the young sigma Orionis cluster, which is considered a unique site for studying stellar and substellar formation. The nine sources were selected because of some peculiar properties, such as extremely red infrared colours or too strong Halpha emission for their blue optical colours. Methods: We took high-quality, low-resolution spectroscopy (R ~ 500) of the nine targets with ALFOSC at the Nordic Optical Telescope. We also re-analyzed [24]-band photometry from MIPS/Spitzer and compiled the best photometry available at the ViJHKs passbands and the four IRAC/Spitzer channels for constructing accurate spectral energy distributions covering from 0.55 to 24 mum. Results: The nine targets were classified into: one Herbig Ae/Be star with a scatterer edge-on disc, two G-type stars, one X-ray flaring, early-M, young star with chromospheric Halpha emission, one very low-mass, accreting, young spectroscopic binary, two young objects at the brown dwarf boundary with the characteristics of classical T Tauri stars, and two emission-line galaxies, one undergoing star formation, and another one whose spectral energy distribution is dominated by an active galactic nucleus. Besides, we discover three infrared sources associated to overdensities in a cold cloud in the cluster centre. Conclusions: Low-resolution spectroscopy and spectral energy distributions are a vital tool for measuring the physical properties and the evolution of young stars and candidates in the sigma Orionis cluster.

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arXiv2008

Spitzer's Contribution to the AGN Population

(abridged) Infrared selection is a potentially powerful way to identify heavily obscured AGN missed in even the deepest X-ray surveys. Using a 24 micron-selected sample in GOODS-S, we test the reliability and completeness of three infrared AGN selection methods: (1) IRAC color-color selection, (2) IRAC power-law selection, and (3) IR-excess selection; we also evaluate a number of infrared excess approaches. We find that the vast majority of non-power-law IRAC color-selected AGN candidates in GOODS-S have colors consistent with those of star-forming galaxies. Contamination by star-forming galaxies is most prevalent at low 24 micron flux densities (~100 uJy) and high redshifts (z~2), but the fraction of potential contaminants is still high (~50%) at 500 uJy, the highest flux density probed reliably by our survey. AGN candidates selected via a simple, physically-motivated power-law criterion (PLGs), however, appear to be reliable. We confirm that the infrared excess methods successfully identify a number of AGN, but we also find that such samples may be significantly contaminated by star-forming galaxies. Adding only the secure Spitzer-selected PLG, color-selected, IR-excess, and radio/IR-selected AGN candidates to the deepest X-ray-selected AGN samples directly increases the number of known X-ray AGN (84) by 54-77%, and implies an increase to the number of 24 micron-detected AGN of 71-94%. Finally, we show that the fraction of MIR sources dominated by an AGN decreases with decreasing MIR flux density, but only down to f_24 = 300 uJy. Below this limit, the AGN fraction levels out, indicating that a non-negligible fraction (~10%) of faint 24 micron sources (the majority of which are missed in the X-ray) are powered not by star formation, but by the central engine.

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arXiv2008

Extended UV (XUV) Emission in Nearby Galaxy Disks

We summarize the main properties of the extended UV (XUV) emission found in roughly 30% of the nearby spiral galaxies observed by the GALEX satellite. Two different classes of XUV disks are identified, the Type 1 XUV disks where significant, structured UV-bright features are found beyond the "classical" azimuthally-averaged star-formation threshold, and the Type 2 XUV disks, which are characterized by very extended (seven times the area where most of the stellar mass is found), blue [(FUV-K)<5mag] outer disks. These latter disks are extreme examples of galaxies growing inside-out. The few XUV disks studied in detail to date are rich in HI but relatively poor in molecular gas, have stellar populations with luminosity-weighted ages of ~1 Gyr, and ionized-gas metal abundances of ~Zsun/10. As part of the CAHA-XUV project we are in the process of obtaining deep multi-wavelength imaging and spectroscopy of 65 XUV-disk galaxies so to determine whether or not these properties are common among XUV disks.

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arXiv2007

The Host Galaxies and Black Holes of Typical z~0.5-1.4 AGN

We study the stellar and star formation (SF) properties of the host galaxies of 58 X-ray selected AGN in the GOODS portion of the Chandra Deep Field South (CDF-S) region at z~0.5-1.4. The AGN are selected such that their rest-frame UV to near-IR SEDs are dominated by stellar emission, i.e., they show a prominent 1.6micron bump, thus minimizing the AGN emission 'contamination'. This AGN population comprises approximately 50% of the X-ray selected AGN at these redshifts. Using models of stellar and dust emission we model their SEDs to derive stellar masses (M*) and total (UV+IR) star formation rates (SFR). AGN reside in the most massive galaxies at the redshifts probed here. Their characteristic stellar masses (M*~7.8x10^10 Msun and M*~1.2x10^11 Msun at median z of 0.67 and 1.07, respectively) appear to be representative of the X-ray selected AGN population at these redshifts, and are intermediate between those of local type 2 AGN and high redshift (z~2) AGN. The inferred black hole masses (MBH~ 2x10^8 Msun) of typical AGN are similar to those of optically identified quasars at similar redshifts. Since the AGN in our sample are much less luminous (L(2-10keV)<10^44 erg/s) than quasars, typical AGN have low Eddington ratios (eta~0.01-0.001). This suggests that, at least at intermediate z, the cosmic AGN 'downsizing' is due to both a decrease in the characteristic stellar mass of typical host galaxies, and less efficient accretion. Finally there is no strong evidence in AGN host galaxies for either highly suppressed SF (expected if AGN played a role in quenching SF) or elevated SF when compared to mass selected (i.e., IRAC-selected) galaxies of similar stellar masses and redshifts. This may be explained by the fact that galaxies with M*~5x 10^10 - 5x10^11 Msun are still being assembled at the redshifts probed here.

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arXiv2007

Mid-Infrared Spectroscopy of Lensed Galaxies at 1<z<3: The Nature of Sources Near the MIPS Confusion Limit

We present Spitzer/IRS mid-infrared spectra for 15 gravitationally lensed, 24 micron--selected galaxies, and combine the results with 4 additional very faint galaxies with IRS spectra in the literature. The median intrinsic 24 micron flux density of the sample is 130 microJy, enabling a systematic survey of the spectral properties of the very faint 24 micron sources that dominate the number counts of Spitzer cosmological surveys. Six of the 19 galaxy spectra (32%) show the strong mid-IR continuua expected of AGN; X-ray detections confirm the presence of AGN in three of these cases, and reveal AGNs in two other galaxies. These results suggest that nuclear accretion may contribute more flux to faint 24 micron--selected samples than previously assumed. Almost all the spectra show some aromatic (PAH) emission features; the measured aromatic flux ratios do not show evolution from z=0. In particular, the high S/N mid-IR spectrum of SMM J163554.2+661225 agrees remarkably well with low--redshift, lower--luminosity templates. We compare the rest-frame 8 micron and total infrared luminosities of star--forming galaxies, and find that the behavior of this ratio with total IR luminosity has evolved modestly from z=2 to z=0. Since the high aromatic--to--continuum flux ratios in these galaxies rule out a dominant contribution by AGN, this finding implies systematic evolution in the structure and/or metallicity of infrared sources with redshift. It also has implications for the estimates of star forming rates inferred from 24 micron measurements, in the sense that at z ~2, a given observed frame 24 micron luminosity corresponds to a lower bolometric luminosity than would be inferred from low-redshift templates of similar luminosity at the corresponding rest wavelength.

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arXiv2007

On the metallicity dependence of the 24micron luminosity as a star formation tracer

We investigate the use of the rest-frame 24microns luminosity as an indicator of the star formation rate (SFR) in galaxies with different metallicities by comparing it to the (extinction corrected) Halpha luminosity. We carry out this analysis in 2 steps: First, we compare the emission from HII regions in different galaxies with metallicities between 12+log(O/H) = 8.1 and 8.9. We find that the 24microns and the extinction corrected Halpha luminosities from individual HII regions follow the same correlation for all galaxies, independent of their metallicity. Second, the role of metallicity is explored further for the integrated luminosity in a sample of galaxies with metallicities in the range of 12+log(O/H) = 7.2 - 9.1. For this sample we compare the 24microns and Halpha luminosities integrated over the entire galaxies and find a lack of the 24microns emission for a given Halpha luminosity for low metallicity objects, likely reflecting a low dust content. These results suggest that the 24microns luminosity is a good metallicity independent tracer for the SFR in individual HII regions. On the other hand, metallicity has to be taken into account when using the 24microns luminosity as a tracer for the SFR of entire galaxies.

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arXiv2007

The SCUBA HAlf Degree Extragalactic Survey (SHADES) - III. Identification of radio and mid-infrared counterparts to submillimetre galaxies

Determining an accurate position for a submm galaxy (SMG) is the crucial step that enables us to move from the basic properties of an SMG sample - source counts and 2-D clustering - to an assessment of their detailed, multi-wavelength properties, their contribution to the history of cosmic star formation and their links with present-day galaxy populations. In this paper, we identify robust radio and/or IR counterparts, and hence accurate positions, for over two thirds of the SCUBA HAlf-Degree Extragalactic Survey (SHADES) Source Catalogue, presenting optical, 24-um and radio images of each SMG. Observed trends in identification rate have given no strong rationale for pruning the sample. Uncertainties in submm position are found to be consistent with theoretical expectations, with no evidence for significant additional sources of error. Employing the submm/radio redshift indicator, via a parameterisation appropriate for radio-identified SMGs with spectroscopic redshifts, yields a median redshift of 2.8 for the radio-identified subset of SHADES, somewhat higher than the median spectroscopic redshift. We present a diagnostic colour-colour plot, exploiting Spitzer photometry, in which we identify regions commensurate with SMGs at very high redshift. Finally, we find that significantly more SMGs have multiple robust counterparts than would be expected by chance, indicative of physical associations. These multiple systems are most common amongst the brightest SMGs and are typically separated by 2-6", or 15-50/(sin i) kpc at z ~ 2, consistent with early bursts seen in merger simulations.

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arXiv2007

Spitzer Power-law AGN Candidates in the Chandra Deep Field-North

We define a sample of 62 galaxies in the Chandra Deep Field-North whose Spitzer IRAC SEDs exhibit the characteristic power-law emission expected of luminous AGN. We study the multiwavelength properties of this sample, and compare the AGN selected in this way to those selected via other Spitzer color-color criteria. Only 55% of the power-law galaxies are detected in the X-ray catalog at exposures of >0.5 Ms, although a search for faint emission results in the detection of 85% of the power-law galaxies at the > 2.5 sigma detection level. Most of the remaining galaxies are likely to host AGN that are heavily obscured in the X-ray. Because the power-law selection requires the AGN to be energetically dominant in the near- and mid-infrared, the power-law galaxies comprise a significant fraction of the Spitzer-detected AGN population at high luminosities and redshifts. The high 24 micron detection fraction also points to a luminous population. The power-law galaxies comprise a subset of color-selected AGN candidates. A comparison with various mid-infrared color selection criteria demonstrates that while the color-selected samples contain a larger fraction of the X-ray luminous AGN, there is evidence that these selection techniques also suffer from a higher degree of contamination by star-forming galaxies in the deepest exposures. Considering only those power-law galaxies detected in the X-ray catalog, we derive an obscured fraction of 68% (2:1). Including all of the power-law galaxies suggests an obscured fraction of < 81% (4:1).

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arXiv2006

Dusty waves on a starry sea: the mid-infrared view of M31

Mid-infrared observations of the Andromeda galaxy, M31, obtained with the Infrared Array Camera on board the Spitzer Space Telescope, are presented. The image mosaics cover areas of approximate 3.7deg x 1.6deg and include the satellite galaxies M32 and NGC 205. The appearance of M31 varies dramatically in the different mid-infrared bands, from the smooth bulge and disk of the old stellar population seen at 3.6um to the well-known '10 kpc ring' dominating the 8um image. The similarity of the 3.6um and optical isophotes and nearly constant optical-mid-infrared color over the inner 400 arcsec confirms that there is no significant extinction at optical wavelengths in M31's bulge. The nuclear colors indicate the presence of dust but not an infrared-bright nucleus. The integrated 8um non-stellar luminosity implies a star formation rate of 0.4 Msun/yr, consistent with other indicators that show M31 to be a quiescent galaxy.

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arXiv2006

Detecting Obscured AGN in the Distant Universe with Spitzer

We present the results of a Spitzer search for obscured AGN in the Chandra Deep Field-North, using both radio-excess and mid-infrared power-law selection. AGN selected via the former technique tend to lie at z ~ 1, have SEDs dominated by the 1.6 micron stellar bump, and have Seyfert-like X-ray luminosities (when detected in the X-ray). In contrast, the IRAC (3.6-8.0 micron) power-law selected AGN lie at higher redshifts of z ~ 2, and comprise a significant fraction of the most X-ray luminous AGN in the CDF-N. While there is almost no overlap in the AGN samples selected via these two methods, their X-ray detection fractions are very similar. Only 40% and 55% of the radio-excess and power-law samples are detected in the 2 Ms X-ray catalog, respectively. The majority of the AGN selected via both methods are consistent with being obscured (N_H > 10^(22) cm^-2), but not Compton-thick (N_H > 10^(24) cm^-2), although Compton-thick candidates exist in both samples. We place an upper limit of <82% (or < 4:1) on the obscured fraction of the power-law sample, consistent with predictions from the cosmic X-ray background. The sources selected via the power-law criteria comprise a subset of AGN selected via other IRAC color-color cuts. While smaller in number than the color-selected samples in the deep fields, the power-law sample suffers from less contamination by star-forming galaxies.

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arXiv2006

Near-Infrared and Star-forming properties of Local Luminous Infrared Galaxies

We use HST NICMOS continuum and Pa-alpha observations to study the near-infrared and star-formation properties of a representative sample of 30 local (d ~ 35-75Mpc) luminous infrared galaxies (LIRGs, infrared 8-1000um luminosities of L_IR=11-11.9[Lsun]). The data provide spatial resolutions of 25-50pc and cover the central ~3.3-7.1kpc regions of these galaxies. About half of the LIRGs show compact (~1-2kpc) Pa-alpha emission with a high surface brightness in the form of nuclear emission, rings, and mini-spirals. The rest of the sample show Pa-alpha emission along the disk and the spiral arms extending over scales of 3-7kpc and larger. About half of the sample contains HII regions with H-alpha luminosities significantly higher than those observed in normal galaxies. There is a linear empirical relationship between the mid-IR 24um and hydrogen recombination (extinction-corrected Pa-alpha) luminosity for these LIRGs, and the HII regions in the central part of M51. This relation holds over more than four decades in luminosity suggesting that the mid-IR emission is a good tracer of the star formation rate (SFR). Analogous to the widely used relation between the SFR and total IR luminosity of Kennicutt (1998), we derive an empirical calibration of the SFR in terms of the monochromatic 24um luminosity that can be used for luminous, dusty galaxies.

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arXiv2006

The role of the LIRG and ULIRG phases in the evolution of Ks-selected galaxies

We investigate the role of the luminous infrared galaxy (LIRG) and ultra-luminous infrared galaxy (ULIRG) phases in the evolution of Ks-selected galaxies and, in particular, Extremely Red Galaxies (ERGs). With this aim, we compare the properties of a sample of 2905 Ks<21.5 (Vega mag) galaxies in the GOODS/CDFS with the sub-sample of those 696 sources which are detected at 24 microns. We find that LIRGs constitute 30% of the galaxies with stellar mass M>1x10^{11} Msun assembled at redshift z=0.5. A minimum of 65% of the galaxies with M>2.5x10^{11} Msun at z~2-3 are ULIRGs at those redshifts. 60% of the ULIRGs in our sample have the characteristic colours of ERGs. Conversely, 40% of the ERGs with stellar mass M>1.3x10^{11} Msun at 1.5 1x10^{11} Msun) galaxies present in the Universe at a given time, this fraction becomes very significant (>50%) at redshifts z>~2.

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arXiv2006

Spitzer Massive Lensing Cluster Survey

We are currently undertaking a Spitzer GTO program to image ~30 massive lensing clusters at moderate redshift with both IRAC and MIPS. By taking advantage of the gravitatinoal lensing power of these clusters, we will study the population of faint galaxies that are below the nominal Spitzer detection limits. Here, we present a few examples of our science programs.

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