Very faint quasar survey - Procedures and preliminary results

The spectral analysis of the persistent X-ray flux from the bright galactic bulge X-ray source and an X-ray burster X1813−14=GX17+2 is presented. A model with a single thermal bremsstrahlung continuum plus iron emission line at ∼6.7 keV fits the lower and higher intensity state data reasonably well. The line feature observed here is reproduced by a single emission line at ∼6.7 keV with intrinsic line width less than 0.7 keV. The equivalent width of the line ranges between 52 and 43 eV, depending on the intensity state of the source. This implies that the observed line is mostly due to helium-like iron (Fexxv). The properties of the line suggest that line-emitting matter is located far outside the neutron star.

We present the results of the first observations of the emission line galaxies (ELG) of the extended Baryon Oscillation Spectroscopic Survey. From the total 9000 targets, 4600 have been selected from the Dark Energy Survey (DES). In this subsample, the total success rate for redshifts between 0.6 and 1.2 is 71 and 68 per cent for a bright and a faint samples, respectively, including redshifts measured from a single strong emission line. The mean redshift is 0.80 for the bright and 0.87 for the faint sample, while the percentage of unknown redshifts is 15 and 13 per cent, respectively. In both cases, the star contamination is lower than 2 per cent. We evaluate how well the ELG redshifts are measured using the target selection photometry and validating with the spectroscopic redshifts measured by eBOSS. We explore different techniques to reduce the photometric redshift outliers fraction with a comparison between the template fitting, the neural networks and the random forest methods. Finally, we study the clustering properties of the DES SVA1 ELG samples. We select only the most secure spectroscopic redshift in the redshift range 0.6 < z < 1.2, leading to a mean redshift for the bright and faint sample of 0.85 and 0.90, respectively. We measure the projected angular correlation function and obtain a galaxy bias averaging on scales from 1 to 10 Mpc h(-1) of 1.58 +/- 0.10 for the bright sample and 1.65 +/- 0.12 for the faint sample. These values are representative of a galaxy population with M-B - log(h) < -20.5, in agreement with what we measure by fitting galaxy templates to the photometric data.

Context. The usual approach to classify the ionizing source using optical spectroscopy is based on the use of diagnostic diagrams that compare the relative strength of pairs of collisitional metallic lines (e.g., [O III] and [N II]) to recombination hydrogen lines (e.g., Hβ and Hα). Despite it having been accepted as the standard procedure, it presents known problems, including confusion regimes and/or limitations related to the required signal-to-noise (S/N) of the emission lines involved. These problems not only affect our intrinsic understanding of the interstellar medium and its properties, but also the fundamental galaxy properties, such as the star formation rate and the oxygen abundance. This raises key questions related to the fraction of active galactic nuclei and other essential parameters. Aims. We attempt to minimize the problems introduced by the use of these diagrams, in particular, their implementation when the available information is limited due to either the fact that not all lines are available or they do not have the required S/N value. Methods. We explored the existing alternatives in the literature to minimize the confusion among different ionizing sources. We have proposed a new, simple diagram that uses the equivalent width and the velocity dispersion from one single emission line, Hα, to classify the ionizing sources. Results. We used aperture-limited and spatially resolved spectroscopic data from the nearby Universe (z ∼ 0.01) to demonstrate that the new diagram, which we have named WHaD, segregates the different ionizing sources in a more efficient way than earlier procedures. A new set of regions have been defined in this diagram to select among different ionizing sources. Conclusions. The new proposed diagram is well positioned to assist in determining the ionizing source when only Hα is available or when the S/N of the emission lines is too low to obtain reliable fluxes for the weakest emission lines in classical diagnostic diagrams (e.g., Hβ).

We present results from the largest contiguous narrow-band survey in the near-infrared. We have used the wide-field infrared camera/Canada–France–Hawaii Telescope and the lowOH2 filter (1.187 ± 0.005 μm) to survey ≈10 deg2 of contiguous extragalactic sky in the SA22 field. A total of ∼6000 candidate emission-line galaxies are found. We use deep ugrizJK data to obtain robust photometric redshifts. We combine our data with the High-redshift(Z) Emission Line Survey (HiZELS), explore spectroscopic surveys (VVDS, VIPERS) and obtain our own spectroscopic follow-up with KMOS, FMOS and MOSFIRE to derive large samples of high-redshift emission-line selected galaxies: 3471 Hα emitters at z = 0.8, 1343 [O iii] + Hβ emitters at z = 1.4 and 572 [O ii] emitters at z = 2.2. We probe comoving volumes of >106 Mpc3 and find significant overdensities, including an 8.5σ (spectroscopically confirmed) overdensity of Hα emitters at z = 0.81. We derive Hα, [O iii] + Hβ and [O ii] luminosity functions at z = 0.8, 1.4, 2.2, respectively, and present implications for future surveys such as Euclid. Our uniquely large volumes/areas allow us to subdivide the samples in thousands of randomized combinations of areas and provide a robust empirical measurement of sample/cosmic variance. We show that surveys for star-forming/emission-line galaxies at a depth similar to ours can only overcome cosmic-variance (errors <10 per cent) if they are based on volumes >5 × 105 Mpc3; errors on L* and ϕ* due to sample (cosmic) variance on surveys probing ∼104 and ∼105 Mpc3 are typically very high: ∼300 and ∼40–60 per cent, respectively.

We present new optical and near-IR spectroscopy, as well as new high dynamic range, arcsecond-resolution VLA radio maps, of BL Lacertae objects from the complete radio-selected jansky (1 Jy) sample (RBLs) for which such data were not previously available. Redshift information is now available for all but six of the 37 BL Lac objects in the 1 Jy sample. Of the 31 with redshift information, four redshifts are only minimum values based on absorption lines, and four other objects have uncertain redshifts based on the detection of only a single emission line. Unlike BL Lac objects from the complete X-ray–selected Einstein Medium Sensitivity Survey (EMSS) sample (XBLs), most RBLs possess weak but moderately luminous emission lines. The emission-line luminosities of RBLs are several orders of magnitude lower than flat-spectrum radio quasars (FSRQs); however, there is significant overlap in the luminosity distributions of the two classes. All but one object in the 1 Jy sample has now been observed with the VLA, and extended flux was detected for all but three of the observed objects. Whereas nearly all XBLs have extended power levels consistent with FR 1s, more than half of the RBLs have extended radio power levels too luminous to be beamed FR 1 radio galaxies. In fact, we find evidence for and examples of three distinct mechanisms for creating the BL Lac phenomenon in the 1 Jy sample: beamed FR 1s, beamed FR 2s, and possibly a few gravitationally lensed quasars. The V/Vmax value determined for the 1 Jy sample is 0.614 ± 0.047, which is markedly different from the negative evolution seen in the EMSS and other XBL samples. A correlation between logarithmic X-ray–to–radio flux ratio and V/Vmax value is observed across the EMSS and 1 Jy samples from negative evolution in the more extreme XBLs to positive evolution in the more extreme RBLs. There is evidence that the selection criteria chosen by Stickel et al. eliminates some BL Lac objects from the 1 Jy sample, although how many is unknown. In addition, several objects currently in the sample have exhibited strong emission lines in one or more epochs, suggesting that they should be reclassified as FSRQs. However, these selection effects cannot account for the observed discrepancy in XBL and RBL properties. From these observational properties, we conclude that RBLs and XBLs cannot be related by viewing angle alone, and that RBLs are more closely related to FSRQs.

The next generation of wide-field spectroscopic redshift surveys will map the large-scale galaxy distribution in the redshift range 0.7< z<2 to measure baryonic acoustic oscillations (BAO). The primary optical signature used in this redshift range comes from the [OII] emission line doublet, which provides a unique redshift identification that can minimize confusion with other single emission lines. To derive the required spectrograph resolution for these redshift surveys, we simulate observations of the [OII] (3727,3729) doublet for various instrument resolutions, and line velocities. We foresee two strategies about the choice of the resolution for future spectrographs for BAO surveys. For bright [OII] emitter surveys ([OII] flux ~30.10^{-17} erg /cm2/s like SDSS-IV/eBOSS), a resolution of R~3300 allows the separation of 90 percent of the doublets. The impact of the sky lines on the completeness in redshift is less than 6 percent. For faint [OII] emitter surveys ([OII] flux ~10.10^{-17} erg /cm2/s like DESi), the detection improves continuously with resolution, so we recommend the highest possible resolution, the limit being given by the number of pixels (4k by 4k) on the detector and the number of spectroscopic channels (2 or 3).

We review the results from recent deep HST imaging on faint galaxies. Artificial Neural Networks (ANNs) were used to classify faint galaxies based on surface brightness and light profiles. We use available UV + ground-based U-band images of nearby galaxies to address the effects from the uncertain redshifted UV-morphology on the classifications, resulting in ‘restframe ANN’ classifiers. These distinguish quite consistently between E/SO’s, Sabc’s, and Sd/Irr for B ≲ 27 mag. A Fourier-based method is used to quantify higher order morphological features and asymmetries in faint galaxies. We review the faint blue galaxy population as classified with these restframe ANN’s. The median scale-length at B ≲ 27 mag is r hl ≃ 0″.25–0″.3(∼ 1–2kpc at z≃1–2). Although E/SO’s are the reddest and Sd/lrr’s are the bluest galaxies, early and late types are not wel separated in color-magnitude diagrams to B ≲ 28 mag. We discuss the B-band galaxy counts as a function of type for 18.5 ≲ B ≲ 28 mag. We briefly review deep medium-band imaging with HST/WFPC2 in the filter F410M (Lyα at z ≲ 2.4) which yielded 18 faint, compact objects surrounding the radio galaxy 53W002 at z ≲2.39, as well as another 30 in random HST parallel fields, plus ground-based images over a larger area. Deep HST/PC images of 53W002 at ∼ 0″.06 FWHM resolution in BVI + redshifted Lyα suggest that both reflected AGN continuum-light shining through a cone and jet-induced star-formation play a role in its ‘alignment effect’. We discuss the formation and evolution of 53W002 in the context of its many surrounding sub-galactic sized objects. We discuss the nature of a μJy radio source that remained essentially unidentified in HDF flanking fields. The object was identified in deep HST/NICMOS J- and H-band images, and a single emission line at 6595 A was detected, most likely Lyα at z = 4.42. This faint (H = 23.9 mag), compact (re ≃ 0″.2), red (I - K = 2.0 mag) object is most likely a dusty, star-forming young galaxy with an embedded active nucleus. Finally, HST/NICMOS images are presented of some of the oldest known red galaxies at z ≃ 1.5. Both galaxies are dominated by a r 1/4-profile with 5 kpc scale-length, and are amongst the oldest known relaxed systems at that epoch.

The WFC3 Infrared Spectroscopic Parallel Survey uses the Hubble Space Telescope (HST) infrared grism capabilities to obtain slitless spectra of thousands of galaxies over a wide redshift range including the peak of star formation history of the universe. We select a population of very strong emission-line galaxies with rest-frame equivalent widths (EWs) higher than 200 Å. A total of 176 objects are found over the redshift range 0.35 &lt; z &lt; 2.3 in the 180 arcmin^2 area that we have analyzed so far. This population consists of young and low-mass starbursts with high specific star formation rates (sSFR). After spectroscopic follow-up of one of these galaxies with Keck/Low Resolution Imaging Spectrometer, we report the detection at z = 0.7 of an extremely metal-poor galaxy with 12 + log(O/H) =7.47 ± 0.11. After estimating the active galactic nucleus fraction in the sample, we show that the high-EW galaxies have higher sSFR than normal star-forming galaxies at any redshift. We find that the nebular emission lines can substantially affect the total broadband flux density with a median brightening of 0.3 mag, with some examples of line contamination producing brightening of up to 1 mag. We show that the presence of strong emission lines in low-z galaxies can mimic the color-selection criteria used in the z ~ 8 dropout surveys. In order to effectively remove low-redshift interlopers, deep optical imaging is needed, at least 1 mag deeper than the bands in which the objects are detected. Without deep optical data, most of the interlopers cannot be ruled out in the wide shallow HST imaging surveys. Finally, we empirically demonstrate that strong nebular lines can lead to an overestimation of the mass and the age of galaxies derived from fitting of their spectral energy distribution (SED). Without removing emission lines, the age and the stellar mass estimates are overestimated by a factor of 2 on average and up to a factor of 10 for the high-EW galaxies. Therefore, the contribution of emission lines should be systematically taken into account in SED fitting of star-forming galaxies at all redshifts.

We present spectroscopy of 76 emission-line galaxies (ELGs) in CDF-S taken with the LDSS3 spectrograph on Magellan Telescope. These galaxies are selected to have emission lines with ACS grism data in the Hubble Space Telescope Probing Evolution and Reionization Spectroscopically (PEARS) grism Survey. The ACS grism spectra cover the wavelength range 6000-9700 \AA\ and most PEARS grism redshifts are based on a single emission line + photometric redshifts from broad-band colors; the Magellan spectra cover a wavelength range from 4000 {\AA} to 9000 {\AA}, and provide a check on redshifts derived from PEARS data. We find an accuracy of $\sigma_z$ = 0.006 for the ACS grism redshifts with only one catastrophic outlier. We probe for AGN in our sample via several different methods. In total we find 7 AGNs and AGN candidates out of 76 galaxies. Two AGNs are identified from the X-ray full-band luminosity, $L_{X-ray,FB}>10^{43}$ erg$\;$s$^{-1}$, the line widths and the power-law continuum spectra. Two unobscured faint AGN candidates are identified from the X-ray full-band luminosity $L_{X-ray,FB}\sim10^{41}$ erg$\;$s$^{-1}$, the hardness ratio and the column density, and the emission-line and X-ray derived SFRs. Two candidates are classified based on the line ratio of [NII]\lambda6584/H$\alpha$ versus [OIII]$\lambda$5007/H$\beta$ (BPT diagram), which are between the empirical and theoretical demarcation curves, i.e, the transition region from star-forming galaxies to AGNs. One AGN candidate is identified from the high-ionization emission line HeII{\AA}4686.

In this paper, the blue quasar SDSS J105816.19+544310.2 (=SDSS J1058+5443) at redshift 0.479 has been reported as the best true type 2 quasar candidate with the disappearance of central broad-line regions. There are no definite conclusions on the very existence of true type 2 active galactic nuclei (AGN), mainly due to detected optical broad emission lines in high-quality spectra of some previously classified true type 2 AGN candidates. Here, unlike previously reported true type 2 AGN candidates among narrow emission-line galaxies with weak AGN activities but strong stellar lights, the definitely blue quasar SDSS J1058+5443 can be well confirmed as a true type 2 quasar due to apparent quasar-shape blue continuum emissions but an apparent loss of both the optical broad Balmer emission lines and the near-UV (NUV) broad Mg ii emission line. Based on different model functions and the F-test statistical technique, after considering blueshifted optical and UV Fe ii emissions, there are no apparent broad optical Balmer emission lines and/or broad NUV Mg ii lines, and the confidence level is smaller than 1σ in support of broad optical and NUV emission lines. Moreover, assuming the virialization assumption to broad-line emission clouds, the reconstructed broad emission lines strongly indicate that the probable intrinsic broad emission lines, if they exist, cannot be hidden or overwhelmed in the noise of the Sloan Digital Sky Survey spectrum of SDSS J1058+5443. Therefore, SDSS J1058+5443 is so far the best and most robust true type 2 quasar candidate, leading to the clear conclusion of the very existence of true type 2 AGN.

Post-starburst galaxies, or E+A galaxies, are characterized by optical spectra showing strong Balmer absorption lines, indicating a young stellar population, and little or no emission lines, implying no active star formation. These galaxies are interpreted as a transitional population between star-forming, disk-dominated galaxies and spheroidal quiescent, non-star forming galaxies. Here, we present single dish HI 21-cm emission line measurements of a sample of eleven of these galaxies at redshifts z<0.05. We detect H I emission in six of the E+A galaxies. In combination with earlier studies, the total number of E+A galaxies with measured cold gas components is now eleven. Roughly half of the E+As studied so far have detectable HI. The gas fractions of these galaxies, measured with respect to their stellar mass, are between 1 and 10 percent and are at the high end of the gas fractions measured in gas-bearing early type galaxies and typically lower than seen in late-type galaxies with comparable stellar masses. This finding is consistent with the idea that E+As are currently evolving from the blue cloud to the red sequence. However, the question of why the star formation has ceased in these galaxies while a significant gas reservoir is still present can only be answered by higher spatial resolution observations of the cold gas.

Quasar samples remain severely incomplete at low Galactic latitudes because of strong extinction and source confusion. We conduct a systematic search for quasars behind the Galactic plane using X-ray sources from the Chandra Source Catalog (CSC), combined with optical data from Gaia Data Release 3 and mid-infrared data from CatWISE2020. Using spectroscopically confirmed quasars and stellar-type objects from datasets including DESI, the Sloan Digital Sky Survey, and LAMOST, we apply a random forest classifier to identify quasar candidates, with stellar contaminants suppressed using Gaia proper-motion constraints. Photometric redshifts are estimated for the candidates using a random forest regression model. Applying this framework to previously unclassified CSC sources, we identify 7570 quasar candidates, including 1060 Galactic plane quasar (GPQ) candidates at ∣ b ∣ &lt; 20°, of which 551 are high-confidence candidates. Relative to the previously known GPQ sample, our selected GPQs reach lower optical and X-ray fluxes, improving sensitivity to low-flux GPQs. In addition, both the GPQ candidates and known GPQs display harder X-ray spectra than the all-sky quasar sample, consistent with increased absorption through the Galactic plane. Pilot spectroscopy confirms two high-confidence GPQ candidates as quasars at spectroscopic redshifts of z = 1.2582 and z = 1.1313, and further spectroscopic follow-up of the GPQ sample is underway. This work substantially improves the census of GPQs and provides a valuable target sample for future spectroscopic follow-up, enabling the use of GPQs to refine the reference frames for astrometry and probe the Milky Way interstellar and circumgalactic media with the absorption features of GPQs.

The IR Spectrometer and Array Camera (ISAAC) at the VLT has ben used for follow-up spectroscopy of 11 candidate H(alpha) emitting galaxies at z approximately equals 2.2 detected in a preparatory, IR narrow band filter imaging survey. This survey was conducted with Son of ISAAC at the ESO NTT and covered an area of 100 sq. arcmin including the WFPC2 and STIS fields in the HDFS. Line emission within the wavelength range of the 2.1 micrometers narrow band filters is confirmed in 6 of the candidates and in all but one of the 6 detected at &gt;= 4 (sigma) in the survey. Although only a single emission line is observed its identification with H(alpha) is relatively secure and the absence of (NII) (6548,6584 Angstrom) is consistent with high ionization and/or low metallicity systems. Velocity dispersions and one H(alpha) rotation curve imply masses of up to approximately 10<SUP>10</SUP>M. Star formation rates of the individual galaxies derived from the H(alpha) fluxes are 20-35 M yr<SUP>-1</SUP> and the total star formation rate density is 0.12 M Mpc<SUP>-3</SUP>. This is the same as found at z approximately equals 1.3 from H(alpha) observations with NICMOS on HST$_1) and close to that at z equals 3-4.5 derived from the extinction corrected UV continuum fluxes of Lyman Break galaxies<SUP>2</SUP>.

Spectral-line fitting problems are extremely common in all remote-sensing disciplines, solar physics included. Spectra in solar physics are frequently parameterized by using a model for the background and the emission lines, and various computational techniques are used to find values to the parameters given the data. However, the most commonly-used techniques, such as least-squares fitting, are highly dependent on the initial parameter values used and are therefore biased. In addition, these routines occasionally fail because of ill-conditioning. Simulated annealing and Bayesian posterior distribution analysis offer different approaches to finding parameter values through a directed, but random, search of the parameter space. The algorithms proposed here easily incorporate any other available information about the emission spectrum, which is shown to improve the fit. Example algorithms are given and their performance is compared to a least-squares algorithm for test data – a single emission line, a blended line, and very low signal-to-noise-ratio data. It is found that the algorithms proposed here perform at least as well or better than standard fitting practices, particularly in the case of very low signal-to-noise ratio data. A hybrid simulated annealing and Bayesian posterior algorithm is used to analyze a Mg x line contaminated by an O IV triplet, as observed by the Coronal Diagnostic Spectrometer onboard SOHO. The benefits of these algorithms are also discussed.

We present complementary techniques to find emission-line targets and measure their properties in a semi-automated fashion from grism observations obtained with the Advanced Camera for Surveys aboard the Hubble Space Telescope (HST). The first technique is to find all likely sources in a direct image, extract their spectra, and search them for emission lines. The second method is to look for emission-line sources as compact structures in an unsharp masked version of the grism image. Using these methods we identify 46 emission-line targets in the Hubble Deep Field-North using a modest (three-orbit) expenditure of HST observing time. Grism spectroscopy is a powerful tool for efficiently identifying interesting low-luminosity, moderate-redshift emission-line field galaxies. The sources found here have a median i-band (F775W) flux 1.5 mag fainter than the spectroscopic redshift catalog of Cohen et al. They have redshift z ≤ 1.42, and high equivalent widths (typically &gt;100 Å) and are usually less luminous than the characteristic luminosity at the same redshift. The chief obstacle in interpreting the results is line identification, since the majority of sources have a single emission line and the spectral resolution is low. Photometric redshifts are useful for providing a first-guess redshift. However, even at the depth of the state-of-the-art ground-based and HST data used here, photometric errors can result in uncertainties in line identifications, especially for sources with i magnitudes fainter than 24.5 ABmag. Reliable line identification for the faintest emission-line galaxies requires additional ground-based spectroscopy for confirmation. Of particular concern are the faint high-EW [O II] emitters, which could represent a strongly evolving galaxy population if the possibility that they are misidentified lower redshift interlopers can be ruled out.&#13;\n