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Register a new userStrong Field-to-Field Variation of Lyman alpha Nebulae Populations at z~2.3
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Yujin Yang
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Understanding the nature of distant Ly-alpha nebulae (blobs) and connecting them to their present-day descendants requires constraining their number density, clustering, and large-scale environment. To measure these basic quantities, we conduct a deep narrowband imaging survey in four different fields, Chandra Deep Field South (CDFS), Chandra Deep Field North, and two COSMOS subfields, for a total survey area of 1.2deg^2. We discover 25 blobs at z=2.3 with Ly-alpha luminosities of 0.7-8x10^43 erg/s and isophotal areas of Aiso = 10-60 arcsec^2. The transition from compact Ly-alpha emitters (Aiso ~ a few arcsec^2) to extended blobs (Aiso > 10 arcsec^2) is continuous, suggesting a single family perhaps governed by similar emission mechanisms. Surprisingly, most blobs (16/25) are in one survey field, the CDFS. The six brightest, largest blobs with L > 1.5x10^43 erg/s and Aiso > 16 arcsec^2 lie only in the CDFS. These large, bright blobs have a field-to-field variance of sigma_v >~ 1.5 (150%) about their number density n ~ 1.0x10^-5 Mpc^-3. This variance is large, significantly higher than that of unresolved LAEs (sigma_v ~ 0.3 or 30%), and can adversely affect comparisons of blob number densities and luminosity functions among different surveys. We compare the statistics of our blobs with dark matter halos in a 1 Gpc/h cosmological N-body simulation. At z=2.3, the number density (n) implies that each bright, large blob could occupy a halo of M_halo > 10^13 Msun if most halos have detectable blobs. The predicted variance in n is consistent with that observed and corresponds to a bias of ~7. Blob halos lie at the high end of the halo mass distribution at z=2.3 and are likely to evolve into the ~10^14 Msun halos typical of galaxy clusters today. On larger scales of ~10 co-moving Mpc, blobs cluster where compact LAEs do, indicating that blobs lie in coherent, highly overdense structures.
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We study the multi-wavelength properties of a set of 171 Ly-alpha emitting candidates at redshift z = 2.25 found in the COSMOS field, with the aim of understanding the underlying stellar populations in the galaxies. We especially seek to understand what the dust contents, ages and stellar masses of the galaxies are, and how they relate to similar properties of Ly-alpha emitters at other redshifts. The candidates here are shown to have different properties from those of Ly-alpha emitters found at higher redshift, by fitting the spectral energy distributions (SEDs) using a Monte-Carlo Markov-Chain technique and including nebular emission in the spectra. The stellar masses, and possibly the dust contents, are higher, with stellar masses in the range log M_* = 8.5 - 11.0 M_sun and A_V = 0.0 - 2.5 mag. Young population ages are well constrained, but the ages of older populations are typically unconstrained. In 15% of the galaxies only a single, young population of stars is observed. We show that the Ly-alpha fluxes of the best fit galaxies are correlated with their dust properties, with higher dust extinction in Ly-alpha faint galaxies. Testing for whether results derived from a light-weighted stack of objects correlate to those found when fitting individual objects we see that stellar masses are robust to stacking, but ages and especially dust extinctions are derived incorrectly from stacks. We conclude that the stellar properties of Ly-alpha emitters at z = 2.25 are different from those at higher redshift and that they are diverse. Ly-alpha selection appears to be tracing systematically different galaxies at different redshifts.
To obtain an unbiased sample of bright LyA blobs [L(LyA) > 10^43 ergs/s], we have undertaken a blind, wide-field, narrow-band imaging survey in the NOAO Deep Wide Field Survey Bootes field with the Steward Bok-2.3m telescope. After searching over 4.82 sq. degrees at z=2.3, we discover four LyA blobs with L(LyA) = 1.6-5.3 x 10^43 ergs/s, isophotal areas of 28-57 sq. arcsec, and broad LyA line profiles (FWHM = 900-1250 km/s). In contrast with the extended Lyman alpha halos associated with high-z radio galaxies, none of our four blobs are radio-loud. The X-ray luminosities and optical spectra of these blobs are diverse. Two blobs (3 and 4) are X-ray-detected with L_X(2-7 keV) = 2-4 x 10^44 ergs/s and have broad optical emission lines (C IV) characteristic of AGN, implying that 50% of our sample blobs are associated with strong AGN. The other 50% of blobs (1 and 2) are not X-ray or optically-detected as AGN down to similar limits. The number density of the four blobs is ~3 x 10^{-6} Mpc^{-3}, comparable to that of galaxy clusters at similar redshifts and 3x lower than that found in the SSA22 proto-cluster at z=3.1, even after accounting for the over-density of that region. The two X-ray undetected blobs are separated by only 70 (550 kpc) and have almost identical redshifts (< 360 kpc along the line-of-sight), suggesting that they are part of the same system. Given the rarity of the blobs and our discovery of a close pair, we speculate that blobs occupy the highest density regions and thus may be precursors of todays rich cluster galaxies.
We present the first spectroscopic measurements of the [OIII] 5007 A line in two z ~ 3.1 Lyman-alpha emitting galaxies (LAEs) using the new near-infrared instrument LUCIFER1 on the 8.4m Large Binocular Telescope (LBT). We also describe the optical imaging and spectroscopic observations used to identify these Lya emitting galaxies. Using the [OIII] line we have measured accurate systemic redshifts for these two galaxies, and discovered a velocity offset between the [OIII] and Ly-alpha lines in both, with the Lya line peaking 342 and 125 km/s redward of the systemic velocity. These velocity offsets imply that there are powerful outflows in high-redshift LAEs. They also ease the transmission of Lya photons through the interstellar medium and intergalactic medium around the galaxies. By measuring these offsets directly, we can refine both Lya-based tests for reionization, and Lya luminosity function measurements where the Lya forest affects the blue wing of the line. Our work also provides the first direct constraints on the strength of the [OIII] line in high-redshift LAEs. We find [OIII] fluxes of 7 and 36 x 10^-17 erg s^-1 cm^-2 in two z ~ 3.1 LAEs. These lines are strong enough to dominate broad-band flux measurements that include the line (in thiscase, K_s band photometry). Spectral energy distribution fits that do not account for the lines would therefore overestimate the 4000 A (and/or Balmer) break strength in such galaxies, and hence also the ages and stellar masses of such high-z galaxies.
We present a sample of 33 spectroscopically confirmed z ~ 3.1 Ly$alpha$-emitting galaxies (LAEs) in the Cosmological Evolution Survey (COSMOS) field. This paper details the narrow-band survey we conducted to detect the LAE sample, the optical spectroscopy we performed to confirm the nature of these LAEs, and a new near-infrared spectroscopic detection of the [O III] 5007 AA line in one of these LAEs. This detection is in addition to two [O III] detections in two z ~ 3.1 LAEs we have reported on previously (McLinden et al 2011). The bulk of the paper then presents detailed constraints on the physical characteristics of the entire LAE sample from spectral energy distribution (SED) fitting. These characteristics include mass, age, star-formation history, dust content, and metallicity. We also detail an approach to account for nebular emission lines in the SED fitting process - wherein our models predict the strength of the [O III] line in an LAE spectrum. We are able to study the success of this prediction because we can compare the model predictions to our actual near-infrared observations both in galaxies that have [O III] detections and those that yielded non-detections. We find a median stellar mass of 6.9 $times$ 10$^8$ M$_{odot}$ and a median star formation rate weighted stellar population age of 4.5 $times$ 10$^6$ yr. In addition to SED fitting, we quantify the velocity offset between the [O III] and Ly$alpha$ lines in the galaxy with the new [O III] detection, finding that the Ly$alpha$ line is shifted 52 km s$^{-1}$ redwards of the [O III] line, which defines the systemic velocity of the galaxy.
We investigate the stellar populations of Lyman alpha emitters (LAEs) at z=3.1 and 3.7 in 0.65 deg^2 of the Subaru/XMM-Newton Deep Field, based on rest-frame UV-to-optical photometry obtained from the Subaru/XMM-Newton Deep Survey, the UKIDSS/Ultra Deep Survey, and the Spitzer legacy survey of the UKIDSS/UDS. Among a total of 302 LAEs (224 for z=3.1 and 78 for z=3.7), only 11 are detected in the K band, i.e., brighter than K(3sigma)=24.1 mag. Eight of the 11 K-detected LAEs are spectroscopically confirmed. We find that the K-undetected objects, which should closely represent the LAE population as a whole, have low stellar masses of ~ 10^8 - 10^8.5 Msun, modest SFRs of 1 - 100 Msun yr^-1, and modest dust extinction of E(B-V) < 0.2. The K-detected objects are massive, Mstar ~ 10^9 - 10^10.5 Msun, and have significant dust extinction with a median of E(B-V) ~= 0.3. Four K-detected objects with the reddest spectral energy distributions, two of which are spectroscopically confirmed, are heavily obscured with E(B-V) ~ 0.65, and their continua resemble those of some local ULIRGs. Interestingly, they have large Lyman alpha equivalent widths ~= 70 - 250 A. If these four are excluded, our sample has a weak anti-correlation between EW(Lya) and Mstar. We compare the stellar masses and the specific star formation rates (sSFR) of LAEs with those of Lyman-break galaxies, distant red galaxies, submillimetre galaxies, and I- or K-selected galaxies with z_phot ~ 3. We find that the LAE population is the least massive among all the galaxy populations in question, but with relatively high sSFRs, while NIR-detected LAEs have Mstar and sSFR similar to LBGs. Our reddest four LAEs have very high sSFRs in spite of large Mstar, thus occupying a unique region in the Mstar versus sSFR space. (abridged)
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