A Lyα EMITTER WITH AN EXTREMELY LARGE REST-FRAME EQUIVALENT WIDTH OF ∼900 Å ATz= 6.5: A CANDIDATE POPULATION III-DOMINATED GALAXY?

The cluster CL1358+62 displays a prominent red arc in WFPC2 images obtained with the Hubble Space Telescope. Keck spectra of the arc show Ly alpha emission at 7204 Angstrom, a continuum drop blueward of the line, and several absorption lines to the red. We identify the arc as a gravitationally lensed galaxy at a redshift of z=4.92. It is the highest redshift object currently known. A gravitational lens model was used to reconstruct images of the high-redshift galaxy. The reconstructed image is asymmetric, containing a bright knot and a patch of extended emission 0.4 arcsec from the knot. The effective radius of the bright knot is 0.022 arcsec or 130 h^-1 pc. The extended patch is partially resolved into compact regions of star formation. The reconstructed galaxy has I_AB= 24, giving a bolometric luminosity of about 3e11 Lsun. This can be produced by a star formation rate of 36 h^-2 Msun /yr (q0=0.5), or by an instantaneous star burst of 3e8 Msun. The spectral lines show velocity variations on the order of 300 km/s along the arc. The Si II line is blue shifted with respect to the Ly alpha emission, and the Ly alpha emission line is asymmetric with a red tail. These spectral features are naturally explained by an outflow model, in which the blue side of the Ly alpha line has been absorbed by outflowing neutral H I. Evidence from other sources indicates that outflows are common in starburst galaxies at high and low redshift. We have discovered a companion galaxy with a radial velocity only 450 km/s different than the arc's. The serendipitous discovery of these two galaxies suggests that systematic searches may uncover galaxies at even higher redshifts.

?????We present a quantitative investigation of the optical depth through spiral disks using BVRIJHK colors of 15 highly inclined Sab?Sc spirals and new models for radiative transfer in stellar disks. The models include exponential stellar and dust disks, exponential stellar bulge components, multiple scattering, and both homogeneous and clumpy dust distributions. Preliminary comparisons of the observed optical and near-infrared (NIR) color gradients across galaxy dust lanes with predictions from radiative transfer models with slab and spherical shapes underscore the need for these realistic exponential disk geometries. When compared with the extent of reddening predicted by the disk models, the maximum optical and NIR color excesses in galaxy dust lanes imply central face-on optical depths of 0.5?2.0 in the V band. For these highly inclined systems, we find this inferred optical depth to be largely insensitive to the difference between clumpy and homogeneous dust distributions. Comparisons of galaxy color gradients to models with high central optical depths contradict the often-stated claim that spiral disks are opaque out to D25. Our derived optical depths show that the predicted radial color changes caused by attenuation in face-on galaxies are much smaller than the observed color gradients in such systems, which suggests that the observed changes are the result of variations in stellar content. The face-on optical depths also imply that, when viewed edge-on, galaxy dust lanes have optical depths greater than 1.0 even in the K band and, thus, the NIR mass-to-light ratio changes across dust features.

We present and discuss the results of new multiple-scattering radiative transfer calculations for three representative types of galactic environments, filled with either homogeneous or two-phase clumpy dust distributions. Extinction and scattering properties for two types of interstellar dust, similar to those found in the average diffuse medium of the Milky Way (MW) and the Bar of the Small Magellanic Cloud (SMC), are considered. The wavelength coverage extends from 1000 to 30000 A, with particular emphasis on the rest-frame UV. This makes these models especially applicable to starburst galaxies and Lyman-break galaxy samples. The examination of the models concentrates on the study of UV/visual/near-IR reddening effects, the wavelength dependence of attenuation, and on the changes that arise from the transition from homogeneous to clumpy dust distributions in different star/dust geometries. Embedded dust, especially when clumpy, leads to saturation at fairly low reddening values with correspondingly gray attenuation functions. This makes the assessment of the attenuation of the far-UV flux from starburst galaxies difficult, if only UV/visual/near-IR data are available. Existing data for UV-selected starburst galaxies indicate a range of UV attenuation factors of 0-150. Our models reproduce the Attenuation Law, provided one adopts SMC-type dust and a clumpy shell-type dust distribution surrounding the starbursts. The average far-UV attenuation factor for the Calzetti sample is found to be 7.4. The only relatively reliable measure for the UV attenuation factor for single galaxies was found in the ratio of the integrated far-IR flux to the far-UV flux, measured near 1600 A, requiring the measurement of the entire spectral energy distribution of galaxies.

The combined effects of interstellar dust absorption and of scattering by hydrogen atoms may give rise to a Ly(alpha) spectral feature of negative equivalent width, as has been observed in several star-forming galaxies. By considering the transfer of Ly(alpha) line radiation and of neighboring stellar continuum radiation within a dusty galaxy, we find that dust absorption has three effects: (1) it reduces the apparent ultraviolet continuum luminosity at all wavelengths; (2) it preferentially decreases the apparent Ly(alpha) line luminosity from H II regions; and (3) it creates an 'attenuation feature' in the continuum spectrum -- centered at the Ly(alpha) rest frequency -- which occurs because the attenuation of the stellar continuum radiation increases as the Ly(alpha) rest frequency is approached, due to the effects of scattering by hydrogen atoms. For plausible values of the galactic dust content and of the disk thickness, these effects can lead to a negative net Ly(alpha) equivalent width, even for galaxies in which the unattenuated spectrum would show a strong Ly(alpha) emission line.

view Abstract Citations (8) References (36) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Gravitational Lens System Q0957+561 in the Ultraviolet Dolan, J. F. ; Michalitsianos, A. G. ; Thompson, R. W. ; Boyd, P. T. ; Wolinski, K. G. ; Bless, R. C. ; Nelson, M. J. ; Percival, J. W. ; Taylor, M. J. ; Elliot, J. L. ; van Citters, G. W. Abstract Photometric and polarimetric observations of both images of the gravitationally lensed quasar Q0957 + 561 (z_em_ = 1.41) were obtained in the UV in 1993 with the High Speed Photometer on board the Hubble Space Telescope. The images exhibited no significant polarization in a bandpass centered on 2770 A (observer's frame); p <= 3.2% (2 σ upper limit) in each image. The ratio of the flux density in image A to that in image B in late 1993 had a constant value, 1.021 +/- 0.008, in four different UV bandpasses between 1400 A and 3040 A (observer's frame). These results are consistent with the prediction of the gravitational lens interpretation that the photometric ratio of the images measured simultaneously should be independent of frequency. Reprocessed archival spectra of the two images obtained between 1981 and 1983 by the International Ultraviolet Explorer show that the photometric ratio of A to B varies between 0.96 and 2.0 in the Lyα emission line, and between 0.77 and 1.8 in the O VI λ1037 emission line (quasar rest frame). The photometric ratio of A to B at any single epoch is often significantly different in the two emission lines. Accepting the system as a gravitational lens implies that in the quasar the flux in the Lyα emission line can vary independently of the flux in the O VI emission line. Publication: The Astrophysical Journal Pub Date: March 1995 DOI: 10.1086/175425 Bibcode: 1995ApJ...442...87D Keywords: Astronomical Polarimetry; Galaxies; Gravitational Lenses; Lyman Alpha Radiation; Quasars; Ultraviolet Photometry; Flux Density; Hubble Space Telescope; Image Analysis; Iue; Spectrum Analysis; Astrophysics; COSMOLOGY: GRAVITATIONAL LENSING; GALAXIES: QUASARS: INDIVIDUAL ALPHANUMERIC: 0957; 561; POLARIZATION; ULTRAVIOLET: GALAXIES full text sources ADS | data products SIMBAD (2) MAST (2) NED (1) INES (1) ESA (1)

For a source of Ly alpha radiation embedded in a neutral intergalactic medium (IGM) prior to the reionization epoch, the emission line is strongly suppressed by the intervening IGM. The damping wing of the so-called Gunn-Peterson trough can extend to the red side of the emission line, and erase a significant fraction of the total line flux. However, the transmitted fraction increases with the size of the local cosmological HII region surrounding the source, and therefore with the ionizing luminosity and age of the source. Motivated by the recent discovery of a Ly alpha emitting galaxy at a redshift z=6.56 (Hu et al. 2002), possibly prior to the reionization of the IGM, we revisit the effects of a neutral IGM on the Ly alpha emission line. We show that even for faint sources with little ionizing continuum, the emission line can remain observable. In particular, the line detected by Hu et al. is consistent with a source embedded in a neutral IGM. We provide characterizations of the asymmetry and total transmitted flux of the Ly alpha line as functions of the ionizing emissivity of its source. A statistical sample of Ly alpha emitters extending beyond the reionization redshift can be a useful probe of reionization.

Photometric and spectroscopic observations of PC 1158 + 4635, a quasar with a redshift of 4.73, are presented. This object was discovered by the detection of its Ly-alpha emission line in a low-resolution CCD grism survey. The apparent magnitude of the quasar is r = 20.2; its absolute magnitude is similar to that of 3C 273 (for q0 = 0.5). The spectrum of PC 1158 + 4635 reveals prominent emission lines of Ly-alpha and C IV and a sharp, approximately one magnitude drop in the continuum level across the Ly-alpha emission line (features seen in most greater than 4 quasars). The redshift-continuum depression relation for quasars requires a dramatic change in the 'Lymann alpha clouds' (either in number or intrinsic properties) at a redshift of about 3.5 or else the presence of a new source that is responsible for the majority of the absorption at redshifts obove four.

We present a spectroscopic and photometric analysis for eight field galaxies in the direction of the galaxy cluster LCDCS-S001. The spectra were obtained with the GMOS instrument at the Gemini South Observatory. The objects were selected in an i'-band image and the multiobject spectroscopic observations were centred at 7500 A. For the galaxies ID 440 and ID 461, we have determined redshifts of z = 0.7464 and 0.7465, respectively. For the other six galaxies we have confirmed the previous measurements of redshift. The redshifts of the field galaxies are in the range of 0.2201 < z < 0.7784. We determined the blue and visual luminosities and they are brighter than M B = -18.64. The galaxies ID 180, ID 266 and ID 461 follow the Faber-Jackson relation of the Coma and Virgo early-type galaxies, and therefore do not present a brightening of the B luminosity as observed in galaxies at higher redshifts. The stellar velocity dispersion was measured for five galaxies (ID 146, ID 180, ID 266, ID 428 and ID 440) and estimated to be in the range of 200 < a < 346 km s -1 . Lick indices were measured and used to determine the stellar population properties of the galaxies ID 120 and ID 146, by means of spectral synthesis. The first galaxy, ID 120, presents in its spectrum absorption and emission lines, and we have found that the main contribution in the flux at λ5870 A is of a 0.1-Gyr stellar population of solar metallicity. For ID 146, the dominant flux contribution at λ4200 A is provided by a stellar population of 10 Gyr of subsolar metallicity. From stellar population synthesis, we estimated reddening values of E(B - V) = 0.90 and 0.82 for ID 120 and ID 146, respectively. According to classical diagnostic diagrams the emission lines present in the spectrum of ID 120 indicate that it is a starburst galaxy.

We show that unusual Balmer emission line profiles of the quasar OX 169, frequently described as either self-absorbed or double peaked, are actually neither. The effect is an illusion resulting from two coincidences. First, the forbidden lines are quite strong and broad. Consequently, the [N II]6583 line and the associated narrow-line component of H-alpha present the appearance of twin H-alpha peaks. Second, the redshift of 0.2110 brings H-beta into coincidence with Na I D at zero redshift, and ISM absorption in Na I D divides the H-beta emission line. In spectra obtained over the past decade, we see no substantial change in the character of the line profiles, and no indication of intrinsic double-peaked structure. The H-gamma, Mg II, and Ly-alpha emission lines are single peaked, and all of the emission-line redshifts are consistent once they are correctly attributed to their permitted and forbidden-line identifications. A systematic shift of up to 700 km/s between broad and narrow lines is seen, but such differences are common, and could be due to gravitational and transverse redshift in a low-inclination disk. Stockton & Farnham (1991) had called attention to an apparent tidal tail in the host galaxy of OX 169, and speculated that a recent merger had supplied the nucleus with a coalescing pair of black holes which was now revealing its existence in the form of two physically distinct broad-line regions. Although there is no longer any evidence for two broad emission-line regions in OX 169, binary black holes should form frequently in galaxy mergers, and it is still worthwhile to monitor the radial velocities of emission lines which could supply evidence of their existence in certain objects.

Spatially extended Ly-alpha sources which are faint and compact in coninuum are candidates for extremely young galaxies (age of ~<10e+7 yrs) at high redshifts. We carried out a systematic survey for extended Ly-alpha sources, using deep intermediate-band imaging data taken with the Suprime-Cam on the Subaru Telescope. Our survey covers a field of view of 33' * 25' and a redshift range of 3.24<z<4.95, down to a Ly-alpha flux of ~1e-17 ergs/cm^2/s. We identified 41 extended Ly-alpha sources. The redshift distribution of these sources shows that this kind of objects are common in the early universe over the surveyed redshift range. The identified objects have typical sizes of \~10-15 kpc and luminosities of ~10^42 ergs/s. Follow-up spectroscopy made for 7 of the 41 objects showed that our sample suffers from little contamination. All 7 objects have large equivalent widths of Ly-alpha emission line, all but one exceeding 240A in the rest frame. The large equivalent widths suggest that their extended Ly-alpha emissions are unlikely to be due to normal starbursts, but possibly originated from very young galaxies. All 41 objects in our sample have much smaller Ly-alpha luminosities than the two Ly-alpha Blobs (LABs) found by Steidel et al. (2000) in spite of our much larger survey volume. This suggests that large and luminous extended Ly-alpha objects like the two LABs are very rare and are clustered in overdense regions.

Unusual patterns of recombination emission from gas ionized by metal-free stars may distinguish early star-forming galaxies from their present-day counterparts. This pattern arises from the harder ionizing spectrum expected from metal-free stars, which strongly enhances the strength of He II recombination lines. Our calculations indicate that line fluxes of He II 1640 and 4686 are sufficiently large to be detected by narrowband and spectroscopic searches for high-redshift emission-line sources at redshift z ~ 5 using current instruments. An unknown fraction of Lya emitters may harbor low-metallicity or metal-free stars. As the predicted He II 1640 flux is comparable to and may exceed hydrogen Lya, searches for high-redshift galaxies should consider He II recombination lines as possible identifications for single emission lines in observed spectra. Spectra of metal-free stars may show both H I and He II emission lines, improving the constraints on their redshift and identification. We assess the considerable uncertainties that affect our expectations for the detection and identification of true first-generation stars with present search techniques, including the role of stellar mass loss in spectral evolution and the confusion of ionization by primordial stellar sources and AGN in the early universe.

Long gamma-ray bursts (GRBs) have been suggested to occur preferentially in low-metallicity environment. We discuss the possibility and theoretical aspects of using Lyalpha emission properties of long GRB host galaxies as a metallicity indicator of high-redshift GRB environments, where direct metallicity measurements are not easy. We propose to use the fraction of Lyalpha emitters (LAEs) in long GRB host galaxies as a function of UV luminosity, which can be compared with star formation rate weighted LAE fraction of Lyman break galaxies as the standard in the case of no metallicity dependence. There are two important effects of metallicity dependence of long GRB rate to change the LAE fraction of host galaxies. One is the enhancement of intrinsic Lyalpha equivalent width (EW) by stronger ionizing UV luminosity of low-metallicity stellar population, and the other is extinction by interstellar dust to change the observable EW. Based on a latest theoretical model of LAEs that reproduce observations, we argue that the latter is likely to work in the opposite direction to the former, i.e., to decrease LAE fraction if GRBs preferentially occur in low-metallicity environments, because of the clumpy interstellar medium effect. The high LAE fraction of GRB host galaxies indicated by observationsmore » is quantitatively explained by the LAE model if GRBs occur when Z approx< 0.1 Z{sub sun}, although this result is still indicative because of the limited statistics and theoretical uncertainties. This result demonstrates that the LAE statistics of GRB hosts may give us useful information in the future.« less

We present a new formalism to describe the ratios and profiles of emission lines from hydrogen in Balmer-dominated shocks. We use this model to interpret the measured widths and ratios of broad and narrow H-alpha, H-beta and Ly-alpha emission lines in supernova remnants (SNRs). Our model results agree fairly well with those obtained previously by Chevalier, Kirshner & Raymond (1980) and are consistent with observations of several SNRs. The same model fails to account for the ratio of broad to narrow line emission from the reverse shock in SNR 1987A as observed by Heng et al. (2006). We suggest that this discrepancy between theory and observation results from a faulty assumption that Balmer-dominated shocks can be treated as sharp discontinuities. If the spatial structure of the shock transition zone is taken into account, the predicted ratios of broad to narrow line emission in most SNRs will change by modest factors, but the ratio in SNR 1987A will increase substantially. Significantly greater shock velocities will be required to account for the observed full widths at half-maximum of the broad emission lines in most SNRs.

view Abstract Citations (64) References (59) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Seyfert II Nature of the IRAS Source FSC 10214+4724 Elston, Richard ; McCarthy, Patrick J. ; Eisenhardt, Peter ; Dickinson, Mark ; Spinrad, Hyron ; Januzzi, Buell T. ; Maloney, Philip Abstract We have observed the rest-frame optical and UV spectra of the luminous, high redshift IRAS source FSC 10214+4724. We find the optical emission lines to be characterized by ratios similar to those found in Seyfert II galaxies. We support the conclusion of previous work that the UV emission lines are similar to those attributed to AGN rather than to star formation. The ratio Hα/Hβ >= 20 (2σ lower limit) implies substantial reddening of the narrow line region with A_V_ > 5.5, sufficient to hide a broad line region in our Hα observations. Given this large inferred reddening and the strength of the UV continuum and emission lines, we conclude (as have others) that simple screen models of reddening are not appropriate for this object. These properties are very similar to those of the infrared luminous galaxies at lower redshift, suggesting that FSC 10214+4724 is the luminous extreme of the same population. We also present H band (1.6 micron) imaging polarimetry observations and find that the rest-frame optical emission is unpolarized (P = 3.2%+/-2.0%). This deep image of the field shows FSC 10214+4724 to possess an unresolved core, with several companions located within 10" of the point source. We find it unlikely that this group of objects is physically associated with FSC 10214+4724 at z = 2.3, and we argue that their magnitudes and colors are more consistent with those expected for galaxies in a foreground group. While galaxy number counts would suggest that such a projection has a low probability of being observed randomly, a foreground group might gravitational lens the z= 2.3 source, making such random statistics inappropriate, and contribute to the large observed luminosity of FSC 10214+4724. Comparison of H band images taken on two occasions one year apart show that FSC 10214+4724 had varied by 0.16+/-0.03 mag relative to a nearby star during that time. The UV- optical emission line spectra, the bright dereddened continuum magnitude (K <= 12) of the unresolved core, its extremely large bolometric luminosity, strong UV polarization, and possible variability all suggest that FSC 10214+4724 harbors an obscured AGN. Given that we observe no spectral features attributable to star formation, it appears either that star formation in FSC 10214+4724 is more highly obscured at optical and UV wavelengths than the narrow line region of the AGN, or that the AGN (rather than star formation) dominates the large luminosity of this object. Publication: The Astronomical Journal Pub Date: March 1994 DOI: 10.1086/116905 Bibcode: 1994AJ....107..910E Keywords: Active Galactic Nuclei; Astronomical Polarimetry; Astronomical Spectroscopy; Images; Infrared Astronomy; Infrared Astronomy Satellite; Infrared Radiation; Light (Visible Radiation); Seyfert Galaxies; Ultraviolet Radiation; Astronomical Observatories; Charge Coupled Devices; Infrared Spectrometers; Morphology; Polarizers; Spectrographs; Telescopes; Astronomy; GALAXIES: SEYFERT; GALAXIES: INDIVIDUAL: FSC 10214; 4724; INFRARED: GALAXIES full text sources ADS | data products SIMBAD (5) NED (1)

We study the expected properties of starbursts in order to provide the point of reference for interpretation of high-z galaxy surveys and of very metal-poor galaxies. We concentrate mainly on the UV characteristics such as the ionizing spectra, the UV continuum, the Ly alpha and HeII 1640 A line and two-photon continuum emission. We use evolutionary synthesis models covering metallicities from Pop III to solar and a wide range of IMFs. We also combine the synthetic SEDs with the CLOUDY photoionization code for more accurate predictions of nebular emission, and to study possible departures from case B assumed in the synthesis models. The ionizing fluxes, UV continuum properties, and predicted Ly alpha and HeII 1640 A line strengths are presented for synthesis models covering a wider range of parameter space than our earlier calculations. Strong departures from case B predictions are obtained for Ly alpha and two-photon continuum at low metallicities. At low nebular densities both are shown to be enhanced proportionally to the mean energy carried by the Lyman continuum photons emitted by the ionizing source. Larger Ly alpha equivalent widths are therefore predicted at low metallicity. The HeII 1640 A line can be weaker than case B predicts (in terms of flux as well as the equivalent width) due to its ionization parameter dependence and to the enhanced underlying two-photon continuum. Our results have implications for the interpretation of star-forming metal-poor and/or high redshift galaxies, for galaxies among the Ly alpha emitters (LAE) and Lyman Break galaxy (LBG) populations, and for searches of Population III stars in the distant Universe.