Quasi-stellar objects in the ALHAMBRA survey

Abstract

Context. Even the spectroscopic capabilities of today's ground and space-based observatories can not keep up with the enormous flow of detections (>10 5 deg −2 ) unveiled in modern cosmological surveys as: i) would be required enormous telescope time to perform the spectroscopic follow-ups and ii) spectra remain unattainable for the fainter detected population. In the past decade, the typical accuracy of photometric redshift (photo-z) determination has drastically improved. Nowdays, it has become a perfect complement to spectroscopy, closing the gap between photometric surveys and their spectroscopic follow-ups. The photo-z precision for active galactic nuclei (AGN) has always lagged behind that for the galaxy population owing to the lack of proper templates and their intrinsic variability. Aims. Our goal is to characterize the ability of the Advanced Large, Homogeneous Area Medium-Band Redshift Astronomical (ALHAMBRA) survey in assigning accurate photo-z's to broad-line AGN (BLAGN) and quasi-stellar objects (QSOs) based on their ALHAMBRA very-low-resolution optical-near-infrared (NIR) spectroscopy. This will serve as a benchmark for any future compilation of ALHAMBRA selected QSOs and the basis for the statistical analysis required to derive luminosity functions up to z ∼ 5. Methods. We selected a sample of spectroscopically identified BLAGN and QSOs and used a library of templates (including the SEDs of AGN and both normal and starburst galaxies, as well as stars) to fit the 23 photometric data points provided by ALHAMBRA in the optical and NIR (20 medium-band optical filters plus the standard JHKs). Results. We find that the ALHAMBRA photometry is able to provide an accurate photo-z and spectral classification for ∼88% of the 170 spectroscopically identified BLAGN/QSOs over 2.5 deg 2 in different areas of the survey and brighter than m 678 = 23.5 (equivalent to r SLOAN ∼ 24.0). The derived photo-z accuracy is below 1% and is comparable to the most recent results in other cosmological fields that use photometric information over a wider wavelength range. The fraction of outliers (∼12%) is mainly caused by the larger photometric errors for the faintest sources and the intrinsic variability of the BLAGN/QSO population. A small fraction of outliers may have an incorrectly assigned spectroscopic redshift. Conclusions. The definition of the ALHAMBRA survey in terms of the number of filters, filter properties, areal coverage, and depth is able to provide photometric redshifts for BLAGN/QSOs with a precision similar to any previous survey that makes use of mediumband optical photometry. In agreement with previous literature results, our analysis also reveals that, in the 0 < z < 4 redshift interval, very accurate photo-z can be obtained without the use of NIR broadband photometry at the expense of a slight increase in the outliers. The importance of NIR data is expected to increase at higher z (z > 4). These results are relevant for the design of future optical follow-ups of surveys containing a large fractio...

The ALHAMBRA survey: Discovery of a faint QSO atz= 5.41

“…Unfortunately, the i-dropouts and broad-band colours of a z ∼ 5-6 QSO can look very similar to those of evolved/passive galaxies at z ∼ 1 or of local red giants and super-giants. Matute et al (2012) have shown that using the 23 bands of the ALHAMBRA photometric catalogue makes it possible to efficiently classify a high percentage (∼90%) of the z = [0, 3] BLAGN/QSO population and measure highly accurate photometric redshifts.…”

“…We have applied the methodology described by Matute et al (2012) to the search for high-z (z > 5) QSOs using the ALHAMBRA photometric database. The candidates were selected over ≈1 deg 2 in the two ALHAMBRA fields (out of eight; ALH-2/DEEP2 and ALH-8/SDSS) observable from the Roque de los Muchachos observatory in La Palma during the 2011B semester according to the following criteria: i) located in the fully exposed areas of the images and in regions not contaminated by bright sources flux or artefacts (e.g.…”

“…(i) ALHAMBRA field name; (ii, iii) Object coordinates; (iv) observed magnitude in the ALHAMBRA A830M filter; (v, vi, vii, viii) derived photo-z, template, intrinsic extinction and extinction law from the best fit to the ALHAMBRA photometry (for details see Matute et al 2012); (ix) spectroscopic redshift; (x) rest-frame Lyα equivalent width (Å) from our modellisation of the spectrum continuum and emission lines (see Sect. 4.5); (xi) rest-frame monochromatic flux at 1450 Å in units of erg s −1 cm −2 Å −1 ; (xii) absolute magnitude at 1450 Å; (xiii) log 10 of the bolometric luminosity [erg s −1 ]; (xiv) black hole mass (M ) assuming it radiates at the Eddington limit; (xv) optical continuum power law index ( f ∝ ν −α ).…”

“…The survey actually covers ∼4 deg 2 over eight different regions of sky and provides a photometric dataset over 20 contiguous, equal-width, nonoverlapping, medium-band optical filters (3500-9700 Å) plus the three standard broad-band near-infrared (NIR) filters JHK S (Benítez et al 2009;Aparicio Villegas et al 2010). The capabilities of the ALHAMBRA survey to select and classify extragalactic sources with its very low-resolution-spectra have been explored for galaxies and AGN by Molino et al (2013) and Matute et al (2012), respectively. The photometric redshift precision achieved is ∆z/(1 + z) ∼ 0.011 and 0.009 for galaxies and AGNs, respectively.…”

Identification and photometric redshifts for type-I quasars with medium- and narrow-band filter surveys

“…We can note there is a large number of outliers in the sample. In Fig.2.7 we show the photometric redshift estimation for a sample of 170 quasars detected in the ALHAMBRA fields (Matute et al 2012). The analysis was performed with LePhare, using all of the 23 band filters and 50 templates (see more details in Chapter 5).…”