Right to Serve, Right to Lead: Lives and Legacies of the USCT

This is a catalog for an exhibit that follows the evolution of African-American participation in the Civil War, from slaves, to contrabands, to soldiers of the United States Colored Troops (USCT), as well as the lives of black veterans beyond the war, and their ultimate military and social legacy. Using a variety of period items, it creates a narrative that stretches from the Antebellum Period to the current day. In doing so, the exhibit shows how black sacrifice on the battlefield redefined the war\u27s purpose throughout the divided nation, how Jim Crowe suppressed the memory of black participation after Reconstruction, and how the illustrious African-American military tradition left by the USCT endures to this day in their modern heirs

S-CANDELS: The Spitzer-Cosmic Assembly Near-Infrared Deep Extragalactic Survey. Survey Design, Photometry, and Deep IRAC Source Counts

The Spitzer-Cosmic Assembly Deep Near-Infrared Extragalactic Legacy Survey (S-CANDELS; PI G. Fazio) is a Cycle 8 Exploration Program designed to detect galaxies at very high redshifts (z > 5). To mitigate the effects of cosmic variance and also to take advantage of deep coextensive coverage in multiple bands by the Hubble Space Telescope Multi-Cycle Treasury Program CANDELS, S-CANDELS was carried out within five widely separated extragalactic fields: the UKIDSS Ultra-Deep Survey, the Extended Chandra Deep Field South, COSMOS, the HST Deep Field North, and the Extended Groth Strip. S-CANDELS builds upon the existing coverage of these fields from the Spitzer Extended Deep Survey (SEDS) by increasing the integration time from 12 hours to a total of 50 hours but within a smaller area, 0.16 square degrees. The additional depth significantly increases the survey completeness at faint magnitudes. This paper describes the S-CANDELS survey design, processing, and publicly-available data products. We present IRAC dual-band 3.6+4.5 micron catalogs reaching to a depth of 26.5 AB mag. Deep IRAC counts for the roughly 135,000 galaxies detected by S-CANDELS are consistent with models based on known galaxy populations. The increase in depth beyond earlier Spitzer/IRAC surveys does not reveal a significant additional contribution from discrete sources to the diffuse Cosmic Infrared Background (CIB). Thus it remains true that only roughly half of the estimated CIB flux from COBE/DIRBE is resolved.Comment: 23 pages, 19 figures, accepted by ApJ

A Substantial Population of Red Galaxies at z > 2: Modeling of the Spectral Energy Distributions of an Extended Sample

We investigate the nature of the substantial population of high-z galaxies with Js-Ks>2.3 discovered as part of our FIRES survey. This colour cut efficiently isolates z>2 galaxies with red rest-frame optical colors ("Distant Red Galaxies" or DRGs). We select objects in the 2.5'x2.5' HDF-South (HDF-S) and 5'x5' field around the MS1054-03 cluster; the surface densities at Ks<21 are 1.6+-0.6 and 1.0+-0.2 arcmin^-2. We discuss the 34 DRGs at 2<z<3.5: 11 at Ks<22.5 in HDF-S and 23 at Ks<21.7 in the MS1054-03 field. We analyze the SEDs constructed from our deep near-infrared (NIR) and optical imaging from the ESO VLT and HST. We develop diagnostics involving I-Js, Js-H, and H-Ks to argue that the red NIR colors of DRGs cannot be attributed solely to extinction and require for many an evolved stellar population with prominent Balmer/4000A break. In the rest-frame, the optical colours of DRGs fall within the envelope of normal nearby galaxies and the UV colours suggest a wide range in star formation activity and/or extinction. This contrasts with the much bluer and more uniform SEDs of Lyman break galaxies (LBGs). From evolutionary synthesis models with constant star formation, solar metallicity, Salpeter IMF, and Calzetti et al. extinction law, we derive for the HDF-S (MS1054-03 field) DRGs median ages of 1.7(2.0) Gyr, A_V = 2.7(2.4) mag, stellar masses 0.8(1.6)x10^11 Msun, M/L_V = 1.2(2.3) Msun/LVsun, and SFR = 120(170) Msun/yr. Models assuming declining SFRs with e-folding timescales of 10Myr-1Gyr generally imply younger ages, lower A_V's and SFRs, but similar stellar masses within a factor of two. Compared to LBGs at similar redshifts and rest-frame L_V's, DRGs are older, more massive, and more obscured for any given star formation history. [ABRIDGED]Comment: Accepted for publication in the Astrophysical Journal. 27 pages, 14 b/w figure

Predicting Quiescence: The Dependence of Specific Star Formation Rate on Galaxy Size and Central Density at 0.5<z<2.5

In this paper, we investigate the relationship between star formation and structure, using a mass-complete sample of 27,893 galaxies at $0.5<z<2.5$ selected from 3D-HST. We confirm that star-forming galaxies are larger than quiescent galaxies at fixed stellar mass (M$_{\star}$). However, in contrast with some simulations, there is only a weak relation between star formation rate (SFR) and size within the star-forming population: when dividing into quartiles based on residual offsets in SFR, we find that the sizes of star-forming galaxies in the lowest quartile are 0.27$\pm$0.06 dex smaller than the highest quartile. We show that 50% of star formation in galaxies at fixed M$_{\star}$ takes place within a narrow range of sizes (0.26 dex). Taken together, these results suggest that there is an abrupt cessation of star formation after galaxies attain particular structural properties. Confirming earlier results, we find that central stellar density within a 1 kpc fixed physical radius is the key parameter connecting galaxy morphology and star formation histories: galaxies with high central densities are red and have increasingly lower SFR/M$_{\star}$, whereas galaxies with low central densities are blue and have a roughly constant (higher) SFR/M$_{\star}$ at a given redshift. We find remarkably little scatter in the average trends and a strong evolution of $>$0.5 dex in the central density threshold correlated with quiescence from $z\sim0.7-2.0$. Neither a compact size nor high-$n$ are sufficient to assess the likelihood of quiescence for the average galaxy; rather, the combination of these two parameters together with M$_{\star}$ results in a unique quenching threshold in central density/velocity.Comment: 20 pages, 15 figures, and 2 tables; Accepted for publication in the Astrophysical Journa

The Color Magnitude Distribution of Field Galaxies to z~3: the evolution and modeling of the blue sequence

Using deep NIR VLT/ISAAC and optical HST/WFPC2 imaging in the fields of the HDFS and MS1054-03, we study the rest-frame UV-to-optical colors and magnitudes of galaxies to z~3. While there is no evidence for a red sequence at z~3, there does appear to be a well-defined color-magnitude relation (CMR) for blue galaxies at all redshifts, with more luminous galaxies having redder U-V colors. The slope of the blue CMR is independent of redshift d(U-V)/dMV = -0.09 (0.01) and can be explained by a correlation of dust-reddening with luminosity. The average color at fixed luminosity reddens strongly \Delta(U-V) = 0.75 from z~3 to z=0, much of which can be attributed to aging of the stars. The color scatter of the blue sequence is relatively small sigma(U-V) = 0.25 (0.03) and constant to z~3, but notably asymmetrical with a sharp blue ridge and a wing towards redder colors. We explore sets of star formation histories to study the constraints placed by the shape of the scatter at z=2-3. One particular set of models, episodic star formation, reproduces the detailed properties very well. For a two-state model with high and low star formation, the duty cycle is constrained to be > 40% and the contrast between the states must be a factor > 5 (or a scatter in log(SFR) of > 0.35 dex around the mean). However, episodic models do not explain the observed tail of very red galaxies, primarily Distant Red Galaxies (DRGs), which may have ceased star formation altogether or are more heavily obscured. Finally, the relative number density of red, luminous MV < -20.5 galaxies increases by a factor of ~ 6 from z = 2.7 to z = 0.5, as does their contribution to the total rest-frame V-band luminosity density. We are likely viewing the progressive formation of red, passively evolving galaxies.Comment: 29 pages, 24 figures, in emulateapj style. Abstract is abridged. Some postscript figures are compressed. accepted for publication in ApJ (scheduled for August 20, 2007, v665n 2 issue

X-Ray Properties of K-Selected Galaxies at 0.5 Less than z Less than 2.0: Investigating Trends with Stellar Mass, Redshift and Spectral Type

We examine how the total X-ray luminosity correlates with stellar mass, stellar population, and redshift for a K-band limited sample of approximately 3500 galaxies at 0.5 < z < 2.0 from the NEWFIRM Medium Band Survey in the COSMOS field. The galaxy sample is divided into 32 different galaxy types, based on similarities between the spectral energy distributions. For each galaxy type, we further divide the sample into bins of redshift and stellar mass, and perform an X-ray stacking analysis using the Chandra COSMOS data. We find that full band X-ray luminosity is primarily increasing with stellar mass, and at similar mass and spectral type is higher at larger redshifts. When comparing at the same stellar mass, we find that the X-ray luminosity is slightly higher for younger galaxies (i.e., weaker 4000 angstrom breaks), but the scatter in this relation is large. We compare the observed X-ray luminosities to those expected from low- and high-mass X-ray binaries (XRBs). For blue galaxies, XRBs can almost fully account for the observed emission, while for older galaxies with larger 4000 angstrom breaks, active galactic nuclei (AGN) or hot gas dominate the measured X-ray flux. After correcting for XRBs, the X-ray luminosity is still slightly higher in younger galaxies, although this correlation is not significant. AGN appear to be a larger component of galaxy X-ray luminosity at earlier times, as the hardness ratio increases with redshift. Together with the slight increase in X-ray luminosity this may indicate more obscured AGNs or higher accretion rates at earlier times

Réalisation d’un laser à faible courant de seuil, avec des boites quantiques InAs/InP organisées et couplées latéralement

Nous présentons ici la réalisation d’un laser à faible courant de seuil avec des boites quantiques (QDs) organisées et couplées InAs/InP sur subsstrat (311)B pour une émission à 1.55 m. En effet, pour des hautes densités de QDs, une organisation périodique apparaît dans le plan. Cette organisation renforce le couplage latéral inter-boites. Des expériences de magnéto-photoluminescence permettent de mettre en évidence ces effets de couplage. Ce couplage améliore l’injection des porteurs. Une émission laser avec des faibles courants de seuil est obtenue avec de telles boites

The Rest-Frame Optical Luminosity Density, Color, and Stellar Mass Density of the Universe from z=0 to z=3

We present the evolution of the rest-frame optical luminosity density, of the integrated rest-frame optical color, and of the stellar mass density for a sample of Ks-band selected galaxies in the HDF-S. We derived the luminosity density in the rest-frame U, B, and V-bands and found that the luminosity density increases by a factor of 1.9+-0.4, 2.9+-0.6, and 4.9+-1.0 in the V, B, and U rest-frame bands respectively between a redshift of 0.1 and 3.2. We derived the luminosity weighted mean cosmic (U-B)_rest and (B-V)_rest colors as a function of redshift. The colors bluen almost monotonically with increasing redshift; at z=0.1, the (U-B)_rest and (B-V)_rest colors are 0.16 and 0.75 respectively, while at z=2.8 they are -0.39 and 0.29 respectively. We derived the luminosity weighted mean M/LV using the correlation between (U-V)_rest and log_{10} M/LV which exists for a range in smooth SFHs and moderate extinctions. We have shown that the mean of individual M/LV estimates can overpredict the true value by ~70% while our method overpredicts the true values by only ~35%. We find that the universe at z~3 had ~10 times lower stellar mass density than it does today in galaxies with LV>1.4 \times 10^{10} h_{70}^-2 Lsol. 50% of the stellar mass of the universe was formed by $z~1-1.5. The rate of increase in the stellar mass density with decreasing redshift is similar to but above that for independent estimates from the HDF-N, but is slightly less than that predicted by the integral of the SFR(z) curve.Comment: 19 pages, 12 figures, Accepted for Publication in the Dec. 20, 2003 edition of the Astrophysical Journal. Minor changes made to match the accepted version including short discussions on the effects of clustering and on possible systematic effects resulting from photometric redshift error

Evidence for Ubiquitous, High-EW Nebular Emission in z~7 Galaxies: Towards a Clean Measurement of the Specific Star Formation Rate using a Sample of Bright, Magnified Galaxies

Growing observational evidence now indicates that nebular line emission has a significant impact on the rest-frame optical fluxes of z~5-7 galaxies observed with Spitzer. This line emission makes z~5-7 galaxies appear more massive, with lower specific star formation rates. However, corrections for this line emission have been very difficult to perform reliably due to huge uncertainties on the overall strength of such emission at z>~5.5. Here, we present the most direct observational evidence yet for ubiquitous high-EW [OIII]+Hbeta line emission in Lyman-break galaxies at z~7, while also presenting a strategy for an improved measurement of the sSFR at z~7. We accomplish this through the selection of bright galaxies in the narrow redshift window z~6.6-7.0 where the IRAC 4.5 micron flux provides a clean measurement of the stellar continuum light. Observed 4.5 micron fluxes in this window contrast with the 3.6 micron fluxes which are contaminated by the prominent [OIII]+Hbeta lines. To ensure a high S/N for our IRAC flux measurements, we consider only the brightest (H_{160}<26 mag) magnified galaxies we have identified in CLASH and other programs targeting galaxy clusters. Remarkably, the mean rest-frame optical color for our bright seven-source sample is very blue, [3.6]-[4.5]=-0.9+/-0.3. Such blue colors cannot be explained by the stellar continuum light and require that the rest-frame EW of [OIII]+Hbeta be greater than 637 Angstroms for the average source. The bluest four sources from our seven-source sample require an even more extreme EW of 1582 Angstroms. Our derived lower limit for the mean [OIII]+Hbeta EW could underestimate the true EW by ~2x based on a simple modeling of the redshift distribution of our sources. We can also set a robust lower limit of >~4 Gyr^-1 on the specific star formation rates based on the mean SED for our seven-source sample. (abridged)Comment: 9 pages, 6 figures, 1 table, submitted to the Astrophysical Journa