arXiv subjects

Junxian Wang

Publications and source records attributed to Junxian Wang.

At least 73 records · Page 4Linked to original sources

arXiv2017

Ly$α$ profile, dust, and prediction of Ly$α$ escape fraction in Green Pea Galaxies

We studied Lyman-$α$ (Ly$α$) escape in a statistical sample of 43 Green Peas with HST/COS Ly$α$ spectra. Green Peas are nearby star-forming galaxies with strong [OIII]$λ$5007 emission lines. Our sample is four times larger than the previous sample and covers a much more complete range of Green Pea properties. We found that about 2/3 of Green Peas are strong Ly$α$ line emitters with rest-frame Ly$α$ equivalent width $>20$ Å. The Ly$α$ profiles of Green Peas are diverse. The Ly$α$ escape fraction, defined as the ratio of observed Ly$α$ flux to intrinsic Ly$α$ flux, shows anti-correlations with a few Ly$α$ kinematic features -- both the blue peak and red peak velocities, the peak separations, and FWHM of the red portion of the Ly$α$ profile. Using properties measured from SDSS optical spectra, we found many correlations -- Ly$α$ escape fraction generally increases at lower dust reddening, lower metallicity, lower stellar mass, and higher [OIII]/[OII] ratio. We fit their Ly$α$ profiles with the HI shell radiative transfer model and found Ly$α$ escape fraction anti-correlates with the best-fit $N_{HI}$. Finally, we fit an empirical linear relation to predict Ly$α$ escape fraction from the dust extinction and Ly$α$ red peak velocity. The standard deviation of this relation is about 0.3 dex. This relation can be used to isolate the effect of IGM scatterings from Ly$α$ escape and to probe the IGM optical depth along the line of sight of each $z>7$ Ly$α$ emission line galaxy in the JWST era.

astro-ph.GA↗

arXiv2017

First Results from the Lyman Alpha Galaxies in the Epoch of Reionization (LAGER) Survey: Cosmological Reionization at z ~ 7

We present the first results from the ongoing LAGER project (Lyman Alpha Galaxies in the Epoch of Reionization), which is the largest narrowband survey for $z \sim$ 7 galaxies to date. Using a specially built narrowband filter NB964 for the superb large-area Dark-Energy Camera (DECam) on the NOAO/CTIO 4m Blanco telescope, LAGER has collected 34 hours NB964 narrowband imaging data in the 3 deg$^2$ COSMOS field. We have identified 23 Lyman Alpha Emitter (LAE) candidates at $z$ = 6.9 in the central 2-deg$^2$ region, where DECam and public COSMOS multi-band images exist. The resulting luminosity function can be described as a Schechter function modified by a significant excess at the bright end (4 galaxies with $L_{Lyα} \sim $ 10$^{43.4\pm0.2}$ erg s$^{-1}$). The number density at $L_{Lyα}\sim$ 10$^{43.4\pm0.2}$ erg s$^{-1}$ is little changed from z= 6.6, while at fainter $L_{Lyα}$ it is substantially reduced. Overall, we see a fourfold reduction in Ly$α$ luminosity density from $z$ = 5.7 to 6.9. Combined with a more modest evolution of the continuum UV luminosity density, this suggests a factor of $\sim 3$ suppression of Ly$α$ by radiative transfer through the $z \sim$ 7 intergalactic medium (IGM). It indicates an IGM neutral fraction $x_{HI}$ $\sim$ 0.4--0.6 (assuming Ly$α$ velocity offsets of 100-200 km s$^{-1}$). The changing shape of the Ly$α$ luminosity function between $z\lesssim 6.6$ and $z=6.9$ supports the hypothesis of ionized bubbles in a patchy reionization at $z\sim$ 7.

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arXiv2017

An adaptive BDDC algorithm in variational form for mortar discretizations

A balancing domain decomposition by constraints (BDDC) algorithm with adaptive primal constraints in variational form is introduced and analyzed for high-order mortar discretization of two-dimensional elliptic problems with high varying and random coefficients. Some vector-valued auxiliary spaces and operators with essential properties are defined to describe the variational algorithm, and the coarse space is formed by using a transformation operator on each interface. Compared with the adaptive BDDC algorithms for conforming Galerkin approximations, our algorithm is more simple, because there is not any continuity constraints at subdomain vertices in the mortar method involved in this paper. The condition number of the preconditioned system is proved to be bounded above by a user-defined tolerance and a constant which is dependent on the maximum number of interfaces per subdomain, and independent of the mesh size and the contrast of the given coefficients. Numerical results show the robustness and efficiency of the algorithm for various model problems.

math.NA↗

arXiv2017

An imperfectly passive nature: Bright sub-millimeter emission from dust-obscured star formation in the z=3.717 "passive" system, ZF20115

The identification of high-redshift massive galaxies with old stellar populations may pose challenges to some models of galaxy formation. However, to securely classify a galaxy as quiescent, it is necessary to exclude significant ongoing star formation, something that can be challenging to achieve at high redshift. In this letter, we analyse deep ALMA/870um and SCUBA-2/450um imaging of the claimed "post-starburst" galaxy ZF-20115 at z=3.717 that exhibits a strong Balmer break and absorption lines. The far-infrared imaging reveals a luminous starburst located 0.4+/-0.1 arcsec (~3kpc in projection) from the position of the rest-frame ultra-violet/optical emission, with an obscured star-formation rate of 100 Mo/yr. This star-forming component is undetected in the rest-frame ultraviolet but contributes significantly to the lower angular resolution photometry at restframe wavelengths >3500A, significantly complicating the determination of a reliable stellar mass. Importantly, in the presence of dust obscuration, strong Balmer features are not a unique signature of a post-starburst galaxy and are indeed frequently observed in infrared-luminous galaxies. We conclude that the ZF20015 system does not pose a challenge to current models of galaxy formation and that deep sub-/millimeter observations are a prerequisite for any claims of quiescence. The multi-wavelength observations of ZF20115 unveil a complex system with an intricate and spatially-varying star-formation history. ZF20115 demonstrates that understanding high-redshift obscured starbursts will only be possible with multi-wavelength studies that include high-resolution observations, available with the JWST, at mid-infrared wavelengths.

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arXiv2017

Central Engine and Host Galaxy of RXJ 1301.9+2747: A Multiwavelength View of a Low-mass Black Hole Active Galactic Nuclei with Ultra-soft X-ray Emission

RXJ 1301.9+2747 is an optically identified very low mass AGN candidate with M_BH~1x10^6M_sun, which shows extremely soft X-ray emission and unusual X-ray variability in the form of short-lived flares. We present an analysis of multiwavelength observations of RXJ 1301.9+2747 in order to study the properties of the active nucleus and its host galaxy. The UV-to-X-ray spectrum in the quiescent state can be well and self-consistently described by a thermal and a Comptonized emission from accretion disk, with blackbody dominating ~70% of the X-rays in the 0.2-2 keV. The same model can describe the X-ray spectrum in the flare state but the Comptonized component becomes dominant (~80%). The best-fit implies an Eddington ratio of ~0.14, and a black hole mass of 1.7-2.8x10^6M_sun, in agreement with the estimation from the optical data within errors. However, the best-fitting model under-predicts the optical flux for the HST point source by a factor of ~2. The excess of nuclear optical emission could be attributed to a nuclear stellar cluster which is frequently seen in low mass AGNs. The X-ray to optical spectral slope (alpha_ox) is lower than in most other active galaxies, which may be attributed to intrinsically X-ray weakness due to very little hot and optically thin coronal emission. We performed a pilot search for weak or hidden broad emission lines using optical spectropolarimetry observations, but no any polarized broad lines are detected. The host galaxy appears to be a disk galaxy with a boxy pseudobulge or nuclear bar accounting for ~15% of the total starlight, which is consistent with the general characteristics of the host of low mass AGNs.

astro-ph.HE↗

arXiv2016

The deepest X-ray view of high-redshift galaxies: constraints on low-rate black-hole accretion

We exploit the 7 Ms \textit{Chandra} observations in the \chandra\,Deep Field-South (\mbox{CDF-S}), the deepest X-ray survey to date, coupled with CANDELS/GOODS-S data, to measure the total X-ray emission arising from 2076 galaxies at $3.5\leq z < 6.5$. This aim is achieved by stacking the \textit{Chandra} data at the positions of optically selected galaxies, reaching effective exposure times of $\geq10^9\mathrm{s}$. We detect significant ($>3.7σ$) X-ray emission from massive galaxies at $z\approx4$. We also report the detection of massive galaxies at $z\approx5$ at a $99.7\%$ confidence level ($2.7σ$), the highest significance ever obtained for X-ray emission from galaxies at such high redshifts. No significant signal is detected from galaxies at even higher redshifts. The stacking results place constraints on the BHAD associated with the known high-redshift galaxy samples, as well as on the SFRD at high redshift, assuming a range of prescriptions for X-ray emission due to X- ray binaries. We find that the X-ray emission from our sample is likely dominated by processes related to star formation. Our results show that low-rate mass accretion onto SMBHs in individually X-ray-undetected galaxies is negligible, compared with the BHAD measured for samples of X-ray detected AGN, for cosmic SMBH mass assembly at high redshift. We also place, for the first time, constraints on the faint-end of the AGN X-ray luminosity function ($\mathrm{logL_X\sim42}$) at $z>4$, with evidence for fairly flat slopes. The implications of all of these findings are discussed in the context of the evolution of the AGN population at high redshift.

astro-ph.GA↗

arXiv2016

BDDC and FETI-DP algorithms with adaptive coarse spaces for three-dimensional elliptic problems with oscillatory and high contrast coefficients

BDDC and FETI-DP algorithms are developed for three-dimensional elliptic problems with adaptively enriched coarse components. It is known that these enriched components are necessary in the development of robust preconditioners. To form the adaptive coarse components, carefully designed generalized eigenvalue problems are introduced for each faces and edges, and the coarse components are formed by using eigenvectors with their corresponding eigenvalues larger than a given tolerance $λ_{TOL}$. Upper bounds for condition numbers of the preconditioned systems are shown to be $C λ_{TOL}$, with the constant $C$ depending only on the maximum number of edges and faces per subdomain, and the maximum number of subdomains sharing an edge. Numerical results are presented to test the robustness of the proposed approach.

math.NA↗

arXiv2016

Green Pea Galaxies Reveal Secrets of Ly$α$ Escape

We analyze archival Ly$α$ spectra of 12 "Green Pea" galaxies observed with the Hubble Space Telescope, model their Ly$α$ profiles with radiative transfer models, and explore the dependence of Ly$α$ escape fraction on various properties. Green Pea galaxies are nearby compact starburst galaxies with [OIII]$λ$5007 equivalent widths of hundreds of Å. All 12 Green Pea galaxies in our sample show Ly$α$ lines in emission, with a Ly$α$ equivalent width distribution similar to high redshift Ly$α$ emitters. Combining the optical and UV spectra of Green Pea galaxies, we estimate their Ly$α$ escape fractions and find correlations between Ly$α$ escape fraction and kinematic features of Ly$α$ profiles. The escape fraction of Ly$α$ in these galaxies ranges from 1.4% to 67%. We also find that the Ly$α$ escape fraction depends strongly on metallicity and moderately on dust extinction. We compare their high-quality Ly$α$ profiles with single HI shell radiative transfer models and find that the Ly$α$ escape fraction anti-correlates with the derived HI column densities. Single shell models fit most Ly$α$ profiles well, but not the ones with highest escape fractions of Ly$α$. Our results suggest that low HI column density and low metallicity are essential for Ly$α$ escape, and make a galaxy a Ly$α$ emitter.

astro-ph.GA↗

arXiv2014

CLASH: Extreme Emission Line Galaxies and Their Implication on Selection of High-Redshift Galaxies

We utilize the CLASH (Cluster Lensing And Supernova survey with Hubble) observations of 25 clusters to search for extreme emission-line galaxies (EELGs). The selections are carried out in two central bands: F105W (Y105) and F125W (J125), as the flux of the central bands could be enhanced by the presence of [O III] 4959, 5007 at redshift of about 0.93-1.14 and 1.57-1.79, respectively. The multi-band observations help to constrain the equivalent widths of emission lines. Thanks to cluster lensing, we are able to identify 52 candidates down to an intrinsic limiting magnitude of 28.5 and to a rest-frame [O III] 4959,5007 equivalent width of about 3737 angstrom. Our samples include a number of EELGs at lower luminosities that are missed in other surveys, and the extremely high equivalent width can be only found in such faint galaxies. These EELGs can mimic the dropout feature similar to that of high redshift galaxies and contaminate the color-color selection of high redshift galaxies when the S/N ratio is limited or the band coverage is incomplete. We predict that the fraction of EELGs in the future high redshift galaxy selections cannot be neglected.

astro-ph.GA↗

arXiv2013

UM 625 Revisited: Multiwavelength Study of A Seyfert 1 Galaxy with a Low-mass Black Hole

UM 625, previously identified as a narrow-line active galactic nucleus (AGN), actually exhibits broad \ha\ and \hb\ lines whose width and luminosity indicate a low black hole mass of $1.6 \times 10^6$ \msun. We present a detailed multiwavelength study of the nuclear and host galaxy properties of UM 625. Analysis of \chandra\ and \xmm\ observations suggests that this system contains a heavily absorbed and intrinsically X-ray weak ($\aox=-1.72$) nucleus. Although not strong enough to qualify as radio-loud, UM 625 does belong to a minority of low-mass AGNs detected in the radio. The broad-band spectral energy distribution constrains the bolometric luminosity to $\lbol\approx(0.5-3)\times10^{43}$ \lum\ and $\lratio\approx0.02-0.15$. A comprehensive analysis of Sloan Digital Sky Survey and {\it Hubble Space Telescope}\ images shows that UM 625 is a nearly face-on S0 galaxy with a prominent, relatively blue pseudobulge (\sersic\ index $n = 1.60$) that accounts for $\sim$60% of the total light in the $R$ band. The extended disk is featureless, but the central $\sim150-400$ pc contains a conspicuous semi-ring of bright, blue star-forming knots, whose integrated ultraviolet luminosity suggests a star formation rate of $\sim$0.3 \msun yr$^{-1}$. The mass of the central black hole roughly agrees with the value predicted from its bulge velocity dispersion but is significantly lower than that expected from its bulge luminosity.

astro-ph.GA↗

arXiv2012

The X-ray Nature of Nucleus in Seyfert 2 Galaxy NGC 7590

We present the result of the Chandra high-resolution observation of the Seyfert~2 galaxy NGC 7590. This object was reported to show no X-ray absorption in the low-spatial resolution ASCA data. The XMM observations show that the X-ray emission of NGC 7590 is dominated by an off-nuclear ultra-luminous X-ray source (ULX) and an extended emission from the host galaxy, and the nucleus is rather weak, likely hosting a Compton-thick AGN. Our recent Chandra observation of NGC 7590 enables to remove the X-ray contamination from the ULX and the extended component effectively. The nuclear source remains undetected at ~4x10^{-15} erg/s/cm^-2 flux level. Although not detected, Chandra data gives a 2--10 keV flux upper limit of ~6.1x10^{-15} erg/s/cm^-2 (at 3 sigma level), a factor of 3 less than the XMM value, strongly supporting the Compton-thick nature of the nucleus. In addition, we detected five off-nuclear X-ray point sources within the galaxy D25 ellipse, all with 2 -- 10 keV luminosity above 2x10^{38} erg/s (assuming the distance of NGC 7590). Particularly, the ULX previously identified by ROSAT data was resolved by Chandra into two distinct X-ray sources. Our analysis highlights the importance of high spatial resolution images in discovering and studying ULXs.

astro-ph.HE↗

arXiv2012

A possible ultra strong and broad Fe Ka emission line in Seyfert 2 galaxy IRAS 00521-7054

We present XMM-Newton spectra of the Seyfert 2 Galaxy IRAS 00521-7054. A strong feature at ~6 keV (observer's frame) can be formally fitted with a strong (EW=1.3+-0.3 keV in the rest frame) and broad Fe Ka line, extending down to 3 keV. The underlying X-ray continuum could be fitted with an absorbed powerlaw (with Gamma = 1.8+-0.2 and N_H = 5.9 x 10^22 cm^-2) plus a soft component. If due to relativistically smeared reflection by an X-ray illuminated accretion disk, the spin of the supermassive black hole is constrained to be 0.97^+0.03_-0.13 (errors at 90% confidence level for one interesting parameter), and the accretion system is viewed at an inclination angle of 37+-4 degree. This would be the first type 2 AGN reported with strong red Fe Ka wing detected which demands a fast rotating SMBH. The unusually large EW would suggest that the light bending effect is strong in this source. Alternatively, the spectra could be fitted by a dual absorber model (though with a global chi^2 higher by ~ 6 for 283 d.o.f) with N_H1 = 7.0 x 10^22 cm^-2 covering 100% of the X-ray source, and N_H2 = 21.7 x 10^22 cm^-2 covering 71%, which does not require an extra broad Fe Ka line.

astro-ph.HE↗

arXiv2011

On the X-ray Baldwin Effect in Active Galactic Nuclei Observed by the Chandra High Energy Grating

Using Chandra High Energy Grating (HEG) observations of 32 AGNs, we present a systematic study of the X-ray Baldwin effect (XBE, i.e. the anti-correlation between narrow Fe Kα line EW and X-ray continuum luminosity for AGN samples) with the highest spectral resolution currently available. We have previously reported an anti-correlation with EW\proptoL(2-10 keV)^-0.22 in a HEG sample, and the correlation is much weaker after averaging multiple observations of individual AGNs (EW\proptoL(2-10 keV)^-0.13). This indicates that rapid variation in X-ray continuum plays an important role in producing the XBE, and such an effect should also be visible in individual AGNs. In this paper, by normalizing the line EWs and continuum luminosities to the time-averaged values for each AGN in our sample with multiple HEG observations, we find a strong anti-correlation between EW and Lx (EW/ \propto (L/ )^-0.82+/-0.10), consistent with the XBE expected in an individual AGN if the narrow line flux remains constant while the continuum varies. This is first observational evidence that the Fe Kα line flux in a large sample of AGNs lacks of a corresponding response to the continuum variation, supporting that the narrow Fe-K line emission originates from a region far from the nucleus. We then performed Monte-Carlo simulations to address whether the global XBE can be produced by X-ray continuum variation solely, and found that such interpretation of XBE cannot be ruled out statistically. One thus should be very cautious before reaching any scientific conclusion based on an observed XBE.

astro-ph.CO↗

arXiv2011

Chandra High Energy Grating Observations of the Fe Ka Line Core in Type 2 Seyfert Galaxies: A Comparison with Type 1 Nuclei

We present a study of the core of the Fe Ka emission line at ~6.4 keV in a sample of type II Seyfert galaxies observed by the Chandra High Energy Grating (HEG). The sample consists of 29 observations of 10 unique sources. We present measurements of the Fe Ka line parameters with the highest spectral resolution currently available. In particular, we derive the most robust intrinsic line widths for some of the sources in the sample to date. We obtained a weighted mean FWHM of 2000 \pm 160 km/s for 8 out of 10 sources (the remaining sources had insufficient signal-to-noise). From a comparison with the optical emission-line widths obtained from spectropolarimetric observations, we found that the location of Fe Ka line-emitting material is a factor of ~0.7-11 times the size of the optical BLR. Furthermore, compared to 13 type I AGNs for which the best Fe Ka line FWHM constraints were obtained, we found no difference in the FWHM distribution or the mean FWHM, and this conclusion is independent of the central black hole mass. This result suggests that the bulk of the Fe Ka line emission may originate from a universal region at the same radius with respect to the gravitational radius, ~30,000 Rg on average. By examining the correlation between the Fe Ka luminosity and the [O IV] line luminosity, we found a marginal difference in the Fe K line flux between type I and type II AGNs, but the spread in the ratio of L(Fe) to L([O IV]) is about two orders of magnitude. Our results confirm the theoretical expectation that the Fe Ka emission-line luminosity cannot trivially be used as a proxy of the intrinsic AGN luminosity, unless a detailed comparison of the data with proper models is applied.

astro-ph.CO↗

arXiv2011

Weighing Super-Massive Black Holes with Narrow Fe K$α$ Line

It has been suggested that the narrow cores of the Fe K$α$ emission lines in Active Galactic Nuclei (AGNs) are likely produced in the torus, the inner radius of which can be measured by observing the lag time between the $V$ and $K$ band flux variations. In this paper we compare the virial products of the infrared time lags and the narrow Fe K$α$ widths for 10 type 1 AGNs with the black hole masses from other techniques. We find the narrow Fe K$α$ line width is in average 2.6$^{+0.9}_{-0.4}$ times broader than expected assuming an isotropic velocity distribution of the torus at the distance measured by the infrared lags. We propose the thick disk model of the torus could explain the observed larger line width. Another possibility is the contamination by emission from the broad line region or the outer accretion disk. Alternatively, the narrow iron line might originate from the inner most part of the obscuring torus within the sublimation radius, while the infrared emission from outer cooler part. We note the correlation between the black hole masses based on this new technique and those based on other known techniques is statistically insignificant. We argue that this could be attributed to the small sample size and the very large uncertainties in the measurements of iron K line widths. The next generation of X-ray observatories could help verify the origin of the narrow iron K$α$ line and the reliability of this new technique.

astro-ph.HE↗

arXiv2011

Towards Detecting The 2175-Å Dust Feature Associated With Strong High Redshift Mg ßII Absorption Lines

We report detections of 39 2175-Å dust extinction bump candidates associated with strong Mg II absorption lines at z$\sim$ 1--1.8 on quasar spectra in Sloan Digital Sky Survey (SDSS) DR3. These strong Mg II absorption line systems are detected among 2,951 strong Mg II absorbers with the rest equivalent width $W_r\lambda2796 >$ 1.0Å at $1.0 < z < 1.86$, which is part of a full sample of 7,421 strong Mg II absorbers compiled by Prochter et al. (2006). The redshift range of the absorbers is chosen to allow the 2175-Å extinction features to be completely covered within the SDSS spectrograph operation wavelength range. An upper limit of the background quasar emission redshift at z$=$2.1 is set to prevent the Ly$α$ forest lines from contaminating the sensitive spectral region for the 2175-Å bump measurements. The FM90 (Fitzpatrick & Massa 1990) parameterization is applied to model the Optical/UV extinction curve in the rest frame of Mg II absorbers of the 2175-Å bump candidates. The simulation technique developed by Jiang et al. (2010a, b) is used to derive the statistical significance of the candidate 2175-Å bumps. A total of 12 absorbers are detected with 2175-Å bumps at a 5$σ$ level of statistical significance, 10 are detected at a 4$σ$ level and 17 are detected at a 3$σ$ level. Most of the candidate bumps in this work are similar to the relatively weak 2175-Å bumps observed in the Large Magellanic Clouds (LMC) LMC2 supershell rather than the strong ones observed in the Milky Way (MW). This sample has greatly increased the total number of 2175-Å extinction bumps measured on SDSS quasar spectra. Follow-up observations may rule out some of possible false detections and reveal the physical and chemical natures of 2175-Å quasar absorbers.

astro-ph.CO↗

arXiv2010

The Luminosity Function of Lyman alpha Emitters at Redshift z=7.7

Lyman alpha (Lya) emission lines should be attenuated in a neutral intergalactic medium (IGM). Therefore the visibility of Lya emitters at high redshifts can serve as a valuable probe of reionization at about the 50% level. We present an imaging search for z=7.7 Lya emitting galaxies using an ultra-narrowband filter (filter width= 9A) on the NEWFIRM imager at the Kitt Peak National Observatory. We found four candidate Lya emitters in a survey volume of 1.4 x 10^4 Mpc^3, with a line flux brighter than 6x10^-18 erg/cm^2/s (5 sigma in 2" aperture). We also performed a detailed Monte-Carlo simulation incorporating the instrumental effects to estimate the expected number of Lya emitters in our survey, and found that we should expect to detect one Lya emitter, assuming a non-evolving Lya luminosity function (LF) between z=6.5 and z=7.7. Even if one of the present candidates is spectroscopically confirmed as a z~8 Lya emitter, it would indicate that there is no significant evolution of the Lya LF from z=3.1 to z~8. While firm conclusions would need both spectroscopic confirmations and larger surveys to boost the number counts of galaxies, we successfully demonstrate the feasibility of sensitive near-infrared (1.06 um) narrow-band searches using custom filters designed to avoid the OH emission lines that make up most of the sky background.

astro-ph.CO↗

arXiv2010

On the scattering enhancement of Nv1240 emission line of quasi-stellar objects

The Nv emission line of active galactic nuclei shows peculiar behavior in the line--continuum correlation, which may be indicative of an extra line component in addition to that from the normal broad emission line region. In this paper, we investigate possible contribution to the Nv emission via resonant scattering of both continuum and Ly alpha in a broad absorption line (BAL) outflow, by performing the Sobolev Monte Carlo simulations. The contribution is dependent on the covering factor, optical depth and velocity profile of the outflow, as well as the equivalent width (EW) of Ly alpha. Adopting model parameters constrained by observations, we find that the measured Nv EW in the spectra of non-BAL quasi-stellar objects (QSOs) could have been enhanced by a factor of 1.82~2.73 on average, while there is only moderate absorption of Ly alpha along the BAL outflow direction. Our model can produce a relatively narrow scattering line profile. About 80% of the total scattered flux falls within the central +-4500km/s. We find that the resonant scattering can produce a prominent polarized emission line around Nv. Both the broad excess emission and the unusually large polarized flux observed around Nv in BAL QSOs are considered as strong evidence for the scattering enhancement. Future spectropolarimetric observations and spectroscopic monitoring of luminous QSOs may offer crucial tests for this interpretation, and provide useful information on the physical and geometrical properties of QSO outflows. We argue that the scattering offers a promising and robust process for producing the peculiar behavior of Nv emission compared to the other processes proposed previously.

astro-ph.CO↗