J/AJ/157/99 Photometric redshifts of emission-line galaxies (Lesser+, 2019)
Photometric redshifts of emission-line galaxies using ramp filters.
Lesser R.W., Moody J.W., Steele J., Bohman J., McNeff M., Joner M.D.,
Barnes J.
<Astron. J., 157, 99 (2019)>
=2019AJ....157...99L 2019AJ....157...99L (SIMBAD/NED BibCode)
ADC_Keywords: Galaxies, nearby ; Galaxies, photometry ; Galaxies, Seyfert ;
Redshifts ; Equivalent widths
Keywords: large-scale structure of universe - techniques: photometric -
techniques: radial velocities
Abstract:
Broadband photometric redshifts are routinely obtained for galaxies to
estimate their distances. While effective for many uses, the common
resolution in z of 0.01-0.02 is too coarse for detailed large-scale
structure mapping, particularly in low-density volumes where the galaxy
distribution is least understood. To map galaxies in these low-density
volumes, and noting that the percentage of galaxies having emission tends
to rise as number density decreases, we have designed a filter system
to photometrically measure the redshifts of galaxies with emission.
The system consists of two "ramp" filters that cover a common wavelength
range with transmission curves sloping from blue to red and from red
to blue respectively. This causes the intensity of the image through
either filter to be a function of the wavelength of the emission line.
A third filter with a bandpass to the side is used to measure and remove
the continuum. We have obtained a set of such filters that are tuned
for isolating Hα in the redshift range of 3000-9000 km/s. Simulated
photometry, applied to spectra of 197 emission-line galaxies from the
SDSS, shows the accuracy of the method to be between 250 and 620 km/s,
depending on line strength. Actual photometry of a sample of 16 active
galaxies measured their redshifts with an accuracy of 573 km/s. This
is approximately an order of magnitude more accurate than broadband
photometric redshifts. We discuss the errors inherent in this method
and present ways to modify the filter set to further improve accuracy.
Description:
We designed two filters with one-sided overlapping wings that we refer
to as "ramps". Since we want to map the nearby galaxy distribution, the
filter ramps were chosen to span 655-685 nm, which isolates Hα
for 0<cz<12000 km/s. Ideally, the red-sloping (RS) filter ramp would slope
from 0% transmission at 655 nm linearly up to 100% transmission at 685 nm,
then drop immediately back to 0%. An ideal blue-sloping (BS) filter ramp
would have a mirror image slope in the opposite sense between the same
wavelength limits.
To quantify redshift accuracy, we first obtained 197 spectra from
SDSS DR7 (Cat. II/294). We then created synthetic photometric indices
from these spectra using our filter profiles and CCD response curve.
The results are given in Table 1.
In addition to the SDSS data, we observed 16 nearby active galaxies with
the BYU West Mountain Observatory 0.9 m telescope in 2015 and 2016.
Although a small number, it is still sufficient to demonstrate how well
this technique works with actual data. These data are plotted as red
diamonds in Figure 4, and are listed in Table 2.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 82 197 SDSS data used to estimate redshift accuracy
table2.dat 42 16 Imaged active galaxies
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See also:
VII/250 : The 2dF Galaxy Redshift Survey (2dFGRS) (2dFGRS Team, 1998-2003)
II/294 : The SDSS Photometric Catalog, Release 7 (Adelman-McCarthy+, 2009)
J/ApJS/201/31 : Emission-line galaxies from SDSS. I. (Ge+, 2012)
J/MNRAS/421/1043 : Emission-line galaxies from SDSS DR7 (Shirazi+, 2012)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 19 A19 --- SDSS SDSS identifier (JHHMMSS.ss+DDMMSS.s)
21- 32 F12.8 deg RAdeg Right Ascension in decimal degrees (J2000)
34- 45 F12.9 deg DEdeg Declination in decimal degrees (J2000)
47- 54 F8.6 --- z [0.01001/0.02962] SDSS redshift in z
56- 59 I4 km/s cz [3004/8886] SDSS redshift in cz
61- 67 F7.5 --- BS/RS [0.082/1.405] Blue-sloping (BS) to red-sloping
(RS) flux ratio
69- 71 I3 0.1nm EWHa [5/292] Hα equivalent width (Å)
73- 76 I4 km/s Dcz [3098/8527] cz derived from BS/RS ratio
78- 82 I5 km/s Deltacz [-1744/1284] SDSS cz minus derived cz, Δcz
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Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
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1- 9 A9 --- Galaxy Galaxy identifier
11- 15 A5 --- Type Seyfert type (1)
17- 21 F5.2 mag Bmag [13.1/15.6] B-band magnitude
23- 26 I4 km/s cz [3765/8564] Published cz from Simbad
28- 32 F5.3 --- BS/RS [0.317/1.243] Blue-sloping (BS) to red-sloping
(RS) flux ratio
34- 37 I4 km/s Dcz [4047/8297] cz derived from BS/RS ratio
39- 42 I4 km/s Deltacz [-951/754] Published cz minus derived cz,
Δcz
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Note (1): Seyfert type as follows:
Sey 1 = Type I Seyfert galaxy;
Sey 2 = Type II Seyfert galaxy;
LINER = Low-ionization nuclear emission region.
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History:
From electronic version of the journal
(End) Prepared by [AAS], Tiphaine Pouvreau [CDS] 03-Jun-2019