A very bright quasar in the ‘redshift desert’ discovered by LAMOST
2 Target selection and Observation
In order to test whether our newly proposed quasar selection criterion in the vs. diagram is efficient in identifying quasars, we selected many candidates in several sky fields overlapped between UKIDSS and SDSS survey area with RA from 0 to 9 hours for the LAMOST commissioning observations in the winter of 2009. Although LAMOST has met many problems during these commissioning observations, such as the low accuracy of fiber positioning and poor dom seeing condition, we were still able to identify some new quasars including one reported here.
SDSS J085543.40-001517.7 is a relatively bright source among our quasar candidates. After the correction of Galactic extinction using the map of Schlegel et al. (1998), Its SDSS magnitudes (in AB system) are 17.67, 16.87, 16.62, 16.44, 16.20, respectively and its UKIDSS magnitudes (in Vega system) are 15.61, 15.24, 14.60, 13.84, respectively. The offset between its SDSS ad UKIDSS positions is 0.05. Fig. 1 shows its SDSS finding chart (obtained from http://cas.sdss.org/dr7/en/tools/chart/chart.asp). Obviously SDSS J085543.40-001517.7 is a bright point source, surrounding by several other fainter sources with the offsets from 8 to 20. In Fig. 2 we show the location of this source in 3 optical color-color diagrams and the vs. diagram, in comparison with the 8996 SDSS-UKIDSS stars (Wu & Jia 2010). Note that in the vs. diagram the magnitude of and have been converted to the magnitudes in Vega system by using the scalings (Hewett et al. 2006): and . It is clear that SDSS J085543.40-001517.7 locates in the stellar locus in three optical color-color diagrams, but is well separated from stars in the vs. diagram and meets the selection criterion, , proposed by Wu & Jia (2010). This also explains why this source was not selected as a quasar candidate in SDSS, although it is bright enough.
The spectroscopy of SDSS J085543.40-001517.7 was obtained by LAMOST during the commissioning observations on December 18, 2009, with the spectral resolution of R1000 and the exposure time of 30 minutes. The spectrum was processed using a preliminary version of LAMOST spectral pipeline. In the left panel of Fig. 3 we show the LAMOST spectrum of SDSS J085543.40-001517.7 (some sky light emissions were not well subtracted). From the spectrum we can clearly observe at least four strong emission lines, namely, Ly , Si IV , C IV and C III] . With these four lines we derived an average redshift of for this new quasar. Three weak emission lines, N V , O I and C II , can be also seen between Ly and Si IV lines. The complicated feature around 5900 is due to the problem in combining the LAMOST blue and red spectra, which overlap with each other from 5700 to 6000. In this figure we also compare the LAMOST spectrum with the scaled SDSS composite quasar spectrum (Vanden Berk et al. 2001). It is clearly that both match well with each other, except in the red end.
On March 9, 2010, we also used the NAOC/Xinglong 2.16m telescope to do spectroscopy of this new quasar. Because the seeing condition was bad (4-5), we took two 40-minute exposures on this quasar and obtained the median spectrum, which is shown in the right panel of Fig. 3 in comparison with the scaled SDSS composite quasar spectrum. Although its signal to noise ratio is lower than the LAMOST spectrum, four strong emission lines can still be clearly observed. Moreover, its continuum shape matches the SDSS composite quasar spectrum better than the LAMOST spectrum, especially in the red end.
3 Properties of SDSS J085543.40-001517.7
With the magnitude of 16.44 and redshift of 2.427, SDSS J085543.40-001517.7 is undoubtedly a very bright quasar. We compared it with other SDSS quasars in the redshift range from 2 to 3.2 and found the new quasar is indeed very bright. In Fig. 4 we show the location of the new quasar in the magnitude-redshift diagram in comparison with other SDSS quasars, as well as the histogram of the redshift distribution of SDSS quasars. The redshift distribution clearly shows the presence of ‘redshift desert’ in the redshift range from 2.2 to 3. The new quasar is apparently the brightest one in the redshift range from 2.3 to 2.7. Its absolute magnitude is -30.0 if the cosmological parameters =70 Mpc, and are adopted. Clearly this quasar belongs to the most luminous quasars in the universe.
We also searched the counterparts of SDSS J085543.40-001517.7 in other wavelength bands. From the VLA/FIRST radio catalog (White et al. 1997) we did not find any radio counterpart within 20 from its SDSS position. The closest radio source is 121.5 far away. Therefore, this quasar is a radio-quiet one, which is also another reason why it is missed by the SDSS spectroscopy. We also searched the ROSAT X-ray source catalog (Voges et al. 1999) and did not find any counterpart within 1’. The closest X-ray source is 23 away. From GALEX catalog (Morrissey et al, 2007) we failed to find any ultraviolet counterpart within 5. One GALEX source is 27 away (in the south-western direction) from the optical position of SDSS J085543.40-001517.7, and is clearly the counterpart of another fainter extended source in the SDSS image. Therefore, we believe that SDSS J085543.40-001517.7 is faint in radio, UV and X-ray bands, although it is very luminous in optical and near-IR bands.
From the spectral properties we can estimate the black hole mass and bolometric luminosity of this new quasar. After doing the redshift correction, Galactic extinction correction using the reddening map of Schelegal et al. (1998), continuum fitting and Fe II subtraction using the template from Vestergaard & Wilkes (2001), we measured the C IV line width (FWHM, the Full Width at Half Maximum) and the rest frame 1350 continuum flux from the spectrum. Because we did not make the absolute flux calibration of the LAMOST spectrum of SDSS J085543.40-001517.7, we used the ultraviolet continuum window 1320 – 1330 to calibrate the LAMOST spectrum with the spectrum taken by the 2.16m telescope. The C IV FWHM values measured from the LAMOST and 2.16m spectra are 8520and 11040, respectively. Due to the lower signal to noise ratio of the 2.16m spectrum (see the right panel of Fig. 3), we took the C IV FWHM value from the LAMOST spectrum in the following calculation. The black hole mass estimation was done with two similar formula proposed by Kong et al. (2006) and Vestergaard & Peterson (2006), both involving the C IV line width and continuum luminosity. The first one gives and the latter one gives . Using a scaling between 1350 luminosity and bolometric luminosity, , given by Vestergaard (2004) based on the SED of radio-quiet quasars (Elvis et al. 1994), we estimated the bolometric luminosity of this new quasar as , which is about times of the Eddington luminosity if the above estimated black hole mass is adopted. Obviously, this quasar is intrinsically very bright, and is accreting matters with the accretion rate around the Eddington limit.
Quasars with redshifts in the range from 2.2 to 3 are very important for studying their cosmological evolution, and the relation between quasar activity and star formation activity which peaks at redshift between 1 and 2 (Madau et al. 1998). However, because these quasars have similar optical colors as normal stars, it is very difficult for find them in previous quasar surveys. The low efficiency of finding quasars in the ‘redshift desert’ has led to the obvious incompleteness of quasar sample in this redshift range and serious problems in constructing the luminosity function for quasars around the redshift peak (between 2 and 3) of quasar activity (Richards et al. 2006; Jiang et al. 2006).
In this paper we have presented a case study to find a very bright new quasar in the redshift desert by the LAMOST commissioning observation. The spectroscopic identification of an source, SDSS J085543.40-001517.7, as a quasar gives us confidence to discover more missing quasars in the future LAMOST quasar survey. This discovery also supports the idea that by combining the UKIDSS near-IR colors with the SDSS optical colors we are able to efficiently recover these missing quasars. In the winter of 2009, LAMOST has made test observations on several sky fields and we are now searching for more quasars from the spectra taken in these fields. The discovery of more new quasars in these fields will be reported in the future works. We hope that in the next a few months great progress will be made in improving the capability of LAMOST spectroscopy and the spectral processing pipeline. As long as LAMOST can reach its designed capability after the commissioning phase, we expect to find several hundred-thousands of quasars in the LAMOST quasar survey. This will form the largest quasar sample in the world and play a leading role in the quasar study of the next decade.
This work was supported by the National Natural Science Foundation of China (10525313), the National Key Basic Research Science Foundation of China (2007CB815405), and the Open Project Program of the Key Laboratory of Optical Astronomy, NAOC, CAS. The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) is a National Major Scientific Project built by the Chinese Academy of Sciences. Funding for the project has been provided by the National Development and Reform Commission. The LAMOST is operated and managed by the National Astronomical Observatories, Chinese Academy of Sciences. We acknowledge the use of LAMOST and the NAOC/Xinglong 2.16m telescope, as well as the archive data from SDSS, UKIDSS, FIRST, ROSAT and GALEX. We thank Marianne Vestergaard for kindly providing us the Fe II template, which was used in this work.
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