Rest-frame Optical Emission Lines Research Articles

C iii] Emission in Star-forming Galaxies at z ∼ 1

Abstract The C iii]λλ1907, 1909 rest-frame UV emission doublet has recently been detected in galaxies during the epoch of reionization (z > 6), with a high equivalent width (EW; 10 Å, rest frame). Currently, it is possible to obtain much more detailed information for star-forming galaxies at significantly lower redshift. Accordingly, studies of their far-UV spectra are useful for understanding the factors modulating the strength of C iii] emission. We present the first statistical sample of C iii] emission measurements in star-forming galaxies at z ∼ 1. Our sample is drawn from the DEEP2 survey and spans the redshifts 0.64 z 1.35 ( ). We find that the median EW of individual C iii] detections in our sample (1.30 Å) is much smaller than the typical value observed thus far at z > 6. Furthermore, out of 184 galaxies with coverage of C iii], only 40 have significant detections. Galaxies with individual C iii] detections have bluer colors and lower luminosities on average than those without, implying that strong C iii] emitters are in general young and low-mass galaxies without significant dust extinction. Using stacked spectra, we further investigate how C iii] strength correlates with multiple galaxy properties (M B , U − B, M *, star formation rate, specific star formation rate) and rest-frame near-UV (Fe ii* and Mg ii) and optical ([O iii] and Hβ) emission line strengths. These results provide a detailed picture of the physical environment in star-forming galaxies at z ∼ 1, and motivate future observations of strong C iii] emitters at similar redshifts.

The SINFONI survey of powerful radio galaxies at z ~ 2: Jet-driven AGN feedback during the Quasar Era

We present VLT/SINFONI imaging spectroscopy of the rest-frame optical emission lines of warm ionized gas in 33 powerful radio galaxies at redshifts z ≳ 2, which are excellent sites to study the interplay of rapidly accreting active galactic nuclei and the interstellar medium of the host galaxy in the very late formation stages of massive galaxies. Our targets span two orders of magnitude in radio size (2−400 kpc) and kinetic jet energy (a few 1046– almost 1048 erg s-1). All sources have complex gas kinematics with broad line widths up to ~1300 km s-1. About half have bipolar velocity fields with offsets up to 1500 km s-1 and are consistent with global back-to-back outflows. The others have complex velocity distributions, often with multiple abrupt velocity jumps far from the nucleus of the galaxy, and are not associated with a major merger in any obvious way. We present several empirical constraints that show why gas kinematics and radio jets seem to be physically related in all galaxies of the sample. The kinetic energy in the gas from large scale bulk and local outflow or turbulent motion corresponds to a few 10-3 to 10-2 of the kinetic energy output of the radio jet. In galaxies with radio jet power ≳ 1047 erg s-1, the kinetic energy in global back-to-back outflows dominates the total energy budget of the gas, suggesting that bulk motion of outflowing gas encompasses the global interstellar medium. This might be facilitated by the strong gas turbulence, as suggested by recent analytical work. We compare our findings with recent hydrodynamic simulations, and discuss the potential consequences for the subsequent evolution of massive galaxies at high redshift. Compared with recent models of metal enrichment in high-z AGN hosts, we find that the gas-phase metallicities in our galaxies are lower than in most low-z AGN, but nonetheless solar or even super-solar, suggesting that the ISM we see in these galaxies is very similar to the gas from which massive low-redshift galaxies formed most of their stars. This further highlights that we are seeing these galaxies near the end of their active formation phase.

Novel calibrations of virial black hole mass estimators in active galaxies based on X-ray luminosity and optical/near-infrared emission lines

Accurately weigh the masses of SMBH in AGN is currently possible for only a small group of local and bright broad-line AGN through reverberation mapping (RM). Statistical demographic studies can be carried out considering the empirical scaling relation between the size of the BLR and the AGN optical continuum luminosity. However, there are still biases against low-luminosity or reddened AGN, in which the rest-frame optical radiation can be severely absorbed/diluted by the host and the BLR emission lines could be hard to detect. Our purpose is to widen the applicability of virial-based SE relations to reliably measure the BH masses also for low-luminosity or intermediate/type 2 AGN that are missed by current methodology. We achieve this goal by calibrating virial relations based on unbiased quantities: the hard X-ray luminosities, in the 2-10 keV and 14-195 keV bands, that are less sensitive to galaxy contamination, and the FWHM of the most important rest-frame NIR and optical BLR emission lines. We built a sample of RM AGN having both X-ray luminosity and broad optical/NIR FWHM measurements available in order to calibrate new virial BH mass estimators. We found that the FWHM of the H$\alpha$, H$\beta$ and NIR lines (i.e. Pa$\alpha$, Pa$\beta$ and HeI$\lambda$10830) all correlate each other having negligible or small offsets. This result allowed us to derive virial BH mass estimators based on either the 2-10 keV or 14-195 keV luminosity. We took also into account the recent determination of the different virial coefficients $f$ for pseudo and classical bulges. By splitting the sample according to the bulge type and adopting separate $f$ factors we found that our virial relations predict BH masses of AGN hosted in pseudobulges $\sim$0.5 dex smaller than in classical bulges. Assuming the same average $f$ factor for both populations, a difference of $\sim$0.2 dex is still found.

THE FMOS-COSMOS SURVEY OF STAR-FORMING GALAXIES AT z ∼ 1.6. IV. EXCITATION STATE AND CHEMICAL ENRICHMENT OF THE INTERSTELLAR MEDIUM

ABSTRACT We investigate the physical conditions of ionized gas in high-z star-forming galaxies using diagnostic diagrams based on the rest-frame optical emission lines. The sample consists of 701 galaxies with an Hα detection at , from the Fiber Multi-Object Spectrograph (FMOS)-COSMOS survey, that represent the normal star-forming population over the stellar mass range , with those at being well sampled. We confirm an offset of the average location of star-forming galaxies in the Baldwin–Phillips–Terlevich (BPT) diagram ( versus ), primarily toward higher , compared with local galaxies. Based on the [S ii] ratio, we measure an electron density ( ), which is higher than that of local galaxies. Based on comparisons to theoretical models, we argue that changes in emission-line ratios, including the offset in the BPT diagram, are caused by a higher ionization parameter both at fixed stellar mass and at fixed metallicity, with additional contributions from a higher gas density and possibly a hardening of the ionizing radiation field. Ionization due to active galactic nuclei is ruled out as assessed with Chandra. As a consequence, we revisit the mass–metallicity relation using and a new calibration including as recently introduced by Dopita et al. Consistent with our previous results, the most massive galaxies ( ) are fully enriched, while those at lower masses have metallicities lower than local galaxies. Finally, we demonstrate that the stellar masses, metallicities, and star formation rates of the FMOS sample are well fit with a physically motivated model for the chemical evolution of star-forming galaxies.

Unveiling the nature of brightz ≃ 7galaxies with theHubble Space Telescope

We present new Hubble Space Telescope/Wide Field Camera 3 imaging of 25 extremely luminous (-23.2 < M_ UV < -21.2) Lyman-break galaxies (LBGs) at z ~ 7. The sample was initially selected from 1.65 deg^2 of ground-based imaging in the UltraVISTA/COSMOS and UDS/SXDS fields, and includes the extreme Lyman-alpha emitters, `Himiko' and `CR7'. A deconfusion analysis of the deep Spitzer photometry available suggests that these galaxies exhibit strong rest-frame optical nebular emission lines (EW_0(H_beta + [OIII]) > 600A). We find that irregular, multiple-component morphologies suggestive of clumpy or merging systems are common (f_multi > 0.4) in bright z ~ 7 galaxies, and ubiquitous at the very bright end (M_UV < -22.5). The galaxies have half-light radii in the range r_1/2 ~ 0.5-3 kpc. The size measurements provide the first determination of the size-luminosity relation at z ~ 7 that extends to M_UV ~ -23. We find the relation to be steep with r_1/2 ~ L^1/2. Excluding clumpy, multi-component galaxies however, we find a shallower relation that implies an increased star-formation rate surface density in bright LBGs. Using the new, independent, HST/WFC3 data we confirm that the rest-frame UV luminosity function at z ~ 7 favours a power-law decline at the bright-end, compared to an exponential Schechter function drop-off. Finally, these results have important implications for the Euclid mission, which we predict will detect > 1000 similarly bright galaxies at z ~ 7. Our new HST imaging suggests that the vast majority of these galaxies will be spatially resolved by Euclid, mitigating concerns over dwarf star contamination.

A HIGH FRACTION OF Lyα EMITTERS AMONG GALAXIES WITH EXTREME EMISSION LINE RATIOS AT z ∼ 2*

ABSTRACT Star-forming galaxies form a sequence in the [O iii] λ5007/ versus [N ii] λ6584/ diagnostic diagram, with low-metallicity, highly ionized galaxies falling in the upper left corner. Drawing from a large sample of UV-selected star-forming galaxies at with rest-frame optical nebular emission line measurements from Keck-MOSFIRE, we select the extreme ∼5% of the galaxies lying in this upper left corner, requiring log([N ii]/ ) and log([O iii]/ ) . These cuts identify galaxies with , when oxygen abundances are measured via the O3N2 diagnostic. We study the properties of the resulting sample of 14 galaxies. The mean (median) rest-frame equivalent width is 39 (36) Å, and 11 of the 14 objects (79%) are emitters (LAEs) with . We compare the equivalent width distribution of a sample of 522 UV-selected galaxies at identified without regard to their optical line ratios; this sample has mean (median) equivalent width −1 (−4) Å, and only 9% of these galaxies qualify as LAEs. The extreme galaxies typically have lower attenuation at than those in the comparison sample and have ∼50% lower median oxygen abundances. Both factors are likely to facilitate the escape of : in less dusty galaxies photons are less likely to be absorbed during multiple scatterings, while the harder ionizing spectrum and higher ionization parameter associated with strong, low-metallicity star formation may reduce the covering fraction or column density of neutral hydrogen, further easing escape. The use of nebular emission line ratios may prove useful in the identification of galaxies with low opacity to photons across a range of redshifts.

Mean Hα+[N ii]+[S ii] EW inferred for star-forming galaxies atz ∼ 5.1–5.4 using high-qualitySpitzer/IRAC photometry

Recent Spitzer/InfraRed Array Camera (IRAC) photometric observations have revealed that rest-frame optical emission lines contribute signficantly to the broadband fluxes of high-redshift galaxies. Specifically, in the narrow redshift range z~5.1-5.4 the [3.6]-[4.5] color is expected to be very red, due to contamination of the 4.5-micron band by the dominant Halpha line, while the 3.6-micron filter is free of nebular emission lines. We take advantage of new reductions of deep Spitzer/IRAC imaging over the GOODS-North+South fields (Labbe+2015) to obtain a clean measurement of the mean Halpha equivalent width from the [3.6]-[4.5] color in the redshift range z=5.1-5.4. The selected sources either have measured spectroscopic redshifts (13 sources) or lie very confidently in the redshift range z=5.1-5.4 based on the photometric redshift likelihood intervals (11 sources). Our z_{phot}=5.1-5.4 sample and z_{spec}=5.10-5.40 spectroscopic sample have a mean [3.6]-[4.5] color of 0.31+/-0.05 mag and 0.35+/-0.07 mag, implying a rest-frame equivalent width EW(Halpha+[NII]+[SII]) of 665+/-53 Angstroms and 707+/-74 Angstroms, respectively, for sources in these samples. These values are consistent albeit slightly higher than derived by Stark+2013 at z~4, suggesting an evolution to higher values of the Halpha+[NII]+[SII] EW at z>2. Using the 3.6micron band, which is free of emission line contamination, we perform robust SED fitting and find a median specific star formation rate of sSFR = 17_{-5}^{+2} Gyr^{-1}, 7_{-2}^{+1}x higher than at z~2. We find no strong correlation (<2sigma) between the Halpha+[NII]+[SII] EW and the stellar mass of sources. Before the advent of JWST, improvements in these results will come through an expansion of current spectroscopic samples and deeper Spitzer/IRAC measurements.

ISM EXCITATION AND METALLICITY OF STAR-FORMING GALAXIES AT Z ≃ 3.3 FROM NEAR-IR SPECTROSCOPY

ABSTRACT We study the relationship between stellar mass, star formation rate (SFR), ionization state, and gas-phase metallicity for a sample of 41 normal star-forming galaxies at 3 ≲ z ≲ 3.7. The gas-phase oxygen abundance, ionization parameter, and electron density of ionized gas are derived from rest-frame optical strong emission lines measured on near-infrared spectra obtained with Keck/Multi-Object Spectrograph for Infra-Red Exploration. We remove the effect of these strong emission lines in the broadband fluxes to compute stellar masses via spectral energy distribution fitting, while the SFR is derived from the dust-corrected ultraviolet luminosity. The ionization parameter is weakly correlated with the specific SFR, but otherwise the ionization parameter and electron density do not correlate with other global galaxy properties such as stellar mass, SFR, and metallicity. The mass–metallicity relation (MZR) at z ≃ 3.3 shows lower metallicity by ≃0.7 dex than that at z = 0 at the same stellar mass. Our sample shows an offset by ≃0.3 dex from the locally defined mass–metallicity–SFR relation, indicating that simply extrapolating such a relation to higher redshift may predict an incorrect evolution of MZR. Furthermore, within the uncertainties we find no SFR–metallicity correlation, suggesting a less important role of SFR in controlling the metallicity at high redshift. We finally investigate the redshift evolution of the MZR by using the model by Lilly et al., finding that the observed evolution from z = 0 to z ≃ 3.3 can be accounted for by the model assuming a weak redshift evolution of the star formation efficiency.

DUST ATTENUATION OF THE NEBULAR REGIONS OF z ∼ 2 STAR-FORMING GALAXIES: INSIGHT FROM UV, IR, AND EMISSION LINES

ABSTRACT We use a sample of 149 spectroscopically confirmed UV-selected galaxies at z ∼ 2 to investigate the relative dust attenuation of the stellar continuum and the nebular emission lines. For each galaxy in the sample, at least one rest-frame optical emission line (Hα/[N ii] λ6583 or [O iii] λ5007) measurement has been taken from the litterature, and 41 galaxies have additional Spitzer/MIPS 24 μm observations that are used to infer infrared luminosities. We use a spectral energy distribution (SED) fitting code that predicts nebular line strengths when fitting the stellar populations of galaxies in our sample, and we perform comparisons between the predictions of our models and the observed/derived physical quantities. We find that on average our code is able to reproduce all the physical quantities (e.g., UV β slopes, infrared luminosities, emission line fluxes), but we need to apply a higher dust correction to the nebular emission compared to the stellar emission for the largest star formation rate (SFR) (log SFR/M ⊙ yr−1 &gt; 1.82, Salpeter initial mass function). We find a correlation between SFR and the difference in nebular and stellar color excesses, which could resolve the discrepant results regarding nebular dust correction at z ∼ 2 from previous studies.

REST-FRAME OPTICAL EMISSION LINES IN z ∼ 3.5 LYMAN-BREAK-SELECTED GALAXIES: THE UBIQUITY OF UNUSUALLY HIGH [O III]/Hβ RATIOS AT 2 Gyr* †

ABSTRACT We present K-band spectra of rest-frame optical emission lines for 24 star-forming galaxies at z ∼ 3.2–3.7 using MOSFIRE on the Keck I telescope. Strong rest-frame optical [O iii] and Hβ emission lines were detected in 18 Lyman break galaxies (LBGs). The median flux ratio of [O iii]λ5007 to Hβ is 5.1 &minus; 0.5 &plus; 0.5 ?> . This is a factor of 5–10 times higher than in local galaxies with similar stellar masses. None of our sources are detected in deep X-ray stacks, ruling out significant contamination by active galactic nuclei. Combining our sample with a variety of LBGs from the literature, including 49 galaxies selected in a very similar manner, we find a high median ratio of [O iii]/Hβ = 4.8 &minus; 1.7 &plus; 0.8 ?> . This high ratio seems to be a ubiquitous feature of z ∼ 3–4 LBGs, very different from typical local star-forming galaxies at similar stellar masses. The only comparable systems at z ∼ 0 are those with similarly high specific star formation rates (SSFRs), though ∼5 times lower stellar masses. High SSFRs may result in a higher ionization parameter, higher electron density, or harder ionizing radiation, which, combined different elemental abundances, result in a much higher [O iii]/Hβ line ratio. This implies a strong relation between a global property of a galaxy, the SSFR, and the local conditions of ISM in star-forming regions.

THE DUST ATTENUATION CURVE VERSUS STELLAR MASS FOR EMISSION LINE GALAXIES ATz∼ 2

We derive the mean wavelength dependence of stellar attenuation in a sample of 239 high redshift (1.90 < z < 2.35) galaxies selected via Hubble Space Telescope (HST) WFC3 IR grism observations of their rest-frame optical emission lines. Our analysis indicates that the average reddening law follows a form similar to that derived by Calzetti et al. for local starburst galaxies. However, over the mass range 7.2 < log M/Msolar < 10.2, the slope of the attenuation law in the UV is shallower than that seen locally, and the UV slope steepens as the mass increases. These trends are in qualitative agreement with Kriek & Conroy, who found that the wavelength dependence of attenuation varies with galaxy spectral type. However, we find no evidence of an extinction "bump" at 2175 A in any of the three stellar mass bins, or in the sample as a whole. We quantify the relation between the attenuation curve and stellar mass and discuss its implications.

REST-FRAME OPTICAL EMISSION LINES IN FAR-INFRARED-SELECTED GALAXIES AT z &lt; 1.7 FROM THE FMOS-COSMOS SURVEY

We have used FMOS on Subaru to obtain near-infrared spectroscopy of 123 far-infrared-selected galaxies in COSMOS and the key rest-frame optical emission lines. This is the largest sample of infrared galaxies with near-infrared spectroscopy at these redshifts. The far-infrared selection results in a sample of galaxies that are massive systems that span a range of metallicities in comparison with previous optically selected surveys, and thus has a higher active galactic nucleus (AGN) fraction and better samples the AGN branch. We establish the presence of AGNs and starbursts in this sample of (U)LIRGs selected as Herschel-PACS and Spitzer-MIPS detections in two redshift bins (z ~ 0.7 and z ~ 1.5) and test the redshift dependence of diagnostics used to separate AGNs from star formation dominated galaxies. In addition, we construct a low-redshift (z ~ 0.1) comparison sample of infrared-selected galaxies and find that the evolution from z ~ 1.5 to today is consistent with an evolving AGN selection line and a range of ISM conditions and metallicities from the models of Kewley et al. We find that a large fraction of (U)LIRGs are BPT-selected AGNs using their new redshift-dependent classification line. We compare the position of known X-ray-detected AGNs (67 in total) with the BPT selection and find that the new classification line accurately selects most of these objects (>70%). Furthermore, we identify 35 new (likely obscured) AGNs not selected as such by their X-ray emission. Our results have direct implications for AGN selection at higher redshift with either current (MOSFIRE, KMOS) or future (PFS, MOONS) spectroscopic efforts with near-infrared spectral coverage.

A SPECTROSCOPIC REDSHIFT MEASUREMENT FOR A LUMINOUS LYMAN BREAK GALAXY AT z = 7.730 USING KECK/MOSFIRE

We present a spectroscopic redshift measurement of a very bright Lyman break galaxy at using the Keck/Multi-Object Spectrometer for Infra-Red Exploration. The source was pre-selected photometrically in the EGS field as a robust z ∼ 8 candidate with H = 25.0 mag based on optical non-detections and a very red Spitzer/IRAC [3.6]–[4.5] broad-band color driven by high equivalent width [O iii]+Hβ line emission. The Lyα line is reliably detected at and shows an asymmetric profile as expected for a galaxy embedded in a relatively neutral intergalactic medium near the Planck peak of cosmic reionization. The line has a rest-frame equivalent width of Å and is extended with km s−1. The source is perhaps the brightest and most massive z ∼ 8 Lyman break galaxy in the full CANDELS and BoRG/HIPPIES surveys, having already assembled of stars at only 650 Myr after the Big Bang. The spectroscopic redshift measurement sets a new redshift record for galaxies. This enables reliable constraints on the stellar mass, star formation rate, and formation epoch, as well as combined [O iii]+Hβ line equivalent widths. The redshift confirms that the IRAC [4.5] photometry is very likely dominated by line emission with EW0([O iii]+Hβ) Å. This detection thus adds to the evidence that extreme rest-frame optical emission lines are a ubiquitous feature of early galaxies promising very efficient spectroscopic follow-up in the future with infrared spectroscopy using the James Webb Space Telescope and, later, ELTs.

THE MOSDEF SURVEY: EXCITATION PROPERTIES OFz∼ 2.3 STAR-FORMING GALAXIES

We present results on the excitation properties of z~2.3 galaxies using early observations from the MOSFIRE Deep Evolution Field (MOSDEF) Survey. With its coverage of the full suite of strong rest-frame optical emission lines, MOSDEF provides an unprecedented view of the rest-frame optical spectra of a representative sample of distant star-forming galaxies. We investigate the locations of z~2.3 MOSDEF galaxies in multiple emission-line diagnostic diagrams. These include the [OIII]/Hb vs. [NII]/Ha and [OIII]/Hb vs. [SII]/Ha "BPT" diagrams, as well as the O_32 vs. R_23 excitation diagram. We recover the well-known offset in the star-forming sequence of high-redshift galaxies in the [OIII]/Hb vs. [NII]/Ha BPT diagram relative to SDSS star-forming galaxies. However, the shift for our rest-frame optically selected sample is less significant than for rest-frame-UV selected and emission-line selected galaxies at z~2. Furthermore, we find that the offset is mass-dependent, only appearing within the low-mass half of the z~2.3 MOSDEF sample, where galaxies are shifted towards higher [NII]/Ha at fixed [OIII]/Hb. Within the [OIII]/Hb vs. [SII]/Ha and O_32 vs. R_23 diagrams, we find that z~2.3 galaxies are distributed like local ones, and therefore attribute the shift in the [OIII]/Hb vs. [NII]/Ha BPT diagram to elevated N/O abundance ratios among lower-mass (M_*<10^10 M_sun) high-redshift galaxies. The variation in N/O ratios calls into question the use at high redshift of oxygen abundance indicators based on nitrogen lines, but the apparent invariance with redshift of the excitation sequence in the O_32 vs. R_23 diagram paves the way for using the combination of O_32 and R_23 as an unbiased metallicity indicator over a wide range in redshift. This indicator will allow for an accurate characterization of the shape and normalization of the mass-metallicity relationship over more than 10 Gyr.

THE MOSDEF SURVEY: MASS, METALLICITY, AND STAR-FORMATION RATE ATz∼ 2.3

We present results on the z~2.3 mass-metallicity relation (MZR) using early observations from the MOSFIRE Deep Evolution Field (MOSDEF) survey. We use an initial sample of 87 star-forming galaxies with spectroscopic coverage of H\beta, [OIII]\lambda 5007, H\alpha, and [NII]\lambda 6584 rest-frame optical emission lines, and estimate the gas-phase oxygen abundance based on the N2 and O3N2 strong-line indicators. We find a positive correlation between stellar mass and metallicity among individual z~2.3 galaxies using both the N2 and O3N2 indicators. We also measure the emission-line ratios and corresponding oxygen abundances for composite spectra in bins of stellar mass. Among composite spectra, we find a monotonic increase in metallicity with increasing stellar mass, offset ~0.15-0.3 dex below the local MZR. When the sample is divided at the median star-formation rate (SFR), we do not observe significant SFR dependence of the z~2.3 MZR among either individual galaxies or composite spectra. We furthermore find that z~2.3 galaxies have metallicities ~0.1 dex lower at a given stellar mass and SFR than is observed locally. This offset suggests that high-redshift galaxies do not fall on the local "fundamental metallicity relation" among stellar mass, metallicity, and SFR, and may provide evidence of a phase of galaxy growth in which the gas reservoir is built up due to inflow rates that are higher than star-formation and outflow rates. However, robust conclusions regarding the gas-phase oxygen abundances of high-redshift galaxies await a systematic reappraisal of the application of locally calibrated metallicity indicators at high redshift.

THE Lyα PROPERTIES OF FAINT GALAXIES ATz∼ 2-3 WITH SYSTEMIC REDSHIFTS AND VELOCITY DISPERSIONS FROM KECK-MOSFIRE

We study the Lyα profiles of 36 spectroscopically detected Lyα-emitters (LAEs) at z ∼ 2–3, using Keck MOSFIRE to measure systemic redshifts and velocity dispersions from rest-frame optical nebular emission lines. The sample has a median optical magnitude , and ranges from to , corresponding to rest-frame UV absolute magnitudes MUV ≃ −22 to MUV > −18.2. Dynamical masses range from Mdyn < 1.3 × 108 M☉ to Mdyn = 6.8 × 109 M☉, with a median value of Mdyn = 6.3 × 108 M☉. Thirty of the 36 Lyα emission lines are redshifted with respect to the systemic velocity with at least 1σ significance, and the velocity offset with respect to systemic ΔvLyα is correlated with the -band magnitude, MUV, and the velocity dispersion measured from nebular emission lines with >3σ significance: brighter galaxies with larger velocity dispersions tend to have larger values of ΔvLyα. We also make use of a comparison sample of 122 UV-color-selected galaxies at z ∼ 2, all with Lyα emission and systemic redshifts measured from nebular emission lines. Using the combined LAE and comparison samples for a total of 158 individual galaxies, we find that ΔvLyα is anti-correlated with the Lyα equivalent width with 7σ significance. Our results are consistent with a scenario in which the Lyα profile is determined primarily by the properties of the gas near the systemic redshift; in such a scenario, the opacity to Lyα photons in lower mass galaxies may be reduced if large gaseous disks have not yet developed and if the gas is ionized by the harder spectrum of young, low metallicity stars.

THE DYNAMICAL MASSES, DENSITIES, AND STAR FORMATION SCALING RELATIONS OF Lyα GALAXIES

We present the first dynamical mass measurements for Lyman alpha galaxies at high redshift, based on velocity dispersion measurements from rest-frame optical emission lines and size measurements from HST imaging, for a sample of nine galaxies drawn from four surveys. These measurements enable us to study the nature of Lyman alpha galaxies in the context of galaxy scaling relations. The resulting dynamical masses range from 1e9 to 1e10 solar masses. We also fit stellar population models to our sample, and use them to plot the Lyman alpha sample on a stellar mass vs. line width relation. Overall, the Lyman alpha galaxies follow well the scaling relation established by observing star forming galaxies at lower redshift (and without regard for Lyman alpha emission), though in 1/3 of the Lyman alpha galaxies, lower-mass fits are also acceptable. In all cases, the dynamical masses agree with established stellarmass-linewidth relation. Using the dynamical masses as an upper limit on gas mass, we show that Lyman alpha galaxies resemble starbursts (rather than "normal" galaxies) in the relation between gas mass surface density and star formation activity, in spite of relatively modest star formation rates. Finally, we examine the mass densities of these galaxies, and show that their future evolution likely requires dissipational ("wet") merging. In short, we find that Lyman alpha galaxies are low mass cousins of larger starbursts.

THE SPECTRAL ENERGY DISTRIBUTIONS OF z ∼ 8 GALAXIES FROM THE IRAC ULTRA DEEP FIELDS: EMISSION LINES, STELLAR MASSES, AND SPECIFIC STAR FORMATION RATES AT 650 MYR

Using new ultradeep Spitzer/IRAC photometry from the IRAC Ultradeep Field program (IUDF), we investigate the stellar populations of a sample of 63 Y-dropout galaxy candidates at z~8, only 650Myr after the Big Bang. The sources are selected from HST/ACS+WFC3/IR data over the Hubble Ultra Deep Field (HUDF), two HUDF parallel fields, and wide area data over the CANDELS/GOODS-South. The new Spitzer/IRAC data increase the coverage at 3.6 micron and 4.5 micron to ~120h over the HUDF reaching depths of ~28 (AB,1 sigma). The improved depth and inclusion of brighter candidates result in direct >3 sigma IRAC detections of 20/63 sources, of which 11/63 are detected at > 5 sigma. The average [3.6]-[4.5] colors of IRAC detected galaxies at z~8 are markedly redder than those at z~7, observed only 130Myr later. The simplest explanation is that we witness strong rest-frame optical emission lines (in particular [OIII]4959,5007+Hbeta) moving through the IRAC bandpasses with redshift. Assuming that the average rest-frame spectrum is the same at both z~7 and z~8 we estimate a rest-frame equivalent width of W([OIII]4959,5007+Hbeta) = 670 (+260,-170) Angstrom contributing 0.56 (+0.16,-0.11) mag to the 4.5 micron filter at z~8. The corresponding W(Halpha) = 430 (+160,-110) Angstrom implies an average specific star formation rate of sSFR = 11 (+11,-5) Gyr^-1 and a stellar population age of 100 (+100,-50) Myr. Correcting the spectral energy distribution for the contribution of emission lines lowers the average best-fit stellar masses and mass-to-light ratios by x3, decreasing the integrated stellar mass density to rho*(z=8,MUV<-18)=0.6 (+0.4,-0.3) x 10^6 Msun Mpc^-3.

THE SPECTRALLY RESOLVED Lyα EMISSION OF THREE Lyα-SELECTED FIELD GALAXIES ATz∼ 2.4 FROM THE HETDEX PILOT SURVEY

We present new results on the spectrally resolved Lyman-alpha (LyA) emission of three LyA emitting field galaxies at z~2.4 with high LyA equivalent width (>100 Angstroms) and LyA luminosity (~10^43 erg/s). At 120 km/s (FWHM) spectral resolution, the prominent double-peaked LyA profile straddles the systemic velocity, where the velocity zero-point is determined from spectroscopy of the galaxies' rest-frame optical nebular emission lines. The average velocity offset from systemic of the stronger redshifted emission component for our sample is 176 km/s while the average total separation between the redshifted and main blueshifted emission components is 380 km/s. These measurements are a factor of ~2 smaller than for UV continuum-selected galaxies that show LyA in emission with lower LyA equivalent width. We compare our LyA spectra to the predicted line profiles of a spherical "expanding shell" LyA radiative transfer grid that models large-scale galaxy outflows. Specifically blueward of the systemic velocity where two galaxies show a weak, highly blueshifted (by ~1000 km/s) tertiary emission peak, the model line profiles are a relatively poor representation of the observed spectra. Since the neutral gas column density has a dominant influence over the shape of the LyA line profile, we caution against equating the observed LyA velocity offset with a physical outflow velocity, especially at lower spectral resolution where the unresolved LyA velocity offset is a convoluted function of several degenerate parameters. Referring to rest-frame ultraviolet and optical Hubble Space Telescope imaging, we find that galaxy-galaxy interactions may play an important role in inducing a starburst that results in copious LyA emission, as well as perturbing the gas distribution and velocity field which have strong influence over the LyA emission line profile.

A REST-FRAME OPTICAL VIEW ONz∼ 4 GALAXIES. I. COLOR AND AGE DISTRIBUTIONS FROM DEEP IRAC PHOTOMETRY OF THE IUDF10 AND GOODS SURVEYS

We present a study of rest-frame UV-to-optical color distributions for z~4 galaxies based on the combination of deep HST/ACS+WFC3/IR data with Spitzer/IRAC imaging. In particular, we use new, ultra-deep data from the IRAC Ultradeep Field program (IUDF10). Our sample contains a total of ~2600 galaxies selected as B-dropout Lyman Break Galaxies (LBGs) in the HUDF and one of its deep parallel fields, the HUDF09-2, as well as GOODS-North and South. This sample is used to investigate the UV continuum slopes beta and Balmer break colors (J_125-[4.5]) as a function of rest-frame optical luminosity. The [4.5] filter is chosen to avoid potential contamination by strong rest-frame optical emission lines. We find that galaxies at M_z<-21.5 (roughly corresponding to L*[z~4]) are significantly redder than their lower luminosity counterparts. The UV continuum slopes and the J_125-[4.5] colors are well correlated. The most simple explanation for this correlation is that the dust reddening at these redshifts is better described by an SMC-like extinction curve, rather than the typically assumed Calzetti reddening. After correcting for dust, we find that the galaxy population shows mean stellar population ages in the range 10^8.5 to 10^9 yr, with a dispersion of ~0.5 dex, and only weak trends as a function of luminosity. In contrast to some results from the literature, we find that only a small fraction of galaxies shows Balmer break colors which are consistent with extremely young ages, younger than 100 Myr. Under the assumption of smooth star-formation histories, this fraction is only 12-19% for galaxies at M_z<-19.75. Our results are consistent with a gradual build-up of stars and dust in galaxies at z>4, with only a small fraction of stars being formed in short, intense bursts of star-formation.