Camilla Pacifici

3D-HST WFC3-selected Photometric Catalogs in the Five CANDELS/3D-HST Fields: Photometry, Photometric Redshifts and Stellar Masses

The 3D-HST and CANDELS programs have provided WFC3 and ACS spectroscopy and photometry over ≈900 arcmin2 in five fields: AEGIS, COSMOS, GOODS-North, GOODS-South, and the UKIDSS UDS field. All these fields have a wealth of publicly available imaging data sets in addition to the Hubble Space Telescope (HST) data, which makes it possible to construct the spectral energy distributions (SEDs) of objects over a wide wavelength range. In this paper we describe a photometric analysis of the CANDELS and 3D-HST HST imaging and the ancillary imaging data at wavelengths 0.3-8 μm. Objects were selected in the WFC3 near-IR bands, and their SEDs were determined by carefully taking the effects of the point-spread function in each observation into account. A total of 147 distinct imaging data sets were used in the analysis. The photometry is made available in the form of six catalogs: one for each field, as well as a master catalog containing all objects in the entire survey. We also provide derived data products: photometric redshifts, determined with the EAZY code, and stellar population parameters determined with the FAST code. We make all the imaging data that were used in the analysis available, including our reductions of the WFC3 imaging in all five fields. 3D-HST is a spectroscopic survey with the WFC3 and ACS grisms, and the photometric catalogs presented here constitute a necessary first step in the analysis of these grism data. All the data presented in this paper are available through the 3D-HST Web site (http://3dhst.research.yale.edu).

THE 3D-HST SURVEY: HUBBLE SPACE TELESCOPE WFC3/G141 GRISM SPECTRA, REDSHIFTS, AND EMISSION LINE MEASUREMENTS FOR ∼100,000 GALAXIES

NASA [NAS5-26555]; NASA through Hubble Fellowship - Space Telescope Science Institute [HST-HF-51318.001, HST-HF2-51368]; 3D-HST Treasury Program [GO 12177, 12328]; NASA/ESA HST [GO 11600, GO 13420]

Empirical determination of the shape of dust attenuation curves in star‐forming galaxies

We present a systematic study of the shape of the dust attenuation curve in star-forming galaxies from the far-ultraviolet (far-UV) to the near-infrared (NIR; ∼0.15–2 μm), as a function of specific star formation rate (ψ S) and axial ratio (b/a), for galaxies with and without a significant bulge. Our sample comprises 23 000 (15 000) galaxies with a median redshift of 0.07, with photometric entries in the Sloan Digital Sky Survey (SDSS), UKIRT Infrared Deep Sky SurveyLarge Area Survey and Galaxy Evolution Explorer-All-Sky Imaging Survey catalogues and emission-line measurements from the SDSS spectroscopic survey. We develop a new pairmatching technique to isolate the dust attenuation curves from the stellar continuum emission. The main results are: (i) the slope of the attenuation curve in the optical varies weakly with ψ S, strongly with b/a, and is significantly steeper than the Milky Way extinction law in bulgedominated galaxies; (ii) the NIR slope is constant and matches the slope of the Milky Way extinction law; (iii) the UV has a slope change consistent with a dust bump at 2175 A which is evident in all samples and varies strongly in strength with b/a in the bulge-dominated sample; (iv) there is a strong increase in emission-line-to-continuum dust attenuation (τ V,line/τ V,cont) with both decreasing ψ S and increasing b/a; and (v) radial gradients in dust attenuation increase strongly with increasing ψ S, and the presence of a bulge does not alter the strength of the gradients. These results are consistent with the picture in which young stars are surrounded by dense ‘birth clouds’ with low covering factor which disperse on time-scales of ∼10 7 yr and the diffuse interstellar dust is distributed in a centrally concentrated disc with a smaller scaleheight than the older stars that contribute the majority of the red and NIR light. Within this model, the path-length of diffuse dust, but not of birth-cloud dust, increases with increasing inclination and the apparent optical attenuation curve is steepened by the differential effect of larger dust opacity towards younger stars than towards older stars. Additionally, our findings suggest that: (i) galaxies with higher star formation rates per unit stellar mass have a higher fraction of diffuse dust, which is more centrally concentrated; (ii) the observed strength of the 2175-A dust feature is affected predominantly by global geometry; and (iii) only highly inclined discs are optically thick. We provide new empirically derived attenuation curves for correcting the light from star-forming galaxies for dust attenuation.

Active galactic nuclei emission line diagnostics and the mass-metallicity relation up to redshift z ∼ 2: The impact of selection effects and evolution

Emission line diagnostic diagrams probing the ionization sources in galaxies, such as the Baldwin-Phillips-Terlevich (BPT) diagram, have been used extensively to distinguish active galactic nuclei (AGN) from purely star-forming galaxies. However, they remain poorly understood at higher redshifts. We shed light on this issue with an empirical approach based on a z ~ 0 reference sample built from ~300,000 Sloan Digital Sky Survey galaxies, from which we mimic selection effects due to typical emission line detection limits at higher redshift. We combine this low-redshift reference sample with a simple prescription for luminosity evolution of the global galaxy population to predict the loci of high-redshift galaxies on the BPT and Mass-Excitation (MEx) diagnostic diagrams. The predicted bivariate distributions agree remarkably well with direct observations of galaxies out to z ~ 1.5, including the observed stellar mass-metallicity (MZ) relation evolution. As a result, we infer that high-redshift star-forming galaxies are consistent with having normal interstellar medium (ISM) properties out to z ~ 1.5, after accounting for selection effects and line luminosity evolution. Namely, their optical line ratios and gas-phase metallicities are comparable to that of low-redshift galaxies with equivalent emission-line luminosities. In contrast, AGN narrow-line regions may show a shift toward lower metallicities at higher redshift. While a physical evolution of the ISM conditions is not ruled out for purely star-forming galaxies and may be more important starting at z 2, we find that reliably quantifying this evolution is hindered by selections effects. The recipes provided here may serve as a basis for future studies toward this goal. Code to predict the loci of galaxies on the BPT and MEx diagnostic diagrams and the MZ relation as a function of emission line luminosity limits is made publicly available.

Relative merits of different types of rest-frame optical observations to constrain galaxy physical parameters

We present a new approach to constrain galaxy physical parameters from the combined interpretation of stellar and nebular emission in wide ranges of observations. This approach relies on the Bayesian analysis of any type of galaxy spectral energy distribution using a comprehensive library of synthetic spectra assembled using state-of-the-art models of star formation and chemical enrichment histories, stellar population synthesis, nebular emission and attenuation by dust. We focus on the constraints set by 5-band ugriz photometry and low- and medium-resolution spectroscopy at rest wavelengths =3600‐7400 ˚ A on a few physical parameters of galaxies: the observer-frame absolute r-band stellar mass-to-light ratio, M =Lr; the fraction of a current galaxy stellar mass formed during the last 2.5 Gyr,fSFH; the specific star formation rate, S; the gas-phase oxygen abundance, 12 + log (O/H); the total effective V -band absorption optical depth of the dust, ^ V ; and the fraction of this arising from dust in the ambient interstellar medium, . Since these parameters cannot be known a priori for any galaxy sample, we assess the accuracy to which they can be retrieved from observations by simulating ‘pseudo-observations’ using models with known parameters. Assuming that these models are good approximations of true galaxies, we find that the combined analysis of stellar and nebular emission in low-resolution (50 ˚ A FWHM) galaxy spectra provides valuable constraints on all physical parameters. The typical uncertainties in high-quality spectra are about 0.13 dex for M =Lr, 0.23 for fSFH, 0.24 dex for S, 0.28 for 12 + log (O/H), 0.64 for ^ V and 0.16 for . The uncertainties in 12 + log (O/H) and ^ V tighten by about 20 percent for galaxies with detectable emission lines and by another 45 percent when the spectral resolution is increased to 5 ˚ A FWHM. At this spectral resolution, the analysis of the combined stellar and nebular emission in the high-quality spectra of 12,660 SDSS star-forming galaxies using our approach yields likelihood distributions of M?, 12 + log (O/H), ^ V and S similar to those obtained in previous separate analyses of the stellar and nebular emission at the original (twice higher) SDSS spectral resolution. Meanwhile, rest-frame ugriz photometry provides competitive constraints on M =Lr. We show that the constraints derived on galaxy physical parameters from these different types of observations depend sensitively on signal-to-noise ratio. Our approach can be extended to the analysis of any type of observation across the wavelength range covered by spectral evolution models.

The Nature of Extreme Emission Line Galaxies at z=1-2: Kinematics and Metallicities from Near-Infrared Spectroscopy

We present near-infrared spectroscopy of a sample of 22 Extreme Emission Line Galaxies at redshifts 1.3 < z < 2.3, confirming that these are low-mass (M* = 108-109M(circle dot)) galaxies undergoing intense starburst episodes (M*/SFR similar to 10-100 Myr). The sample is selected by [O iii] or Ha emission line flux and equivalent width using near-infrared grism spectroscopy from the 3D-HST survey. High-resolution NIR spectroscopy is obtained with LBT/LUCI and VLT/X-SHOOTER. The [Oiii]/H line ratio is high (greater than or similar to 5) and [N ii]/Ha is always significantly below unity, which suggests a low gas-phase metallicity. We are able to determine gas-phase metallicities for seven of our objects using various strong-line methods, with values in the range 0.05-0.30 Z similar to and with a median of 0.15 Z similar to; for three of these objects we detect [O iii].4363, which allows for a direct constraint on the metallicity. The velocity dispersion, as measured from the nebular emission lines, is typically similar to 50 km s-1. Combined with the observed star-forming activity, the Jeans and Toomre stability criteria imply that the gas fraction must be large (fgas - 2/3), consistent with the difference between our dynamical and stellar mass estimates. The implied gas depletion timescale (several hundred Myr) is substantially longer than the inferred mass-weighted ages (similar to 50 Myr), which further supports the emerging picture that most stars in low-mass galaxies form in short, intense bursts of star formation.

Keck-I MOSFIRE Spectroscopy of Compact Star- Forming Galaxies at z≳ 2: High Velocity Dispersions in Progenitors of Compact Quiescent Galaxies

We present Keck-I MOSFIRE near-infrared spectroscopy for a sample of 13 compact star-forming galaxies (SFGs) at redshift 2 ≤ z ≤ 2.5 with star formation rates of SFR ~ 100 M_☉ yr^(–1) and masses of log(M/M_☉) ~10.8. Their high integrated gas velocity dispersions of σ_(int_ =230^(+40)_(-30) km s^(–1), as measured from emission lines of Hα and [O III], and the resultant M_* -σ_(int) relation and M_*-M_(dyn) all match well to those of compact quiescent galaxies at z ~ 2, as measured from stellar absorption lines. Since log(M*/M_(dyn)) =–0.06 ± 0.2 dex, these compact SFGs appear to be dynamically relaxed and evolved, i.e., depleted in gas and dark matter (<13^(+17)_(-13)%), and present larger σ_(int) than their non-compact SFG counterparts at the same epoch. Without infusion of external gas, depletion timescales are short, less than ~300 Myr. This discovery adds another link to our new dynamical chain of evidence that compact SFGs at z ≳ 2 are already losing gas to become the immediate progenitors of compact quiescent galaxies by z ~ 2.

How Dead are Dead Galaxies? Mid-infrared Fluxes of Quiescent Galaxies at Redshift 0.3 z < 2.5: Implications for Star Formation Rates and Dust Heating

We investigate star formation rates (SFRs) of quiescent galaxies at high redshift (0.3 1.5 marginally consistent with the ones expected from gas recycling (assuming that mass loss from evolved stars refuels star formation) and well below that at lower redshifts.

BREAKING THE CURVE WITH CANDELS: A BAYESIAN APPROACH TO REVEAL THE NON-UNIVERSALITY OF THE DUST-ATTENUATION LAW AT HIGH REDSHIFT

Dust attenuation affects nearly all observational aspects of galaxy evolution, yet very little is known about the form of the dust-attenuation law in the distant Universe. Here, we model the spectral energy distributions (SEDs) of galaxies at z = 1.5--3 from CANDELS with rest-frame UV to near-IR imaging under different assumptions about the dust law, and compare the amount of inferred attenuated light with the observed infrared (IR) luminosities. Some individual galaxies show strong Bayesian evidence in preference of one dust law over another, and this preference agrees with their observed location on the plane of infrared excess (IRX,

Hubble Space Telescope Grism Spectroscopy of Extreme Starbursts Across Cosmic Time: The Role of Dwarf Galaxies in the Star Formation History of the Universe

L_{\text{TIR}}/L_{\text{UV}}