Advanced
THE INFRARED MEDIUM-DEEP SURVEY. V. A NEW SELECTION STRATEGY FOR QUASARS AT z > 5 BASED ON MEDIUM-BAND OBSERVATIONS WITH SQUEAN
THE INFRARED MEDIUM-DEEP SURVEY. V. A NEW SELECTION STRATEGY FOR QUASARS AT z > 5 BASED ON MEDIUM-BAND OBSERVATIONS WITH SQUEAN
Journal of The Korean Astronomical Society. 2016. Feb, 49(1): 25-35
Copyright © 2016, Korean Astronomical Society
  • Received : September 01, 2015
  • Accepted : January 10, 2016
  • Published : February 29, 2016
Download
PDF
e-PUB
PubReader
PPT
Export by style
Article
Author
Metrics
Cited by
About the Authors
Yiseul Jeon
Center for the Exploration of the Origin of the Universe (CEOU), Astronomy Program, Department of Physics & Astronomy, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151742 Korea
Myungshin Im
Center for the Exploration of the Origin of the Universe (CEOU), Astronomy Program, Department of Physics & Astronomy, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151742 Korea
Soojong Pak
School of Space Research and Institute of Natural Sciences, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin, Gyeonggi 446701, Korea
Minhee Hyun
Center for the Exploration of the Origin of the Universe (CEOU), Astronomy Program, Department of Physics & Astronomy, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151742 Korea
Sanghyuk Kim
School of Space Research and Institute of Natural Sciences, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin, Gyeonggi 446701, Korea
Yongjung Kim
Center for the Exploration of the Origin of the Universe (CEOU), Astronomy Program, Department of Physics & Astronomy, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151742 Korea
Hye-In Lee
School of Space Research and Institute of Natural Sciences, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin, Gyeonggi 446701, Korea
Woojin Park
School of Space Research and Institute of Natural Sciences, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin, Gyeonggi 446701, Korea

Abstract
Multiple color selection techniques are successful in identifying quasars from wide-field broadband imaging survey data. Among the quasars that have been discovered so far, however, there is a redshift gap at 5 ≲ z ≲ 5.7 due to the limitations of filter sets in previous studies. In this work, we present a new selection technique of high redshift quasars using a sequence of medium-band filters: nine filters with central wavelengths from 625 to 1025 nm and bandwidths of 50 nm. Photometry with these medium-bands traces the spectral energy distribution (SED) of a source, similar to spectroscopy with resolution R ~ 15. By conducting medium-band observations of high redshift quasars at 4.7 ≤ z ≤ 6.0 and brown dwarfs (the main contaminants in high redshift quasar selection) using the SED camera for QUasars in EArly uNiverse (SQUEAN) on the 2.1-m telescope at the McDonald Observatory, we show that these medium-band filters are superior to multi-color broad-band color section in separating high redshift quasars from brown dwarfs. In addition, we show that redshifts of high redshift quasars can be determined to an accuracy of Δz/(1 + z) = 0.002 – 0.026. The selection technique can be extended to z ~ 7, suggesting that the medium-band observation can be powerful in identifying quasars even at the re-ionization epoch.
Keywords
1. INTRODUCTION
Recently, more than 0.4 million quasars have been discovered (Flesch 2015) , among which 70 lie at z ≳ 6 (e.g., Fan et al. 2003 , 2006 ; Willott et al. 2007 , 2010 ; Jiang et al. 2008 , 2009 , 2015 ; Mortlock et al. 2009 , 2011) . Using these high redshift quasars, we can study the evolution of supermassive black holes (SMBHs; e.g., Jiang et al. 2007 ; Kurk et al. 2007 ; Jun et al. 2015) and the metal enrichment history of the early universe when the age of the universe was less than 1 Gyr (e.g., Jiang et al. 2007 ; Kurk et al. 2007 ; Juarez et al. 2009 ; De Rosa et al. 2011 , 2014) . In addition, they allow us to examine the line-of-sight condition of the intergalactic medium (IGM), such as the re-ionization state (e.g., Fan et al. 2006 ; Carilli et al. 2010 ; McGreer et al. 2011) .
However, there exist redshift gaps in current high redshift quasar surveys, especially in the range of 4.8 ≤ z ≥ 5.7. Although some quasar surveys were performed at z ~ 5 or z ~ 6 (e.g., Zheng et al. 2000 ; Sharp et al. 2001 ; Schneider et al. 2001 ; Fan et al. 2003 , 2006 ; Mahabal et al. 2005 ; Cool et al. 2006 ; Willott et al. 2007 , 2010 ; Jiang et al. 2008 , 2009 , 2015 ; Wu & Jia 2010 ; Matute et al. 2013 ; McGreer et al. 2013) , they could not detect quasars at 5 ≲ z ≲ 5.7 efficiently (Figure 16 in Jun et al. 2015) . This was due to the limitations of the filter systems currently employed by these surveys as described below.
Most of the quasars at z ~ 5 were discovered by the Sloan Digital Sky Survey (SDSS; Richards et al. 2002 ; McGreer et al. 2013) , using the r i color for selecting r -dropouts and the i z color for removing brown dwarfs. Also Ikeda et al. (2012) used the r i vs. i z color-color diagram and discovered only one type-2 quasar at z = 5.07. However, these studies were unsuccessful in finding quasars at 5 ≲ z ≲ 5.7 because the r -dropout quasars, starting from z ~ 4.6, blend with brown dwarfs on color-color diagrams for z ~ 5.1 (Jeon et al. in preparation). Wu & Jia (2010) adopted new color cuts, J K vs. i Y to find high redshift quasar candidates, but their method is limited to z < 5.3. On the other hand, for quasar surveys at z ~ 6, i z vs. z J color-color diagrams are usually used. However the i -dropouts start from z ~ 5.7; thus, most surveys for z ~ 6 are limited to z > 5.7. Therefore we need a new method and a new filter system which exploits the wavelength range between conventional filters to find quasars at redshift between 5.1 and 5.7.
To discover quasars at 5 ≲ z ≲ 5.7, we have been conducting an imaging survey as a part of the Infrared Medium-deep Survey (IMS; Im et al. in preparation). This imaging survey has used the Camera for QUasars in EArly uNiverse (CQUEAN; Park et al. 2012 ; Kim et al. 2011 ; Lim et al. 2013) , with custom-designed is and iz filters that compensate for previous filter systems. Since the central wavelengths of these filters are located between r and i , and between i and z , respectively, it was expected that these filters will be effective in selecting high redshift quasars at 5 < z < 6, where SDSS or other filter systems cannot explore (Jeon et al. in preparation). The quasar candidates were selected from r -dropout objects of z < 19.5 mag over 3,400 deg 2 using multi-wavelength data such as SDSS, the United Kingdom Infrared Telescope Infrared Deep Sky Survey (UKIDSS; Lawrence et al. 2007) Large Area Survey (LAS), the Two Micron All Sky Survey (2MASS), and theWide-field Infrared Survey Explorer ( WISE ). So far, 6 r -dropouts in 4.7 ≤ z ≤ 5.4, and 2 i -dropouts in 5.9 ≤ z ≤ 6.1 have been confirmed as quasars with optical spectroscopic observations (Jeon et al. in preparation); these are referred to as IMS quasars. The discovery of 6 r -dropout quasars at 4.7 ≤ z ≤ 5.4 is encouraging, but the same technique has not been successful at uncovering quasars at 5.4 < z < 5.7. This is mostly because the adopted color cuts cannot yet remove contaminants such as late type stars or brown dwarfs effectively. Therefore, it is highly desirable to have a new method based on a new filter system for selecting quasars in this range.
Matute et al. (2013) adopted a spectral energy distribution (SED) fitting method and discovered a faint quasar at z = 5.41 from the Advance Large Homogeneous Area Medium Band Redshift Astronomical (ALHAMBRA; Moles et al. 2008) survey over ~1 deg 2 . Their survey included a filter set composed of 20 optical medium-band and 3 broad-band J/H/Ks filters, and an extensive library of SEDs including those of active galaxies. Despite the fact that high redshift quasar candidates are contaminated by many low redshift galaxies and late type stars, they were able to select the quasar at z = 5.41 via SED fitting. This demonstrates that low resolution spectroscopy with medium-band filters can be an effective way to select high redshift quasars.
As an attempt to improve the selection of high red-shift quasars, we devised a set of 9 medium-band filters to better trace SEDs of various sources and modified CQUEAN to allow the usage as many as 20 filters. We now call this modified CQUEAN the SED camera for QUasars in EArly uNiverse (SQUEAN; Kim et al. in preparation; Choi et al. 2015) . Like CQUEAN, SQUEAN operates on the 2.1 m Otto Struve Telescope at the McDonald Observatory. To explore the potential of selecting high redshift quasars using our mediumband filters, we observed known high redshift quasars and brown dwarfs, and conducted SED fitting to distinguish these two populations. This paper reports the result of the study. Section 2 describes the overview of our new medium-band filters and two test observations. In Sections 3 and 4, we describe the performance from the result of medium-band photometry including SED fitting results. We discuss the feasibility of using medium-band filters on small or medium-class telescopes for future high redshift quasar selection in Section 5. We summarize our results in the final section. Throughout this paper, we use a cosmology with Ω M = 0.3, Ω = 0.7, and H 0 = 70 km s −1 Mpc −1 , and the AB magnitude system.
2. OBSERVATIONS
- 2.1. SQUEAN Medium-band Filters
The SQUEAN medium-band filter set consists of nine filters, m 625, m 675, m 725, m 775, m 825, m 875, m 925 s , m 975, and m 1025, where ”m” stands for ”medium” and the number indicates the central wavelength in nm ( Table 1 ). The bandwidths of these filters are fixed to 50 nm. In addition, SQUEAN has an m 925 n filter, whose bandwidth is 25 nm and has a central wavelength identical to that of m 925 s . These filters are off-the-shelf products from Edmund Optics, Inc.. The optical characteristics of the filters are described in a separated SQUEAN instrument paper (Kim et al. in preparation).
Medium-band filter characteristics
PPT Slide
Lager Image
a Kim et al. in preparation
Color cuts and redshift ranges for quasar selection inFigure 6
PPT Slide
Lager Image
Color cuts and redshift ranges for quasar selection in Figure 6
Figure 1 shows filter transmission curves of the medium-band filters (thin solid lines) and other SQUEAN filters (dashed lines) with the detector quantum efficiency (QE; thick solid line) taken into account. The figure shows that the medium-band filter bandwidths are several times narrower than those of broadband filters, such as SDSS r/i/z .
PPT Slide
Lager Image
The QE of SQUEAN (thick solid line) and the combined response function of the detector QE and filters. The thin solid lines are for our medium-band filters and the dashed lines are for the CQUEAN g/r/i/z/Y.
- 2.2. SQUEAN Medium-band Observations
We carried out test observations with a subset of medium-band filters installed in CQUEAN in 2014 November and with a complete set of medium-band filters in SQUEAN in 2015 February. During the two observing runs, we observed 6 IMS quasars at 4.7 ≤ z ≥ 6.0 and 5 brown dwarfs ( Table 3 ) with m 675, m 725, m 775, m 825 and m 875 filters. The observations were carried out under mostly clear sky conditions. However, both observations were executed during bright time, and seeing values varied from 1.3” to 4.0”.
Information of targets
PPT Slide
Lager Image
Note. a Integration times of the m675,m725,m775,m825, and m875 filters respectively, for the test observations b Seeing values in the m675,m725,m775,m825, and m875 filters respectively, for the test observations c Yi et al. (2015) provide a more accurate redshift, zspec = 4.76. d Jiang et al. (2015) provide a more accurate redshift, zspec = 5.923 ± 0.003.
Preprocessing, such as bias subtraction, dark subtraction and flat fielding were conducted with usual data reduction procedures using the IRAF 1 noao.imred.ccdred package. Then we combined images of each field and each filter in average. We used the ccmap task of IRAF and SCAMP (Bertin 2006) to derive astrometric solutions.
SExtractor (Bertin & Arnouts 1996) was used for source detection and photometry. We derived auto-magnitudes which are taken as the total magnitudes. We estimated the limiting magnitudes for each medium-band filter using the data taken on 2015 February 8. The 5 σ detection limits of a point source with 1 hour integration time are 23.7 mag for m 675 and m 725, 23.3 mag for m 775 and m 825, and 22.7 mag for m 875-bands.
- 2.3. Photometric Calibration
For photometric calibration, we used stars that have SDSS photometry and are in the same field as our target. Using SDSS r/i/z magnitudes of a star, we did a X 2 fitting to the SED of the object to find the best spectral template among stellar spectral templates. We used the stellar templates from Gunn & Stryker (1983) , that contain 175 spectra with various stellar types. From the best-fit template, we calculated model magnitudes for each medium-band filter by convolving their filter transmission curves and defined these values as their standard magnitudes ( M std ) of each filter. Figure 2 shows examples of the X 2 fitting of stars, with their r/i/z magnitudes (boxes) and the best-fit templates (solid lines). Then, the M std of the stellar sources were compared with their instrument magnitudes ( m ins ; m ins = − 2.5 log(DN)), and the differences between the M std and the m ins were used to estimate the zero-point ( Zp ). During the Zp calculation, faint objects or objects with large reduced
PPT Slide
Lager Image
values were rejected: we chose stars of
PPT Slide
Lager Image
and instrument magnitude error < 0.05 mag for each band. Figure 3 shows an example of the Zp calculations for each filter in the case of IMS J143704.82+070808.3. The crosses are for all sources in the field and the boxes are for sources that satisfy the conditions explained above. After 3 σ clipping, we calculated the Zp as the average value of the magnitude differences. For the Zp uncertainty, we used the standard deviation value of the Zp values. The average zero-point error is about 0.05 mag. The standard magnitudes of our targets were calculated as M std = Zp + m ins .
PPT Slide
Lager Image
Examples of the X2 fitting of SDSS stars. The solid line shows the best-fit template and the boxes are from SDSS r/i/z photometry.
PPT Slide
Lager Image
The Zp calculations of IMS J143704.82+070808.3 field for each filter. The crosses are for all sources in the field and the boxes are sources that satisfy the conditions described in Section 2.3.
This calibration technique was compared to results from standard star observations. We used data from the observation of 2015 February 5. BD+33 2642 was observed with the 5 filters at two different airmasses. To calculate the standard magnitudes of the star for each filter, we used its spectrum obtained from Oke (1990) , and the filter transmission curves. The Zp values from the standard star data are found to be consistent with the values derived from SDSS stars.
3. PROPERTIES OF MEDIUM-BAND PHOTOMETRY
- 3.1. Filter Response Curves with Quasar Spectra
We plot the filter response curves of medium-band filters (thin solid lines) with quasar SED templates at different redshifts (thick solid lines) in Figure 4 . For the quasar SEDs, we used the median composite spectrum of SDSS quasars from Vanden Berk et al. (2001) and adopted the IGM attenuation model from Madau et al. (1996) . We also plot the model magnitudes from the SED templates in each filter with squares. We can notice that the Lyman α (Ly α ) emission line is located in the m 725-band at z = 5.0, in the m 775-band at z = 5.5 and between the m 825 and m 875-bands at z = 6.0, and the model magnitudes show a sharp drop in flux shortward of these filters. We can see that the photometry with these medium-bands trace the SED of sources, similar to spectroscopy with resolution R ~ 15.
PPT Slide
Lager Image
Filter response curves (thin solid lines) with quasar spectra (thick solid lines) redshifted to z = 5.0, 5.5, and 6.0. The model magnitudes in each filter (squares) trace the SEDs of the sources similar to spectroscopy with R ∼15.
- 3.2. Medium-band Spectral Energy Distributions
Table 4 lists the medium-band magnitudes of our targets with g/r/i/z/Y/J/H/K -band photometry from SDSS and UKIDSS LAS. The magnitude errors of the medium-band filters are computed by combining the standard SExtractor estimates based on Poisson statistics and the zero-point errors from Section 2.3.
Photometric information of targets
PPT Slide
Lager Image
Photometric information of targets
The medium-band filters trace SEDs of high redshift quasars and brown dwarfs differently. In Figure 5 , we show the medium-band SEDs of quasars and brown dwarfs, where the red squares are the medium-band data and the blue squares are the SDSS r/i/z magnitudes. The gray lines in the upper two rows are quasar templates from Vanden Berk et al. (2001) redshifted appropriately, with IGM attenuation (Madau et al. 1996) . The gray lines in the lower two rows are model brown dwarf spectra with a temperature of 2,200K Burrows et al. (2006) . Note that IMS J012247.33+121623.9 is a broad absorption line (BAL) quasar, which shows a deep absorption feature near the wavelengths of the m 775-band. High redshift quasars show a sharp drop in flux in the filter that covers the wavelength range blueward from the redshifted Ly α emission line. On the other hand, SEDs of brown dwarfs smoothly increases from short to long wavelengths. Such a difference in SEDs is much less pronounced in the r/i/z broad-band photometry. The drastic magnitude drop at the redshifted Ly α is key to distinguishing high red-shift quasars from brown dwarfs, and the medium-band filters offer sufficient spectral resolution for such a purpose. These dropouts can also be recognized in colorcolor diagrams.
PPT Slide
Lager Image
Medium-band SEDs of quasars and brown dwarfs from SQUEAN observations (red squares). Also we plot the SDSS r/i/z data (blue squares), the redshifted quasar SEDs (gray lines in upper two rows), and the model brown dwarf spectra (gray lines in lower two rows). This figure demonstrates that high redshift quasar SEDs and brown dwarf SEDs can be distinguished easily thanks to the fine wavelength sampling of the medium-band filters.
- 3.3. Color-Color Diagrams
Figure 6 shows 7 color-color diagrams from combinations of all nine medium-band filters (black lines are quasar redshift tracks within specific redshift ranges, light green triangles are model brown dwarfs from Burrows et al. (2006), dark green boxes with error bars or arrows are IMS brown dwarfs, and red circles with error bars or arrows are IMS quasars). Figure 6 (a) is for m 625− m 675, vs. m 675− m 725, (b) is for m 675− m 725, vs. m 725 − m 775, (c) is for m 725 − m 775 vs. m 775 − m 825, (d) is for m 775− m 825 vs. m 825− m 875, (e) is for m 825− m 875 vs. m 875− m 925, (f) is for m 875− m 925 vs. m 925 − m 975, and (g) is for m 925 − m 975 vs. m 975 − m 1025. Figure 6 (b), (c), and (d) contain the medium-band colors from our observations. From Figure 6 (b), we can deduce that the m 675-dropout is found for IMS J012247.33+121623.9 (z=4.86) and IMS J143704.82+070808.3 (z=4.97). On the other hand, it is difficult to select IMS J032407.70+042613.3 (z=4.71) as a high redshift quasar, because its Ly α drop is located at the m 675-band and its colors using these 3 filters are similar to those of brown dwarfs. Note that the BAL quasar at z=4.86, IMS J012247.33+121623.9, is off from the redshift tracks due to broad absorption lines in its continuum. For similar reasons, in Figure 6 (c), IMS J222514.39+033012.6 (z=5.35) and IMS J015533.28+041506.8 (z=5.39) also show red m 725 − m 775 colors relative to IMS J032407.70+042613.3 (z=4.71), IMS J012247.33+121623.9 (z=4.86), and IMS J143704.82+070808.3 (z=4.97) due to the m 725-dropout. We can see that Figure 6 (b), (c), and (d) can be used for selecting quasars at 4.8 < z < 5.8 because the positions of the quasars are significantly different from those of the brown dwarfs, distanced by ≳ 1 mag from the quasar tracks. For the quasar selection at z > 5.8, Figure 6 (e), (f), and (g) show that the quasars at z > 5.8 can be separated from brown dwarfs clearly. Therefore we can use these color-color diagrams from medium-band photometry for quasar candidate selections at 4.7 < z < 7. Table 2 lists the color cuts (blue solid lines in Figure 6 ) and the redshift ranges for quasar selection. Since Lyman break galaxies (LBGs) at high redshift have ultraviolet SEDs similar to those of quasars, these selection criteria can also be used for selecting high redshift star-forming galaxies. On the other hand, we expect that the objects satisfying the color cuts at bright magnitude (< 21 AB mag; e.g., Shim et al. 2007) are predominantly quasars.
PPT Slide
Lager Image
Color-color diagrams of successive combinations of nine medium-band filters. The black lines are quasar redshift tracks with diamond symbols indicating redshifts at a Δz = 0.2 interval, except the last redshift point is chosen to be the redshift where Lyα is located at 12.5 nm above the central wavelength of the second filter. The light green triangles are brown dwarfs, the dark green boxes are IMS brown dwarfs, and the red circles are IMS quasars.
For comparison, we plot various color-color diagrams using SDSS and UKIDSS LAS filters in Figure 7 . Since the r -band has wavelengths similar to those of m 675, we plot the r m 725 vs. m 725− m 775 color-color diagram in Figure 7 (a). In Figure 7 (a), the colors of the model quasar redshifted to 4.7 < z < 5.1 are not different from those of brown dwarfs, contrary to Figure 6 (b). This is due to the difference in wavelength coverage between the r and m 675-bands. Since the m 675 filter has a narrower bandwidth compared to the r filter, the m 675 − m 725 color can measure the Ly α dropout feature clearly, without being affected by other spectral features. For example, IMS J143704.82+070808.3 at z = 4.97 shows a higher flux in the r -band than in the m 675-band because the r filter also covers the Lyman β emission line at this redshift. Therefore, the dropout effect between r and m 725 is not strong enough to distinguish quasars from brown dwarfs.
PPT Slide
Lager Image
Similar to Figure 6, but including SDSS and UKIDSS LAS filters.
To check the possibility of selecting quasars at z ~ 6 with our medium-bands and near-infrared bands, we adopted the Y or J -bands ((b) and (c) of Figure 7 ). The z Y or z J colors of z ~ 6 quasars are different from those of brown dwarfs, which is used in a traditional selection method to select z ~ 6 quasars, such as the i z vs. z J color-color diagram (e.g., Fan et al. 2001 , 2003 , 2004 , 2006 ; Jiang et al. 2008 , 2009 ; Willott et al. 2007 , 2009 , 2010) . Therefore, using these three diagrams, we are able to select quasars at z ~ 6 that are separated from the brown dwarfs.
In Figure 7 (d), the z m 975 vs. m 975− J color-color diagram shows a clear separation between the quasar redshift track and brown dwarfs, similar to Figure 6 (g), which is also suitable for selecting z ~ 7 quasar candidates.
4. SED FITTING FROM MEDIUM-BAND PHOTOMETRY
To estimate the efficiency of the medium-band filters, we performed SED fitting using templates of the SDSS quasar composite spectrum from Vanden Berk et al. (2001) with IGM attenuation (Madau et al. 1996) . SEDs from the medium-bands or other bands are compared to the quasar templates redshifted to various redshifts and
PPT Slide
Lager Image
values between the SEDs and the quasar templates were calculated. We found the best-fit template of a source using the minimization of
PPT Slide
Lager Image
values and estimated a photometric redshift (z phot ) from the template.
We used several filter sets for the SED fitting, combining our medium-bands and SDSS filters, to compare fitting performances while using different filter sets. We define the filter sets as follows:
Filterset1={ m 675, m 725, m 775, m 825, m 875}
Filterset2={ r , m 725, m 775, m 825, m 875}
Filterset3={ m 675, m 725, m 775, m 825, z }
Filterset4={ r , m 725, m 775, m 825, z }
Filterset5={ r, i, z }
- 4.1. Best-fitValues of Quasars and Brown Dwarfs
After finding the best-fit templates for each target, we compared the best-fit
PPT Slide
Lager Image
values for quasars and brown dwarfs for a specific filter set. Figure 8 shows the average values of the best-fit
PPT Slide
Lager Image
values for quasars and brown dwarfs for each filter set. We exclude IMS J012247.33+121623.9, since it is a BAL quasar, whose spectra are known to be clearly different from normal quasar spectra. The ranges of the best-fit
PPT Slide
Lager Image
values are denoted with error bars. We can see that the best-fit
PPT Slide
Lager Image
values from the quasar and the brown dwarfs are quite different, although some brown dwarfs show small
PPT Slide
Lager Image
values comparable to those of quasars. On average, for Filterset1 – 4, which include 3 medium-bands that substitute the i -band wavelengths ( m 725, m 775, m 825), the best-fit
PPT Slide
Lager Image
values for the quasars are 2 – 13 times smaller than those for the brown dwarfs. Therefore the SED fitting from photometry using SDSS filters and these 3 medium-band filters can be used for quasar candidate selection at 5 < z < 6 efficiently: we can choose robust quasar candidates among large samples selected from broad-band photometry, rejecting contaminants that have large
PPT Slide
Lager Image
values. On the other hand, in the case of Filterset5, which contains only r/i/z -bands without medium-bands, the best-fit
PPT Slide
Lager Image
values of the brown dwarfs are instead smaller than those of the quasars. Therefore we might misclassify brown dwarfs as actual quasars via SED fitting if we use photometry from the r/i/z -bands only. We tested with another filter set { r,m 725, m 775, z } and found that this filter set shows poor ability to separate quasars from brown dwarfs using their best-fit
PPT Slide
Lager Image
values.
PPT Slide
Lager Image
The average values of the best-fit values for quasars and brown dwarfs from each filter set. We exclude IMS J012247.33+121623.9, since it is a BAL quasar. The ranges of the best-fit values are denoted with error bars.
- 4.2. Photometric Redshifts of Quasars
Using the 5 medium-band filters from m 675 to m 875, we estimated z phot via X 2 fitting. Figure 10 shows the
PPT Slide
Lager Image
distributions as a function of redshift for each quasar. The z phot values are estimated from redshifts where c has a minimum value. The red dashed lines indicate z spec and the blue solid lines are for z phot . The errors of the z phot (blue) and z spec (red) are denoted in the gray boxes on each plot.
The uncertainties of z phot (Δz phot ) are estimated from the 1 σ deviations (confidence level of 68.3%) of the
PPT Slide
Lager Image
distribution, where the
PPT Slide
Lager Image
takes on a value one greater than its minimum. The z spec uncertainties are estimated as discussed in Jeon et al. (in preparation). We can see that z phot agree with z spec within their redshift uncertainties. The Δz phot values are less than 0.07, which shows the power of the medium-band photometry to estimate accurate z phot of high red-shift quasars via X 2 fitting of SEDs. The z phot values and their errors are provided in Table 3 . Figure 9 shows z phot versus z spec , with the solid line indicating z spec =z phot , and this figure also demonstrates the excellent agreement between the two values. The average absolute deviation of z phot − z spec (Δz) from this comparison is only 0.012 in Δz/(1+z) (dotted lines), which is better than the value we get from the formal error of the fit.
PPT Slide
Lager Image
Photometric redshifts from medium-band photometry versus spectoroscopic redshifts of high redshift quasars. The dotted lines indicate Δz/(1+z) = 0.012. The solid line indicates the case where zspec = zphot. The comparison shows that medium-band data can provide photometric redshifts with an accuracy of ∼1%.
PPT Slide
Lager Image
The values as a function of redshift for each quasar when we use Filterset1. The red dashed lines indicate the zspec values and the blue solid lines indicate for the zphot values. The errors of the zphot (blue) and zspec (red) are denoted in the gray boxes.
Richards et al. (2009 , 2015) provide zphot values for 3 of the 6 quasars, IMS J012247.33+121623.9, IMS J143704.82+070808.3, and IMS J222514.39+033012.6, estimated from broad-band photometry, u/g/r/i/z/J/H/K . Their estimate for IMS J012247.33+121623.9 (
PPT Slide
Lager Image
from Richards et al. (2015) ) does not agree with our spectroscopic redshift, although IMS J143704.82+070808.3 (
PPT Slide
Lager Image
from Richards et al. (2009) or
PPT Slide
Lager Image
from Richards et al. (2015) ) and IMS J222514.39+033012.6 (
PPT Slide
Lager Image
from Richards et al. (2015) ) are in agreement with our estimates. Also these estimates show large Δz phot values compared to those from our medium-band photometry.
5. OBSERVATION STRATEGY
In this section, we discuss whether medium-band filters can be used on small to medium-sized telescopes for high redshift quasar selection. We showed that three medium-bands ( m 725, m 775, and m 825 filters) are sufficient to remove brown dwarfs through the SED fitting when SDSS photometric information is provided. Considering the depths of the medium-bands and the average magnitude of our candidates ( i = 19.9 mag and z = 19.3 mag), we need 6, 6, and 3 minutes for m 725, m 775, and m 825-band observations with the McDonald 2.1-m telescope and SQUEAN, respectively, to reach a S/N ~ 25 (corresponding photometric error of 0.04 mag) for detection or a 5 σ upper limit at 6 μ Jy for dropouts in m 725 and m 775. In this case we need 15 minutes per target for photometry in these three medium-bands. If a similar filter set is used on a 0.5-m class telescope (e.g., LSGT; Im et al. 2015) , several hours of observing time is necessary for each target.
It is possible to extend a similar selection method to higher redshifts (z = 6.0, 6.5 and 7.0; see Figures 6 (e), (f), and (g)). A particularly interesting point is of the redshift range at z > 6.4 where the lack of wide-field near-infrared imaging survey data and severe brown dwarf contamination make quasar selection difficult. A luminous quasar at z = 7.085 (Mortlock et al. 2011) has Y = 20.3 mag and Ly α at 985 nm. Therefore, a combination of m 925 s , Y , and J filters or m 925 s , m 975, and m 1025 filters can determine whether such an object is a promising quasar candidate. To securely identify the break, we need to reach m 925 s ~ 22.3 mag at 5 σ , which is possible with a few hours of exposure time on a 2-m class telescope under nominal observing conditions.
6. SUMMARY
We show the results from the medium-band observations of 6 high redshift quasars at 4.7 ≤ z ≤ 6.0 and 5 brown dwarfs. The SEDs of the quasars and brown dwarfs from the medium-band photometry show discrepancies that are much more significant than when using broad-band photometry, and their distant loci on various color-color diagrams can effectively distinguish the two populations. The best-fit
PPT Slide
Lager Image
values of quasars and brown dwarfs are quite different even with information from only 3 medium-bands and SDSS photometry. These results from the medium-bands cannot be reproduced solely with SDSS photometry. Also, the X 2 fitting of the SEDs can estimate accurate z phot within an accuracy of Δz/(1+z) ~ 0.012 if we use the 5 mediumband filters discussed in this paper.
Therefore, although previous quasar surveys are incapable of covering the redshift range between 5 and 6 due to the limitations of classic filter sets, our new technique using the medium-bands will be a powerful method for selecting high redshift quasars in this red-shift gap. Two meter class telescopes, such as the Mc-Donald 2.1-m telescope with SQUEAN, can conduct imaging follow-up observations of high redshift quasar candidates of z < 19.3 mag in these 3 medium-band filters within 15 minutes per target. For higher red-shift quasars, larger telescopes are capable of carrying out the quasar survey at z > 6 using longer bandpass medium-band filters with a high efficiency.
Acknowledgements
This work was supported by the National Research Foundation of Korea (NRF) grant, No. 2008-0060544, funded by the Korean government (MSIP). M.H. acknowledges the support from Global PH.D Fellowship Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2013H1A2A1033110). This paper includes data taken at The McDonald Observatory of The University of Texas at Austin.
References
Bertin E. 2006 Automatic Astrometric and Photometric Calibration with SCAMP ADASS XV 351 112 -
Bertin E. , Arnouts S. 1996 SExtractor: Software for source extraction A&AS 117 393 -    DOI : 10.1051/aas:1996164
Burrows A. , Sudarsky D. , Hubeny I. 2006 L and T Dwarf Models and the L to T Transition ApJ 640 1063 -    DOI : 10.1086/500293
Carilli C. L. , Wang R. , Fan X. 2010 Ionization Near Zones Associated with Quasars atz∼ 6 ApJ 714 834 -    DOI : 10.1088/0004-637X/714/1/834
Choi N. , Park W.-K. , Lee H.-I. 2015 A New AutoGuiding System for CQUEAN JKAS 48 177 -
Cool R. J. , Kochanek C. S. , Eisenstein D. J. 2006 The Discovery of Three Newz> 5 Quasars in the AGN and Galaxy Evolution Survey ApJ 132 823 -    DOI : 10.1086/505535
De Rosa G. , Decarli R. , Walter F. 2011 Evidence for Non-evolving Fe II/Mg II Ratios in Rapidly Accretingz∼ 6 QSOs ApJ 739 56 -    DOI : 10.1088/0004-637X/739/2/56
De Rosa G. , Venemans B. P. , Decarli R. 2014 Black Hole Mass Estimates and Emission-Line Roperties of a Sample of Redshiftz> 6.5 Quasars ApJ 790 145 -    DOI : 10.1088/0004-637X/790/2/145
Fan X. , Narayanan V. K. , Lupton R. H. 2001 A Survey ofz> 5.8 Quasars in the Sloan Digital Sky Survey. I. Discovery of Three New Quasars and the Spatial Density of Luminous Quasars atz∼ 6 ApJ 122 2833 -    DOI : 10.1086/324111
Fan X. , Strauss M. A. , Schneider D. P. 2003 A Survey ofz> 5.7 Quasars in the Sloan Digital Sky Survey. II. Discovery of Three Additional Quasars atz> 6 ApJ 125 1649 -    DOI : 10.1086/368246
Fan X. , Hennawi J. F. , Richards G. T. 2004 A Survey ofz> 5.7 Quasars in the Sloan Digital Sky Survey. III. Discovery of Five Additional Quasars ApJ 128 515 -    DOI : 10.1086/422434
Fan X. , Strauss M. A. , Richards G. T. 2006 A Survey ofz> 5.7 Quasars in the Sloan Digital Sky Survey. IV. Discovery of Seven Additional Quasars ApJ 131 1203 -    DOI : 10.1086/500296
Fan X. , Strauss M. A. , Becker R. H. 2006 Constraining the Evolution of the Ionizing Background and the Epoch of Reionization withz∼ 6 Quasars. II. A Sample of 19 Quasars ApJ 132 117 -    DOI : 10.1086/504836
Flesch E. W. 2015 The Half Million Quasars (HMQ) Catalogue PASA 32 e010 -    DOI : 10.1017/pasa.2015.10
Gunn J. E. , Stryker L. L. 1983 Stellar spectrophotometric atlas, wavelengths from 3130 to 10800 A ApJS 52 121 -    DOI : 10.1086/190861
Im M. , Choi C. , Kim K. 2015 Lee Sang Gak Telescope (LSGT): A Remotely Operated Robotic Telescope for Education and Research at Seoul National University JKAS 48 207 -
Ikeda H. , Nagao T. , Matsuoka K. 2012 Constraints on the Faint End of the Quasar Luminosity Function atz∼ 5 in the COSMOS Field ApJ 756 160 -    DOI : 10.1088/0004-637X/756/2/160
Jiang L. , Fan X. , Vestergaard M. 2007 Gemini Near-Infrared Spectroscopy of Luminousz∼ 6 Quasars: Chemical Abundances, Black Hole Masses, and Mg II Absorption ApJ 134 1150 -    DOI : 10.1086/520811
Jiang L. , Fan X. , Annis J. 2008 A Survey ofz∼ 6 Quasars in the Sloan Digital Sky Survey Deep Stripe. I. A Flux-Limited Sample atzAB< 21 ApJ 135 1057 -    DOI : 10.1088/0004-6256/135/3/1057
Jiang L. , Fan X. , Bian F. 2009 A Survey ofz∼ 6 Quasars in the Sloan Digital Sky Survey Deep Stripe. II. Discovery of Six Quasars atzAB>21 ApJ 138 305 -    DOI : 10.1088/0004-6256/138/1/305
Jiang L. , McGreer I. D. , Fan X. 2015 Discovery of Eightz∼ 6 Quasars in the Sloan Digital Sky Survey Overlap Regions ApJ 149 188 -    DOI : 10.1088/0004-6256/149/6/188
Juarez Y. , Maiolino R. , Mujica R. 2009 The metallicity of the most distant quasars A&A 494 L25 -    DOI : 10.1051/0004-6361:200811415
Jun H. D. , Im M. , Lee H. M. 2015 Rest-Frame Optical Spectra and Black Hole Masses of 3    DOI : 10.1088/0004-637X/806/1/109
Kim E. , Park W.-K. , Jeong H. 2011 Auto-Guiding System for CQUEAN (Camera for Quasars in Early Universe) JKAS 44 115 -
Kurk J. D. , Walter F. , Fan X. 2007 Black Hole Masses and Enrichment ofz∼ 6 SDSS Quasars ApJ 669 32 -    DOI : 10.1086/521596
Lawrence A. , Warren S. J. , Almaini O. 2007 The UKIRT Infrared Deep Sky Survey (UKIDSS) MNRAS 379 1599 -    DOI : 10.1111/j.1365-2966.2007.12040.x
Lim J. , Chang S. , Pak S. 2013 Focal Reducer for CQUEAN (Camera for QUasars in EArly uNiverse) JKAS 46 161 -
Madau P. , Ferguson H. C. , Dickinson M. E. 1996 High-Redshift Galaxies in the Hubble Deep Field: Colour Selection and Star Formation History toz∼ 4 MNRAS 283 1388 -    DOI : 10.1093/mnras/283.4.1388
Mahabal A. , Stern D. , Bogosavljević M. , Djorgovski S. G. , Thompson D. 2005 Discovery of an Optically Faint Quasar atz= 5.70 and Implications for the Faint Endof the Quasar Luminosity Function ApJ 634 L9 -    DOI : 10.1086/498847
Matute I. , Masegosa J. , Márquez I. 2013 The ALHAMBRA Survey: Discovery of a Faint QSO atz= 5.41 A&A 557 A78 -    DOI : 10.1051/0004-6361/201321920
McGreer I. D. , Mesinger A. , Fan X. 2011 The First (nearly) Model-Independent Constraint on the Neutral Hydrogen Fraction atz∼ 6 MNRAS 415 3237 -    DOI : 10.1111/j.1365-2966.2011.18935.x
McGreer I. D. , Jiang L. , Fan X. 2013 Thez= 5 Quasar Luminosity Function from SDSS Stripe 82 ApJ 768 105 -    DOI : 10.1088/0004-637X/768/2/105
Moles M. , Benítez N. , Aguerri J. A. L. 2008 The Alhambra Survey: a Large Area Multimedium-Band Optical and Near-Infrared Photometric Survey ApJ 136 1325 -    DOI : 10.1088/0004-6256/136/3/1325
Mortlock D. J. , Patel M. , Warren S. J. 2009 Discovery of a Redshift 6.13 Quasar in the UKIRT Infrared Deep Sky Survey A&A 505 97 -    DOI : 10.1051/0004-6361/200811161
Mortlock D. J. , Warren S. J. , Venemans B. P. 2011 A Luminous Quasar at a Redshift ofz= 7.085 Nature 474 616 -    DOI : 10.1038/nature10159
Oke J. B. 1990 Faint Spectrophotometric Standard Stars ApJ 99 1621 -    DOI : 10.1086/115444
Park W.-K. , Pak S. , Im M. 2012 Camera for Quasars in Early Universe (CQUEAN) PASP 124 839 -    DOI : 10.1086/667390
Richards G. T. , Fan X. , Newberg H. J. 2002 Spectroscopic Target Selection in the Sloan Digital Sky Survey: The Quasar Sample ApJ 123 2945 -    DOI : 10.1086/340187
Richards G. T. , Myers A. D. , Gray A. G. 2009 Efficient Photometric Selection of Quasars from the Sloan Digital Sky Survey. II. 1,000,000 Quasars from Data Release 6 ApJS 180 67 -    DOI : 10.1088/0067-0049/180/1/67
Richards G. T. , Myers A. D. , Peters C. M. 2015 Bayesian High-Redshift Quasar Classification from Optical and Mid-IR Photometry, arXiv:1507.07788
Schneider D. P. , Fan X. , Strauss M. A. 2001 High-Redshift Quasars Found in Sloan Digital Sky Survey Commissioning Data. V. Hobby-Eberly Telescope Observations ApJ 121 1232 -    DOI : 10.1086/319422
Sharp R. G. , McMahon R. G. , Irwin M. J. , Hodgkin S. T. 2001 First Results from the Isaac Newton Telescope Wide Angle Survey: thez> 5 Quasar Survey MNRAS 326 L45 -    DOI : 10.1111/j.1365-2966.2001.04845.x
Shim H. , Im M. , Choi P. , Yan L. , Storrie-Lombardi L. 2007 Massive Lyman Break Galaxies atz∼ 3 in the Spitzer Extragalactic First Look Survey ApJ 669 749 -    DOI : 10.1086/522105
Vanden Berk D. E. , Richards G. T. , Bauer A. 2001 Composite Quasar Spectra from the Sloan Digital Sky Survey ApJ 122 549 -    DOI : 10.1086/321167
Willott C. J. , Delorme P. , Omont A. 2007 Four Quasars above Redshift 6 Discovered by the Canada-France High-z Quasar Survey ApJ 134 2435 -    DOI : 10.1086/522962
Willott C. J. , Delorme P. , Reylé C. 2009 Six More Quasars at Redshift 6 Discovered by the Canada-France High-z Quasar Survey ApJ 137 3541 -    DOI : 10.1088/0004-6256/137/3/3541
Willott C. J. , Delorme P. , Reylé C. 2010 The Canada-France High-z Quasar Survey: Nine New Quasars and the Luminosity Function at Redshift 6 ApJ 139 906 -    DOI : 10.1088/0004-6256/139/3/906
Wu X.-B. , Jia Z. 2010 Quasar Candidate Selection and Photometric Redshift Estimation Based on SDSS and UKIDSS Data MNRAS 406 1583 -
Yi W. , Wu X. , Wang F. 2015 Discovery of Two Broad Absorption Line Quasars at Redshift about 4.75 Using the Lijiang 2.4 m telescope Science China Physics, Mechanics, and Astronomy 58 5685 -
Zheng W. , Tsvetanov Z. I. , Schneider D. P. 2000 Five High-Redshift Quasars Discovered in Commissioning Imaging Data of the Sloan Digital Sky Survey ApJ 120 1607 -    DOI : 10.1086/301570