---
abstract: 'We quadruple the number of quasars known behind the Large Magellanic Cloud (LMC) from 55 (42 in the LMC fields of the third phase of the Optical Gravitational Lensing Experiment (OGLE)) to 200 by spectroscopically confirming [169 ]{}([144 ]{}new) quasars from a sample of [845 ]{}observed candidates in four $\sim$3 deg$^2$ Anglo-Australian Telescope/AAOmega fields south of the LMC center. The candidates were selected based on their [*Spitzer*]{} mid-infrared colors, X-ray emission, and/or optical variability properties in the database of the OGLE microlensing survey. The contaminating sources can be divided into 115 young stellar objects (YSOs), 17 planetary nebulae (PNe), 39 Be and 24 blue stars, 68 red stars, and 12 objects classed as either YSO/PN or blue star/YSO. There are also 402 targets with either featureless spectra or too low signal-to-noise ratio for source classification. Our quasar sample is 50% (30%) complete at $I=18.6$ mag (19.3 mag). The newly discovered active galactic nuclei (AGNs) provide many additional reference points for proper motion studies of the LMC, and the sample includes 10 bright AGNs ($I<18$ mag) potentially suitable for absorption line studies. Their primary use, however, is for detailed studies of quasar variability, as they all have long-term, high cadence, continuously growing light curves from the microlensing surveys of the LMC. Completing the existing Magellanic Quasars Survey fields in the LMC and Small Magellanic Cloud should yield a sample of $\sim$700 well-monitored AGNs, and expanding it to the larger regions covered by the OGLE-IV survey should yield a sample of $\sim$3600 AGNs.'
author:
- 'Szymon Koz[ł]{}owski, C. S. Kochanek, A. M. Jacyszyn, A. Udalski, M. K. Szyma[ń]{}ski, R. Poleski, M. Kubiak, I. Soszy[ń]{}ski, G. Pietrzy[ń]{}ski, [Ł]{}. Wyrzykowski, K. Ulaczyk, and P. Pietrukowicz'
title: 'The Magellanic Quasars Survey. II. Confirmation of [144 ]{}New Active Galactic Nuclei Behind the Southern Edge of the Large Magellanic Cloud'
---

Introduction
============

[@2009ApJ...698..895K], [@2010ApJ...708..927K], and [@2010ApJ...721.1014M] introduced, developed, and applied a new approach to quantitatively modeling the variability of individual quasars, describing the light curves as damped random walks (DRW) characterized by two parameters, a timescale and an amplitude. These parameters are then found to be correlated with the physical properties of the quasar – wavelength, luminosity, and black hole mass. Unfortunately, accurately measuring the parameters of individual quasars requires long ($\gtrsim$decade) well-sampled light curves that are generally not available (see @2011ApJ...728...26M). While this may ultimately be solved by projects such as the Panoramic Survey Telescope and Rapid Response System (PanSTARRS; @2002SPIE.4836..154K) and the Large Synoptic Survey Telescope (LSST; @2008arXiv0805.2366I), their cadences are not ideal and it still requires a decade of operations to accumulate the light curves. This makes it challenging to expand on this promising new approach to probing quasar physics.

There are, however, quasars[^1] with almost two decades of well-sampled light curves – the quasars lying in the regions surveyed by microlensing projects such as the Optical Gravitational Lensing Experiment[^2] (OGLE; e.g., @2008AcA....58...69U), MACHO (e.g., @2000ApJ...542..281A), and the Microlensing Observations in Astrophysics (MOA; e.g., @2002MNRAS.330..137N). For example, the combined phases of the OGLE survey (OGLE-II to OGLE-IV) span 15 years with up to 1000 epochs in $I$-band and a few dozen in $V$-band for approximately 40 million sources with $I<21$ mag in the Magellanic Clouds, and the light curves continue to be extended by the OGLE-IV survey.

The challenge is identifying the rare quasars in these dense stellar fields. A combination of serendipity (e.g., @1994PASP..106..843S [@2003IAUC.8258....1C]), X-ray (@1997AJ....114.2353C [@2002ApJ...569L..15D; @2003AJ....126..734D]), and crude variability measures () had succeeded in identifying only 55 and 28 quasars in the LMC and SMC, respectively, as of 2010. In [@2009ApJ...701..508K], we showed that the mid-IR selection techniques developed for extragalactic surveys (@2005ApJ...631..163S) could also be applied to dense stellar regions, albeit with some contamination from the relatively rare dusty stars (e.g., young stellar objects (YSOs), planetary nebulae (PNe), etc.). In [@2010ApJ...708..927K], we showed that the DRW variability parameters of quasars differed from those of variable stars, providing a more quantitative approach to variability-selecting quasars that has been expanded on in [@2011ApJ...728...26M] and [@2011AJ....141...93B]. In a single test observation under poor conditions, [@2011ApJS..194...22K] doubled the number of known quasars behind the SMC from 28 to 57.

Apart from studying variability, quasars in these fields are important tools for deriving more precise proper motions of the LMC, SMC and, potentially, the Galactic Bulge using the [*Hubble Space Telescope (HST)*]{} or, in the future, the [*James Webb Space Telescope (JWST)*]{}. [@2006ApJ...638..772K] and [@2008AJ....135.1024P] based their LMC proper motions on 21 quasars. Further improvements require better measuring the internal motions of the Clouds, and this requires larger numbers of quasars well-distributed over the LMC disk. Based on [@2008AJ....135.1024P], the newly confirmed LMC quasars we discuss here could be used to reduce the proper motion uncertainties by a factor of three, since our new quasars are distributed over a large $6 \times 4$ deg$^2$ area of the LMC disk. We will also identify any bright quasars suitable for absorption studies of the LMC’s interstellar medium (ISM), as we expect 1, 10, and 74 quasars brighter than $I<17$, 18, and 19 mag in the four observed fields.

The goal of our Magellanic Quasars Survey (MQS) is to identify and characterize all these quasars, where we estimate a final sample of roughly $700$ quasars in a complete survey of the OGLE-III project regions. Spectroscopy is essential not only for confirmation of the candidates but also to determine the luminosity, rest-wavelength and black hole masses (through the emission line widths) of the quasars, so that the variability parameters can be related to other physical properties. Relatively large numbers of quasars will be needed to explore this (minimally) three dimensional parameter space quantitatively. While the sample will be smaller than the $\sim 9000$ quasars we considered in [@2010ApJ...721.1014M], the longer, more densely sampled light curves should let us examine the physical correlations with less “blurring” from the noisy parameter estimates obtained from the sparsely sampled Sloan Digital Sky Survey (SDSS) light curves, making it far easier to determine the intrinsic parameter distributions. In this paper, we report our preliminary results for the LMC, sadly reduced in scope by hurricane Yasi. In Section \[sec:agnsel\], we describe the selection methods leading to a sample of 2434 quasar candidates behind LMC. Section \[sec:data\] describes the observation plan, the observations, and the data analysis. Section \[sec:newquasars\] introduces the new quasars and Section \[sec:contamination\] discusses the contaminating sources. We discuss the results in Section \[sec:discussion\] and summarize the paper in Section \[sec:summary\].

AGN candidate selection {#sec:agnsel}
=======================

We selected targets based on their mid-IR, X-ray and/or photometric variability characteristics and we describe each method in detail below. The observations were designed for AAOmega, a 400-fiber spectrograph mounted on the 3.9 m Anglo-Australian Telescope (AAT; @2006SPIE.6269E..14S). The AAOmega instrument has many more fibers than the expected number of quasars in its field of view, so we kept candidates satisfying any of the selection criteria, thereby aiming for higher completeness at the price of higher contamination. There is no point in having empty fibers even if it means observing more contaminating stars.

Mid-IR-selected AGN Candidates {#sec:mIRsel}
------------------------------

[@2005ApJ...631..163S] showed that active galactic nuclei (AGNs) have redder mid-IR colors than galaxies and stars, and that they form a distinctive group in the mid-IR color-color plane (see @2004ApJS..154..166L and @2004ApJS..154...48E, and also @2010ApJ...713..970A, @2008ApJ...679.1040G, and @2010ApJ...716..530K for the mid-IR colors of X-ray and variability selected AGNs). The red mid-IR colors of a typical AGN are not due to dust emission, so mid-IR-selected AGNs are generally normal $z\gtorder 1$ broad-line quasars (see @2010ApJ...713..970A, @2011ApJ...728...56A). There is also no significant tendency for them to be Type II quasars. This is particularly true with the addition of a magnitude limit for the optical spectroscopy, since at most redshifts the emission from the disk is required to render the source bright enough to be observed.

In [@2009ApJ...701..508K], we used this approach to identify quasars in the LMC and SMC from the [*Spitzer/IRAC*]{} four-band photometry (i.e., 3.6, 4.5, 5.8, and 8.0$\micron$) of the Surveying the Agents of a Galaxy’s Evolution project (SAGE; @2006AJ....132.2268M). The 5000 AGN candidates were divided into several categories. First, all the objects must be located inside the “Stern AGN wedge” (@2005ApJ...631..163S). The locus of cool blackbodies passes through the wedge so the quasar sample is divided into group “A” if they are far from the blackbody locus, and “B” if they can be contaminated with cold stars. Second, the mid-IR color-magnitude diagram (CMD) was divided into regions heavily contaminated by YSOs and one that should principally contain quasars, based on mid-IR properties of YSOs from [@2008AJ....136...18W] and the mid-IR data from the 9 deg$^2$ of the Spitzer Deep Wide-Field Survey (SDWFS; @2009ApJ...701..428A) of spectroscopically confirmed quasars from the AGN and Galaxy Evolution Survey (AGES; @2011arXiv1110.4371K). Finally, the candidates were classed as “a” if they had the optical-to-mid-IR colors of quasars in AGES and “b” if they did not.

The most interesting candidates are classed as QSO-Aa (quasar region, away from blackbody locus with QSO optical-to-mid-IR colors), but given the large number of AAOmega fibers, we targeted all classes of objects, excluding only those marked as “outside” (of the OGLE-III fields) or “faint” (not detected on the OGLE-III template images).

X-Ray-selected AGN Candidates
-----------------------------

We searched for the OGLE-III counterparts to the 758 X-ray sources distributed over $10\times10$ deg$^2$ of the LMC from . The typical positional uncertainly for these X-ray sources is several arcseconds, so we cannot effectively target them without some additional selection criterion given the stellar densities. As a second criterion we chose the most optically variable source within $3\sigma$ of the X-ray position. In this case, variability simply means that the light curve is inconsistent with a constant flux, without any of the more quantitative variability selection criteria we discuss in the next section. This led to a sample of 205 X-ray candidates, of which 160 were also mid-IR-selected candidates and 99 satisfy the variability cuts presented in Section \[sec:varsel\].

Variability-selected AGN Candidates {#sec:varsel}
-----------------------------------

The OGLE-III database (@2008AcA....58...69U [@2008AcA....58...89U]) contains the data for 9 years (2001–2009) of continuously monitoring $\sim$35 million objects toward the LMC. We used this database to search for variable objects that are likely quasars based on the DRW model of their light curves (e.g., @2009ApJ...698..895K [@2010ApJ...708..927K; @2010ApJ...721.1014M]).

We prepared and analyzed the light curves as described in [@2010ApJ...708..927K]. We use the timescale $\tau$, scaled amplitude $\hat{\sigma}^2=2\sigma^2/\tau$, and the likelihood ratio $\ln L_{\rm best}/\ln L_{\rm noise}$ between the best fitting model and a white noise model corresponding to simply expanding the photometric errors. We also fit a power-law structure function (SF) and estimated its slope $\gamma$ between 30 days and 2 years, and an amplitude $A$ defined by the magnitude difference between the first and third quartile of the sorted light curve.

To remove unwanted variable sources, we use the following cuts: [*Cut 1.*]{} The average light curve magnitude is $I<19.5$ mag; [*Cut 2.*]{} $\ln L_{\rm best}>\ln L_{\rm noise}+2$; [*Cut 3.*]{} The SF slope $0.1<\gamma<0.9$ (e.g., see @2010ApJ...714.1194S); [*Cut 4.*]{} The $I$-band amplitude $A<0.4$ mag (this removes large amplitude variable stars); [@2010ApJ...721.1014M] report that some of $\sim$9000 SDSS quasars occupy areas outside the $\tau$–$\hat{\sigma}$ cut of [@2010ApJ...708..927K], therefore we decided to loosen the cut on the $\tau$ parameter and to ignore $\hat{\sigma}$ parameter, so [*Cut 5*]{} is $1<\log(\tau)<5$. When simply applied to the OGLE light curve database, this process yields $\sim 24,000$ candidates. The vast majority are due to two known systematic issues. First, bright variable stars typically generate several fake, fainter variable stars in their wings, and these “ghost” variables show long term irregular variability. Second, there can be small magnitude shifts between seasons, and (for one field in particular) the shifts were being misinterpreted as quasar-like variability. We carried out a quick visual inspection of all candidates, but this was over kill and would certainly be automated in any subsequent analysis. We were left with 1063 variability-selected candidates.

![Venn diagram for all 2434 LMC MQS quasar candidates. The upper numbers give the number of candidates in each selection region. Priority 9 is given to AGN candidates located in the three overlapping circles (84 objects), priority 8 to objects in any two overlapping circles (715 objects), and priority 7 to sources in the non-overlapping areas (1635 objects). The lower numbers, in parenthesis, give the number of objects in the four observed fields followed by the number of spectroscopically confirmed AGN.[]{data-label="fig:VennCand"}](MQS_fig1.eps){width="8cm"}

Final AGN Candidates Sample {#sec:finalsample}
---------------------------

The total number of candidates is 2434, distributed over $\sim$30 deg$^2$ in the LMC. There are 2049 mid-IR-selected candidates and 385 non-mid-IR-selected candidates. Of the 2049 mid-IR (385 non-mid-IR) objects, 708 (355) are also variable and 160 (45) have associated X-ray emission. There are 1063 objects selected as variable, of which 708 are mid-IR sources, 99 are X-ray sources, and 84 both mid-IR and X-ray sources. Figure \[fig:VennCand\] shows how the samples overlap. Note that the limited overlap of the mid-IR and X-ray-selected AGNs is also seen in the spectroscopically confirmed samples of AGNs in the NDWFS field (e.g., @2011ApJ...728...56A). The AAOmega fiber allocation software allows to prioritize targets from 1 (the lowest priority) to 9. We gave priorities of 9, 8, and 7 to sources selected by all three methods (84 sources), sources selected by only two methods (715 sources), and sources selected by only one method (1635), respectively.

Data Preparation and Observations {#sec:data}
=================================

[lccccr]{} LMC 01 & 04:41:43 & $-$69:50:29 & 220 & $\cdots$ & $\cdots$\
LMC 02 & 04:43:19 & $-$68:19:18 & 197 & $\cdots$ & $\cdots$\
LMC 03 & 04:56:52 & $-$67:07:48 & 213 & $\cdots$ & $\cdots$\
LMC 04 & 05:00:51 & $-$70:27:49 & 221 & 41 & 1480\
LMC 05 & 05:01:42 & $-$68:49:26 & 269 & $\cdots$ & $\cdots$\
LMC 06 & 05:14:27 & $-$67:35:00 & 189 & $\cdots$ & $\cdots$\
LMC 07 & 05:19:43 & $-$69:31:07 & 307 & 36 & 5400\
LMC 08 & 05:21:54 & $-$71:02:48 & 247 & 60 & 3600\
LMC 09 & 05:32:54 & $-$68:31:01 & 230 & $\cdots$ & $\cdots$\
LMC 10 & 05:41:31 & $-$71:36:00 & 201 & 37 & 2550\
LMC 11 & 05:41:56 & $-$70:11:07 & 275 & $\cdots$ & $\cdots$\
LMC 12 & 05:52:39 & $-$68:57:51 & 183 & $\cdots$ & $\cdots$\

We are mostly interested in quasars located in the overlapping areas of the SAGE and OGLE-III surveys. We divided this LMC area into 12 fields (see Figure \[fig:MQS\_LMC\_fields\]), each $\sim$3 deg$^2$ and corresponding to the field of view of AAOmega. The basic information for each field is given in Table \[tab:MQSfields\]. The fields overlap slightly to avoid having gaps between them, so some of the quasar candidates were observed up to three times if they happened to fall into such an overlapping region. The total number of targeted objects in all 12 fields was 2678, of which several (less than 244) were targeted at least twice. On average there were 220 objects in a field, corresponding to $\sim$70 sources deg$^{-2}$.

We observed the LMC fields on 2011 February 1–3. Unfortunately, due to arrival of tropical storm Yasi, we were completely clouded out on the night of February 2 and partly on February 1 and 3. Of the 12 planned fields, we were able to execute only four (see Table \[tab:MQSfields\] and Figure \[fig:MQS\_LMC\_fields\]), and only one with the desired 1.5 hr (5400 s) exposure time. Nevertheless, the data are sufficient to confirm a large number of quasars. We observed [845 ]{}candidates of which we confirm [169 ]{}([144 ]{}new) in this paper. The yields divided by the AGN selection method are presented in Table \[tab:selectionresults\].

![image](MQS_fig2.eps){width="15cm"}

We followed the spectral identification procedures from Paper I (@2011ApJS..194...22K). To find as many quasar emission lines in a spectrum as possible, we decided to use the widest available spectral coverage of $\sim$5100Å (3750–8850Å) and a resolution of $R=1300$. Each spectrum consists of a blue (580V) and a red (385R) channel that were spliced together at 5700Å. Some of the spectra appear to have a “wide broad line” around the splicing wavelength, mimicking the MgII (2800Å) line at a redshift of $z\approx1.04$. We carefully checked all our $z\approx 1$ quasars for possible misidentifications. Our initial goal was to obtain three exposures of $1800$ s per field, resulting in a median signal-to-noise ratio (S/N) of $\sim$25 at $I=19.5$ mag (calculated from obtained spectra). To subtract the sky flux contribution from each spectrum, we initially selected $\sim$50 and finally used $\sim$25 sky fibers, each positioned to avoid stellar emission based on the OGLE-III LMC catalogs (@2008AcA....58...89U). These catalogs were also used to find $\sim$20 single and well separated bright guide stars for each field, of which 4–8 were used for guiding. The AAOmega [FLD]{} files were created with the [Configure]{} software, and the data reduction was performed using the [2dfdr]{} software (@1996ASPC..101..195T).

As in Paper I, we searched each spectrum for the common (redshifted) quasar emission lines (e.g., @2001AJ....122..549V) Ly$\alpha$ at 1216Å, H$\delta$ at 4101Å, H$\gamma$ at 4340Å, H$\beta$ at 4861Å, H$\alpha$ at 6563Å, magnesium MgII at 2800Å, carbon CIV at 1549Å and CIII\] at 1909Å, as well as the narrow forbidden lines of oxygen \[O II\] at 3727Å, \[O III\] at 4959Å or 5007Å. In general, we required the identification of two lines, except in the redshift range $0.7 < z < 1.2$, where we can only find MgII despite the broad spectral coverage.

New Quasars {#sec:newquasars}
===========

We identified [144 ]{}new quasars behind the LMC. Their basic parameters (including the identified lines) are presented in Table 3. We also confirmed the [25 ]{}known quasars that were in our target list (Table 4). Of these known quasars, we had flagged four only as probable quasars because their spectra were too noisy for reliable classification. We missed one known quasar, MQS J051140.7$-$710032.8, that had extremely low S/N in our data, although we verified its existence. These five sources are reported at the bottom of Table 4.

The newly discovered quasars cover the range of redshifts $z=0.15$–$3.35$ as shown in Figure \[fig:hist\]. The deficit at $z\approx 0.8$ is probably due to problems in convincingly identifying quasars based on only MgII in the region of the dichroic split. We show ten selected spectra of the newly confirmed AGNs in Figure \[fig:spectra1\] and also show several spectra of common contaminating sources in Figure \[fig:contamination\]. The remaining spectra are discussed in Section \[sec:contamination\], and are either not quasars or quasars with too low S/N for clear detection. As we discuss in Section \[sec:discussion\], a fair number of the fainter sources ($I \gtrsim 19$ mag) should be quasars.

![Redshift distribution of the confirmed quasars in $z=0.1$ bins.[]{data-label="fig:hist"}](MQS_fig3.eps){width="8cm"}

![image](MQS_fig4.eps){width="15cm"}

![image](MQS_fig5.eps){width="15cm"}

One target, MQS J050155.46$-$700210.1, shows emission lines corresponding to redshifts of $z=0.228$ and $0.341$, as shown in Figure \[fig:doubleAGN\]. This could be a chance coincidence or a gravitational lens. In the OGLE-III reference image (Figure \[fig:doubleAGNchart\]), we find that the AGN candidate was a faint source ($V=21.3$, $I=20.5$) in the wings of a brighter extended source. The standard OGLE catalogs may not include all extended sources, so it is likely that the faint source was mistargeted and the brighter galaxy is the AGN candidate. If we assume the bright galaxy is acting as a gravitational lens, then we estimate that it has an Einstein radius of $R_{\rm E}<0.4$ arcsec, which is very small because of the small redshift difference. Higher resolution images and spatially resolved spectra are required to clarify the nature of this source.

![Spectrum for MQS J050155.46$-$700210.1. It displays two sets of the same emission lines at two redshifts, $z=0.228$ and $z=0.341$. We did not mask the LMC’s ISM lines \[OI\] (6300Å), H$\alpha$ (6563Å), and HeI (6678Å).[]{data-label="fig:doubleAGN"}](MQS_fig6.eps){width="8cm"}

![Finding chart for MQS J050155.46$-$700210.1. The circle has a diameter of 2 arcsec, corresponding to the AAOmega fiber size. The plus symbol inside the fiber marks the targeted source, while the plus symbol outside the fiber marks the galaxy, most likely hosting an AGN. The image is $10 \times 10$ arcsec. North is up, east is to the left.[]{data-label="fig:doubleAGNchart"}](MQS_fig7.eps){width="8cm"}

Contaminants {#sec:contamination}
============

For the sources that were not quasars, we used our spectra of the known sources to build template YSO, PN, blue/Be, and red star spectra, respectively, and fit them to the individual spectra. Apart from the automatic fits, we also visually inspected each spectrum. Table 5 reports the object type for each of the sources and Figure \[fig:contamination\] shows examples of some of these sources. We briefly describe each type below.

[*Planetary nebulae.*]{} We classified sources as candidate PNe if they had roughly equally strong H$\alpha$, H$\beta$, and/or \[OIII\] emission lines along with several additional lines such as HeI at 5876Å, 6680Å and 7065Å, HeII at 4686Å, \[ArIII\] at 7136Å, \[NII\] at 6548Å and 6584Å, \[SII\] 6716Å and 6731Å, and/or \[ArIV\] at 4711Å and 4740Å (see, e.g., @1991ApJS...75..407M). Of the 17 we classified as PNe based on our spectra, 7 were classified as such in SIMBAD and 5 were classified as YSOs.

[*Young stellar objects.*]{} We classified sources as candidate YSOs if they had significantly weaker \[OIII\] and H$\beta$ lines relative to H$\alpha$ combined with relatively strong \[SII\] emission lines at 6716Å and 6731Å, \[OI\] at 6300Å, and/or \[NII\] at 6548Å and 6584Å (see, e.g., @1998AJ....115.2491K). Of the 115 objects we classified as YSOs, 18 were classified as such by SIMBAD and 1 was classified as a PN, leaving 96 new candidate YSOs. There were also four additional sources we identify as YSO/PN where we were uncertain as to the correct classification. SIMBAD has two of these four classified as YSOs.

[*Be and blue stars*]{}. Blue stars had blue spectra with strong hydrogen Balmer series absorption. They were then further classed as Be stars if they had strong hydrogen H$\alpha$ (H$\beta$) emission lines. We identified 63 blue stars of which 39 appear to be Be stars. While there is little ambiguity about the Be stars, SIMBAD labels two of the blue stars without emission lines as PNe, one as a Mira-type star and one only as a Two Micron All Sky Survey (2MASS) infrared source (@2003yCat.2246....0C). We also found eight objects that could be either blue stars or YSOs. The median OGLE-III color of these blue stars is $(V-I)\simeq0.1$ mag.

[*Red stars*]{}. These are stars with red spectra sometimes showing molecular absorption bands. We identified 68 red sources, where SIMBAD classifies 4 as infrared sources (one YSO from @2009ApJS..184..172G and three 2mass sources), 1 as a PN, 1 as a YSO, 1 as an asymptotic giant branch star, and 1 as a galaxy. The median OGLE-III color of these sources is $(V-I)\simeq1.1$ mag.

Discussion {#sec:discussion}
==========

The yield of our survey is determined by a combination of contamination and depth, where it is difficult to fully characterize the effects of depth because of the large variation in integration time created by the weather. For discussion, we simply combine the four fields (Table \[tab:selectionresults\]). We know from the surface density of candidates compared to quasars (@2011arXiv1110.4371K, @2006AJ....131.2766R), shown in Figure \[fig:cumul\], that the level of contamination is high, but much of this is by design because of the large number of available fibers.

[lccc]{} Mid-IR QSO-Aa & 577 & 136 (137$^*$) & 24\
Mid-IR QSO-Ab & 10 & 2 & 20\
Mid-IR QSO-Ba & 62 & 12 & 19\
Mid-IR QSO-Bb & 1 & 0 & 0\
Mid-IR YSO-Aa & 32 & 4 & 13\
Mid-IR YSO-Ab & 16 & 1 & 6\
Mid-IR YSO-Ba & 1 & 0 & 0\
Mid-IR YSO-Bb & 3 & 0 & 0\
Mid-IR (any) & 702 & 155 (156$^*$) & 22\
X-ray (any) & 102 & 30 & 29\
Var. (any) & 350 & 107 & 31\
X-ray + Mid-IR (any) & 77 & 25 & 32\
Mid-IR + Var. (any) & 228 & 95 & 42\
Var. + X-ray (any) & 39 & 26 & 67\
\
\
Mid-IR (only) & 431 & 57 (58$^*$) & 13\
X-ray (only) & 21 & 1 & 5\
Var. (only) & 117 & 8 & 7\
\
\
X-ray + Mid-IR (only) & 43 & 3 & 7\
Mid-IR + Var. (only) & 194 & 73 & 38\
Var. + X-ray (only) & 5 & 4 & 80\
\
\
All three & 34 & 22 & 65

In Figure \[fig:VennCand\] and Table \[tab:selectionresults\], we present our yields from this observing run, divided by the selection method. The mid-IR-selected sources were divided into several classes (QSO-Aa, QSO-Ab, etc.; see Section \[sec:mIRsel\]). As expected the purest class, QSO-Aa, has the highest confirmation rate, at 24%. Next, the two classes QSO-Ab and QSO-Ba had confirmation rates of 20% and 19%, respectively. The YSO-Aa and YSO-Ab groups have lower rates of 13% and 6%, as expected from the original division into YSO and QSO classes. Variability selection had a yield of 31% and X-ray selection 30%.

Priority 7 objects, where the source was flagged based on only one of the mid-IR, variability or X-ray criteria, had confirmation rates of 13% (57 new AGNs), 7% (8), and 5% (1), respectively. Of the priority 8 objects selected by any two methods but not the third, the combination of X-ray plus variability yielded the highest confirmation rate, at 80% (4 new AGNs), most likely due to low number statistics. The mid-IR and variability criteria had a yield of 38% (73 AGNs) and X-ray plus mid-IR criteria had a yield of only 7%, again with low number statistics. Not surprisingly, combining all three selection methods was the most efficient with a yield of 65% (22 new AGNs).

Figure \[fig:cumul\] shows the cumulative distribution of our targets and then their division into quasars, stars, and unclassified objects. We also show the cumulative distribution of $z>0.5$ quasars in AGES (@2011arXiv1110.4371K). These quasars are primarily mid-IR and X-ray selected, like much of the present sample, but with a much deeper optical spectroscopic limit. We have adjusted the AGES magnitudes for the $\Delta E(B-V)\simeq 0.14$ mag extinction difference between AGES and a typical LMC site line (@1999AcA....49..201U), although this may underestimate the correction necessary for a true background population. The completeness of our present sample relative to AGES is roughly 50% (30%) at 18.6 mag (19.3 mag). Figure \[fig:cumul\] also shows the cumulative distribution of the SDSS quasars with $0.3 < z < 2.2$ from [@2006AJ....131.2766R]. Given the change in bandpass (the $(i-I) \simeq 0.5$ mag color term varies significantly with redshift, see Figure \[fig:AGNcolor\]) and the additional line width and luminosity selection criteria in [@2006AJ....131.2766R] it is harder to make a direct comparison, but after matching as best possible, we obtain similar completeness estimates. As expected from filling the fibers with targets, most targets are peculiar stars of various types, particularly at the brighter magnitudes. Unclassifiable spectra dominate at the fainter magnitudes, and with the weather-limited integration times, there was an effective magnitude limit of roughly $19.5$–$20$ mag.

![Cumulative distributions of MQS targets. The upper solid curve shows the distribution of all the targets, which is then decomposed into stars (dashed), unclassified spectra (dotted), and quasars (solid, divided into $z<0.5$ and $z>0.5$ sources). The red solid (dashed) curves show the distribution of $z>0.5$ AGES (SDSS $0.3 < z < 2.2$) quasars for comparison. Our selection methods and bands are more comparable to AGES than to SDSS. []{data-label="fig:cumul"}](MQS_fig8.eps){width="8cm"}

![$V-I$ colors of the MQS LMC quasars (filled squares) and other confirmed LMC and SMC quasars (open circles) as a function of redshift. Overplotted is the color for a pure AGN spectrum from [@2001AJ....122..549V] convolved with the $V$ and $I$ filters (solid line). The same spectrum was also convolved with the $i$ and $I$ filters in order to convert OGLE magnitudes into SDSS $i$-band magnitudes—the $(i-I)$ color is shown as dotted line.[]{data-label="fig:AGNcolor"}](MQS_fig9.eps){width="8cm"}

Figure \[fig:AGNcolor\] shows the colors of the confirmed quasars as a function of redshift. While we imposed no optical color selection criteria, the quasars generally follow the colors of SDSS quasars except at very low $z$, where the host dominates the colors. Adding an optical color constraint has no leverage on the selection of quasars, as the colors of quasars are indistinguishable from the colors of many LMC stars.

In Figures \[fig:tau-sigma\]–\[fig:KKO9\], we examine the selection spaces used to select the candidates. First, we inspect the $\tau$–$\hat{\sigma}$ variability plane from [@2010ApJ...708..927K] and [@2010ApJ...721.1014M]. [@2010ApJ...708..927K] proposed a narrow and long trapezoidal region to select high purity sample of AGN candidates. Indeed, if we had restricted our variability selection to this region, we would have a yield of 70% at the price of a significant reduction in completeness. In fact, the distribution of the variability parameters of all quasars follows the distribution found by [@2010ApJ...721.1014M] for SDSS quasars, extending outside the [@2010ApJ...708..927K] selection boundaries, particularly for long $\tau$ where there is also increased contamination from irregularly varying stars. The left panel of Figure \[fig:tau-sigma\] shows the distribution of objects classified as non-quasars (PNe, YSOs, etc.) or where we failed to classify the spectrum in this variability plane. We note that the vast majority of classified non-quasars lie outside the [@2010ApJ...708..927K] region.

![image](MQS_fig10.eps){width="15cm"}

![image](MQS_fig11.eps){width="15cm"}

In Figure \[fig:Schmidt\], we make the same comparison between the MQS quasars and the other candidates in the $A$–$\gamma$ variability plane of [@2010ApJ...714.1194S]. We used a cut in this plane to remove more variable sources (shown as contours) other than quasars that we included as we greatly relaxed the DRW selection criteria of [@2010ApJ...708..927K] in these dense stellar fields. A fair number of common variable stars are periodic with the index of the power-law SF of $\gamma < 0.1$. On the other hand, objects with very high amplitude variations are unlikely to be quasars as quasar variability amplitudes on long time-scales are expected to be $A\approx 0.3$ mag (e.g., @2010ApJ...721.1014M). Our simple cut (solid lines) allows higher contamination than that of @2010ApJ...714.1194S [dotted lines], but also returns the higher number of confirmed quasars.

We also examined the distribution of the confirmed MQS quasars and quasar candidates relative to the mid-IR selection criteria of @2009ApJ...701..508K [Figure \[fig:KKO9\]]. The quasars lie in the proposed selection regions confirming the basic principles outlined in [@2009ApJ...701..508K]. The remaining quasar candidates (shown in the left panels) are either quasars with low S/N or contaminating sources. In the mid-IR color-color plot (top panels in Figure \[fig:KKO9\]), the confirmed quasars (shown in the right panel) and the faint candidates (gray squares in the left panel) occupy nearly identical regions, while the blue and red stars tend to clump in the lower-left region of the “AGN wedge” area. Also, YSOs tend to have a somewhat larger color spread than the more centrally clumped AGNs. In the mid-IR CMD (middle panels of Figure \[fig:KKO9\]), the confirmed quasars seem to be on average brighter than the remaining faint sources. The red stars tend to have bluer mid-IR $[3.6]-[8.0]$ colors than the other sources. In the bottom panels of Figure \[fig:KKO9\], we show the optical-to-mid-IR colors of our observed targets. As expected the vast majority of our quasars occupy the “a” (QSO-like optical-to-mid-IR colors) region, but then so do the most of other sources. The red stars again have bluer optical-to-mid-IR colors than the other sources.

![image](MQS_fig12.eps){width="15cm"}\
![image](MQS_fig13.eps){width="15cm"}\
![image](MQS_fig14.eps){width="15cm"}

As expected from earlier surveys of mid-IR-selected quasars (@2005ApJ...631..163S [@2010ApJ...713..970A]), we find that 88% of the mid-IR-selected quasars show optical broad lines and are not Type II quasars. Similarly, of the 155 confirmed, mid-IR-selected quasars, 95 (61%) are clearly variable sources.

Summary {#sec:summary}
=======

We observed [845 ]{}AGN candidates in four $\sim$3 deg$^2$ LMC fields with AAOmega to confirm [169 ]{}quasars, including [144 ]{}new ones. Given the 55 previously known quasars behind the LMC, including [25 ]{}in our present fields, we have quadrupled (to 200) the number of known quasars behind the LMC. By observing the eight remaining fields, we should find another $\sim$300 new quasars, although the yield would increase if all data were obtained with the full integration time.

We observed the quasar candidates based on the priorities set by the number of the selection methods that picked them. If they were selected by only one method, either mid-IR colors, X-ray emission, or photometric variability, the average confirmation yield is low, at $\sim$9%. If the candidate was selected based on two methods but not the third, the average confirmation rate is at 40%. The highest yields come with the simultaneous selection by all three methods, with the yield at 65%. Note, however, that we deliberately chose to fill all the AAOmega fibers with candidates in order to achieve higher completeness, guaranteeing high contamination rates because the surface density of quasars is well below the surface density of fibers.

Our biggest problem given the availability of fibers is not contamination. We could assign rough classifications to many contaminating sources, including 115 YSOs (96 new), 17 PNe (5 new), 39 new Be and 24 blue stars, 68 red stars, and also 12 objects that are either YSO/PN or blue star/YSO. The big problem is that we could not classify 402 spectra due to low S/N, what is also a major problem for making black hole mass estimates from the line widths of the quasar emission lines.

Nonetheless, tripling the number of quasars in the LMC and SMC using only 5 hr on sky is a strong vindication of the general approach and the advantages created by the large numbers of AAOmega fibers. With no improvements in yields due to actually obtaining our target integration times, we estimate that completing the OGLE-III LMC and SMC fields would yield $\sim$700 quasars and completing the OGLE-IV fields would yield $\sim$3600 quasars. These remain the best fields for extending studies of quasar variability independent of their additional uses for understanding the Galaxy through the proper motions of the LMC or probing the local ISM.

We thank Sarah Brough, our support astronomer, for help during the observing run. We thank Micha[ł]{} Jaroszy[ń]{}ski for the helpful discussion on MQS J050155.46$-$700210.1. We also thank the anonymous referee, whose comments helped us to improve the manuscript. This research was made based on observations with the Anglo-Australian Telescope, for which the observing time was granted by the Optical Infrared Coordination Network for Astronomy (OPTICON). This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France. This research has also made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory (JPL), California Institute of Technology (Caltech), under contract with the National Aeronautics and Space Administration (NASA).

Support for this work was provided by the Polish Ministry of Science and Higher Education (MNiSW) through the program “Iuventus Plus”, award number IP2010 020470 to S.K., A.M.J., and A.U. This work has been supported by NSF grants AST-0708082 and AST-1009756 to C.S.K. The OGLE is supported by the European Research Council under the European Community’s Seventh Framework Programme (FP7/2007-2013), ERC grant agreement no. 246678 to A.U.

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![image](table1.eps)

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[^1]: Quasars, AGNs and QSOs will be used interchangeably throughout this paper.

[^2]: http://ogle.astrouw.edu.pl/
