1. The document describes a new method for selecting dust-reddened quasars that are radio quiet using data from the Wide-Field Infrared Survey Explorer (WISE). The selection process involves identifying bright objects in WISE that have color ratios consistent with quasars, and then visually inspecting archival images to identify point sources. 2. This new selection method identified 12 potential radio-quiet, dust-reddened quasars. Optical and infrared spectra were obtained for these objects using the Keck and NASA IRTF telescopes. Redshifts were calculated by identifying emission lines and their wavelengths. 3. BPT diagrams using emission line ratios identified 5 of the 8 objects as quasars, providing a sample of

1. New Selection Criteria for Dust-Reddened Quasars
Milena Crnogorˇcevi´c*, Henry Daniels-Koch**, and Professor Eilat Glikman*
*Middlebury College
**Bowdoin College
1. Introduction
A
S two or more galaxies merge to create one massive galaxy,
their black holes combine to form a supermassive black hole.
These mergers are followed by a period of intense star forma-
tion in which large amount of gas and dust are released. Super-
massive black holes actively accrete gas from their surroundings
to form accretion disks, the defining characteristic of an Active
Galactic Nuclei (AGN). Quasars, shown in Fig 1, are the most
luminous type of AGN.
Figure 1: Anatomy of Quasar. Gases accrete around a black
hole in the accretion disc as radio jets of light are shot out of
both ends of a radio-loud quasar. Radiation from the quasar can
be emitted by fast moving clouds causing broad-line emissions
or by slower moving clouds causing narrow-line emissions. Dust
around the quasar (not shown) absorbs blue light, reddening the
image of the quasar.
The accretion in a quasar excites the gas particles, causing
quasars to radiate at nearly all wavelengths. However, the gas
particles surrounding a quasar absorb and scatter a significant
amount of the blue light emitted by that quasar. This results in
a decrease of the observed luminosity and the reddening of the
source in the optical region. Initially, it was thought all quasars
emit light in the radio region of the EM spectrum, allowing them
to be easily identified by radio telescopes. However, it was shown
that only about 10% of all quasars emit in the radio region (Keller-
mann et al. 1989). Glikman et al. (2007, 2012) showed that
about 20% of all radio-loud quasars are reddened. It remains un-
known if radio-quiet, reddened quasars constitute 20% of radio-
quiet quasars. In order to decouple reddened quasars and radio
loud quasars, we need to utilize methods of selecting radio-quiet,
but reddened quasars.
2. Selection Process
Using recently released Infrared data from the Wide Field In-
frared Survey Explorer (WISE), we utilize new selection tech-
niques from Lacy et al.(2004, 2007) and Stern et al. (2005)
for selecting radio quiet, highly reddened quasars. As shown
in our selection criteria flow chart (Fig 2), we select bright
objects from WISE in Stripe 82 that have color-to-color ra-
tios specific to quasars shown in Fig 3. We examine
archival images of these objects in visible wavelengths from
the SDSS database and dispose of images that do not look
like point sources giving us a final list of 12 quasars.
Select WISE objects in Stripe 82
-50 < RA < 59, -1.25 < DEC < 1.25
(3428 objects)
Select brightest objects in WISE
W4 ≤ 6.6mag
Select objects using color-to-color
ratios: 2MASS[J − K] > 0.8,
W1−W2 > 0.7
W2−W3 > 2, W3−W4 > 1.9
Select brightest objects in 2MASS
K≤15.1 mag
Select objects without spetra in SDSS
48 objects
Remove artifacts by
visual inspection (12 objects)
Figure 2: Quasar Selection Process Flow Chart. Utilizes criteria
from WISE, 2MASS, and SDSS to select objects that are bright,
have color-to-color ratios indicative of quasars, and do not con-
tain spectra in SDSS.
Figure 3: Wright et al. (2010). Color-to-color ratios of astro-
physical objects with infrared emissions. Red dots are the WISE
colors of previously selected radio-loud, dust reddened quasars.
Our selection criteria for radio-quiet, reddened quasars is based
on color to color of QSO region shown.
3. Data Reduction
The optical data used in this research was obtained from the
world's largest optical telescope at Keck Observatory and the in-
frared observations were conducted via NASA's IRTF telescope.
The raw data from Keck consists of a table of fluxes at specified
wavelengths in the optical spectrum. First, we reduce the data by
subtracting background radiation from the sky and remove any
additive effects from the imperfections of the instruments. Next,
we extract the spectra from the reduced image. Finally, we com-
bine the optical and infrared regions to produce spectra, some of
which are shown in Fig 4.
4. Redshift Calculation and Gaussian Fitting
The light from the objects we observed takes between 1 and 8
billion light years to travel from the source to the observer, de-
pending on their distance. While the light is travelling to Earth,
the expansion of the universe causes a redshift in the emit-
ted wavelengths. Thus, emission lines corresponding to certain
gases coming from these sources will be shifted to higher wave-
lengths compared to the emission lines recorded in a laboratory.
We identify wavelengths of peaks in emission spectra and match
them to the wavelengths of emissions from gases found in a lab-
oratory. We calculate the redshift z by using the following expres-
sion:
1 + z =
λobs
λemit
,
where λobs is the observed wavelength of emission line from
quasar, and λemit is the emitted wavelength observed in
a laboratory. Fig 4 shows a plot of six emission spec-
tra with their corresponding redshifts. Next, we deter-
mine fluxes by fitting Gaussian curves to emission spec-
tra. An example of such fitting is shown in Fig 5.
Figure 4: Combined optical and infrared spectra of 6 quasar can-
didates. Peaks correspond to emissions by gases around the
quasar. By matching peaks in the spectra with emission wave-
lengths in labs on Earth, we calculate redshifts.
Figure 5: Gaussian Fits. We fit Gaussian curves to peaks in or-
der to find the total flux emitted by a specific gas. Fluxes were
used to create BPT diagrams.
5. BPT Diagrams
BPT diagrams are used to classify extragalactic objects based
on various emission-line intensity ratios according to the principal
excitation mechanism. We use the fluxes found from Gaussian
curves to calculate the ratios and plot the results shown in Fig 6.
Object log(
O(III)
Hβ ) log(
N(II)
Hα ) log(
S(II)
Hα ) log(
O(I)
Hα ) Type (I) Type (II) Type (III)
2152 -0.73 -1.38 -2.48 NaN starb starb AGN
2057 1.03 -0.60 NaN -0.13 AGN AGN AGN
2355 0.35 -0.69 -0.98 -1.86 starb starb starb
0103 1.12 NaN NaN NaN - - -
2054 0.73 -0.12 -0.13 -0.57 AGN AGN AGN
0306 1.75 -0.63 -0.47 0.78 AGN AGN AGN
2252 0.17 -0.19 -0.33 -0.68 starb starb starb
2246 0.30 -0.10 -0.48 -1.33 AGN starb starb
0349 1.41 0.02 -0.37 -0.87 AGN AGN AGN
Figure 6: BPT Table and Diagrams. We found flux ratios of O(III)
Hβ ,
N(II)
Hα , S(II)
Hα , and O(I)
Hα shown in the table above. BPT diagrams show
that objects with large ratios (above and to the right of the lines
shown) are quasars. We conclude that 5 out of 8 plottable objects
are quasars.
6. Conclusion
In combination with the X-ray observations our sample of lumi-
nous obscured quasars is providing, for the first time, a chance
to study the most extreme quasars that have been missed by
traditional quasar selection methods. This small sample of low
redshift, yet very luminous obscured quasars is an ideal sample
for follow up studies with high resolution imaging with the Hubble
Space Telescope to look for direct evidence of mergers as has
been done for the radio-selected red quasars at higher redshifts.
References
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1989, AJ, 98, 1195
[4] Lacy, M. et al. 2007, ApJS, 133, 186
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Spring Symposium 2016, Middlebury College