arXiv subjects

Garth D. Illingworth

Publications and source records attributed to Garth D. Illingworth.

At least 73 records · Page 4Linked to original sources

arXiv2008

Recent Structural Evolution of Early-Type Galaxies: Size Growth from z=1 to z=0

Strong size and internal density evolution of early-type galaxies between z~2 and the present has been reported by several authors. Here we analyze samples of nearby and distant (z~1) galaxies with dynamically measured masses in order to confirm the previous, model-dependent results and constrain the uncertainties that may play a role. Velocity dispersion measurements are taken from the literature for 50 morphologically selected 0.8<z<1.2 field and cluster early-type galaxies with typical masses 2e11 Msol. Sizes are determined with ACS imaging. We compare the distant sample with a large sample of nearby (0.04<z<0.08) early-type galaxies extracted from the SDSS for which we determine sizes, masses, and densities in a consistent manner, using simulations to quantify systematic differences between the size measurements of nearby and distant galaxies. We find a highly significant structural difference between the nearby and distant samples, regardless of sample selection effects. The implied evolution in size at fixed mass between z=1 and the present is a factor of 1.97(0.15). This is in qualitative agreement with semianalytic models; however, the observed evolution is much faster than the predicted evolution. Our results reinforce and are quantitatively consistent with previous, photometric studies that found size evolution of up to a factor of 5 since z~2. A combination of structural evolution of individual galaxies through the accretion of companions and the continuous formation of early-type galaxies through increasingly gas-poor mergers is one plausible explanation of the observations.

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arXiv2008

The HD 163296 Circumstellar Disk in Scattered Light: Evidence of Time-Variable Self-Shadowing

We present the first multi-color view of the scattered light disk of the Herbig Ae star HD 163296, based on coronagraphic observations from the Hubble Space Telescope Advanced Camera for Surveys (ACS). Radial profile fits of the surface brightness along the disk's semi-major axis indicates that the disk is not continuously flared, and extends to 540 AU. The disk's color (V-I)=1.1 at a radial distance of 3.5 arcseconds is redder than the observed stellar color (V-I)=0.15. This red disk color might be indicative of either an evolution in the grain size distribution (i.e. grain growth) and/or composition, both of which would be consistent with the observed non-flared geometry of the outer disk. We also identify a single ansa morphological structure in our F435W ACS data, which is absent from earlier epoch F606W and F814W ACS data, but corresponds to one of the two ansa observed in archival HST STIS coronagraphic data. Following transformation to similar band-passes, we find that the scattered light disk of HD 163296 is 1 mag arcsec$^{-2}$ fainter at 3.5 arcseconds in the STIS data than in the ACS data. Moreover, variations are seen in (i) the visibility of the ansa(e) structures, in (ii) the relative surface brightness of the ansa(e) structures, and in (iii) the (known) intrinsic polarization of the system. These results indicate that the scattered light from the HD 163296 disk is variable. We speculate that the inner disk wall, which Sitko et al. suggests has a variable scale height as diagnosed by near-IR SED variability, induces variable self-shadowing of the outer disk. We further speculate that the observed surface brightness variability of the ansa(e) structures may indicate that the inner disk wall is azimuthally asymmetric.

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arXiv2008

z~7-10 Galaxies in the HUDF and GOODS fields, and their UV Luminosity Functions

We use all available deep optical ACS and near-IR data over both the HUDF and the two GOODS fields to search for star-forming galaxies at z>~7 and constrain the UV LF within the first 700 Myrs. Our data set includes ~23 arcmin^2 of deep NICMOS J+H data and ~248 arcmin^2 of ground-based (ISAAC+MOIRCS) data, coincident with ACS optical data of greater or equal depths. In total, we find 8 ~7.3 z-dropouts in our search fields, but no z~9 J-dropout candidates. A careful consideration of a wide variety of different contaminants suggest an overall contamination level of just ~12% for our z-dropout selection. After performing detailed simulations to accurately estimate the selection volumes, we derive constraints on the UV LFs at z~7 and z~9. For a faint-end slope alpha=-1.74, our most likely values for M*(UV) and phi* at z~7 are -19.8+/-0.4 mag and 1.1_{-0.7}^{+1.7} x 10^{-3} Mpc^{-3}, respectively. Our search results for z~9 J-dropouts set a 1 sigma lower limit on M*(UV) of -19.6 mag assuming that phi* and alpha are the same as their values at slightly later times. This lower limit on M*(UV) is 1.4 mag fainter than our best-fit value at z~4, suggesting that the UV LF has undergone substantial evolution over this time period. No evolution is ruled out at 99% confidence from z~7 to z~6 and at 80% confidence from z~9 to z~7. The inferred brightening in M*(UV) with redshift (i.e., M*(UV) = (-21.02+/-0.09) + (0.36+/-0.08)(z - 3.8)) matches the evolution expected in the halo mass function, if the mass-to-light ratio of halos evolves as ~(1+z)**{-1}. Finally, we consider the shape of the UV LF at z>~5 and discuss the implications of the Schechter-like form of the observed LFs, particularly the unexpected abrupt cut-off at the bright end.

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arXiv2008

The Detection of a Red Sequence of Massive Field Galaxies at z~2.3 and its Evolution to z~0

The existence of massive galaxies with strongly suppressed star formation at z~2.3, identified in a previous paper, suggests that a red sequence may already be in place beyond z=2. In order to test this hypothesis, we study the rest-frame U-B color distribution of massive galaxies at 2 3 sigma) red sequence, which hosts ~60% of the stellar mass at the high-mass end. The red-sequence galaxies have little or no ongoing star formation, as inferred from both emission-line diagnostics and stellar continuum shapes. Their strong Balmer breaks and their location in the rest-frame U-B, B-V plane indicate that they are in a post-starburst phase, with typical ages of ~0.5-1.0 Gyr. In order to study the evolution of the red sequence, we compare our sample with spectroscopic massive galaxy samples at 0.02 10^11 Msol) of the red sequence grow by factors of ~8 and ~6, respectively. We explore simple models to explain the observed evolution. Passive evolution models predict too strong d(U-B), and produce z~0 galaxies that are too red. More complicated models that include aging, galaxy transformations, and red mergers can explain both the number density and color evolution of the massive end of the red sequence between z~2.3 and the present.

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arXiv2008

A Near-Infrared Spectroscopic Survey of K-selected Galaxies at z~2.3: Redshifts and Implications for Broadband Photometric Studies

Using the Gemini Near-InfraRed Spectrograph (GNIRS), we have completed a near-infrared spectroscopic survey for K-bright galaxies at z~2.3, selected from the MUSYC survey. We derived spectroscopic redshifts from emission lines or from continuum features and shapes for all 36 observed galaxies. The continuum redshifts are driven by the Balmer/4000 Angstrom break, and have an uncertainty in dz/(1+z) of <0.019. We use this unique sample to determine, for the first time, how accurately redshifts and other properties of massive high-redshift galaxies can be determined from broadband photometric data alone. We find that the photometric redshifts of the galaxies in our sample have a systematic error of 0.08 and a random error of 0.13 in dz/(1+z). The systematic error can be reduced by using optimal templates and deep photometry; the random error, however, will be hard to reduce below 5%. The spectra lead to significantly improved constraints for stellar population parameters. For most quantities this improvement is about equally driven by the higher spectral resolution and by the much reduced redshift uncertainty. Properties such as the age, A_V, current star formation rate, and the star formation history are generally very poorly constrained with broadband data alone. Interestingly stellar masses and mass-to-light ratios are among the most stable parameters from broadband data. Nevertheless, photometric studies may overestimate the number of massive galaxies at 2<z<3, and thus underestimate the evolution of the stellar mass density. Finally,the spectroscopy supports our previous finding that red galaxies dominate the high-mass end of the galaxy population at z=2-3.

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arXiv2007

UV Luminosity Functions at z~4, 5, and 6 from the HUDF and other Deep HST ACS Fields: Evolution and Star Formation History

We use the ACS BViz data from the HUDF and all other deep HST ACS fields (including the GOODS fields) to find large samples of star-forming galaxies at z~4 and z~5 and to extend our previous z~6 sample. These samples contain 4671, 1416, and 627 B, V, and i dropouts, respectively, and reach to extremely low luminosities (0.01-0.04 L* or M(UV)~-16 to -17), allowing us to determine the rest-frame UV luminosity function (LF) and faint-end slope alpha at z~4-6 to high accuracy. We find faint-end slopes alpha of -1.73+/-0.05 at z~4, -1.66+/-0.09 at z~5, and -1.74+/-0.16 at z~6 -- suggesting that the faint-end slope is very steep and shows little evolution with cosmic time. We find that M*(UV) brightens considerably in the 0.7 Gyr from z~6 to z~4 (by ~0.7 mag from M*=-20.24+/-0.19 to M*=-20.98+/-0.10). The observed increase in the characteristic luminosity over this range is almost identical to that expected for the halo mass function -- suggesting that the observed evolution is likely due to the hierarchical coalescence and merging of galaxies. The evolution in phi* is not significant. The UV luminosity density at z~6 is modestly lower (0.45+/-0.09 times) than that at z~4 (integrated to -17.5 AB mag) though a larger change is seen in the dust-corrected star-formation rate density. We thoroughly examine published LF results and assess the reasons for their wide dispersion. We argue that the results reported here are the most robust available. The extremely steep faint-end slopes alpha found here suggest that lower luminosity galaxies play a significant role in reionizing the universe. Finally, we consider recent search results for galaxies at z~7-8 and use them to extend our estimates of the evolution in M* from z~7-8 to z~4.

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arXiv2007

The Origin of Line Emission in Massive z~2.3 Galaxies: Evidence for Cosmic Downsizing of AGN Host Galaxies

Using the Gemini Near-InfraRed Spectrograph (GNIRS), we have assembled a complete sample of 20 K-selected galaxies at 2.0<z<2.7 with high quality near-infrared spectra. As described in a previous paper, 9 of these 20 galaxies have strongly suppressed star formation and no detected emission lines. The present paper concerns the 11 galaxies with detected Halpha emission, and studies the origin of the line emission using the GNIRS spectra and follow-up observations with SINFONI on the VLT. Based on their [NII]/Halpha ratios, the spatial extent of the line emission and several other diagnostics, we infer that four of the eleven emission-line galaxies host narrow line active galactic nuclei (AGNs). The AGN host galaxies have stellar populations ranging from evolved to star-forming. Combining our sample with a UV-selected galaxy sample at the same redshift that spans a broader range in stellar mass, we find that black-hole accretion is more effective at the high-mass end of the galaxy distribution (~2.9x10^11 Msun) at z~2.3. Furthermore, by comparing our results with SDSS data, we show that the AGN activity in massive galaxies has decreased significantly between z~2.3 and z~0. AGNs with similar normalized accretion rates as those detected in our K-selected galaxies reside in less massive galaxies (~4.0x10^10 Msun) at low redshift. This is direct evidence for downsizing of AGN host galaxies. Finally, we speculate that the typical stellar mass-scale of the actively accreting AGN host galaxies, both at low and at high redshift, might be similar to the mass-scale at which star-forming galaxies seem to transform into red, passive systems.

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arXiv2007

A Keck Spectroscopic Survey of MS1054-03 (z=0.83): Forming the Red Sequence

Using a magnitude-limited, spectroscopic survey of the X-ray luminous galaxy cluster MS1054-03, we isolate 153 cluster galaxies and measure MS1054's redshift and velocity dispersion to be z=0.8307 and sigma_z=1156 km/s. The absorption-line, post-starburst (``E+A''), and emission-line galaxies respectively make up 63%, 15%, and 23% of the cluster population. With photometry from HST/ACS, we find that the absorption-line members define an exceptionally tight red sequence over a span of ~3.5 magnitudes in i775: their intrinsic scatter in (V-i) color is only 0.048, corresponding to a (U-B)_z scatter of 0.041. Their color scatter is comparable to that of the ellipticals (sigma_Vi=0.055), but measurably smaller than that of the combined E+S0 sample (sigma_Vi=0.072). The color scatter of MS1054's absorption-line population is approximately twice that of the ellipticals in Coma; this difference is consistent with passive evolution where most of the absorption-line members (>75%) formed by z~2, and all of them by z~1.2. For red members, we find a trend (>95% confidence) of weakening Hdelta absorption with redder colors that we conclude is due to age: in MS1054, the color scatter on the red sequence is driven by differences in mean stellar age of up to ~1.5 Gyr. We also generate composite spectra and estimate that the average S0 in MS1054 is ~0.5-1 Gyr younger than the average elliptical; this difference in mean stellar age is mainly due to a number of S0s that are blue (18%) and/or are post-starburst systems (21%).

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arXiv2006

Clustering of i-dropout galaxies at z=6 in GOODS and the UDF

We measured the angular clustering at z~6 from a large sample of i-dropout galaxies (293 with z<27.5 from GOODS and 95 with z<29.0 from the UDF). Our largest and most complete subsample (having L>0.5L*) shows the presence of clustering at 94% significance. For this sample we derive a (co-moving) correlation length of r_0=4.5^{+2.1}_{-3.2} h_{72}^{-1} Mpc and bias b=4.1^{+1.5}_{-2.6}, using an accurate model for the redshift distribution. No clustering could be detected in the much deeper but significantly smaller UDF, yielding b<4.4 (1 sigma). We compare our findings to Lyman break galaxies at z=3-5 at a fixed luminosity. Our best estimate of the bias parameter implies that i-dropouts are hosted by dark matter halos having masses of ~10^11 M_sun, similar to that of V-dropouts at z~5. We evaluate a recent claim that at z>5 star formation might have occurred more efficiently compared to that at z=3-4. This may provide an explanation for the very mild evolution observed in the UV luminosity density between z=6 and z=3. Although our results are consistent with such a scenario, the errors are too large to find conclusive evidence for this.

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arXiv2006

Comparing Dynamical and Photometric Mass Estimates of Low- and High-Redshift Galaxies: Random and Systematic Uncertainties

We determine the importance of redshift-dependent systematic effects in the determination of stellar masses from broad band spectral energy distributions (SEDs), using high quality kinematic and photometric data of early-type galaxies at z~1 and z~0. We find that photometric masses of z~1 galaxies can be systematically different, by up to a factor of 2, from photometric masses of z~0 galaxies with the same dynamical mass. The magnitude of this bias depends on the choice of stellar population synthesis model and the rest-frame wavelength range used in the fits. The best result, i.e., without significant bias, is obtained when rest-frame optical SEDs are fitted with models from Bruzual&Charlot (2003). When the SEDs are extended to the rest-frame near-IR, a bias is introduced: photometric masses of the z~1 galaxies increase by a factor of 2 relative to the photometric masses of the z~0 galaxies. When we use the Maraston (2005) models, the photometric masses of the z~1 galaxies are low relative to the photometric masses of the z~0 galaxies by a factor of ~1.8. This offset occurs both for fits based on rest-frame optical SEDs, and fits based on rest-frame optical+near-IR SEDs. The results indicate that model uncertainties produce uncertainties as high as a factor of 2.5 in mass estimates from rest-frame near-IR photometry, independent of uncertainties due to unknown star formation histories.

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arXiv2006

Line Strengths in Early-Type Cluster Galaxies at z=0.33: Implications for alpha/Fe, Nitrogen and the Histories of E/S0s

[Heavily Abbreviated] In this paper we analyze previously published spectra with high signal-to-noise ratios of E/S0 galaxies in the rich cluster CL1358+62 at z=0.33, and introduce techniques for fitting stellar population models to the data. Here we focus on the 19 E and S0 galaxies with an homogeneous set of eight blue Lick indices. We explore the galaxy properties using six-parameter stellar population models from the literature, and describe an approach for fitting the models differentially, such that the largest systematic errors are avoided. We find: (1) no differences between the stellar population parameters of Es and S0s, at fixed sigma; (2) the stars in the Es and S0s are uniformly old, consistent with previously published results using M/L ratios; (3) a significant correlation of [Z/H] with sigma, in a manner consistent with the observed B-V colors of the galaxies; (4) no significant correlation of [alpha/Fe] with sigma; and (5) a significant anti-correlation of [alpha/N] with [Z/H], which we interpret as the signature of secondary nitrogen. Neither [alpha/C], nor [alpha/Ca] shows significant variation. While the differences between our conclusions and the current view of stellar populations may point to serious deficiencies, our deduced correlation of mean metallicity with sigma does reproduce the B-V colors of the galaxies, as well as the slope of the local Mg-sigma relation. In matching the inferred population trends with published data on nearby galaxies, the line strength-line width relations match well, save for the narrow iron indices. Taken together, these results reduce early-type galaxies in clusters to a family with one-parameter, velocity dispersion, greatly simplifying scenarios for their formation and evolution.

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arXiv2006

BUCS: An Engine For Generating Realistic Imaging Data for Deep Galaxy Fields

Today's deep high resolution multiwavelength surveys contain a wealth of information about galaxies at different epochs. To fully exploit this information, it is useful to be able to produce highly realistic simulations to compare with the observations. Here, we describe one such simulator for producing imaging data for deep galaxy fields. Based upon a pixel-by-pixel modelling of object SEDs and their selection volumes, this simulator allows users to make realistic multicolor simulations of galaxy fields from galaxy samples at all redshifts. Input samples for this simulator range from z~0 samples selected from the Sloan Digital Sky Survey to z~1-6 samples selected from the Great Observatories Origins Deep Survey. Users can specify the passbands, noise, and PSFs, or equivalently the exposure times on well-known instruments like HST or ground-based telescopes.

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arXiv2006

Evolution of the Color-Magnitude relation in High-Redshift Clusters: Early-type Galaxies in the Lynx Supercluster at z~1.26

Color-magnitude relations have been derived in the clusters RX J0849+4452 and RX J0848+4453 at z~1.26. The color-magnitude relation was determined from Advanced Camera for Surveys imaging in the WFC F775W (i_775) and F850LP (z_850) filters combined with ground-based spectroscopy. Early-type cluster candidates have been identified according to the Postman et al. morphological classification. In both clusters the bright red early-type population defines a tight color-magnitude relation very similar in color, although the two clusters present different X-ray luminosities and shapes, with RX J0849+4452 being three times more X-ray luminous and more compact, and having a temperature two times higher. The elliptical galaxy color-magnitude relations (CMR) in RX J0849+4452 and RX J0848+4453 show an intrinsic (i_775-z_850) color scatter of 0.026 +/- 0.012 mag and 0.024 +/- 0.015 mag, respectively, within 2 arcminutes (~1Mpc at z=1.26) from the cluster X-ray emission centers. Simple modeling of the scatters using stellar population models from Bruzual and Charlot, gives a mean luminosity-weighted age t > 2.5 Gyr (z_f > 2.75). S0 galaxies follow the elliptical CMR; they show larger scatters about the CMR. The intrinsic scatter decreases and the CMR slopes are steeper at smaller radii, within both clusters. We conclude that old stellar populations in cluster elliptical galaxies are already in place at z=1.26, both in the more evolved cluster RX J0849+4452, and in its less evolved companion RX J0848+4453. Even at a lookback time of 9 Gyr, in the early merging and buildup of massive clusters, the bulk of the stellar content of the bright elliptical galaxy population was in place - apparently formed some 2.5~Gyr earlier at z~3

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arXiv2006

Luminosity Functions and Star Formation Rates at z~6-10: Galaxy Buildup in the Reionization Age

HST ACS and NICMOS data are now of sufficient depth and areal coverage to place strong constraints on the formation and evolution of galaxies during the first 1-2 Gyrs of the universe. Of particular interest are galaxies at z~6 since they represent the earliest epoch accessible by current high-efficiency optical instrumentation. To this end, we have constructed a sample of 506 z~6 objects from all the deepest wide-area HST data (UDF, UDF-Parallel, and GOODS fields). They have been used to construct an optimal determination of the rest-frame continuum UV LF at z~6. Our LF extends to over 3 magnitudes below L*, fainter than has been done at z~3. Over the interval z~6 to z~3, we find strong evidence for evolution in the UV LF. Though constraints on the faint-end slope remain modest (and are consistent with no-evolution), the characteristic luminosity appears to have approximately doubled over the interval z~6 to z~3, consistent with hierarchical expectations. Remarkably, this shift to lower luminosities extends to even higher redshifts. Using all deep J+H NICMOS observations (800 orbits in total), we have been able to demonstrate that the bright end of the LF (>0.3L*) is at least 5 times lower at z~10 than at z~4, with a similar deficit being established from our recent detections and first statistical sample of z~7-8 galaxies using our UDF NICMOS data. In these proceedings, we discuss what is known about the UV LF and UV luminosity density at z~6-10 from current data and its evolution relative to z~3. We also describe several exciting prospects for advance in this area over the next year.

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arXiv2005

Spectroscopic Confirmation of Multiple Red Galaxy-Galaxy Mergers in MS1054-03 (z=0.83)

We present follow-up spectroscopy of the galaxy cluster MS1054-03 (z=0.83) confirming that at least six of the nine merging galaxy pairs identified by van Dokkum et al. (1999) are indeed bound systems: they have projected separations of R_s<10 kpc and relative line-of sight velocities of dv<165 km/s. For the remaining three pairs, we were unable to obtain redshifts of both constituent galaxies. To identify a more objective sample of merging systems, we select bound red galaxy pairs (R_s<=30 kpc, dv<=300 km/s) from our sample of 121 confirmed cluster members: galaxies in bound red pairs make up 15.7+/-3.6% of the cluster population. The (B-K_s) color-magnitude diagram shows that the pair galaxies are as red as the E/S0 members and have a homogeneous stellar population. The red pair galaxies span a large range in luminosity and internal velocity dispersion to include some of the brightest, most massive members (L>L*, sigma>200 km/s); these bound galaxy pairs must evolve into E/S0 members by z~0.7. These results combined with MS1054's high merger fraction and reservoir of likely future mergers indicates that most, if not all, of its early-type members evolved from (passive) galaxy-galaxy mergers at z<~1.

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arXiv2005

Constraints on z~10 Galaxies from the Deepest HST NICMOS Fields

We use all available fields with deep NICMOS imaging to search for J dropouts (H<28) at z~10. Our primary data set for this search were the two J+H NICMOS parallel fields taken with the ACS HUDF. The 5 sigma limiting mags were 28.6 in J and 28.5 in H. Several shallower fields were also used: J+H NICMOS frames available over the HDF North, the HDF South NICMOS parallel, and the ACS HUDF. The primary selection criterion was (J-H)>1.8. 11 such sources were found in all search fields using this criterion. 8 of these were clearly ruled out as credible z~10 sources, either as a result of detections (>2 sigma) blueward of J or their colors redward of the break (H-K~1.5). The nature of the 3 remaining sources could not be determined from the data. The number appears consistent with the expected contamination from low-z interlopers. Analysis of the stacked images for the 3 candidates also suggests contamination. Regardless of their true redshifts, the actual number of z~10 sources must be <=3. To assess the significance of these results, two lower redshift samples (a z~3.8 B-dropout and z~6 i-dropout sample) were projected to z~8-12 using a (1+z)^{-1} size scaling. They were added to the image frames, and the selection repeated, giving 15.6 and 4.8 J-dropouts, respectively. This suggests that to the limit of this probe (0.3 L*) there has been evolution from z~3.8 and possibly from z~6. This is consistent with the strong evolution already noted at z~6 and z~7.5 relative to z~3-4. Even assuming that 3 sources from this probe are at z~10, the rest-frame continuum UV (~1500 A) luminosity density at z~10 (integrated down to 0.3 L*) is just 0.19_{-0.09}^{+0.13}x that at z~3.8 (or 0.19_{-0.10}^{+0.15}x including cosmic variance). However, if none of our sources is at z~10, this ratio has a 1 sigma upper limit of 0.07. (abridged)

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arXiv2005

The DEEP Groth Strip Survey. I. The Sample

The Deep Extragalactic Exploratory Probe (DEEP) is a multi-phase research program dedicated to the study of the formation and evolution of galaxies and of large scale structure in the distant Universe. This paper describes the first five-year phase, denoted DEEP1. A series of ten DEEP1 papers will discuss a range of scientific topics (e.g., the study of photometric and spectral properties of a general distant galaxy survey, the evolution observed in galaxy populations of varied morphologies). The observational basis for these studies is the Groth Survey Strip field, a 127 square arcminute region which has been observed with the Hubble Space Telescope in both broad I-band and V-band optical filters and with the Low Resolution Imaging Spectrograph on the Keck Telescopes. Catalogs of photometric and structural parameters have been constructed for 11,547 galaxies and stars at magnitudes brighter than 29, and spectroscopy has been conducted for a magnitude-color weighted subsample of 818 objects. We evaluate three independent techniques for constructing an imaging catalog for the field from the HST data, and discuss the depth and sampling of the resultant catalogs. The selection of the spectroscopic subsample is discussed, and we describe the multifaceted approach taken to prioritizing objects of interest for a variety of scientific subprograms. A series of Monte Carlo simulations then demonstrates that the spectroscopic subsample can be adequately modeled as a simple function of magnitude and color cuts in the imaging catalog.

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arXiv2004

The Nature of Blue Cores in Spheroids: a Possible Connection with AGN and Star Formation

We investigate the physical nature of blue cores in early-type galaxies through the first multi-wavelength analysis of a serendipitously discovered field blue-nucleated spheroid in the background of the deep ACS/WFC griz multicolor observations of the cluster Abell 1689. The resolved g-r, r-i and i-z color maps reveal a prominent blue core identifying this galaxy as a ``typical'' case study, exhibiting variations of 0.5-1.0 mag in color between the center and the outer regions, opposite to the expectations of reddened metallicity induced gradients in passively evolved ellipticals. From a Magellan-Clay spectrum we secure the galaxy redshift at $z=0.624$. We find a strong X-ray source coincident with the spheroid galaxy. Spectral features and a high X-ray luminosity indicate the presence of an AGN in the galaxy. However, a comparison of the X-ray luminosity to a sample derived from the Chandra Deep Field South displays Lx to be comparable to Type I/QSO galaxies while the optical flux is consistent with a normal star-forming galaxy. We conclude that the galaxy's non-thermal component dominates at high-energy wavelengths while we associate the spheroid blue light with the stellar spectrum of normal star-forming galaxies. We argue about a probable association between the presence of blue cores in spheroids and AGN activity.

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