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.
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 spaceŃsuch 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).
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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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