Multi-Object Spectrometer For Infra-Red Exploration Research Articles

The MOSDEF-KCWI Survey: Spectral Properties of Lyα Halos around z ∼ 2 Star-forming Galaxies

We present Keck Cosmic Web Imager integral field observations of extended Lyα emission in the circumgalactic medium of 27 typical star-forming galaxies at z ∼ 2, drawn from the Multi-Object Spectrometer for Infra-Red Exploration (MOSFIRE) Deep Evolution Field (MOSDEF) survey. Using composite spectra in two bins of star formation rate (SFR), star formation rate surface density (ΣSFR), and other galactic properties, we measure spatial variations in the Lyα profile across three regions in the Lyα halo. We find single-peaked, redshifted profiles are ubiquitous within a central 7 kpc radius region. Further out in the halo (7–14 and 14–21 kpc), the Lyα profile of the resonantly scattered emission exhibits more diversity, either transitioning to a double-peaked profile or remaining single peaked across the halo. We find a shorter scale length of the Lyα halo surface brightness profile for composite halos with faster winds. The composites have a similar average inclination, suggesting those with faster winds clear channels in the interstellar medium (ISM), reducing the fraction of Lyα photons resonantly scattered to large radii. A uniform expanding shell radiative transfer model reproduces the shape but not the normalization of the observed double-peaked Lyα profiles. Models that adopt a more realistic, clumpy ISM are likely needed to reproduce both the shape and normalization of the Lyα profiles.

An AGN with an Ionized Gas Outflow in a Massive Quiescent Galaxy in a Protocluster at z = 3.09

We report the detection of an ionized gas outflow from an X-ray active galactic nucleus hosted in a massive quiescent galaxy in a protocluster at z = 3.09 (J221737.29+001823.4). It is a type-2 QSO with broad (W 80 > 1000 km s−1) and strong (/erg s−1) ≈ 43.4) [O iii]λ λ 4959,5007 emission lines detected by slit spectroscopy in three-position angles using Multi-Object Infra-Red Camera and Spectrograph (MOIRCS) on the Subaru telescope and the Multi-Object Spectrometer For Infra-Red Exploration (MOSFIRE) on the Keck-I telescope. In the all slit directions, [O iii] emission is extended to ∼15 physical kpc and indicates a powerful outflow spreading over the host galaxy. The inferred ionized gas mass outflow rate is 22 ± 3 M ⊙ yr−1. Although it is a radio source, according to the line diagnostics using Hβ, [O ii], and [O iii], photoionization by the central QSO is likely the dominant ionization mechanism rather than shocks caused by radio jets. On the other hand, the spectral energy distribution of the host galaxy is well characterized as a quiescent galaxy that has shut down star formation several hundred Myr ago. Our results suggest a scenario that QSOs are powered after the shutdown of the star formation and help complete the quenching of massive quiescent galaxies at high redshift.

The Physical Properties of Luminous z ≳ 8 Galaxies and Implications for the Cosmic Star Formation Rate Density from ∼0.35 deg2 of (Pure-)Parallel HST Observations*

Abstract We present the largest systematic, Hubble Space Telescope (HST)–based search to date for luminous z ≳ 8 galaxy candidates using ∼1267 arcmin2 of (pure-)parallel observations from a compilation of 288 random sightlines with Advanced Camera for Surveys and Wide Field Camera 3 observations, derived from the (Super)Brightest of Reionizing Galaxies (SuperBoRG) data set and together representing a factor ∼1.12× larger than existing space-based data sets. Using near-infrared (NIR) color cuts and careful photo-z analyses, we find 31 z ≳ 8 galaxy candidates over 29 unique sightlines, and derive global galaxy properties such as UV magnitudes and continuum slopes, sizes, and rest-frame optical properties (e.g., star formation rates (SFRs), stellar masses, A v). Taking advantage of the (pure-)parallel nature of our data set—making it one of the most representative thus far—and derived SFRs, we evaluate the cosmic SFR density for the bright end of the UV luminosity function (LF) at z ∼ 8–10 and test the validity of luminosity-function-derived results using a conversion factor. We find that our method yields comparable results to those derived with LFs. Furthermore, we present follow-up observations of five (Super)BoRG targets with the Keck Multi-Object Spectrometer For Infra-Red Exploration (MOSFIRE) instrument, finding no evidence of Lyα in >3 hr of Y-band observations in either, consistent with a largely neutral medium at z ∼ 8. Our results offer a definitive HST legacy on the bright end of the LF and provide a valuable benchmark as well as targets for follow-up with the James Webb Space Telescope.

Revisiting the gas kinematics in SSA22 Lyman-α Blob 1 with radiative transfer modelling in a multiphase, clumpy medium

ABSTRACT We present new observations of Lyman-α (Lyα) Blob 1 (LAB1) in the SSA22 protocluster region (z = 3.09) using the Keck Cosmic Web Imager and Keck Multi-object Spectrometer for Infrared Exploration. We have created a narrow-band Lyα image and identified several prominent features. By comparing the spatial distributions and intensities of Lyα and Hβ, we find that recombination of photo-ionized H i gas followed by resonant scattering is sufficient to explain all the observed Lyα/Hβ ratios. We further decode the spatially resolved Lyα profiles using both moment maps and radiative transfer modelling. By fitting a set of multiphase, ‘clumpy’ models to the observed Lyα profiles, we manage to reasonably constrain many parameters, namely the H i number density in the interclump medium (ICM), the cloud volume filling factor, the random velocity and outflow velocity of the clumps, the H i outflow velocity of the ICM, and the local systemic redshift. Our model has successfully reproduced the diverse Lyα morphologies, and the main results are: (1) the observed Lyα spectra require relatively few clumps per line of sight as they have significant fluxes at the line centre; (2) the velocity dispersion of the clumps yields a significant broadening of the spectra as observed; (3) the clump bulk outflow can also cause additional broadening if the H i in the ICM is optically thick; (4) and the H i in the ICM is responsible for the absorption feature close to the Lyα line centre.

Three Dusty Star-forming Galaxies at z ∼ 1.5: Mergers and Disks on the Main Sequence

Abstract The main sequence of galaxies, a correlation between the star formation rates (SFRs) and stellar masses of galaxies, has been observed out to z ∼ 4. Galaxies within the scatter of the correlation are typically interpreted to be secularly evolving while galaxies with SFRs elevated above the main sequence are interpreted to be undergoing interactions or to be Toomre-unstable disks with starbursting clumps. In this paper, we investigate the recent merger histories of three dusty star-forming galaxies, identified by their bright submillimeter emission at z ∼ 1.5. We analyze rest-frame optical and UV imaging, rest-frame optical emission line kinematics using slit spectra obtained with the Multi-Object Spectrometer For Infra-Red Exploration (MOSFIRE) on Keck I, and calculate Gini and M 20 statistics for each galaxy and conclude two are merger driven while the third is an isolated disk galaxy. The disk galaxy lies ∼4× above the main sequence, one merger lies within the scatter of the main sequence, and one merger lies ∼4× below the main sequence. This hints that the location of a galaxy with respect to the main sequence may not be a useful discriminator of the recent star formation history of high-M galaxies at z ∼ 1.

The Detection of [O iii] λ4363 in a Lensed, Dwarf Galaxy at z = 2.59: Testing Metallicity Indicators and Scaling Relations at High Redshift and Low Mass*

Abstract We present Keck/MOSFIRE (Multi-Object Spectrometer for InfraRed Exploration) and Keck/LRIS (Low Resolution Imaging Spectrometer) spectroscopy of A1689-217, a lensed (magnification ∼7.9), star-forming (SFR ∼ 16 M ☉ yr−1), dwarf (log(M */M ☉) = 8.07–8.59) Lyα emitter (EW0 ∼ 138 Å) at z = 2.5918. Dwarf galaxies similar to A1689-217 are common at high redshift and likely responsible for reionization, yet few have been studied with detailed spectroscopy. We report a 4.2σ detection of the electron-temperature-sensitive [O iii] λ4363 emission line and use this line to directly measure an oxygen abundance of 12 + log(O/H) = 8.06 ± 0.12 (∼1/4 Z ☉). A1689-217 is the lowest-mass galaxy at z > 2 with an [O iii] λ4363 detection. Using the rest-optical emission lines, we measure A1689-217’s other nebular conditions, including electron temperature (T e ([O iii]) ∼ 14,000 K), electron density (n e ∼ 220 cm−3), and reddening ( ∼ 0.39). We study relations between strong-line ratios and direct metallicities with A1689-217 and other galaxies with [O iii] λ4363 detections at z ∼ 0–3.1, showing that the locally calibrated, oxygen-based, strong-line relations are consistent from z ∼ 0 to 3.1. We also show additional evidence that the O32 versus R23 excitation diagram can be utilized as a redshift-invariant, direct-metallicity-based, oxygen abundance diagnostic out to z ∼ 3.1. From this excitation diagram and the strong-line ratio–metallicity plots, we observe that the ionization parameter at fixed O/H is consistent with no redshift evolution. Although A1689-217 is metal-rich for its M * and star formation rate, we find it to be consistent within the large scatter of the low-mass end of the fundamental metallicity relation.

On the lack of correlation between [O iii]/[O ii] and Lyman continuum escape fraction

We present the first results of our pilot study of 8 photometrically selected Lyman continuum (LyC) emitting galaxy candidates from the COSMOS field and focus on their optical emission line ratios. Observations were performed in the H and K bands using the Multi-Object Spectrometer for Infra-Red Exploration (MOSFIRE) instrument at the Keck Observatory, targeting the [OII], H$\beta$, and [OIII] emission lines. We find that photometrically selected LyC emitting galaxy candidates have high ionization parameters, based on their high [OIII]/[OII] ratios (O32), with an average ratio for our sample of 2.5$\pm$0.2. Preliminary results of our companion Low Resolution Imaging Spectrometer (LRIS) observations, targeting LyC and Ly$\alpha$, show that those galaxies with the largest O32 are typically found to also be Ly$\alpha$ emitters. High O32 galaxies are also found to have tentative non-zero LyC escape fractions ($f_{esc}(LyC)$) based on $u$ band photometric detections. These results are consistent with samples of highly ionized galaxies, including confirmed LyC emitting galaxies from the literature. We also perform a detailed comparison between the observed emission line ratios and simulated line ratios from density bounded H$_{\textrm{II}}$ regions modeled using the photoionization code MAPPINGS V. Estimates of $f_{esc}(LyC)$ for our sample fall in the range from 0.0-0.23 and suggest possible tension with published correlations between O32 and $f_{esc}(LyC)$, adding weight to dichotomy of arguments in the literature. We highlight the possible effects of clumpy geometry and mergers that may account for such tension.

ALMA OBSERVATIONS OF Lyα BLOB 1: HALO SUBSTRUCTURE ILLUMINATED FROM WITHIN

ABSTRACT We present new Atacama Large Millimeter/Submillimeter Array (ALMA) 850 μm continuum observations of the original Lyα Blob (LAB) in the SSA22 field at z = 3.1 (SSA22-LAB01). The ALMA map resolves the previously identified submillimeter source into three components with a total flux density of S 850 = 1.68 ± 0.06 mJy, corresponding to a star-formation rate of ∼150 M ⊙ yr−1. The submillimeter sources are associated with several faint (m ≈ 27 mag) rest-frame ultraviolet sources identified in Hubble Space Telescope Imaging Spectrograph (STIS) clear filter imaging (λ ≈ 5850 Å). One of these companions is spectroscopically confirmed with the Keck Multi-Object Spectrometer For Infra-Red Exploration to lie within 20 projected kpc and 250 km s−1 of one of the ALMA components. We postulate that some of these STIS sources represent a population of low-mass star-forming satellites surrounding the central submillimeter sources, potentially contributing to their growth and activity through accretion. Using a high-resolution cosmological zoom simulation of a 1013 M ⊙ halo at z = 3, including stellar, dust, and Lyα radiative transfer, we can model the ALMA+STIS observations and demonstrate that Lyα photons escaping from the central submillimeter sources are expected to resonantly scatter in neutral hydrogen, the majority of which is predicted to be associated with halo substructure. We show how this process gives rise to extended Lyα emission with similar surface brightness and morphology to observed giant LABs.

Correlation between star formation activity and electron density of ionized gas at z = 2.5

In the redshift interval of 2 < z < 3, the physical conditions of the interstellar medium (ISM) in star-forming galaxies are likely to be different from those in the local Universe because of lower gaseous metallicities, higher gas fractions, and higher star formation activities. In fact, observations suggest that higher electron densities, higher ionization parameters, and harder UV radiation fields are common. In this paper, based on the spectra of H α-selected star-forming galaxies at z = 2.5 taken with Multi-Object Spectrometer for InfraRed Exploration on Keck-1 telescope, we measure electron densities (n_e) using the oxygen line ratio ([O II] λλ3726,3729), and investigate the relationships between the electron density of ionized gas and other physical properties. As a result, we find that the specific star formation rate (sSFR) and the surface density of SFR (Σ_(SFR)) are correlated with the electron density at z = 2.5 for the first time. The Σ_(SFR)–n_e relation is likely to be linked to the star formation law in H II regions (where star formation activity is regulated by interstellar pressure). Moreover, we discuss the mode of star formation in those galaxies. The correlation between sSFR and Σ_(SFR) suggests that highly star-forming galaxies (with high sSFR) tend to be characterized by higher surface densities of star formation (Σ_(SFR)) and thus higher n_e values as well.

SPECTROSCOPIC OBSERVATION OF Lyα EMITTERS ATz∼ 7.7 AND IMPLICATIONS ON RE-IONIZATION

We present spectroscopic follow-up observations on two bright Ly$\alpha$ emitter (LAE) candidates originally found by Krug et al. (2012) at a redshift of z~7.7 using the Multi-Object Spectrometer for Infra-Red Exploration (MOSFIRE) at Keck. We rule out any line emission at the >5$\sigma$ 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 Ly$\alpha$ luminosity function (LF) and suggest a drop in the Ly$\alpha$ luminosity function 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 Ly$\alpha$ optical depth characterized by (1+z)^(2.2 $\pm$ 0.5) beyond z=6 indicative of a strong evolution of the IGM. 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.