Newly Recognized QSO/Galaxy Pairs at Small Impact Parameters for Low Redshift Galaxies
Jean M. Quashnock1, S. Kuttruff1, M. Bishof2, D.G. York2, M. Subbarao2, G.T. Richards3, D.E. Vanden Berk4, P.
Hall5, D. Bowen6, T. Heckman7, P. Khare8, V. Kulkarni9, J. Meiring9, M. Strauss6
1Carthage
College, 2University of Chicago, 3Drexel University, 4Pennsylvania State University, 5York University, Canada, 6Princeton University, 7Johns Hopkins University, 8Utkal University, India, 9University of South
Carolina
ABSTRACT
SDSS J101015.74-010038.0
SDSS 101105.67+061917.2
METHOD
A search for emission lines in foreground galaxies, in QSO spectra (zgal << zQSO) of
the SDSS data release 5, reveals 21 examples of QSOs shining through low redshift,
foreground galaxies at low impact parameters (<10 kpc). About 74,000 QSO spectra were
examined, by searching for narrow H! emission lines at z < 0.295, at a flux level greater
than 5 x 10-17 ergs/cm2·sec, then confirming that other expected emission lines of the H II
regions in the galaxy are detected. The measurements show that the galaxies have
abundances of solar or above and star formation rates between 10 and 1000 solar masses
per year. The galaxies were deblended from the QSOs to get colors and shapes. For cases
that allow the galaxy and the QSO to be deblended, the galaxies are blue [1.0 < (u-r) < 1.9].
Extinction through the galaxies is determined from the color excesses of the QSOs. The color
excesses range from 0 to 0.25 magnitudes. These extinction values are compared with the
flux ratios of H! and H!, which reflect the extinction for an undetermined fraction of the
sightline through each galaxy. Measurement or limits on galactic, interstellar Ca II and Na I
absorption are given, from the QSO spectrum. The technique used is compared with other
ways of finding QSO/galaxy pairs, for the purpose of designing an experiment to characterize
the absorption properties of gas in galaxies at distances from the galaxy centers < 10 kpc.
•  QSO DR5 catalogue [Schneider et al. 2007] inspected for narrow H!
emission peaks within the 3” bound of SDSS spectroscopic fibers
H! line flux > 5 x 10-17 ergs cm-2 s-1
•  Confirmed additional, expected galactic emissions at the same redshift
as H! to weed out false positives:
H!, H!, [O III], [O II], [N II], [S II]
•  Emission line search produced 21 examples of QSOs overlapped by
foreground low redshift galaxies (QSO/Galaxy pairs).
SDSS J001637.08+135653.4
SDSS J123844.79+105622.2
PHOTOMETRY
•  IDL and IDP3 software used to de-blend the overlapped QSO/Galaxy pairs
QSO Name
SDSS J104257.58+074850.5
SDSS J101015.74-010038.0
SDSS J101105.67+061917.2
SDSS J001637.08+135653.4
SDSS J123844.79+105622.2
QSO z
2.665
0.228
1.946
2.564
1.305
Galaxy z
0.03321
0.02115
0.08778
0.09086
0.11823
Offset ( '' )
2.32
2.88
1.11
1.30
0.52
Offset ( kpc )
1.49
1.20
1.77
2.12
1.07
Gal. Length ( '' )
14.3
11.5
17.7
10.8
9.7
Gal. Width ( '' )
13.6
9.1
3.5
9.4
7.8
Table 1: Redshifts and Physical Characteristics of QSO/Galaxy Pairs.
•  Color magnitudes for each galaxy obtained by subtracting an adjusted PSF
to remove the paired QSO
•  Color magnitudes for each QSO obtained by measuring the magnitudes of
the fitted PSF
MEASURED PROPERTIES
  QSO g and i-band magnitudes
  Galaxy u and r-band magnitudes
  QSO !(g-i) [observer-frame color excess]
Results shown and ordered for the 5 most visibly apparent and separable Pairs.
  QSO E(B-V)g-i
(Order descending from best Pair)
[absorber-frame color excess; extinction measure]
  Star Formation Rate
  H!/H! Flux Ratio
QSO Name
SDSS J104257.58+074850.5
SDSS J101015.74-010038.0
SDSS J101105.67+061917.2
SDSS J001637.08+135653.4
SDSS J123844.79+105622.2
QSO i
19.07
19.03
18.95
19.66
18.91
QSO g
19.24
20.07
19.73
19.98
19.55
∆(g-i)
-0.09
0.28
0.54
0.06
0.36
E(B-V)g-i
-0.01
0.15
0.10
0.01
0.09
H /H
3.03
10.13
6.84
4.36
4.25
Galaxy r
17.91
17.91
18.23
17.38
18.42
Galaxy (u-r)
1.58
1.47
1.2
1.49
1.48
SFR
123.70
1.91
129.51
941.69
42.79
 QSO/Galaxy centroid offset [impact parameter]
  Galactic length, width, and orientation
QSOALS Expectations
•  Each QSO/Galaxy pair spectrum was searched for expected
absorption features due to the foreground galaxy.
•  Ca II and Na I lines were identified and measured for equivalent
widths and errors using IRAF.
Table 2: Photometric Data for the 5 best QSO/Galaxy Pairs
•  Table 3 shows the relevant absorption features for best pairs.
Figure 1: The SDSS spectrum of QJ1042+0748, a z=2.665 QSO with a
superimposed spectrum of a (narrow-line) galaxy from an object that
falls in the SDSS fiber.
QSO Name
SDSS J104257.58+074850.5
SDSS J101015.74-010038.0
SDSS J101105.67+061917.2
SDSS J001637.08+135653.4
SDSS J123844.79+105622.2
Ca Abs.
(3933)
4063.6
4016.2
4280.6
4291.1
4399.6
Ca II K
Width / (error)
N/A (0.4)
N/A (0.4)
1.2 (0.2)
3.8 (0.4)
0.8 (0.2)
Ca Abs.
(3969)
4100.8
4052.9
4318.4
4326.9
4440.8
Ca II H
Width
N/A
N/A
0.5
7.3
3.6
Na Abs.
(5890)
6085.6
6014.5
6408.7
6428.3
6588.8
Na I D2
Width / (error)
N/A (0.2)
N/A (0.3)
1.0 (0.2)
0.6 (0.2)
2.2 (0.3)
Table 3: Measured Absorption Features in QSO/Galaxy Pairs.
Equivalent widths were measured with the original SDSS spectra, using IRAF.
Errors were measured in the local continuum. The same errors apply to the two Ca
II lines. Likewise, the same errors apply to the two Na I lines.
Na Abs.
(5896)
6091.8
6020.7
6415.2
6434.6
6597.6
Na I D1
Width
N/A
N/A
0.2
0.6
1.4
CONCLUSIONS
•  21 QSO/Galaxy pairs discovered via narrow galactic H! emission lines
superimposed on QSO spectra (H! flux > 5 x 10-17 ergs cm-2 s-1) and
verified by additional H!, [O III], [O II], [N II], and [S II] emission
lines.
•  Impact parameter offsets are low (<10 kpc): galaxies directly overlap
background QSOs.
•  Measured Star Formation Rates range from ~ 10 to 1000 Msun/yr.
•  Color excesses, E(B-V)g-i , vary from 0 to .25 magnitudes.
•  H!/H! Flux Ratios range from 2.1 - 11.7.
References
Figure 2: Composite SDSS image of QJ1042+0748 (the blue object just
above center).
The QSO is at z=2.665. Emission lines (Figure 1) from the
spectrum of the overlaying galaxy are at z=0.03321.
Schneider, D. P., et al. 2007, AJ, 134, 102 (DR5 QSO catalog)
York, D.G., Bishof, M., Kuttruff, S. et al. 2008 (in preparation) and references therein
•  Photometric de-blending reveals all galaxies are late-type and blue:
(1.0<(u-r)<1.9)
•  Ca II and Na I absorption features from foreground galaxy usually
found in SDSS spectrum when the QSO is bright (i <~ 19).
•  Ongoing goal is to characterize absorption properties of galactic gas at
distances < 10 kpc from galaxy center.