We constrain the slope of the star formation rate (log Ψ) to stellar mass (log M ⋆ ) relation down to log(M ⋆ /M ⊙ ) = 8.4 (log(M ⋆ /M ⊙ ) = 9.2) at z = 0.5 (z = 2.5) with a mass-complete sample of 39,106 star-forming galaxies selected from the 3D-HST photometric catalogs, using deep photometry in the CANDELS fields. For the first time, we find that the slope is dependent on stellar mass, such that it is steeper at low masses (log Ψ ∝ log M ⋆ ) than at high masses (log Ψ ∝ (0.3 − 0.6) log M ⋆ ). These steeper low mass slopes are found for three different star formation indicators: the combination of the ultraviolet (UV) and infrared (IR), calibrated from a stacking analysis of Spitzer/MIPS 24µm imaging; β-corrected UV SFRs; and Hα SFRs. The normalization of the sequence evolves differently in distinct mass regimes as well: for galaxies less massive than log(M ⋆ /M ⊙ ) < 10 the specific SFR (Ψ/M ⋆ ) is observed to be roughly self-similar with Ψ/M ⋆ ∝ (1 + z) 1.9 , whereas more massive galaxies show a stronger evolution with Ψ/M ⋆ ∝ (1 + z) 2.2−3.5 for log(M ⋆ /M ⊙ ) = 10.2 − 11.2. The fact that we find a steep slope of the star formation sequence for the lower mass galaxies will help reconcile theoretical galaxy formation models with the observations.

The Nuclear Spectroscopic Telescope Array (NuSTAR) mission, launched on 2012 June 13, is the first focusing high-energy X-ray telescope in orbit. NuSTAR operates in the band from 3 to 79 keV, extending the sensitivity of focusing far beyond the ∼10 keV high-energy cutoff achieved by all previous X-ray satellites. The inherently low background associated with concentrating the X-ray light enables NuSTAR to probe the hard X-ray sky with a more than 100-fold improvement in sensitivity over the collimated or coded mask instruments that have operated in this bandpass. Using its unprecedented combination of sensitivity and spatial and spectral resolution, NuSTAR will pursue five primary scientific objectives: (1) probe obscured active galactic nucleus (AGN) activity out to the

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We present a first analysis of deep 24 m observations with the Spitzer Space Telescope of a sample of nearly 1500 galaxies in a thin redshift slice, 0:65 z < 0:75. We combine the infrared data with redshifts, rest-frame luminosities, and colors from COMBO-17 and with morphologies from Hubble Space Telescope images collected by the Galaxy Evolution from Morphology and SEDs (GEMS) and Great Observatories Origins Deep Survey (GOODS) projects. To characterize the decline in star formation rate (SFR) since z $ 0:7, we estimate the total thermal IR luminosities, SFRs, and stellar masses for the galaxies in this sample. At z $ 0:7, nearly 40% of intermediate-and high-mass galaxies (with stellar masses !2 ; 10 10 M ) are undergoing a period of intense star formation above their past-averaged SFR. In contrast, less than 1% of equally massive galaxies in the local universe have similarly intense star formation activity. Morphologically undisturbed galaxies dominate the total infrared luminosity density and SFR density: at z $ 0:7, more than half of the intensely star-forming galaxies have spiral morphologies, whereas less than $30% are strongly interacting. Thus, a decline in major merger rate is not the underlying cause of the rapid decline in cosmic SFR since z $ 0:7. Physical properties that do not strongly affect galaxy morphology-for example, gas consumption and weak interactions with small satellite galaxies-appear to be responsible.

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

We investigate the nature of a sample of 92 Spitzer MIPS 24 m-selected galaxies in the CDF-S, showing powerlaw-like emission in the Spitzer IRAC 3.6-8 m 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 AGNs 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 SEDs of the X-ray-detected galaxies are almost equally divided between a BLAGN SED class (similar to an optically selected QSO) and an NLAGN SED (similar to the BLAGN SED but with an obscured UV/optical continuum). A small fraction of SEDs resemble warm ULIRGs (e.g., Mrk 231). 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 m) selected and optically bright ( VVDS I AB 24) star-forming galaxies whose SEDs show a very prominent stellar bump at 1.6 m. The galaxies detected in X-rays have 2-8 keV rest-frame luminosities typical of AGNs. 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 AGNs. If all these sources are actually obscured AGNs, we would observe a ratio of obscured to unobscured 24 m-detected AGNs of 2:1, whereas models predict a ratio of up to 3:1. Additional studies using Spitzer to detect X-ray-quiet AGNs are likely to find more such obscured sources.

We present a search for "" weak ÏÏ Mg II absorbers [those with in the HIRES/Keck W r (2796) \ 0.3 A ] spectra of 26 QSOs. We found 30, of which 23 are newly discovered. The spectra are 80% complete to and have a cumulative redshift path of D17.2 for the redshift range 0.4 ¹ z ¹ 1.4. The W r (2796) \ 0.02 A number of absorbers per unit redshift, dN/dz, is seen to increase as the equivalent width threshold is decreased ; we obtained dN/dz \ 1.74^0.10 for our 0.02 sample. The equivalent width W r (2796) \ 0.3 A distribution follows a power law, N(W ) P W~d, with d D 1.0 ; there is no turnover down to W r (2796) \ 0.02 at SzT \ 0.9. Weak absorbers comprise at least 65% of the total Mg II absorption population, A which outnumbers Lyman limit systems (LLSs) by a factor of 3.8^1.1 ; the majority of weak Mg II absorbers must arise in sub-LLS environments. Tentatively, we predict that D5% of the Lya forest clouds with will have detectable Mg II absorption to and that this W r (Lya) º 0.1 A W r min(2796) \ 0.02 A is primarily a high-metallicity selection e †ect This implies that Mg II absorbing struc-. tures Ðgure prominently as tracers of sub-LLS environments where gas has been processed by stars. We compare the number density of absorbers with that of both high and low surface W r (2796) º 0.02 A brightness galaxies and Ðnd a Ðducial absorber size of 35 h~1È63 h~1 kpc, depending upon the assumed galaxy population and their absorption properties. The individual absorbing "" clouds ÏÏ have W r (2796) ¹ 0.15 and their narrow (often unresolved) line widths imply temperatures of D25,000 K. We measured A , from C IV in Faint Object Spectrograph/Hubble Space T elescope archival spectra and, based W r (1548) upon comparisons with Fe II, found a range of ionization conditions (low, high, and multiphase) in absorbers selected by weak Mg II.