Zheng Cai

Probing Very Bright End of Galaxy Luminosity Function at z 7 Using Hubble Space Telescope Pure Parallel Observations

We report the first results from the Hubble Infrared Pure Parallel Imaging Extragalactic Survey, which utilizes the pure parallel orbits of the Hubble Space Telescope to do deep imaging along a large number of random sightlines. To date, our analysis includes 26 widely separated fields observed by the Wide Field Camera 3, which amounts to 122.8 arcmin^2 in total area. We have found three bright Y (098)-dropouts, which are candidate galaxies at z ≳ 7.4. One of these objects shows an indication of peculiar variability and its nature is uncertain. The other two objects are among the brightest candidate galaxies at these redshifts known to date (L > 2L^∗). Such very luminous objects could be the progenitors of the high-mass Lyman break galaxies observed at lower redshifts (up to z ~ 5). While our sample is still limited in size, it is much less subject to the uncertainty caused by “cosmic variance” than other samples because it is derived using fields along many random sightlines. We find that the existence of the brightest candidate at z ≈ 7.4 is not well explained by the current luminosity function (LF) estimates at z ≈ 8. However, its inferred surface density could be explained by the prediction from the LFs at z ≈ 7 if it belongs to the high-redshift tail of the galaxy population at z ≈ 7.

PROBING POPULATION III STARS IN GALAXY IOK-1 AT z = 6.96 THROUGH He II EMISSION

The He II ?1640 emission line has been suggested as a direct probe of Population III (Pop?III) stars at high redshift, since it can arise from highly energetic ionizing photons associated with hot, metal-free stars. We use the Hubble Space Telescope WFC3/F130N IR narrowband filter to probe He II ?1640 emission in galaxy IOK-1 at z = 6.96. The sensitivity of this measurement is 5? deeper than for previous measurements. From this deep narrowband imaging, combined with broadband observations in the F125W and F160W filters, we find the He II flux to be (1.2 ? 1.0) ? 10?18 erg s?1 cm?2, corresponding to a 1? upper limit on the Pop?III star formation rate (SFR) of ~0.5 M ? yr?1 for the case of a Salpeter initial mass function (IMF) with 50 M/M ? 1000 and mass loss. Given that the broadband measurements can be fit with a UV-continuum spectral flux density of ~4.85 ? 10?10 ? ??2.46 erg s?1 cm?2 ??1, which corresponds to an overall SFR of ~16+2.6 ? 2.6 M ? yr?1, massive Pop?III stars represent 6% of the total star formation. This measurement places the strongest limit yet on metal-free star formation at high redshift, although the exact conversion from He II luminosity to Pop?III SFR is highly uncertain due to the unknown IMF, stellar evolution, and photoionization effects. Although we have not detected He II ?1640 at more than the 1.2? level, our work suggests that a 3? level detection is possible with the James Webb Space Telescope.

Discovery of an Enormous Lyα Nebula in a Massive Galaxy Overdensity at z = 2.3

Enormous Lyα nebulae (ELANe), unique tracers of galaxy density peaks, are predicted to lie at the nodes and intersections of cosmic filamentary structures. Previous successful searches for ELANe have focused on wide-field narrowband surveys or have targeted known sources such as ultraluminous quasi-stellar objects (QSOs) or radio galaxies. Utilizing groups of coherently strong Lyα absorptions, we have developed a new method to identify high-redshift galaxy overdensities and have identified an extremely massive overdensity, BOSS1441, at z = 2-3. In its density peak, we discover an ELAN that is associated with a relatively faint continuum. To date, this object has the highest diffuse Lyα nebular luminosity of L_(nebula) = 5.1 ± 0.1 x 10^(44) erg s^(−1). Above the 2σ surface brightness limit of SB Lyα = 4.8 x 10^(-18) erg s^(−1) cm^(−2) arcsec^(−2), this nebula has an end-to-end spatial extent of 442 kpc. This radio-quiet source also has extended C IV λ1549 and He II λ1640 emission on ≳ 30 kpc scales. Note that the Lyα, He II, and C IV emissions all have double-peaked line profiles. Each velocity component has an FWHM of ≈700–1000 km s^(−1). We argue that this Lyα nebula could be powered by shocks due to an active galactic nucleus–driven outflow or photoionization by a strongly obscured source.

Discovery of Eight z ~ 6 Quasars in the Sloan Digital Sky Survey Overlap Regions

We present the discovery of eight quasars at z~6 identified in the Sloan Digital Sky Survey (SDSS) overlap regions. Individual SDSS imaging runs have some overlap with each other, leading to repeat observations over an area spanning >4000 deg^2 (more than 1/4 of the total footprint). These overlap regions provide a unique dataset that allows us to select high-redshift quasars more than 0.5 mag fainter in the z band than those found with the SDSS single-epoch data. Our quasar candidates were first selected as i-band dropout objects in the SDSS imaging database. We then carried out a series of follow-up observations in the optical and near-IR to improve photometry, remove contaminants, and identify quasars. The eight quasars reported here were discovered in a pilot study utilizing the overlap regions at high galactic latitude (|b|>30 deg). These quasars span a redshift range of 5.86<z<6.06 and a flux range of 19.3<z_AB<20.6 mag. Five of them are fainter than z_AB=20 mag, the typical magnitude limit of z~6 quasars used for the SDSS single-epoch images. In addition, we recover eight previously known quasars at z~6 that are located in the overlap regions. These results validate our procedure for selecting quasar candidates from the overlap regions and confirming them with follow-up observations, and provide guidance to a future systematic survey over all SDSS imaging regions with repeat observations.

A SURVEY OF LUMINOUS HIGH-REDSHIFT QUASARS WITH SDSS AND WISE. I. TARGET SELECTION AND OPTICAL SPECTROSCOPY

High-redshift quasars are important tracers of structure and evolution in the early universe. However, they are very rare and difficult to find when using color selection because of contamination from late-type dwarfs. High-redshift quasar surveys based on only optical colors suffer from incompleteness and low identification efficiency, especially at

MAPPING the MOST MASSIVE OVERDENSITY THROUGH HYDROGEN (MAMMOTH). I. METHODOLOGY

z\gtrsim4.5

AN ULTRA-LUMINOUS QUASAR AT z = 5.363 WITH A TEN BILLION SOLAR MASS BLACK HOLE AND A METAL-RICH DLA AT z ∼ 5

. We have developed a new method to select

Gaia17biu/SN 2017egm in NGC 3191: The Closest Hydrogen-poor Superluminous Supernova to Date Is in a “Normal,” Massive, Metal-rich Spiral Galaxy

4.7\lesssim z \lesssim 5.4

Mapping the Most Massive Overdensities through Hydrogen (MAMMOTH). II. Discovery of the Extremely Massive Overdensity BOSS1441 at z = 2.32

quasars with both high efficiency and completeness by combining optical and mid-IR Wide-field Infrared Survey Explorer (WISE) photometric data, and are conducting a luminous

CONSTRAINING VERY HIGH MASS POPULATION III STARS THROUGH He II EMISSION IN GALAXY BDF-521 AT z = 7.01

z\sim5