arXiv subjects

P. G. Perez-Gonzalez

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

At least 55 records · Page 3Linked to original sources

arXiv2006

Spitzer Observations of the Brightest Galaxies in X-ray-Luminous Clusters

We have studied the infrared properties of the brightest cluster galaxies (BCGs) located in the cores of X-ray-luminous clusters at 0.15 < z < 0.35. The majority of the BCGs are not particularly infrared-luminous compared with other massive early-type galaxies, suggesting that the cluster environment has little influence on the infrared luminosities of the BCGs. The exceptions, however, are the BCGs in the three X-ray-brightest clusters in the sample, A1835, Z3146, and A2390. These BCGs have a prominent far-infrared peak in their spectral energy distributions (SEDs), and two of them (those in A1835 and Z3146) can be classified as luminous infrared galaxies (LIRGs: L_{IR} > 10^{11} L_{sun}). Although radio AGNs are found to be prevalent among the BCGs, the infrared luminosities of these three BCGs, judged from the infrared SED signatures, are likely to be powered by star formation. Considering the overall trend that clusters with shorter radiative gas cooling times harbor more infrared-luminous BCGs, the enhanced star formation may be caused by the cooling cluster gas accreting onto the BCGs.

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arXiv2006

Morphology of Spitzer 24um-Detected Galaxies in the UDF: the Links between the Star Formation and Galaxy Morphology

We have studied the morphologies of luminous infrared galaxies (LIRGs; L_IR(8-1000um)>10^11 Lsun) at 0.3< z<1.4 in the HST Ultradeep Field (UDF) by calculating concentration and asymmetry indices and comparing the results with similar calculations for: (1) galaxies at similar redshift that are less infrared-active; and (2) local LIRGs. We find that the high-redshift samples are dominated by galaxies with concentrations similar to local late-type disk galaxies; however, they are significantly more asymmetric than most local galaxies but are similar in both regards to local LIRGs. On average, the high-redshift infrared-active galaxies are slightly more asymmetric than the less-active ones, although they do include a significantly higher portion of highly asymmetric (merging?) systems and a lower portion of more concentrated, symmetric ones. The morphological similarity of infrared-active and typical infrared-inactive galaxies at high-redshift suggests that they may be from the same parent population, but are in different stages of an episodic star formation process. The similarity between high-redshift and local LIRGs suggests that a certain level of asymmetry is generally associated with LIRG-level activity.

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arXiv2006

Ultraviolet-to-Far Infrared Properties of Lyman Break Galaxies and Luminous Infrared Galaxies at z ~ 1

We present the first large, unbiased sample of Lyman Break Galaxies (LBGs) at z ~ 1. Far ultraviolet-dropout (1530 A) galaxies in the Chandra Deep Field South have been selected using GALEX data. This first large sample in the z ~ 1 universe provides us with a high quality reference sample of LBGs. We analyzed the sample from the UV to the IR using GALEX, SPITZER, ESO and HST data. The morphology (obtained from GOODS data) of 75 % of our LBGs is consistent with a disk. The vast majority of LBGs with an IR detection are also Luminous Infrared Galaxies (LIRGs). As a class, the galaxies not detected at 24 microns are an order of magnitude fainter relative to the UV compared with those detected individually, suggesting that there may be two types of behavior within the sample. For the IR-bright galaxies, there is an apparent upper limit for the UV dust attenuation and this upper limit is anti-correlated with the observed UV luminosity. Previous estimates of dust attenuations based on the ultraviolet slope are compared to new ones based on the FIR/UV ratio (for LBGs detected at 24 microns), which is usually a more reliable estimator. Depending on the calibration we use to estimate the total IR luminosity, beta-based attenuations A_{FUV} are larger by 0.2 to 0.6 mag. than the ones estimated from FIR/UV ratio. Finally, for IR-bright LBGs, median estimated beta-based SFRs are 2-3 times larger than the total SFRs estimated as SFR_{TOT} = SFR_{UV} + SFR_{IR} while IR-based SFRs provide values below SFR_{TOT} by 15 - 20 %. We use a stacking method to statistically constrain the 24 microns flux of LBGs non individually detected. The results suggest that these LBGs do not contain large amounts of dust.

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arXiv2006

Mid-infrared properties of X-ray sources in the Extended Groth Strip

Mid-infrared observations of Active Galactic Nuclei (AGN) are important for understanding of the physical conditions around the central accretion engines. Chandra and XMM-Newton X-ray observations of a 300 arcmin^2 region in the Extended Groth Strip are used to select a sample of~150 AGN. The Spitzer instruments IRAC and MIPS detect 68-80% of these sources, which show a wide range of mid-infrared properties. About 40% of the sources have red power-law spectral energy distributions (f_nu ~ nu^alpha, alpha<0) in the 3.6-8 um IRAC bands. In these sources the central engine dominates the emission at both X-ray and IR wavelengths. Another 40% of the sources have blue mid-IR spectral energy distributions (alpha>0) with their infrared emission dominated by the host galaxy; the remaining 20% are not well-fit by a power law. Published IRAC color criteria for AGN select most of the red sources, but only some of the blue sources. As with all other known methods, selecting AGN with mid-IR colors will not produce a sample that is simultaneously complete and reliable. The IRAC SED type does not directly correspond to X-ray spectral type (hard/soft). The mid-IR properties of X-ray-detected Lyman-break, radio, submillimeter, and optically-faint sources vary widely and, for the most part, are not distinct from those of the general X-ray/infrared source population. X-ray sources emit 6-11% of the integrated mid--IR light, making them significant contributors to the cosmic infrared background.

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arXiv2005

Infrared power-law galaxies in the Chandra Deep Field South: AGN and ULIRGs

We investigate the nature of a sample of 92 Spitzer/MIPS 24 micron selected galaxies in the CDFS, showing power law-like emission in the Spitzer/IRAC 3.6-8 micron bands. The main goal is to determine whether the galaxies not detected in X-rays (47% of the sample) are part of the hypothetical population of obscured AGN not detected even in deep X-ray surveys. The majority of the IR power-law galaxies are ULIRGs at z>1, and those with LIRG-like IR luminosities are usually detected in X-rays. The optical to IR spectral energy distributions (SEDs) of the X-ray detected galaxies are almost equally divided between a BLAGN SED class (similar to an optically selected QSO) and a NLAGN SED (similar to the BLAGN SED but with an obscured UV/optical continuum). A small fraction of SEDs resemble warm ULIRG galaxies (e.g., Mrk231). Most galaxies not detected in X-rays have SEDs in the NLAGN+ULIRG class as they tend to be optically fainter, and possibly more obscured. Moreover, the IR power-law galaxies have SEDs significantly different from those of high-z (z_sp>1) IR (24 micron) selected and optically bright (VVDS I_AB<=24) star-forming galaxies whose SEDs show a very prominent stellar bump at 1.6 micron. The galaxies detected in X-rays have 2-8 keV rest-frame luminosities typical of AGN. The galaxies not detected in X-rays have global X-ray to mid-IR SED properties that make them good candidates to contain IR bright X-ray absorbed AGN. If all these sources are actually obscured AGN, we would observe a ratio of obscured to unobscured 24 micron detected AGN of 2:1, whereas models predict a ratio of up to 3:1. Additional studies using Spitzer to detect X-ray-quiet AGN are likely to find more such obscured sources.

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arXiv2005

Spitzer Observations of Massive Red Galaxies at High Redshift

We investigate the properties of massive galaxies at z=1-3.5 using HST observations, ground-based near-IR imaging, and Spitzer Space Telescope observations at 3-24 micron. We identify 153 distant red galaxies (DRGs) with J-K > 2.3 mag (Vega) in the southern GOODS field. This sample is approximately complete in stellar mass for passively evolving galaxies above 10^11 solar masses and z < 3. The galaxies identified by this selection are roughly split between objects whose optical and near-IR rest-frame light is dominated by evolved stars combined with ongoing star formation, and galaxies whose light is dominated by heavily reddened starbursts. Very few of the galaxies (< 10%) have no indication of current star formation. Using SFR estimates that include the reradiated IR emission, the DRGs at z=1.5-3 with stellar masses > 10^11 solar masses have specific SFRs (SFRs per unit stellar mass) ranging from 0.2 to 10 Gyr^-1, with a mean value of ~2.4 Gyr^-1. The DRGs with stellar masses > 10^11 solar masses and 1.5 < z < 3 have integrated specific SFRs greater the global value over all galaxies. In contrast, we find that galaxies at z = 0.3-0.75 with these stellar masses have integrated specific SFRs less than the global value, and more than an order of magnitude lower than that for massive DRGs at z = 1.5-3. At z < 1, lower-mass galaxies dominate the overall cosmic mass assembly. This suggests that the bulk of star formation in massive galaxies occurs at early cosmic epochs and is largely complete by z~1.5. [Abridged]

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arXiv2005

Linking stellar mass and star formation in Spitzer/MIPS 24 micron galaxies

We present deep Ks<21.5 (Vega) identifications, redshifts and stellar masses for most of the sources composing the bulk of the 24 micron background in the GOODS/CDFS. Our identified sample consists of 747 Spitzer/MIPS 24 micron objects, and includes ~94% of all the 24 micron sources in the GOODS-South field which have fluxes Snu(24)>83 microJy (the 80% completeness limit of the Spitzer/GTO 24 micron catalog). 36% of our galaxies have spectroscopic redshifts (mostly at z<1.5) and the remaining ones have photometric redshifts of very good quality, with a median of |dz|=|zspec-zphot|/(1+zspec)=0.02. We find that MIPS 24 micron galaxies span the redshift range z~0-4, and that a substantial fraction (28%) lie at high redshifts z>1.5. We determine the existence of a bump in the redshift distribution at z~1.9, indicating the presence of a significant population of galaxies with PAH emission at these redshifts. Massive (M>10^11 Msun) star-forming galaxies at redshifts 2 500 Msun/yr), and some of them are able to construct a mass of 10^10-10^11 Msun in a single burst lifetime (~0.01-0.1 Gyr). At lower redshifts z<2, massive star-forming galaxies are also present, but appear to be building their stars on long timescales, either quiescently or in multiple modest burst-like episodes. At redshifts z~1-2, the ability of the burst-like mode to produce entire galaxies in a single event is limited to some lower (M<7x10^10 Msun) mass systems, and it is basically negligible at z<1. Our results support a scenario where star-formation activity is differential with assembled stellar mass and redshift, and where the relative importance of the burst-like mode proceeds in a down-sizing way from high to low redshifts. (abridged)

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arXiv2005

Understanding Radio-Selected Thermal Sources in M 33: Ultraviolet, Optical, Near-Infrared, Spitzer Mid-Infrared, and Radio Observations

We present ultraviolet, optical, near-infrared, Spitzer mid-infrared, and radio images of 14 radio-selected objects in M 33. These objects are thought to represent the youngest phase of star cluster formation. We have detected the majority of cluster candidates in M 33 at all wavelengths. From the near-IR images, we derived ages 2-10 Myr, K_S-band extinctions (A_K_S) of 0-1 mag, and stellar masses of 10^3-10^4 M_solar. We have generated spectral energy distributions (SEDs) of each cluster from 0.1 micron to 160 microns. From these SEDs, we have modeled the dust emission around these star clusters to determine the dust masses (1-10^3 M_solar) and temperatures (40-90 K) of the clusters' local interstellar medium. Extinctions derived from the JHK_S, Halpha, and UV images are similar to within a factor of 2 or 3. These results suggest that eleven of the fourteen radio-selected objects are optically-visible young star clusters with a surrounding H II region, that two are background objects, possibly AGN, and that one is a Wolf-Rayet star with a surrounding H II region.

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arXiv2005

Infrared Luminous Lyman Break Galaxies: A Population that Bridges LBGs and SCUBA Galaxies

A deep mid- and far-infrared survey in the Extended Groth Strip (EGS) area gives 3.6 to 8\micron flux densities or upper limits for 253 Lyman Break Galaxies (LBGs). The LBGs are a diverse population but with properties correlated with luminosity. The LBGs show a factor of 30 range in indicated stellar mass and a factor of 10 in apparent dust content relative to stellar mass. About 5% of LBGs are luminous at all wavelengths with powerful emission at rest 6\micron. In the rest 0.9 to 2\micron spectral range these galaxies have stellar spectral slopes with no sign of an AGN power law component, suggesting that their emission is mainly powered by intensive star formation. Galaxies in this luminous population share the infrared properties of cold SCUBA sources: both are massive and dusty starburst galaxies at $2<z<3$; their stellar mass is larger than $10^{11} M_{\odot}$. We suggest that these galaxies are the progenitors of present-day giant elliptical galaxies, with a substantial fraction of their stars already formed at $z \approx 3$.

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arXiv2005

Unveiling a Population of X-ray Non-Detected AGN

We define a sample of 27 radio-excess AGN in the Chandra Deep Field North by selecting galaxies that do not obey the radio/infrared correlation for radio-quiet AGN and star-forming galaxies. Approximately 60% of these radio-excess AGN are X-ray undetected in the 2 Ms Chandra catalog, even at exposures of > 1 Ms; 25% lack even 2-sigma X-ray detections. The absorbing columns to the faint X-ray-detected objects are 10^22 cm^-2 < N_H < 10^24 cm^-2, i.e., they are obscured but unlikely to be Compton thick. Using a local sample of radio-selected AGN, we show that a low ratio of X-ray to radio emission, as seen in the X-ray weakly- and non-detected samples, is correlated with the viewing angle of the central engine, and therefore with obscuration. Our technique can explore the proportion of obscured AGN in the distant Universe; the results reported here for radio-excess objects are consistent with but at the low end of the overall theoretical predictions for Compton-thick objects.

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arXiv2005

Why Optically--Faint AGN Are Faint: The Spitzer Perspective

Optically--faint X-ray sources (those with f_X/f_R > 10) constitute about 20% of X-ray sources in deep surveys, and are potentially highly obscured and/or at high redshift. Their faint optical fluxes are generally beyond the reach of spectroscopy. For a sample of 20 optically--faint sources in CDFS, we compile 0.4--24 um photometry, relying heavily on Spitzer. We estimate photometric redshifts for 17 of these 20 sources. We find that these AGN are optically--faint both because they lie at significantly higher redshifts (median z ~ 1.6) than most X-ray--selected AGN, and because their spectra are much redder than standard AGN. They have 2--8 keV X-ray luminosities in the Seyfert range, unlike the QSO--luminosities of optically--faint AGN found in shallow, wide--field surveys. Their contribution to the X-ray Seyfert luminosity function is comparable to that of z>1 optically--bright AGN.

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arXiv2004

Spitzer and Hubble Constraints on the Physical Properties of the z~7 Galaxy Strongly Lensed by Abell 2218

We report the detection of a z~7 galaxy strongly lensed by the massive galaxy cluster Abell 2218 (z=0.175) at 3.6 and 4.5 um using the Spitzer Observatory and at 1.1 um using the Hubble Space Telescope. The new data indicate a refined photometric redshift in the range of 6.6-6.8 depending on the presence of Ly-alpha emission. The spectral energy distribution is consistent with having a significant Balmer break, suggesting that the galaxy is in the poststarburst stage with an age of at least ~50 Myr and quite possibly a few hundred Myr. This suggests the possibility that a mature stellar population is already in place at such a high redshift. Compared with typical Lyman break galaxies at z~3-4, the stellar mass is an order of magnitude smaller (~10^{9} Msun), but the specific star formation rate (star formation rate/M_{star}) is similarly large (> 10^{-9} yr^{-1}), indicating equally vigorous star-forming activity.

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arXiv2004

Far Infrared Source Counts at 70 and 160 microns in Spitzer Deep Surveys

We derive galaxy source counts at 70 and 160 microns using the Multiband Imaging Photometer for Spitzer (MIPS) to map the Chandra Deep Field South (CDFS) and other fields. At 70 microns, our observations extend upwards about 2 orders of magnitude in flux density from a threshold of 15 mJy, and at 160 microns they extend about an order of magnitude upward from 50 mJy. The counts are consistent with previous observations on the bright end. Significant evolution is detected at the faint end of the counts in both bands, by factors of 2-3 over no-evolution models. This evolution agrees well with models that indicate most ofthe faint galaxies lie at redshifts between 0.7 and 0.9. The new Spitzer data already resolve about 23% of the Cosmic Far Infrared Background at 70 microns and about 7% at 160 microns.

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arXiv2004

Identification of luminous infrared galaxies at 1<z<2.5

We present preliminary results on 24micron detections of luminous infrared galaxies at z>1 with the Multiband Imaging Photometer for Spitzer (MIPS). Observations were performed in the Lockman Hole and the Extended Groth Strip (EGS), and were supplemented by data obtained with the Infrared Array Camera (IRAC) between 3 and 9microns. The positional accuracy of ~2arcsec for most MIPS/IRAC detections provides unambiguous identifications of their optical counterparts. Using spectroscopic redshifts from the Deep Extragalactic Evolutionary Probe survey, we identify 24micron sources at z>1 in the EGS, while the combination of the MIPS/IRAC observations with $BVRIJHK$ ancillary data in the Lockman Hole also shows very clear cases of galaxies with photometric redshifts at 1 =M*) galaxy counterparts. It is the first time that this population of luminous objects is detected up to z~2.5 in the infrared. Our work demonstrates the ability of the MIPS instrument to probe the dusty Universe at very high redshift, and illustrates how the forthcoming Spitzer deep surveys will offer a unique opportunity to illuminate a dark side of cosmic history not explored by previous infrared experiments.

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arXiv2004

Far Infrared Imaging of NGC 55

We present images of the galaxy NGC 55 at 24, 70, and 160 micron obtained with the Multiband Imaging Photometer for Spitzer (MIPS) instrument aboard the Spitzer Space Telescope. The new images display the far infrared emission in unprecedented detail and demonstrate that the infrared morphology differs dramatically from that at shorter wavelengths. The most luminous emission region in the galaxy is marginally resolved at 24 micron and has a projected separation of nearly 520 pc from the peak emission in the optical and near infrared. This region is responsible for ~9% of the total emission at 24 micron and is likely a young star formation region. We show that this and other compact sources account for more than 1/3 of the total 24 micron emission. We compute a total infrared luminosity for NGC 55 of 1.2*10^9 L_sun. The star formation rate implied by our measurements is 0.22 M_sun/yr. We demonstrate that the cold dust is more extended than the warm dust in NGC 55--the minor-axis scale heights are 0.32, 0.43, and 0.49 kpc at 24, 70 and 160 micron, respectively. The dust temperature map shows a range of temperatures that are well-correlated with the 24 micron surface brightness, from 20 K in low-surface-brightness regions to 26 K in high-surface-brightness regions.

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arXiv2004

The nature of luminous X-ray sources with mid-infrared counterparts

We investigate the luminous X-ray sources in the Lockman Hole (LH) and the Extended Groth Strip (EGS) detected at 24microns using MIPS and also with IRAC on board Spitzer. We assemble optical/infrared spectral energy distributions (SEDs) for 45 X-ray/24micron sources in the EGS and LH. Only about 1/4 of the hard X-ray/24micron sources show pure type 1 AGN SEDs. More than half of the X-ray/24micron sources have stellar-emission-dominated or obscured SEDs, similar to those of local type 2 AGN and spiral/starburst galaxies. One-third of the sources detected in hard X-rays do not have a 24micron counterpart. Two such sources in the LH have SEDs resembling those of S0/elliptical galaxies. The broad variety of SEDs in the optical-to-Spitzer bands of X-ray selected AGN means that AGN selected according to the behavior in the optical/infrared will have to be supplemented by other kinds of data (e.g., X-ray) to produce unbiased samples of AGN.

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arXiv2004

Spitzer observations of MAMBO galaxies: weeding out active nuclei in starbursting proto-ellipticals

We present Spitzer observations in five wavebands between 3.6 and 24um of an unbiased sample of 9 luminous, dusty galaxies selected at 1200um by the MAMBO camera on the IRAM 30-m telescope, a population akin to the well-known submm or `SCUBA' galaxies (hereafter SMGs). Owing to the coarse resolution of submm/mm instrumentation, SMGs have traditionally been difficult to identify at other wavelengths. We compare our multi-wavelength catalogs to show that the overlap between 24 and 1200um must be close to complete at these flux levels. We find that all (4/4) of the most secure >=4sigma SMGs have robust >=4sigma counterparts at 1.4GHz, while the fraction drops to 7/9 using all >=3sigma SMGs. We show that combining mid-IR and marginal (>=3sigma) radio detections provides plausible identifications in the remaining cases, enabling us to identify the complete sample. Accretion onto an obscured central engine is betrayed by the shape of the mid-IR continuum emission for several sources, confirming Spitzer's potential to weed out active galaxies. We demonstrate the power of a S(24um)/S(8um) vs S(8um)/S(4.5um) color-color plot as a diagnostic for this purpose. However, we conclude that the majority (~75%) of SMGs have rest-frame mid-/far-IR SEDs commensurate with obscured starbursts. Sensitive 24-um observations are clearly a useful route to identify and characterize reliable counterparts to high-redshift far-IR-bright galaxies, complementing what is possible via deep radio imaging.

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arXiv2004

Spatially Resolved Ultraviolet, H-alpha, Infrared, and Radio Star Formation in M81

We present Multiband Imaging Photometer for Spitzer (MIPS) observations of M81 at 24, 70, and 160 micron. The grand design nature of M81 is clearly seen, showing two well resolved spiral arms containing numerous bright star forming regions. The MIPS images reveal a significant amount of cold dust associated with the spiral arms. We investigate the variation of the ultraviolet (UV), H-alpha, and infrared (IR) luminosities and star formation rate (SFR) indicators across the face of M81 using the MIPS images and archival UV and H-alpha images. For regions in M81, we find that UV and H-alpha SFRs (uncorrected for dust attenuation) are always lower than the IR SFR. The cause of this behavior is dust attenuation and/or using SFR calibrations appropriate for entire galaxies, not regions in galaxies. The characteristics of the dust attenuation for the regions indicate the dust grains and/or geometry are different from those in starburst galaxies. The behavior of the infrared-radio correlation in M81 is seen to vary from the global average, with variations correlated with the morphology of M81.

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