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Below a list of main publication (first, second, or third author papers). For a complete and up-to-date list of publications please visit NASA/ADS or download my publication list as a PDF.

23. The ALPINE-ALMA [CII] Survey: Dust Attenuation Properties and Obscured Star-Formation at z~4.4-5.8   
Y. Fudamoto, P. Oesch, A. Faisst, ... + 34
Submitted to Monthly Notices of the Royal Astronomical Society, April 2020

We present dust attenuation properties of spectroscopically confirmed star forming galaxies on the main sequence at redshift ~4.4-5.8. Our analyses are based on the far infrared continuum observations of 118 galaxies at rest-frame 158μm obtained with the ALMA large program ALPINE. We study the connection between the UV spectral slope (β), stellar mass, and infrared excess (IRX=LIR/LUV). Twenty-three galaxies are individually detected in the continuum at >3.5σ significance. We perform a stacking analysis using both detections and non-detections to study the average dust attenuation properties at z~4.4-5.8. The individual detections and stacks show that the IRX-β relation at z~5 is consistent with a steeper dust attenuation curve than typically found at lower redshifts (z < 4). The attenuation curve is similar to or even steeper than that of the extinction curve of the Small Magellanic Cloud (SMC). This systematic change of the IRX-β relation as a function of redshift suggests an evolution of dust attenuation properties at z > 4. Similarly, we find that our galaxies have lower IRX values up to 1 dex on average at fixed mass compared to previously studied IRX-Mstar relations at z < 4, albeit with significant scatter. This implies a lower obscured fraction of star-formation than at lower redshifts. Our results suggest that dust properties of UV-selected star forming galaxies at z > 4 are characterised by (i) a steeper attenuation curve than at z < 4, and (ii) a rapidly decreasing dust obscured fraction of star formation as a function of redshift. Nevertheless, even among this UV-selected sample, massive galaxies (log(Mstar) > 10) at z~5-6 already exhibit an obscured fraction of star formation of ~45%, indicating a rapid build-up of dust during the epoch of reionization.

22. The ALPINE-ALMA [CII] Survey: Survey Strategy, Observations and Sample Properties of 118 Star-Forming Galaxies at 4 < z < 6   
O. LeFevre, M. Bethermin, A. Faisst, ... + 5
Submitted to Astronomy & Astrophysics, October 2020

The ALMA-ALPINE [CII] survey (A2C2S) aims at characterizing the properties of a sample of normal star-forming galaxies (SFGs). ALPINE, the ALMA Large Program to INvestigate 118 galaxies observed in the [CII]-158μm line and far Infrared (FIR) continuum emission in the period of rapid mass assembly, right after HI reionization ended, at redshifts 4 < z < 6. We present the survey science goals, the observational strategy and the sample selection of the 118 galaxies observed with ALMA, with a typical beam size of about 0.7", or < 6 kpc at the median redshift of the survey. The properties of the sample are described, including spectroscopic redshifts derived from UV-rest frame, stellar masses and star-formation rates obtained from spectral energy distribution (SED) fitting. The observed properties derived from the ALMA data are presented and discussed in terms of the overall detection rate in [CII] and FIR continuum, with the observed signal-to-noise distribution. The sample is representative of the SFG population on the main sequence at these redshifts. The overall detection rate in [CII] is 64%. From a visual inspection of the [CII] data cubes together with the large wealth of ancillary data we find a surprisingly wide range of galaxy types, including 40% mergers, 20% extended and dispersion dominated, 13% compact and 11% rotating discs, the remaining 16% being too faint to be classified. This diversity indicates that a wide array of physical processes must be at work at this epoch, first and foremost galaxy merging. This paper sets a reference sample for the gas distribution in normal SFGs at 4 < z < 6, a key epoch in galaxy assembly, ideally suited for studies with future facilities like the James Webb Space Telescope and extremely large telescopes.

21. The ALPINE-ALMA [CII] Survey: Small Lyα-[CII] Velocity Offsets in Main-Sequence Galaxies at 4.4 < z < 6   
P. Cassata, L. Morselli, A. Faisst, ... + 30
Submitted to Astronomy & Astrophysics, February 2020

The Lyα line in the UV and the [CII] line in the FIR are widely used tools to identify galaxies and to get insights about their ISM properties in the Early Universe. Combining data obtained with ALMA in band 7 as part of the ALMA Large Program to INvestigate [CII] at EarlyTimes (ALPINE) with spectroscopic data from DEIMOS/Keck, VIMOS/VLT and FORS2/VLT we assembled an unique sample of 53 main sequence galaxies at 4.4 < z < 6 in which we detect both the Lya line and the [CII]. We use [CII], observed with ALMA, as a tracer of the systemic velocity, and we exploit the available optical spectroscopy to obtain the Lyα-[CII] and the ISM-[CII] velocity offsets. We find that 90% of the objects have Lyα-[CII] offsets in the range 0 < ΔvLyα-[CII] < 400 km/s, in line with the few measurements available so far in the Early Universe, and much smaller than those observed at lower redshifts. At the same time, we observe ISM-[CII] offsets in the range -500 < ΔvISM-[CII] < 0 km/s, in line with values at all redshifts, that we interpret as evidence for outflows in these galaxies. We find significant anti-correlations between Δv_Lyα-[CII] and the Lya equivalent width EW0(Lyα)(or equivalently fesc(Lyα)), with galaxies that show smaller Δv_Lyα-[CII] having larger EW0(Lyα) and fesc(Lyα). We interpret these results in the framework of the uniform expanding shell model: the escape of Lya photons is determined by the neutral hydrogen column density NHI. In galaxies with slow outflows (0 < vout < 300km/s) when NHI is high, Δv_Lyα-[CII] is high and fesc(Lyα) is low, and vice-versa. In galaxies with fast outflows (vout > 300km/s), the probability of Lya photons to escape is high and Δv_Lyα-[CII] is always small. We suggest that the observed increase of the Lya escape observed in the literature between z~2 and ~6 is not due to a higher incidence of fast outflows at high redshift, but rather to a decrease of NHI in the ISM.

20. The ALPINE-ALMA [CII] Survey: Multiwavelength Ancillary Data and Basic Physical Measurements   
A. L. Faisst, D. Schaerer, B. C. Lemaux, ... + 37
The Astrophysical Journal Supplement Series, 247, 61, April 2020

We present the ancillary data and basic physical measurements for the galaxies in the ALMA Large Program to Investigate C+ at Early Times (ALPINE) survey - the first large multiwavelength survey that aims at characterizing the gas and dust properties of 118 main-sequence galaxies at redshifts 4.4 < z < 5.9 via the measurement of [C II] emission at 158 μm (64% at >3.5σ) and the surrounding far-infrared continuum in conjunction with a wealth of optical and near-infrared data. We outline in detail the spectroscopic data and selection of the galaxies as well as the ground- and space-based imaging products. In addition, we provide several basic measurements including stellar masses, star formation rates (SFR), rest-frame ultra-violet (UV) luminosities, UV continuum slopes (β), and absorption line redshifts, as well as Hα emission derived from Spitzer colors. We find that the ALPINE sample is representative of the 4 < z < 6 galaxy population selected by photometric methods and only slightly biased toward bluer colors (Δ ~ 0.2). Using [C II] as tracer of the systemic redshift (confirmed for one galaxy at z = 4.5 out of 118 for which we obtained optical [O III]λ3727A emission), we confirm redshifted Lyα emission and blueshifted absorption lines similar to findings at lower redshifts. By stacking the rest-frame UV spectra in the [C II] rest frame, we find that the absorption lines in galaxies with high specific SFR are more blueshifted, which could be indicative of stronger winds and outflows.

19. The Redshift Evolution of Rest-UV Spectroscopic Properties to z = 5   
A. J. Pahl, A. Shapley, A. Faisst, ... + 5
Monthly Notices of the Royal Astronomical Society, 493, 3, April 2020

We perform a comprehensive analysis of the redshift evolution of the rest-UV spectra of star-forming galaxies out to z ~ 5. We combine new z ~ 5 measurements of HI Lyα and low- and high-ionization interstellar metal absorption features with comparable measurements at z ∼ 2-4. We measure the equivalent widths of interstellar absorption features using stacked spectra in bins of Lyα equivalent width, performing corrections to Lyα strengths based on a model for the transmission of the intergalactic medium. We find a strong correlation between decreasing low-ionization absorption strength and increasing Lyα emission strength over the redshift range z ~ 2-5, suggesting that both of these quantities are fundamentally linked to neutral gas covering fraction. At the highest Lyα equivalent widths, we observe evolution at z ~ 5 towards greater Lyα emission strength at fixed low-ionization absorption strength. If we interpret the non-evolving relationship of Ly α emission strength and low-ionization line strength at z ~ 2-4 as primarily reflecting the radiative transfer of Lyα photons, this evolution at z ~ 5 suggests a higher intrinsic production rate of Lyα photons than at lower redshift. Our conclusion is supported by the joint evolution of the relationships among Lyα emission strength, interstellar absorption strength, and dust reddening. We perform additional analysis in bins of stellar mass, star formation rate, UV luminosity, and age, examining how the relationships between galaxy properties and Lyα emission evolve towards higher redshift. We conclude that increasing intrinsic Lyα photon production and strong detection of nebular CIV emission (signalling lower metallicity) at z ~ 5 indicate an elevated ionized photon production efficiency (ξion).

18. He II Emission from Wolf-Rayet Stars as a Tool for Measuring Dust Reddening   
C. Leitherer, J. Lee, and A. L. Faisst
The Astrophysical Journal, 158, 192, November 2019

We calibrated a technique to measure dust attenuation in star-forming galaxies. The technique utilizes the stellar-wind lines in Wolf-Rayet stars, which are widely observed in galaxy spectra. The He II 1640 and 4686 features are recombination lines whose ratio is largely determined by atomic physics. Therefore they can serve as a stellar dust probe in the same way as the Balmer lines are used as a nebular probe. We measured the strength of the He II 1640 line in 97 Wolf-Rayet stars in the Galaxy and the Large Magellanic Cloud. The reddening corrected fluxes follow a tight correlation with a fixed ratio of 7.76 for the He II 1640 to 4686 line ratio. Dust attenuation decreases this ratio. We provide a relation between the stellar E(B-V) and the observed line ratio for several attenuation laws. Combining this technique with the use of the nebular Balmer decrement allows the determination of the stellar and nebular dust attenuation in galaxies and can probe its effects at different stellar age and mass regimes, independently of the initial mass function and the star formation history. We derived the dust reddening from the He II line fluxes and compared it to the reddening from the Balmer decrement and from the slope of the ultraviolet continuum in two star-forming galaxies. The three methods result in dust attenuations which agree to within the errors. Future application of this technique permits studies of the stellar dust attenuation compared to the nebular attenuation in a representative galaxy sample.

17. The Recent Burstiness of Star Formation in Galaxies at z~4.5 from Hα Measurements   
A. L. Faisst, P. L. Capak, N. Emami, ... + 2
The Astrophysical Journal, 884, 133, October 2019

The redshift range z=4-6 marks a transition phase between primordial and mature galaxy formation in which galaxies considerably increase their stellar mass, metallicity, and dust content. The study of galaxies in this redshift range is therefore important to understand early galaxy formation and the fate of galaxies at later times. Here, we investigate the burstiness of the recent star-formation history (SFH) of 221 z~4.5 main-sequence galaxies at log(M) > 9.7 by comparing their ultra-violet (UV) continuum, H? luminosity, and Hα equivalent-width (EW). The Hα properties are derived from the Spitzer [3.6μm]-[4.5μm] broad-band color, thereby properly taking into account model and photometric uncertainties. We find a significant scatter between H? and UV-derived luminosities and star-formation rates (SFRs). About half of the galaxies show a significant excess in H? compared to expectations from a constant smooth SFH. We also find a tentative anti-correlation between Hα EW and stellar mass, ranging from 1000A at log(M) < 10 to below 100A at log(M) > 11. Consulting models suggests that most z~4.5 galaxies had a burst of star-formation within the last 50 Myrs, increasing their SFRs by a factor of > 5. The most massive galaxies on the other hand might decrease their SFRs, and may be transitioning to a quiescent stage by z=4. We identify differential dust attenuation (f) between stars and nebular regions as the main contributor to the uncertainty. With local galaxies selected by increasing H? EW (reaching values similar to high-z galaxies), we predict that f approaches unity at z>4 consistent with the extrapolation of measurements out to z=2.

16. Low star formation efficiency in typical galaxies at z = 5-6   
R. Pavesi, D. Riechers, A. L. Faisst, ... +2
The Astrophysical Journal, 882, 168, September 2019

We report the detection of CO(2-1) line emission from a Lyman Break Galaxy at z = 5.7 with the VLA. The CO line luminosity implies a massive molecular gas reservoir of (1.3±0.3)(αCO/4.5M (K km s-1 pc2)-1) × 1011 M, suggesting low star formation efficiency, with a gas depletion timescale of order 1 Gyr. This efficiency is much lower than traditionally observed in z > 5 starbursts, indicating that star forming conditions in Main Sequence galaxies at z = 6 may be comparable to those of normal galaxies probed up to z = 3 to-date, but with rising gas fractions across the entire redshift range. We also obtain a deep CO upper limit for a Main Sequence galaxy at z = 5.3 with ~3 times lower SFR, perhaps implying a high αCO conversion factor, as typically found in low metallicity galaxies. Using ALMA, we find faint [NII] 205μm emission relative to [CII] in all but the most IR-luminous "normal" galaxies at z = 5-6 for a sample including both CO targets, suggesting more intense or harder radiation fields in the ionized gas relative to lower redshift. These radiation properties indicate low metallicity may be common in typical ~1010 M galaxies at z = 5-6, consistent with our CO measurements. Our sample shows evidence for high dust temperatures, and a young starburst producing high radiation intensity and hardness even with substantial dust obscuration. While a fraction of Main Sequence star formation in the first billion years may take place in conditions not dissimilar to lower redshift, lower metallicity may affect the remainder of the population.

15. How to Find Variable Active Galactic Nuclei with Machine Learning   
A. L. Faisst, A. Prakash, P. L. Capak, ... + 1
The Astrophysical Journal Letters, 881, 9, August 2019

Machine-learning (ML) algorithms will play a crucial role in studying the large data sets delivered by new facilities over the next decade and beyond. Here, we investigate the capabilities and limits of such methods in finding galaxies with brightness-variable active galactic nuclei (AGNs). Specifically, we focus on an unsupervised method based on self-organizing maps (SOM) that we apply to a set of nonparametric variability estimators. This technique allows us to maintain domain knowledge and systematics control while using all the advantages of ML. Using simulated light curves that match the noise properties of observations, we verify the potential of this algorithm in identifying variable light curves. We then apply our method to a sample of ~8300 WISE color-selected AGN candidates in Stripe 82, in which we have identified variable light curves by visual inspection. We find that with ML we can identify these variable classified AGN with a purity of 86% and a completeness of 66%, a performance that is comparable to that of more commonly used supervised deep-learning neural networks. The advantage of the SOM framework is that it enables not only a robust identification of variable light curves in a given data set, but it is also a tool to investigate correlations between physical parameters in multidimensional spaceÑsuch as the link between AGN variability and the properties of their host galaxies. Finally, we note that our method can be applied to any time-sampled light curve (e.g., supernovae, exoplanets, pulsars, and other transient events).

14. Panchromatic Study of the First Galaxies with Large ALMA Programs   
A. L. Faisst, M. Bethermin, P. Capak, ... + 5
Submitted to Proceedings IAU Symposium No. 341 2018, January 2019

Thanks to deep optical to near-IR imaging and spectroscopy, significant progress is made in characterizing the rest-frame UV to optical properties of galaxies in the early universe (z > 4). Surveys with Hubble, Spitzer, and ground-based facilities (Keck, Subaru, and VLT) provide spectroscopic and photometric redshifts, measurements of the spatial structure, stellar masses, and optical emission lines for large samples of galaxies. Recently, the Atacama Large (Sub) Millimeter Array (ALMA) has become a major player in pushing studies of high redshift galaxies to far-infrared wavelengths, hence making panchromatic surveys over many orders of frequencies possible. While past studies focused mostly on bright sub-millimeter galaxies, the sensitivity of ALMA now enables surveys like ALPINE, which focuses on measuring the gas and dust properties of a large sample of normal main-sequence galaxies at z > 4. Combining observations across different wavelengths into a single, panchromatic picture of galaxy formation and evolution is currently and in the future an important focus of the astronomical community.

13. Cosmology in the 2020s Needs Precision and Accuracy: The Case for Euclid/LSST/WFIRST Joint Survey Processing   
R. R. Chary, L. Armus, A. Faisst, ... +7
Astro2020 Decadal Survey on Astornomy and Astrophysics science white papers; Bulletin of the American Astronomical Society, 51, 3, May 2018

Cosmological breakthroughs in the 2020s will arise from measuring the redshift evolution of dark energy, resolving the tension in the expansion rate of the Universe, and understanding the impact of dark matter on galaxy substructure. Joint processing of Euclid/LSST/WFIRST data provides the most compelling, economical and tractable path forward.

12. Empirical Modeling of the Redshift Evolution of the [NII]/Hα ratio for Galaxy Redshift Surveys   
A. L. Faisst, D. Masters, Y. Wang, ... +4
The Astrophysical Journal, 855, 2, March 2018

We present an empirical parameterization of the [NII]/Hα flux ratio as a function of stellar mass and redshift valid at 0 < z < 2.7 and 8.5 < log(M) < 11.0. This description can easily be applied to (i) simulations for modeling [NII] line emission, (ii) deblend [NII] and Hα in current low-resolution grism and narrow-band observations to derive intrinsic HHα fluxes, and (iii) to reliably forecast the number counts of Hα emission line galaxies for future surveys such as those planned for Euclid and WFIRST. Our model combines the evolution of the locus on the BPT diagram measured in spectroscopic data out to z ~ 2.5 with the strong dependence of [NII]/Hα on stellar mass and [OIII]/Hβ observed in local galaxy samples. We find large variations in the [NII]/Hα flux ratio at a fixed redshift due to its dependency on stellar mass, hence the assumption of a constant [NII] contamination fraction can lead to a significant under- or over-estimate of Hα luminosities. Specifically, measurements of the intrinsic Hα luminosity function derived from current low-resolution grism spectroscopy assuming a constant 29% contamination of [NII] are likely over-estimated by factors of 2-4 at log(L) > 43.0 and systematically under-estimated by ~50 at log(L) < 42.5 at redshifts z ~ 1.5. This has implications on the prediction of H-alpha emitters for Euclid and WFIRST. We also study the impact of blended Hα and [NII] on the accuracy of measured spectroscopic redshifts.

11. An Alternate Approach to Measure Specific Star Formation Rates at 2 < z < 7   
I. Davidzon, O. Ilbert, A. L. Faisst, M. Sparre, and P. L. Capak
The Astrophysical Journal, 852, 107, January 2018

We trace the specific star formation rate (sSFR) of massive star-forming galaxies (> 1010 M) from z = 2 to 7. Our method is substantially different from previous analyses, as it does not rely on direct estimates of star formation rate, but on the differential evolution of the galaxy stellar mass function (SMF). We show the reliability of this approach by means of semianalytical and hydrodynamical cosmological simulations. We then apply it to real data, using the SMFs derived in the COSMOS and CANDELS fields. We find that the sSFR is proportional to (1+z)1.1±0.2 at z > 2, in agreement with other observations but in tension with the steeper evolution predicted by simulations from z = 4 to 2. We investigate the impact of several sources of observational bias, which, however, cannot account for this discrepancy. Although the SMF of high-redshift galaxies is still affected by significant errors, we show that future large-area surveys will substantially reduce them, making our method an effective tool to probe the massive end of the main sequence of star-forming galaxies.

10. Are high redshift Galaxies hot? - Temperature of z > 5 Galaxies and Implications on their Dust Properties   
A. L. Faisst, P. L. Capak, L. Yan, ... +6
The Astrophysical Journal, 847, 21, September 2017

Recent studies have found a significant evolution and scatter in the IRX-β relation at z > 4, suggesting different dust properties of these galaxies. The total far-infrared (FIR) luminosity is key for this analysis but poorly constrained in normal (main-sequence) star-forming z > 5 galaxies where often only one single FIR point is available. To better inform estimates of the FIR luminosity, we construct a sample of local galaxies and three low-redshift analogs of z > 5 systems. The trends in this sample suggest that normal high-redshift galaxies have a warmer infrared (IR) SED compared to average z < 4 galaxies that are used as prior in these studies. The blue-shifted peak and mid-IR excess emission could be explained by a combination of a larger fraction of the metal-poor inter-stellar medium (ISM) being optically thin to ultra-violet (UV) light and a stronger UV radiation field due to high star formation densities. Assuming a maximally warm IR SED suggests 0.6 dex increased total FIR luminosities, which removes some tension between dust attenuation models and observations of the IRX-β relation at z > 5. Despite this, some galaxies still fall below the minimum IRX-? relation derived with standard dust cloud models. We propose that radiation pressure in these highly star-forming galaxies causes a spatial offset between dust clouds and young star-forming regions within the lifetime of O/B stars. These offsets change the radiation balance and create viewing-angle effects that can change UV colors at fixed IRX. We provide a modified model that can explain the location of these galaxies on the IRX-β diagram.

9. Dust Properties of [CII] detected z ~ 5.5 Galaxies: New HST/WFC3 Near-IR Observations   
I. Barisic*, A. L. Faisst, P. L. Capak, ... +7
* Summer student (2015) at Caltech
The Astrophysical Journal, 845, 41, August 2017

We examine the rest-frame ultra-violet (UV) properties of 10 [C II]λ158μm-detected galaxies at z ~ 5.5 in COSMOS using new HST/WFC3 near-infrared imaging. Together with pre-existing 158μm-continuum and [C II] line measurements by ALMA, we study their dust attenuation properties on the IRX-β diagram, which connects the total dust emission (~IRX = log(LFIR/L1600)) to the line-of-sight dust column (~β). We find systematically bluer UV continuum spectral slopes (β) compared to previous low-resolution ground-based measurements, which relieves some of the tension between models of dust attenuation and observations at high redshifts. While most of the galaxies are consistent with local starburst or Small Magellanic cloud like dust properties, we find galaxies with low IRX values and a large range in β that cannot be explained by models of a uniform dust distribution well mixed with stars. A stacking analysis of Keck/DEIMOS optical spectra indicates that these galaxies are metal-poor with young stellar populations which could significantly alter their spatial dust distribution.

8. Constraints on Quenching of z < 2 Massive Galaxies from the Evolution of the average Sizes of Star-Forming and Quenched Populations in COSMOS   
A. L. Faisst, M. Carollo, P. Capak, ... +5
The Astrophysical Journal, 839, 71, April 2017

We use >9400 log(m/Msun)>10 quiescent and star-forming galaxies at z < 2 in COSMOS/UltraVISTA to study the average size evolution of these systems, with focus on the rare, ultra-massive population at log(m/Msun)>11.4. The large 2-square degree survey area delivers a sample of ~400 such ultra-massive systems. Accurate sizes are derived using a calibration based on high-resolution images from the Hubble Space Telescope. We find that, at these very high masses, the size evolution of star-forming and quiescent galaxies is almost indistinguishable in terms of normalization and power-law slope. We use this result to investigate possible pathways of quenching massive m > M* galaxies at z < 2. We consistently model the size evolution of quiescent galaxies from the star-forming population by assuming different simple models for the suppression of star-formation. These models include an instantaneous and delayed quenching without altering the structure of galaxies and a central starburst followed by compaction. We find that instantaneous quenching reproduces well the observed mass-size relation of massive galaxies at z > 1. Our starburst+compaction model followed by individual growth of the galaxies by minor mergers is preferred over other models without structural change for log(m/Msun) > 11.0 galaxies at z > 0.5. None of our models is able to meet the observations at m > M* and z < 1 with out significant contribution of post-quenching growth of individual galaxies via mergers. We conclude that quenching is a fast process in galaxies with m > 1011 Msun, and that major mergers likely play a major role in the final steps of their evolution.

7. Revisiting the Lyman Continuum Escape Fraction Crisis: Predictions for z>6 from Local Galaxies   
A. L. Faisst
The Astrophysical Journal, 829, 99, September 2016

The intrinsic escape fraction of ionizing Lyman continuum photons (fesc) is crucial to understand whether galaxies are capable of reionizing the neutral hydrogen in the early universe at z>6. Unfortunately, it is not possible to access fesc at z>4 with direct observations and the handful of measurements from low redshift galaxies consistently find fesc < 10%, while at least fesc ~ 10% is necessary for galaxies dominate reionization. Here, we present the first empirical prediction of fesc at z>6 by combining the (sparsely populated) relation between [OIII]/[OII] and fesc with the redshift evolution of [OIII]/[OII] as predicted from local high-z analogs selected by their H? equivalent-width. We find fesc = 5.7 (+8.3) (-3.3)% at z=6 and fesc = 10.4 (+15.5) (-6.3)% at z=9 for galaxies with log(M/Msun) ~ 9.0 (errors given as 1σ). However, there is a negative correlation with stellar mass and we find up to 50% larger fesc per 0.5 dex decrease in stellar mass. The population averaged escape fraction increases according to fesc = fesc,0((1+z)/3)a, with fesc,0=2.3 +/-0.05% and a=1.17 +/-0.02 at z>2 for log(M/Msun) ~ 9.0. With our empirical prediction of fesc (thus fixing an important previously unknown variable) and further reasonable assumption on clumping factor and the production efficiency of Lyman continuum photons, we conclude that the average population of galaxies is just capable to reionize the universe by z~6.

6. A tight Relation between N/O Ratio and Galaxy Stellar Mass can explain the Evolution of strong Emission Line Ratios with Redshift   
D. Masters, A. L. Faisst, and P. Capak
The Astrophysical Journal, 828, 18, September 2016

The offset of high redshift star-forming galaxies in the [OII]/Hβ versus [NII]/Hα (O3N2) diagram in comparison with the local star-forming galaxy sequence is now well established. The physical origin of the shift is the subject of some debate, and has important implications for metallicity measurements based on strong lines at all redshifts. To investigate the origin of the O3N2 offset, we use a sample of ~100,000 star-forming galaxies from SDSS DR12 to probe the empirical correlations between emission line diagnostics and measurable galaxy physical properties. In particular, we examine how surface density of star formation (SFRD), ionization parameter, nitrogen-to-oxygen (N/O) ratio, and stellar mass drive position in two key diagnostic diagrams: O3N2 and [OIII]/Hβ versus [SII]/Hα (O3S2). We show that, at fixed [OIII]/Hβ, galaxies falling closer to the high-redshift locus in O3N2 have higher SFRD, stellar mass and N/O ratios. We also find a tight correspondence in the distributions of stellar mass and N/O in the diagnostic diagrams. This relation, spanning a range of galaxy evolutionary states, suggests that the N/O-Mstar relation is more fundamental than the N/O-metallicity relation. We argue that a tight N/O-Mstar relation is well-motivated physically, and that the observed correlation of N/O with O/H in the local universe is primarily a reflection of the existence of the mass-metallicity relation. Because the mass-metallicity relation evolves much more rapidly with redshift than N/O-Mstar, the N/O ratios of high redshift galaxies are significantly elevated in comparison with local galaxies with the same gas-phase O/H. The O3N2 shift and elevated N/O ratios observed in high redshift galaxies therefore come about as a natural consequence of the N/O-Mstar relation combined with the evolution of the mass-metallicity relation.

5. Rest-UV Absorption Lines as Metallicity Estimator: the Metal Content of Star-Forming Galaxies at z~5   
A. L. Faisst, P. Capak, I. Davidzon, ... +13
The Astrophysical Journal, 822, 29, May 2016

We measure a relation between the depth of four prominent rest-UV absorption complexes and metallicity for local galaxies and verify it up to z~3. We then apply this relation to a sample of 224 galaxies at 3.5 < z < 6.0 (z~4.8) in COSMOS, for which unique UV spectra from DEIMOS and accurate stellar masses from SPLASH are available. The average galaxy population at z~5 and log(M/Msun) > 9 is characterized by 0.3-0.4 dex (in units of 12+log(O/H)) lower metallicities than at z~2, but comparable to z~3.5. We find galaxies with weak/no Ly-alpha emission to have metallicities comparable to z~2 galaxies and therefore may represent an evolved sub-population of z~5 galaxies. We find a correlation between metallicity and dust in good agreement with local galaxies and an inverse trend between metallicity and star-formation rate (SFR) consistent with observations at z~2. The relation between stellar mass and metallicity (MZ relation) is similar to z~3.5, however, there are indications of it being slightly shallower, in particular for the young, Lyα emitting galaxies. We show that, within a "bathtub" approach, a shallower MZ relation is expected in the case of a fast (exponential) build-up of stellar mass with an e-folding time of 100-200 Myr. Due to this fast evolution, the process of dust production and metal enrichment as a function of mass could be more stochastic in the first billion years of galaxy formation compared to later times.

4. A Coherent Study of Emission Lines from Broad-Band Photometry: Specific Star-Formation Rates and [OIII]/Hβ Ratio at 3 < z < 6   
A. L. Faisst, P. Capak, B. C. Hsieh, ... +14
The Astrophysical Journal, 821, 122, April 2016

We measure the H-alpha and [OIII] emission line properties as well as specific star-formation rates (sSFR) of spectroscopically confirmed 3 < z < 6 galaxies in COSMOS from their observed colors vs. redshift evolution. Our model describes consistently the ensemble of galaxies including intrinsic properties (age, metallicity, star-formation history), dust-attenuation, and optical emission lines. We forward-model the measured Hα equivalent-widths (EW) to obtain the sSFR out to z~6 without stellar mass fitting. We find a strongly increasing rest-frame Hα EW that is flattening off above z~2.5 with average EWs of 300-600A at z~6. The sSFR is increasing proportional to (1+z)2.4 at z < 2.2 and (1+z)1.5 at higher redshifts, indicative of a fast mass build-up in high-z galaxies within e-folding times of 100-200Myr at z~6. The redshift evolution at z > 3 cannot be fully explained in a picture of cold accretion driven growth. We find a progressively increasing [OIII]5007/Hβ ratio out to z~6, consistent with the ratios in local galaxies selected by increasing H-alpha EW (i.e., sSFR). This demonstrates the potential of using "local high-z analogs" to investigate the spectroscopic properties and relations of galaxies in the re-ionization epoch.

3. Dust Attenuation in High Redshift Galaxies: 'Diamonds in the Sky'   
N. Z. Scoville, A. L. Faisst, P. Capak, ... +3
The Astrophysical Journal, 800, 108, February 2014

We use observed optical to near-infrared spectral energy distributions (SEDs) of 266 galaxies in the COSMOS survey to derive the wavelength dependence of the dust attenuation at high redshift. All of the galaxies have spectroscopic redshifts in the range z = 2-6.5. The presence of the C IV absorption feature, indicating that the rest-frame UV-optical SED is dominated by OB stars, is used to select objects for which the intrinsic, unattenuated spectrum has a well-established shape. Comparison of this intrinsic spectrum with the observed broadband photometric SED then permits derivation of the wavelength dependence of the dust attenuation. The derived dust attenuation curve is similar in overall shape to the Calzetti curve for local starburst galaxies. We also see the 2175  bump feature which is present in the Milky Way and Large Magellanic Cloud extinction curves but not seen in the Calzetti curve. The bump feature is commonly attributed to graphite or polycyclic aromatic hydrocarbons. No significant dependence is seen with redshift between sub-samples at z = 2-4 and z = 4-6.5. The "extinction" curve obtained here provides a firm basis for color and extinction corrections of high redshift galaxy photometry.

2. Spectroscopic Observations of Lyα Emitters at z=7.7 and Implications on Reionization   
A. L. Faisst, P. Capak, C. M. Carollo, C. Scarlata, and N. Z. Scoville
The Astrophysical Journal, 788, 87, June 2014

We present spectroscopic follow-up observations on two bright Lyα emitter (LAE) candidates originally found by Krug et al. at a redshift of z ~ 7.7 using the Multi-Object Spectrometer for Infra-Red Exploration at Keck. We rule out any line emission at the >5? level for both objects, putting on solid ground a previous null result for one of the objects. The limits inferred from the non-detections rule out the previous claim of no or even reversed evolution between 5.7 < z < 7.7 in the Lyα luminosity function (LF) and suggest a drop in the Lyα LF consistent with that seen in Lyman break galaxy (LBG) samples. We model the redshift evolution of the LAE LF using the LBG UV-continuum LF and the observed rest-frame equivalent width distribution. From the comparison of our empirical model with the observed LAE distribution, we estimate lower limits of the neutral hydrogen fraction to be 50% - 70% at z ~ 7.7. Together with this, we find a strong evolution in the Lyα optical depth characterized by (1 + z)2.2 ± 0.5 beyond z = 6, indicative of a strong evolution of the intergalactic medium. Finally, we extrapolate the LAE LF to z ~ 9 using our model and show that it is unlikely that large area surveys, like UltraVISTA or Euclid, pick up LAEs at this redshift assuming the current depths and area.

1. Keck-I MOSFIRE Spectroscopy of the z~12 Candidate Galaxy UDFj-39546284   
P. Capak, A. L. Faisst, J. D. Vieira, ... +3
The Astrophysical Journal Letters, 773, 14, August 2013

We report the results of deep (4.6 hr) H-band spectroscopy of the well studied z ~ 12 H-band dropout galaxy candidate UDFj-39546284 with MOSFIRE on Keck-I. These data reach a sensitivity of 5-10 * 1e-19 erg s-1 cm-2 per 4.4A resolution element between sky lines. Previous papers have argued that this source could either be a large equivalent width line emitting galaxy at 2 < z < 3.5 or a luminous galaxy at z ~ 12. We find a 2.2σ peak associated with a line candidate in deep Hubble Space Telescope Wide-Field Camera 3 Infrared grism observations, but at a lower flux than expected. After considering several possibilities, we conclude these data cannot conclusively confirm or reject the previous line detection, and significantly deeper spectroscopic observations are required. We also search for low-redshift emission lines in 10 other 7 < z < 10 z, Y, and J-dropout candidates in our mask and find no significant detections.

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