Equivalent Width Distribution Research Articles

Lyman α emitters in cosmological simulations - I. Lyman α escape fraction and statistical properties at z= 3.1

We use very large cosmological smoothed particle hydrodynamics simulations to study the properties of high-redshift Lyman α emitters (LAEs). We identify star-forming galaxies at z= 3.1 in a cosmological volume of 100 h−1 Mpc on a side. We develop a phenomenological model of absorption, scattering and escape of Lyman α photons on the assumption that the clumpiness of the interstellar medium in a galaxy is correlated with the larger scale substructure richness. The radiative transfer effect proposed by Neufeld allows a large fraction of Lyman α photons to escape from a clumpy galaxy even if it contains a substantial amount of dust. Our model reproduces, for the first time, all of the following observed properties of LAEs at z= 3.1: the angular correlation function, ultraviolet (UV) and Lyman α luminosity functions and the equivalent width distribution. A simple model that takes only dust absorption into account fails in reproducing LAEs that are also bright in UV, suggesting that the kind of effect we consider is needed. Our model predicts a bimodal age distribution for LAEs. Most of the galaxies with large Lyman α equivalent widths are young, whereas there are also old, massive and dusty LAEs, similar to recently found high-redshift LAEs. The large LAEs have escape fractions of Lyman α photons of fesc∼ 0.05–0.1.

Revisiting the census of low-luminosity AGN

The aim of this paper is to revisit critically the current census of AGN as derived from optical spectroscopy. We considered the spectra of nearby (z<0.1) galaxies from the Sloan Digital Sky Survey (SDSS). The equivalent width (EW) distribution of the [O III]5007 emission line is strongly clustered around ~0.6 A, extending the validity of the results we obtained for red giant ellipticals. The close connection between emission lines and stellar continuum points to stellar processes as the most likely source of the bulk of the ionizing photons in these galaxies although their emission line ratios are similar to those of active nuclei. Genuine AGN might be sought mainly among the minority (~5-10%) of outliers, i.e., galaxies with EW>~2 A. The galaxies located in the AGN region of the spectroscopic diagnostic diagrams outnumber outliers by a factor 5-10 which casts doubts on the accuracy of the current identification of active galaxies, particularly those of LINERs of low line luminosity, <~ 10^39-10^40 erg/s. This conclusion can be tested by using spectra that covers smaller physical regions such as those that are already available in the literature of the ~500 nearest bright galaxies, with a stellar continuum reduced by a factor of 20-100 with respect to SDSS galaxies. If the emission lines were mainly of AGN origin, their contrast against the continuum should be enhanced. Instead, their EW distribution is similar to that of the SDSS sample, with just an increase of the outlier fraction. We conclude that the number low-luminosity AGN is currently largely overestimated with a sample purity as low as ~10%. As a consequence the properties of low-luminosity AGN should be fundamentally revised.

The micro-structure of the intergalactic medium - I. The 21 cm signature from dynamical minihaloes

A unified description is provided for the 21cm signatures arising from minihaloes against a bright background radio source and against the Cosmic Microwave Background (CMB), within the context of a dynamical collapsing cosmological spherical halo model. The equivalent width distribution of the resulting 21cm forest is computed for LCDM cosmologies, along with the brightness temperature differential relative to the CMB. The effects of an ambient Ly-alpha radiation field and heating of the IGM on the signatures are included. It is shown that the dynamical effects of heating substantially suppress absorption features in the 21cm forest with observed equivalent widths exceeding 0.15 kHz. It is demonstrated how measurements of excess fluctuations beyond detector noise could make a statistical detection of the weaker absorption features against a bright background radio source. It is also shown that a Ly-alpha radiation field only a few percent the thermalisation rate is sufficient to render the minihalo signal against the CMB negligible compared with the signal from the diffuse IGM component. The 21cm signals are found to be very sensitive to the amount of small scale power in the primordial density fluctuation spectrum. The effects of gas cooling via radiative atomic and molecular processes and of star formation on setting the maximum mass of the minihaloes giving rise to a 21cm signal are included, with allowance made for the suppression of molecular hydrogen formation by an ambient UV radiation field.

GRB 070125 and the environments of spectral-line poor afterglow absorbers★

GRB 070125 is among the most energetic bursts detected and the most extensively observed so far. Nevertheless, unresolved issues are still open in the literature on the physics of the afterglow and on the GRB environment. In particular, GRB 070125 was claimed to have exploded in a galactic halo environment, based on the uniqueness of the optical spectrum and the non-detection of an underlying host galaxy. In this work we collect all publicly available data and address these issues by modelling the NIR-to-X-ray spectral energy distribution (SED) and studying the high signal-to-noise VLT/FORS afterglow spectrum in comparison with a larger sample of GRB absorbers. The SED reveals a synchrotron cooling break in the UV, low equivalent hydrogen column density and little reddening caused by a LMC- or SMC-type extinction curve. From the weak MgII absorption at z=1.5477 in the spectrum, we derived logN(MgII)=12.96+0.13-0.18 and upper limits on the ionic column density of several metals. These suggest that the GRB absorber is most likely an LLS with a 0.03Z\odot<Z<1.3Z\odot metallicity. The comparison with other GRB absorbers places GRB070125 at the low end of the absorption line equivalent width distribution, confirming that weak spectral features and spectral-line poor absorbers are not so uncommon in afterglow spectra. Moreover, we show that the effect of photo-ionization on the gas surrounding the GRB, combined with a low N(HI) along a short segment of the line of sight within the host galaxy, can explain the lack of spectral features in GRB070125. Finally, the non-detection of an underlying galaxy is consistent with a faint GRB host galaxy, well within the GRB host brightness distribution. Thus, the possibility that GRB070125 is simply located in the outskirts of a gas-rich, massive star-forming region inside its small and faint host galaxy seems more likely than a gas-poor, halo environment origin.

COMPLETING THE CENSUS OF Lyα EMITTERS AT THE REIONIZATION EPOCH $^,$

We carried out extended spectroscopic confirmations of Ly-alpha emitters (LAEs) at z=6.5 and 5.7 in the Subaru Deep Field. Now, the total number of spectroscopically confirmed LAEs is 45 and 54 at z=6.5 and 5.7, respectively, and at least 81% (70%) of our photometric candidates at z=6.5 (5.7) have been spectroscopically identified as real LAEs. We made careful measurements of the Ly-alpha luminosity, both photometrically and spectroscopically, to accurately determine the Ly-alpha and rest-UV luminosity functions (LFs). The substantially improved evaluation of the Ly-alpha LF at z=6.5 shows an apparent deficit from z=5.7 at least at the bright end, and a possible decline even at the faint end, though small uncertainties remain. The rest-UV LFs at z=6.5 and 5.7 are in good agreement, at least at the bright end, in clear contrast to the differences seen in the Ly-alpha LF. These results imply an increase in the neutral fraction of the intergalactic medium from z=5.7 to 6.5. The rest-frame equivalent width (EW_0) distribution at z=6.5 seems to be systematically smaller than z=5.7, and it shows an extended tail toward larger EW_0. The bright end of the rest-UV LF can be reproduced from the observed Ly-alpha LF and a reasonable EW_0-UV luminosity relation. Integrating this rest-UV LF provides the first measurement of the contribution of LAEs to the photon budget required for reionization. The derived UV LF suggests that the fractional contribution of LAEs to the photon budget among Lyman break galaxies significantly increases towards faint magnitudes. Low-luminosity LAEs could dominate the ionizing photon budget, though this inference depends strongly on the uncertain faint-end slope of the Ly-alpha LF.

The relationship between [O iii]λ5007 Å equivalent width and obscuration in active galactic nuclei

In this paper we study the relationship between the equivalent width (EW) of the [OIII]5007A narrow emission line in AGN and the level of obscuration. To this end, we combine the results of a systematic spectral analysis, both in the optical and in the X-rays, on a statistically complete sample of ~170 X-ray selected AGN from the XMM-Newton Bright Serendipitous Source sample (XBS). We find that the observed large range of [OIII]5007A equivalent widths observed in the sample (from a few A up to 500A) is well explained as a combination of an intrinsic spread, probably due to the large range of covering factors of the Narrow Line Region, and the effect of absorption. The intrinsic spread is dominant for EW below 40-50A while absorption brings the values of EW up to ~100-150A, for moderate levels of absorption (AV~0.5-2 mag) or up to ~500A for AV>2 mag. In this picture, the absorption has a significant impact on the observed EW also in type~1 AGN. Using numerical simulations we find that this model is able to reproduce the [OIII]5007A EW distribution observed in the XBS sample and correctly predicts the shape of the EW distribution observed in the optically selected sample of QSO taken from the SDSS survey.

Emission lines in early-type galaxies: active nuclei or stars?

We selected 27244 nearby, red, giant early-type galaxies (RGEs) from the Sloan Digital Sky Survey (SDSS). In a large fraction (53%) of their spectra the [O III] emission line is detected, with an equivalent width (EW) distribution strongly clustered around ~0.75 A. The vast majority of those RGEs for which it is possible to derive emission line ratios (amounting to about half of the sample) show values characteristic of LINERs. The close connection between emission lines and stellar continuum points to stellar processes as the most likely source of the bulk of the ionizing photons in RGEs, rather than active nuclei. In particular, the observed EW and optical line ratios are consistent with the predictions of models in which the photoionization comes from to hot evolved stars. Shocks driven by supernovae or stellar ejecta might also contribute to the ionization budget. A minority, ~4%, of the galaxies show emission lines with an equivalent that is width a factor of ~2 greater than the sample median. Only among them are Seyfert-like spectra found. Furthermore, 40% of this subgroup have a radio counterpart, compared to ~6% of the rest of the sample. These characteristics argue in favor of an AGN origin for their emission lines. Emission lines diagnostic diagrams do not reveal a distinction between the AGN subset and the other members of the sample, and consequently they are not a useful tool for establishing the dominant source of the ionizing photons, which is better predicted by the EW of the emission lines.

KECK SPECTROSCOPY OF FAINT 3&gt; z &gt;7 LYMAN BREAK GALAXIES: A HIGH FRACTION OF LINE EMITTERS AT REDSHIFT SIX

As Lyman-alpha photons are scattered by neutral hydrogen, a change with redshift in the Lyman-alpha equivalent width distribution of distant galaxies offers a promising probe of the degree of ionization in the intergalactic medium and hence when cosmic reionization ended. This simple test is complicated by the fact that Lyman-alpha emission can also be affected by the evolving astrophysical details of the host galaxies. In the first paper in this series, we demonstrated both a luminosity and redshift dependent trend in the fraction of Lyman-alpha emitters seen within color-selected Lyman-break galaxies (LBGs) over the range 3<z<6; lower luminosity galaxies and those at higher redshift show an increased likelihood of strong emission. Here we present the results from much deeper 12.5 hour exposures with the Keck DEIMOS spectrograph focused primarily on LBGs at z~6 which enable us to confirm the redshift dependence of line emission more robustly and to higher redshift than was hitherto possible. We find 54+/-11% of faint z~6 LBGs show strong (W_0>25 A) emission, an increase of 1.6x from a similar sample observed at z~4. With a total sample of 74 z~6 LBGs, we determine the luminosity-dependent Lyman-alpha equivalent width distribution. Assuming continuity in these trends to the new population of z~7 sources located with the Hubble WFC3/IR camera, we predict that unless the neutral fraction rises in the intervening 200 Myr, the success rate for spectroscopic confirmation using Lyman-alpha emission should be high.

O iii] equivalent width and orientation effects in quasars

The flux of the [OIII] line is considered to be a good indicator of the bolometric emission of quasars. The observed continuum emission from the accretion disc should instead be strongly dependent on the inclination angle theta between the disc axis and the line of sight. Based on this, the equivalent width (EW) of [OIII] should provide a direct measure of theta. Here we analyze the distribution of EW([OIII]) in a sample of ~6,000 SDSS quasars, and find that it can be accurately reproduced assuming a relatively small intrinsic scatter and a random distribution of inclination angles. This result has several implications: 1) it is a direct proof of the disc-like emission of the optical continuum of quasars; 2) the value of EW([OIII]) can be used as a proxy of the inclination, to correct the measured continuum emission and then estimate the bolometric luminosity of quasars; 3) the presence of almost edge-on discs among broad line quasars implies that the accretion disc is not aligned with the circumnuclear absorber, and/or that the covering fraction of the latter is rather small. Finally, we show that a similar analysis of EW distributions of broad lines (Hbeta, Mg II, C IV) provides no evidence of inclination effects, suggesting a disc-like geometry of the broad emission line region.

Star-forming galaxies in low-redshift clusters: comparison of integrated properties of cluster and field galaxies

We investigate the effect of the cluster environment on the star formation properties of galaxies in 8 nearby Abell clusters. Star formation properties are determined for individual galaxies using the equivalent width of H alpha plus [NII] line emission from narrow-band imaging. Equivalent width distributions are derived for each galaxy type in each of 3 environments - cluster, supercluster (outside the cluster virial radius) and field. The effects of morphological disturbance on star formation are also investigated. We identify a population of early-type disk galaxies in the cluster population with enhanced star formation compared to their field counterparts. The enhanced cluster galaxies frequently show evidence of disturbance, and the disturbed galaxies show marginal evidence for a higher velocity dispersion, possibly indicative of an infalling population.

AN ATLAS OFz= 5.7 ANDz= 6.5 Lyα EMITTERS,

We present an atlas of 88 z~5.7 and 30 z~6.5 Ly alpha emitters obtained from a wide-field narrowband survey. We combined deep narrowband imaging in 120A bandpass filters centered at 8150A and 9140A with deep BVRIz broadband imaging to select high-redshift galaxy candidates over an area of 4180 square arcmin. The goal was to obtain a uniform selection of comparable depth over the 7 targeted fields in the two filters. For the GOODS-N region of the HDF-N field, we also selected candidates using a 120A filter centered at 9210A. We made spectroscopic observations with Keck DEIMOS of nearly all the candidates to obtain the final sample of Ly alpha emitters. At the 3.3A resolution of the DEIMOS observations the asymmetric profile for Ly alpha emission with its steep blue fall-off can be clearly seen in the spectra of nearly all the galaxies. We show that the spectral profiles are surprisingly similar for many of the galaxies and that the composite spectral profiles are nearly identical at z=5.7 and z=6.5. We analyze the distributions of line widths and Ly alpha equivalent widths and find that the lines are marginally narrower at the higher redshift, with median values of 0.77A at z=6.5 and 0.92A at z=5.7. The line widths have a dependence on the Ly alpha luminosity of the form L(L alpha)^(0.3). We compare the surface densities and the luminosity functions at the two redshifts and find that there is a multiplicative factor of 2 decrease in the number density of bright Ly alpha emitters from z=5.7 to z=6.5, while the characteristic luminosity is unchanged.

X-RAY PROPERTIES OF THEz∼ 4.5 Lyα EMITTERS IN THE CHANDRA DEEP FIELD SOUTH REGION

We report the first X-ray detection of 113 Lyman-alpha emitters at redshift z ~ 4.5. Only one source (J033127.2-274247) is detected in the Extended Chandra Deep Field South (ECDF-S) X-ray data, and has been spectroscopically confirmed as a z = 4.48 quasar with $L_X = 4.2\times 10^{44}$ erg/s. The single detection gives a Lyman-alpha quasar density consistent with the X-ray luminosity function of quasars. The coadded counts of 22 Lyman-alpha emitters (LAEs) in the central Chandra Deep Field South (CDF-S) region yields a S/N=2.4 (p=99.83%) detection at soft band, with an effective exposure time of ~36 Ms. Further analysis of the equivalent width (EW) distribution shows that all the signal comes from 12 LAE candidates with EW_rest < 400 \AA, and 2 of them contribute about half of the signal. Following-up spectroscopic observations show that the two are a low-redshift emission line galaxy and a Lyman break galaxy at z = 4.4. Excluding these two and combined with ECDF-S data, we derive a 3-sigma upper limit on the average luminosity of $L_{0.5-2 keV}$ $<$ 2.4 $\times 10^{42}$ ergs/s for z ~ 4.5 LAEs. If the average X-ray emission is due to star formation, it corresponds to a star-formation rate (SFR) of < 180--530 M$_\sun$ per yr. We use this SFR_X as an upper limit of the unobscured SFR to constrain the escape fraction of Lyman-alpha photons, and find a lower limit of f_esc > 3-10%. However, our upper limit on the SFR_X is ~7 times larger than the upper limit on SFR_X on z ~ 3.1 LAEs in the same field, and at least 30 times higher than the SFR estimated from Lyman-alpha emission. From the average X-ray to Lyman-alpha line ratio, we estimate that fewer than 3.2% (6.3%) of our LAEs could be high redshift type 1 (type 2) AGNs, and those hidden AGNs likely show low rest frame EWs.

Searching for the earliest galaxies in the 21 cm forest

We use a model developed by Xu et al. (2010) to compute the 21 cm line absorption signatures imprinted by star-forming dwarf galaxies (DGs) and starless minihalos (MHs). The method, based on a statistical comparison of the equivalent width (W_\nu) distribution and flux correlation function, allows us to derive a simple selection criteria for candidate DGs at very high (z >= 8) redshift. We find that ~ 18% of the total number of DGs along a line of sight to a target radio source (GRB or quasar) can be identified by the condition W_\nu < 0; these objects correspond to the high-mass tail of the DG distribution at high redshift, and are embedded in large HII regions. The criterion W_\nu > 0.37 kHz instead selects ~ 11% of MHs. Selected candidate DGs could later be re-observed in the near-IR by the JWST with high efficiency, thus providing a direct probe of the most likely reionization sources.

RADIATIVE TRANSFER MODELING OF Lyα EMITTERS. I. STATISTICS OF SPECTRA AND LUMINOSITY

We combine a cosmological reionization simulation with box size of 100Mpc/h on a side and a Monte Carlo Lyman-alpha (Lya) radiative transfer code to model Lyman Alpha Emitters (LAEs) at z~5.7. The model introduces Lya radiative transfer as the single factor for transforming the intrinsic Lya emission properties into the observed ones. Spatial diffusion of Lya photons from radiative transfer results in extended Lya emission and only the central part with high surface brightness can be observed. Because of radiative transfer, the appearance of LAEs depends on density and velocity structures in circumgalactic and intergalactic media as well as the viewing angle, which leads to a broad distribution of apparent (observed) Lya luminosity for a given intrinsic Lya luminosity. Radiative transfer also causes frequency diffusion of Lya photons. The resultant Lya line is asymmetric with a red tail. The peak of the Lya line shifts towards longer wavelength and the shift is anti-correlated with the apparent to intrinsic Lya luminosity ratio. The simple radiative transfer model provides a new framework for studying LAEs. It is able to explain an array of observed properties of z~5.7 LAEs in Ouchi et al. (2008), producing Lya spectra, morphology, and apparent Lya luminosity function (LF) similar to those seen in observation. The broad distribution of apparent Lya luminosity at fixed UV luminosity provides a natural explanation for the observed UV LF, especially the turnover towards the low luminosity end. The model also reproduces the observed distribution of Lya equivalent width (EW) and explains the deficit of UV bright, high EW sources. Because of the broad distribution of the apparent to intrinsic Lya luminosity ratio, the model predicts effective duty cycles and Lya escape fractions for LAEs.

Lyα-EMITTING GALAXIES ATz= 2.1 IN ECDF-S: BUILDING BLOCKS OF TYPICAL PRESENT-DAY GALAXIES?

We discovered a sample of 250 Ly-Alpha emitting (LAE) galaxies at z=2.1 in an ultra-deep 3727 A narrow-band MUSYC image of the Extended Chandra Deep Field-South. LAEs were selected to have rest-frame equivalent widths (EW) > 20 A and emission line fluxes > 2.0 x 10^(-17)erg /cm^2/s, after carefully subtracting the continuum contributions from narrow band photometry. The median flux of our sample is 4.2 x 10^(-17)erg/cm^2/s, corresponding to a median Lya luminosity = 1.3 x 10^(42) erg/s at z=2.1. At this flux our sample is > 90% complete. Approximately 4% of the original NB-selected candidates were detected in X-rays by Chandra, and 7% were detected in the rest-frame far-UV by GALEX. At luminosity>1.3 x 10^42 erg/s, the equivalent width distribution is unbiased and is represented by an exponential with scale-length of 83+/-10 A. Above this same luminosity threshold, we find a number density of 1.5+/-0.5 x 10^-3 Mpc^-3. Neither the number density of LAEs nor the scale-length of their EW distribution show significant evolution from z=3 to z=2. We used the rest frame UV luminosity to estimate a median star formation rate of 4 M_(sun) /yr. The median rest frame UV slope, parametrized by B-R, is that typical of dust-free, 0.5-1 Gyr old or moderately dusty, 300-500 Myr old populations. Approximately 40% of the sample occupies the z~2 star-forming galaxy locus in the UVR two color diagram. Clustering analysis reveals that LAEs at z=2.1 have r_0=4.8+/-0.9 Mpc and a bias factor b=1.8+/-0.3. This implies that z=2.1 LAEs reside in dark matter halos with median masses Log(M/M_(sun))=11.5^(+0.4)_(-0.5), which are among of the lowest-mass halos yet probed at this redshift. We used the Sheth-Tormen conditional mass function to study the descendants of these LAEs and found that their typical present-day descendants are local galaxies with L* properties, like the Milky Way.

THE EVOLUTION OF QUASAR C IV AND Si IV BROAD ABSORPTION LINES OVER MULTI-YEAR TIMESCALES

We investigate the variability of CIV 1549A broad absorption line (BAL) troughs over rest-frame time scales of up to ~7 yr in 14 quasars at redshifts z>2.1. For 9 sources at sufficiently high redshift, we also compare CIV and SiIV 1400A absorption variation. We compare shorter- and longer-term variability using spectra from up to four different epochs per source and find complex patterns of variation in the sample overall. The scatter in the change of absorption equivalent width (EW), Delta EW, increases with the time between observations. BALs do not, in general, strengthen or weaken monotonically, and variation observed over shorter (<months) time scales is not predictive of multi-year variation. We find no evidence for asymmetry in the distribution of Delta EW that would indicate that BALs form and decay on different time scales, and we constrain the typical BAL lifetime to be >~30 yr. The BAL absorption for one source, LBQS 0022+0150, has weakened and may now be classified as a mini-BAL. Another source, 1235+1453, shows evidence of variable, blue continuum emission that is relatively unabsorbed by the BAL outflow. CIV and SiIV BAL shape changes are related in at least some sources. Given their high velocities, BAL outflows apparently traverse large spatial regions and may interact with parsec-scale structures such as an obscuring torus. Assuming BAL outflows are launched from a rotating accretion disk, notable azimuthal symmetry is required in the outflow to explain the relatively small changes observed in velocity structure over times up to 7 yr.

The Southern 2MASS Active Galactic Nuclei Survey: Spectroscopic Follow-up with Six Degree Field

AbstractThe Two Micron All-Sky Survey (2MASS) has provided a uniform photometric catalog to search for previously unknown red active galactic nuclei (AGN) and Quasi-Stellar Objects (QSOs).We have extended the search to the southern equatorial sky by obtaining spectra for 1182 AGN candidates using the six degree field (6dF) multifibre spectrograph on the UK Schmidt Telescope. These were scheduled as auxiliary targets for the 6dF Galaxy Redshift Survey. The candidates were selected using a single color cut of J – Ks &gt; 2 to Ks ≲ 15.5 and a galactic latitude of lbl &gt; 30°. 432 spectra were of sufficient quality to enable a reliable classification. 116 sources (∼27%) were securely classified as type I AGN, 20 as probable type I AGN, and 57 as probable type II AGN. Most of them span the redshift range 0.05 &lt; z &lt; 0.5 and only 8 (∼6%) were previously identified as AGN or QSOs. Our selection leads to a significantly higher AGN identification rate amongst local galaxies (&gt;20%) than in any previous (mostly blue-selected) galaxy survey. A small fraction of the type I AGN could have their optical colors reddened by optically thin dust with AV &lt; 2 mag relative to optically selected QSOs. A handful show evidence of excess far-infrared (IR) emission. The equivalent width (EW) and color distributions of the type I and II AGN are consistent with AGN unified models. In particular, the EW of the [Oiii] emission line weakly correlates with optical–near-IR color in each class of AGN, suggesting anisotropic obscuration of the AGN continuum. Overall, the optical properties of the 2MASS red AGN are not dramatically different from those of optically-selected QSOs. Our near-IR selection appears to detect the most near-IR luminous QSOs in the local universe to z≃0.6 and provides incentive to extend the search to deeper near-IR surveys.

Lyα EMITTERS IN HIERARCHICAL GALAXY FORMATION. II. ULTRAVIOLET CONTINUUM LUMINOSITY FUNCTION AND EQUIVALENT WIDTH DISTRIBUTION

We present theoretical predictions of UV continuum luminosity function (UV LF) and Lya equivalent width (EW) distribution of Lyman alpha emitters (LAEs) in the framework of the hierarchical clustering model of galaxy formation. The model parameters about LAEs were determined by fitting to the observed Lya LF at z=5.7 in our previous study, and the fit indicates that extinction of Lya photons by dust is significantly less effective than that of UV continuum photons, implying clumpy dust distribution in interstellar medium. We then compare the predictions about UV LFs and EW distributions with a variety of observations at z ~ 3-6, allowing no more free parameters and paying careful attention to the selection conditions of LAEs in each survey. We find that the predicted UV LFs and EW distributions are in nice agreement with observed data, and especially, our model naturally reproduces the existence of large EW LAEs (> 240 A) without introducing Pop III stars or top-heavy initial mass function. We show that both the stellar population (young age and low metallicity) and extinction by clumpy dust are the keys to reproduce large EW LAEs. The evidence of EW enhancement by clumpy dust is further strengthened by the quantitative agreement between our model and recent observations about a positive correlation between EW and extinction. The observed trend that brighter LAEs in UV continuum tend to have smaller mean EW is also reproduced, and the clumpy dust is playing an important role again for this trend. We suggested in our previous study that the transmission of intergalactic medium for Lya emission rapidly decreases from z ~ 6 to 7 by the fitting to Lya LFs, and this evidence is quantitatively strengthened by the comparison with the UV LF and EW distribution at z ~ 6.6.

Star formation indicators and line equivalent width in Lyα galaxies

The equivalent width (EW) of the Lyman Alpha (Lya) line is directly related to the ratio of star formation rates determined from Lya flux and UV flux density [SFR(Lya)/SFR(UV)]. We use published data --in the literature EW and SFR(Lya)/SFR(UV) are treated as independent quantities-- to show that the predicted relation holds for the vast majority of observed Lya emitting galaxies (LAEs). We show that the relation between EW and SFR(Lya)/SFR(UV) applies irrespective of a galaxy's `true' underlying star formation rate, and that its only source of scatter is the variation in the spectral slope of the UV continuum between individual galaxies. The derived relation, when combined with the observed EW distribution, implies that the ratio SFR(UV)/SFR(Lya) is described well by a log-normal distribution with a standard deviation of ~0.3-0.35. This result is useful when modelling the statistical properties of LAEs. We further discuss why the relation between EW and SFR(Lya)/SFR(UV) may help identifying galaxies with unusual stellar populations.

Statistics and characteristics of MgII absorbers along GRB lines of sight observed with VLT-UVES

We analyse the properties of MgII absorption systems detected along the sightlines toward GRBs using a sample of 10 GRB afterglow spectra obtained with VLT-UVES over the past six years. The S/N ratio is sufficiently high that we can extend previous studies to smaller equivalent widths (typically Wr>0.3A). Over a pathlength of Delta(z)~14 the number of weak absorbers detected is similar along GRB and QSO lines of sight, while the number of strong systems is larger along GRB lines of sight with a 2-sigma significance. Using intermediate and low resolution observations reported in the literature, we increase the absorption length for strong systems to Delta(z)=31.5 (about twice the path length of previous studies) and find that the number density of strong MgII systems is a factor of 2.1+/-0.6 higher (about 3-sigma significance) toward GRBs as compared to QSOs, about twice smaller however than previously reported. We divide the sample in three redshift bins and we find that the number density of strong MgII is larger in the low redshift bins. We investigate in detail the properties of strong MgII systems observed with UVES. Both the estimated dust extinction in strong GRB MgII systems and the equivalent width distribution are consistent with what is observed for standard QSO systems. We find also that the number density of (sub)-DLAs per unit redshift in the UVES sample is probably twice larger than what is expected from QSO sightlines which confirms the peculiarity of GRB lines of sight. These results indicate that neither a dust extinction bias nor different beam sizes of the sources are viable explanations for the excess. It is still possible that the current sample of GRB lines of sight is biased by a subtle gravitational lensing effect. More data and larger samples are needed to test this hypothesis. (abridged)