arXiv subjects

C. N. A. Willmer

Publications and source records attributed to C. N. A. Willmer.

At least 37 records · Page 2Linked to original sources

arXiv2012

The Arizona CDFS Environment Survey (ACES): A Magellan/IMACS Spectroscopic Survey of the Chandra Deep Field South

We present the Arizona CDFS Environment Survey (ACES), a recently-completed spectroscopic redshift survey of the Chandra Deep Field South (CDFS) conducted using IMACS on the Magellan-Baade telescope. In total, the survey targeted 7277 unique sources down to a limiting magnitude of R = 24.1, yielding 5080 secure redshifts across the ~30' x 30' extended CDFS region. The ACES dataset delivers a significant increase to both the spatial coverage and the sampling density of the spectroscopic observations in the field. Combined with previously-published, spectroscopic redshifts, ACES now creates a highly-complete survey of the galaxy population at R < 23, enabling the local galaxy density (or environment) on relatively small scales (~1 Mpc) to be measured at z < 1 in one of the most heavily-studied and data-rich fields in the sky. Here, we describe the motivation, design, and implementation of the survey and present a preliminary redshift and environment catalog. In addition, we utilize the ACES spectroscopic redshift catalog to assess the quality of photometric redshifts from both the COMBO-17 and MUSYC imaging surveys of the CDFS.

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arXiv2012

CANDELS Observations of the Structural Properties and Evolution of Galaxies in a Cluster at z=1.62

We discuss the structural and morphological properties of galaxies in a z=1.62 proto-cluster using near-IR imaging data from Hubble Space Telescope Wide Field Camera 3 data of the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS). The cluster galaxies exhibit a clear color-morphology relation: galaxies with colors of quiescent stellar populations generally have morphologies consistent with spheroids, and galaxies with colors consistent with ongoing star formation have disk-like and irregular morphologies. The size distribution of the quiescent cluster galaxies shows a deficit of compact (< 1kpc), massive galaxies compared to CANDELS field galaxies at z=1.6. As a result the cluster quiescent galaxies have larger average effective sizes compared to field galaxies at fixed mass at greater than 90% significance. Combined with data from the literature, the size evolution of quiescent cluster galaxies is relatively slow from z~1.6 to the present, growing as (1+z)^(-0.6+/-0.1). If this result is generalizable, then it implies that physical processes associated with the denser cluster region seems to have caused accelerated size growth in quiescent galaxies prior to z=1.6 and slower subsequent growth at z<1.6 compared to galaxies in the lower density field. The quiescent cluster galaxies at z=1.6 have higher ellipticities compared to lower redshift samples at fixed mass, and their surface-brightness profiles suggest that they contain extended stellar disks. We argue the cluster galaxies require dissipationless (i.e., gas-poor or "dry") mergers to reorganize the disk material and to match the relations for ellipticity, stellar mass, size, and color of early-type galaxies in z<1 clusters.

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arXiv2012

A Chandra view of the z=1.62 galaxy cluster IRC-0218A

Context: Very few z > 1.5 clusters of galaxies are known. It is important to study the properties of galaxies in these clusters and the ICM and, further, to cross-check the reliability of the various mass estimates. This will help to clarify the process of structure formation and how distant clusters may be used to constrain cosmology. AIMS: We present a 84 ks Chandra observation of IRC-0218A, a cluster of galaxies inferred by the presence of a galaxy overdensity in the infrared at a redshift of 1.62 and associated with some XMM emission Methods: Spatial analysis of the Chandra X-ray photon distribution. Results: The Chandra observation of IRC-0218A appears to be entirely dominated by a point-source located at the centroid of the MIR galaxy density. In addition, we detect weak extended emission (2.3 sigma) out to a radius of 25" with a flux of ~ 3 10E-15 erg/s/cm2 in the [0.3-2]keV band. Assuming that clusters evolve similarly, we infer a virial mass of M200 =7.7+/-3.8 10E13Mo. This is marginally compatible with our current estimate of the cluster dynamical mass (based on 10 redshifts), although there is no evidence that the galaxy peculiar velocities correspond to the motions of a virialized structure. The stellar mass enclosed in the inferred X-ray virial radius is estimated to 1-2 10E12 Mo. We provide a detailed account of 28 X-ray point-sources detected in the field.

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arXiv2011

Observational constraints on the physics behind the evolution of AGN since z ~ 1

We explore the evolution with redshift of the rest-frame colours and space densities of AGN hosts (relative to normal galaxies) to shed light on the dominant mechanism that triggers accretion onto supermassive black holes as a function of cosmic time. Data from serendipitous wide-area XMM surveys of the SDSS footprint (XMM/SDSS, Needles in the Haystack survey) are combined with Chandra deep observations in the AEGIS, GOODS-North and GOODS-South to compile uniformly selected samples of moderate luminosity X-ray AGN [L_X(2-10keV) = 1e41-1e44erg/s] at redshifts 0.1, 0.3 and 0.8. It is found that the fraction of AGN hosted by red versus blue galaxies does not change with redshift. Also, the X-ray luminosity density associated with either red or blue AGN hosts remains nearly constant since z=0.8. X-ray AGN represent a roughly fixed fraction of the space density of galaxies of given optical luminosity at all redshifts probed by our samples. In contrast the fraction of X-ray AGN among galaxies of a given stellar mass decreases with decreasing redshift. These findings suggest that the same process or combination of processes for fueling supermassive black holes are in operation in the last 5 Gyrs of cosmic time. The data are consistent with a picture in which the drop of the accretion power during that period (1dex since z=0.8) is related to the decline of the space density of available AGN hosts, as a result of the evolution of the specific star-formation rate of the overall galaxy population. Scenarios which attribute the evolution of moderate luminosity AGN since z \approx 1 to changes in the suppermassive black hole accretion mode are not favored by our results.

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arXiv2011

Oxford SWIFT IFS and multi-wavelength observations of the Eagle galaxy at z=0.77

The `Eagle' galaxy at a redshift of 0.77 is studied with the Oxford Short Wavelength Integral Field Spectrograph (SWIFT) and multi-wavelength data from the All-wavelength Extended Groth strip International Survey (AEGIS). It was chosen from AEGIS because of the bright and extended emission in its slit spectrum. Three dimensional kinematic maps of the Eagle reveal a gradient in velocity dispersion which spans 35-75 +/- 10 km/s and a rotation velocity of 25 +/- 5 km/s uncorrected for inclination. Hubble Space Telescope images suggest it is close to face-on. In comparison with galaxies from AEGIS at similar redshifts, the Eagle is extremely bright and blue in the rest-frame optical, highly star-forming, dominated by unobscured star-formation, and has a low metallicity for its size. This is consistent with its selection. The Eagle is likely undergoing a major merger and is caught in the early stage of a star-burst when it has not yet experienced metal enrichment or formed the mass of dust typically found in star-forming galaxies.

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arXiv2011

Evolution of the dusty infrared luminosity function from z=0 to z=2.3 using observations from Spitzer

We derive the evolution of the infrared (IR) luminosity function (LF) over the last 4/5ths of cosmic time, using deep 24um and 70um imaging of the GOODS North and South fields. We use an extraction technique based on prior source positions at shorter wavelengths to build the 24 and 70um source catalogs. The majority (93%) of the sources have a spectroscopic (39%) or a photometric redshift (54%) and, in our redshift range of interest (i.e., 1.3<z<2.3) ~20% of the sources have a spectroscopic redshifts. To extend our study to lower 70um luminosities we perform a stacking analysis and we characterize the observed L_24/(1+z) vs L_70/(1+z) correlation. Using spectral energy distribution templates which best fit this correlation, we derive the IR luminosity of sources from their 24 and 70 um fluxes. We then compute the IR LF at z=1.55+/-0.25 and z=2.05+/-0.25. The redshift evolution of the IR LF from z=1.3 to z=2.3 is consistent with a luminosity evolution proportional to (1+z)^1.0+/-0.9 combined with a density evolution proportional to (1+z)^-1.1+/-1.5. At z~2, luminous IR galaxies (LIRGs: 10^11Lsun< LIR <10^12Lsun) are still the main contributors to the total comoving IR luminosity density (IR LD) of the Universe. At z~2, LIRGs and ultra-luminous IR galaxies (ULIRGs: 10^12Lsun< LIR) account for ~49% and ~17% respectively of the total IR LD of the Universe. Combined with previous results for galaxies at z<1.3 and assuming a constant conversion between the IR luminosity and star-formation rate (SFR) of a galaxy, we study the evolution of the SFR density of the Universe from z=0 to z=2.3. We find that the SFR density of the Universe strongly increased with redshift from z=0 to z=1.3, but is nearly constant at higher redshift out to z=2.3. As part of the online material accompanying this article, we present source catalogs at 24um and 70um for both the GOODS-North and -South fields.

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arXiv2010

Host galaxy colour gradients and accretion disc obscuration in AEGIS z~1 X-ray-selected active galactic nuclei

We describe the effect of AGN light on host galaxy optical and UV-optical colours, as determined from X-ray-selected AGN host galaxies at z~1, and compare the AGN host galaxy colours to those of a control sample matched to the AGN sample in both redshift and stellar mass. We identify as X-ray-selected AGNs 8.7 +4/-3 per cent of the red-sequence control galaxies, 9.8 +/-3 per cent of the blue-cloud control galaxies, and 14.7 +4/-3 per cent of the green-valley control galaxies. The nuclear colours of AGN hosts are generally bluer than their outer colours, while the control galaxies exhibit redder nuclei. AGNs in blue-cloud host galaxies experience less X-ray obscuration, while AGNs in red-sequence hosts have more, which is the reverse of what is expected from general considerations of the interstellar medium. Outer and integrated colours of AGN hosts generally agree with the control galaxies, regardless of X-ray obscuration, but the nuclear colours of unobscured AGNs are typically much bluer, especially for X-ray luminous objects. Visible point sources are seen in many of these, indicating that the nuclear colours have been contaminated by AGN light and that obscuration of the X-ray radiation and visible light are therefore highly correlated. Red AGN hosts are typically slightly bluer than red-sequence control galaxies, which suggests that their stellar populations are slightly younger. We compare these colour data to current models of AGN formation. The unexpected trend of less X-ray obscuration in blue-cloud galaxies and more in red-sequence galaxies is problematic for all AGN feedback models, in which gas and dust is thought to be removed as star formation shuts down. [See paper for full abstract.]

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arXiv2010

A Spitzer-Selected Galaxy Cluster at z=1.62

We report the discovery of a galaxy cluster at z=1.62 located in the Spitzer Wide-Area Infrared Extragalactic survey XMM-LSS field. This structure was selected solely as an overdensity of galaxies with red Spitzer/IRAC colors, satisfying [3.6]-[4.5] > -0.1 AB mag. Photometric redshifts derived from Subaru XMM Deep Survey (BViz-bands), UKIRT Infrared Deep Survey-Ultra-Deep Survey (UKIDSS-UDS, JK-bands), and from the Spitzer Public UDS survey (3.6-8.0 micron) show that this cluster corresponds to a surface density of galaxies at z ~ 1.6 that is more than 20 sigma above the mean at this redshift. We obtained optical spectroscopic observations of galaxies in the cluster region using IMACS on the Magellan telescope. We measured redshifts for seven galaxies in the range z=1.62-1.63 within 2.8 arcmin (<1.4 Mpc) of the astrometric center of the cluster. A posteriori analysis of the XMM data in this field reveal a weak (4 sigma) detection in the [0.5-2 keV] band compatible with the expected thermal emission from such a cluster. The color-magnitude diagram of the galaxies in this cluster shows a prominent red-sequence, dominated by a population of red galaxies with (z-J) > 1.7 mag. The photometric redshift probability distributions for the red galaxies are strongly peaked at z=1.62, coincident with the spectroscopically confirmed galaxies. The rest-frame (U-B) color and scatter of galaxies on the red-sequence are consistent with a mean luminosity-weighted age of 1.2 +/- 0.1 Gyr, yielding a formation redshift z_f = 2.35 +/- 0.10, and corresponding to the last significant star-formation period in these galaxies.

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arXiv2009

The evolution of the hard X-ray luminosity function of AGN

We present new observational determinations of the evolution of the 2-10keV X-ray luminosity function (XLF) of AGN. We utilise data from a number of surveys including both the 2Ms Chandra Deep Fields and the AEGIS-X 200ks survey, enabling accurate measurements of the evolution of the faint end of the XLF. We combine direct, hard X-ray selection and spectroscopic follow-up or photometric redshift estimates at z<1.2 with a rest-frame UV colour pre-selection approach at higher redshifts to avoid biases associated with catastrophic failure of the photometric redshifts. Only robust optical counterparts to X-ray sources are considered using a likelihood ratio matching technique. A Bayesian methodology is developed that considers redshift probability distributions, incorporates selection functions for our high redshift samples, and allows robust comparison of different evolutionary models. We find that the XLF retains the same shape at all redshifts, but undergoes strong luminosity evolution out to z~1, and an overall negative density evolution with increasing redshift, which thus dominates the evolution at earlier times. We do not find evidence that a Luminosity-Dependent Density Evolution, and the associated flattening of the faint-end slope, is required to describe the evolution of the XLF. We find significantly higher space densities of low-luminosity, high-redshift AGN than in prior studies, and a smaller shift in the peak of the number density to lower redshifts with decreasing luminosity. The total luminosity density of AGN peaks at z=1.2+/-0.1, but there is a mild decline to higher redshifts. We find >50% of black hole growth takes place at z>1, with around half in Lx<10^44 erg/s AGN.

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arXiv2009

The 0.4 < z < 1.3 star formation history of the Universe as viewed in the far-infrared

[Abridged] We use the deepest existing mid- and far-infrared observations (reaching ~3 mJy at 70 um) obtained with Spitzer in the GOODS and FIDEL fields to derive the evolution of the rest-frame 15 um, 35 um, and total infrared luminosity functions of galaxies spanning z < 1.3. In comparison with previous studies, the present one takes advantage of deep 70 um observations that provide a more robust infrared luminosity indicator than 24 um affected by the emission of PAHs at high redshift (z~1), and we use several independent fields to control cosmic variance. We use a new extraction technique based on the well-determined positions of galaxies at shorter wavelengths to extract the 24 and 70 um flux densities of galaxies. Using a combination of photometric and spectroscopic redshifts that exist for ~80% of the sources in our sample, we are able to estimate the rest-frame luminosities of galaxies at 15um and 35um. Then using a careful stacking analysis to validate the infrared bolometric corrections we compute the infrared luminosity function in three redshift bins z~0.55, z~0.85, and z~1.15. We find that the average infrared spectral energy distribution of galaxies over the last 2/3 of the cosmic time is consistent with that of local galaxies, although individual sources do present significant scatter. We also measured both the bright and faint ends of the infrared luminosity functions and find no evidence for a change in the slope of the double power law used to characterize the luminosity function. The redshift evolution of infrared luminous galaxies is consistent with pure luminosity evolution proportional to (1+z)^3.6 up to z~1.3. The comoving number density of infrared luminous galaxies has increased by a factor of ~100 between 0< z <1. By z~1.0, LIRGs produce half of the total comoving infrared luminosity density.

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arXiv2009

Nearby Galaxies in the 2micron All Sky Survey I. K-band Luminosity Functions

Differential K-band luminosity functions (LFs) are presented for a complete sample of 1613 nearby bright galaxies segregated by visible morphology. The LF for late-type spirals follows a power law that rises towards low luminosities whereas the LFs for ellipticals, lenticulars and bulge-dominated spirals are peaked and decline toward both higher and lower luminosities. Each morphological type (E, S0, S0/a-Sab, Sb-Sbc, Sc-Scd) contributes approximately equally to the overall K-band luminosity density of galaxies in the local universe. Type averaged bulge/disk ratios are used to subtract the disk component leading to the prediction that the K-band LF for bulges is bimodal with ellipticals dominating the high luminosity peak, comprising 60% of the bulge luminosity density in the local universe with the remaining 40% contributed by lenticulars and the bulges of spirals. Overall, bulges contribute 30% of the galaxy luminosity density at K in the local universe with spiral disks making up the remainder. If bulge luminosities indicate central black hole masses, then our results predict that the black hole mass function is also bimodal.

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arXiv2009

IRS Spectroscopy and Multi-wavelength Study of Luminous Star-forming Galaxies at z~1.9

We analyze a sample of galaxies chosen to have F24>0.5 mJy and satisfy a certain IRAC color criterion. IRS spectra yield redshifts, spectral types, and PAH luminosities, to which we add multi-wavelength broadband photometry. Stellar population modeling and IRS spectra together demonstrate that the double criteria used to select this sample have efficiently isolated massive star-forming galaxies at z~1.9. This is the first starburst-dominated ULIRG sample at high redshift with total infrared luminosity measured directly from FIR and millimeter photometry, and as such gives us the first accurate view of broadband SEDs for starburst galaxies at extremely high luminosity and at all wavelengths. Similar broadband data are assembled for three other galaxy samples -- local starburst galaxies, local AGN/ULIRGS, and a second 24mu-luminous z~1.9 sample dominated by AGN. L(PAH)/L(IR) for the new z~1.9 starburst sample is the highest ever seen, some three times higher than in local starbursts, whereas in AGNs this ratio is depressed below the starburst trend, often severely. Several pieces of evidence imply that AGNs exist in this starburst dominated sample, except two of which even host very strong AGN, while they still have very strong PAH emission. The ACS images show most objects have very extended morphologies in the rest-frame UV band, thus extended distribution of PAH molecules. Such an extended distribution prevents further destruction PAH molecules by central AGNs. We conclude that objects in this sample are ULIRGs powered mainly by starburst; and the total infrared luminosity density contributed by this type of objects is 0.9-2.6x 10^7 Lsol/Mpc^3.

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arXiv2009

Spitzer Observations of Cold Dust Galaxies

We combine new Spitzer Space Telescope observations in the mid- and far-infrared with SCUBA 850 micron observations to improve the measurement of dust temperatures, masses and luminosities for 11 galaxies of the SCUBA Local Universe Galaxy Survey (SLUGS). By fitting dust models we measure typical dust masses of 10E7.9 M_sol and dust luminosities of ~ 10E10 L_sol, for galaxies with modest star formation rates. The data presented in this paper combined with previous observations show that cold dust is present in all types of spiral galaxies and is a major contributor to their total luminosity. Because of the lower dust temperature of the SCUBA sources measured in this paper, they have flatter Far-IR nu F_nu(160um)/nu F_nu(850um) slopes than the larger Spitzer Nearby Galaxies Survey (SINGS), the sample that provides the best measurements of the dust properties of galaxies in the nearby universe. The new data presented here added to SINGS extend the parameter space that is well covered by local galaxies, providing a comprehensive set of templates that can be used to interpret the observations of nearby and distant galaxies.

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arXiv2009

The Morphological Type Dependence of K-band Luminosity Functions

Differential 2.2um (K-band) luminosity functions are presented for a complete sample of 1570 nearby Vgsr < 3000 km/s, where Vgsr is the velocity measured with respect to the Galactic standard of rest), bright (K < 10 mag), galaxies segregated by visible morphology. The K-band luminosity function for late-type spirals follows a power law that rises towards low luminosities whereas the K-band luminosity functions for ellipticals, lenticulars and bulge-dominated spirals are peaked with a fall off at both high and low luminosities. However, each morphological type (E, S0, S0/a-Sab, Sb-Sbc, Sc-Scd) contributes approximately equally to the overall K-band luminosity density in the local universe, and by inference, the stellar mass density as well.

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arXiv2008

AEGIS-X: The Chandra Deep Survey of the Extended Groth Strip

We present the AEGIS-X survey, a series of deep Chandra ACIS-I observations of the Extended Groth Strip. The survey comprises pointings at 8 separate positions, each with nominal exposure 200ks, covering a total area of approximately 0.67 deg2 in a strip of length 2 degrees. We describe in detail an updated version of our data reduction and point source detection algorithms used to analyze these data. A total of 1325 band-merged sources have been found to a Poisson probability limit of 4e-6, with limiting fluxes of 5.3e-17 erg/cm2/s in the soft (0.5-2 keV) band and 3.8e-16 erg/cm2/s in the hard (2-10 keV) band. We present simulations verifying the validity of our source detection procedure and showing a very small, <1.5%, contamination rate from spurious sources. Optical/NIR counterparts have been identified from the DEEP2, CFHTLS, and Spitzer/IRAC surveys of the same region. Using a likelihood ratio method, we find optical counterparts for 76% of our sources, complete to R(AB)=24.1, and, of the 66% of the sources that have IRAC coverage, 94% have a counterpart to a limit of 0.9 microJy at 3.6 microns (m(AB)=23.8). After accounting for (small) positional offsets in the 8 Chandra fields, the astrometric accuracy of the Chandra positions is found to be 0.8 arcsec RMS, however this number depends both on the off-axis angle and the number of detected counts for a given source. All the data products described in this paper are made available via a public website.

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arXiv2008

Metallicity Effects on Dust Properties in Starbursting Galaxies

We present infrared observations of 66 starburst galaxies over a wide range of oxygen abundances, to measure how metallicity affects their dust properties. The data include imaging and spectroscopy from the Spitzer Space Telescope, supplemented by groundbased near-infrared imaging. We confirm a strong correlation of aromatic emission with metallicity, with a threshold at a metallicity [12+log(O/H)]~8. The large scatter in both the metallicity and radiation hardness dependence of this behavior implies that it is not due to a single effect, but to some combination. We show that the far-infrared color temperature of the large dust grains increases towards lower metallicity, peaking at a metallicity of 8 before turning over. We compute dust masses and compare them to HI masses from the literature to derive the gas to dust ratio, which increases by nearly 3 orders of magnitude between solar metallicity and a metallicity of 8, below which it flattens out. The abrupt change in aromatic emission at mid-infrared wavelengths thus appears to be reflected in the far-infrared properties, indicating that metallicity changes affect the composition of the full range of dust grain sizes that dominate the infrared emission. In addition, we find that the ratio L(8 micron)/L(TIR), important for calibrating 24 micron measurements of high redshift galaxies, increases slightly as the metallicity decreases from ~solar to ~50% of solar, and then decreases by an order of magnitude with further decreases in metallicity. Although the great majority of galaxies show similar patterns of behavior as described above, there are three exceptions, SBS 0335-052E, Haro 11, and SHOC 391. Their infrared SEDs are dominated energetically by the mid-IR near 24 micron rather than by the 60 - 200 micron region. (Abridged)

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arXiv2007

The Environment on few Mpc scales of Infrared Luminous Galaxies at Redshifts z~1

We investigate the environment of infrared luminous galaxies (L$_{IR}$[8-1000 $μ$m$] >10^{11}$L$_{\sun}$). We focus on the redshift range 0.7 $\leq$ z $\leq$ 1, where these galaxies dominate the star formation activity and play a significant role in galaxy evolution. We employ MIPS 24$μ$m data to identify infrared galaxies in the Extended Groth Strip (EGS). We use a local density indicator to probe the environment on few Mpc scales and a group member catalog, both of which make use of the DEEP2 spectroscopic redshift catalog, to quantify the environment of these galaxies. We find that the local environment of LIRGs and ULIRGs is intermediate between that of blue and red galaxies. LIRGs and ULIRGs avoid underdense environments and inhabit local environments that are more dense on average than those of other DEEP2 galaxies at similar redshifts. However, when the comparison sample of the non-IR DEEP2 galaxies is restricted to have the same range of stellar mass, color, or luminosity as the IR--galaxies, there is no longer any significant difference in environment; the IR-galaxies follow the same trends in the color-environment and luminosity-environment relations observed at z$\sim$1. We also find that about 30% of the LIRGs and ULIRGs belong to groups, associated with a minimum dark matter halo of 6$\times10^{12}$M$_{\odot}$h$^{-1}$. The group members constitute 20 % of the sources responsible for the IR star formation rate density and comoving energy density at z$\sim$1.

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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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