Direct Method Gas Phase Oxygen Abundances of 4 Lyman Break Analogs
aa r X i v : . [ a s t r o - ph . GA ] S e p Draft Version of
December 5, 2018
Preprint typeset using L A TEX style emulateapj v. 5/2/11
DIRECT METHOD GAS PHASE OXYGEN ABUNDANCES OF 4 LYMAN BREAK ANALOGS * Jonathan S. Brown , Kevin V. Croxall , Richard W. Pogge Draft Version of December 5, 2018
ABSTRACTWe measure the gas-phase oxygen abundances in 4 Lyman Break Analogs (LBAs) using auroralemission lines to derive direct abundances. The direct method oxygen abundances of these objectsare generally consistent with the empirically-derived strong-line method values, confirming that theseobjects are low oxygen abundance outliers from the Mass-Metallicity (MZ) relation defined by starforming SDSS galaxies. We find slightly anomalous excitation conditions (Wolf-Rayet features) thatcould potentially bias the empirical estimates towards high values if caution is not exercised in theselection of the strong-line calibration used. The high rate of star formation and low oxygen abundanceof these objects is consistent with the predictions of the Fundamental Metallicity Relation (FMR), inwhich the infall of relatively unenriched gas simultaneously triggers an episode of star formation anddilutes ISM of the host galaxy.
Subject headings: galaxies: active – galaxies: high-redshift – galaxies: starburst INTRODUCTION
Identifying and quantifying correlations between fun-damental parameters gives us insight into the phys-ical processes governing the evolution of the objectsunder investigation. In the ΛCDM paradigm, galax-ies accrete mass primarily via heirarchical mergers withother galaxies. This formulation reproduces the phys-ical properties of galaxies we observe in the nearbyuniverse (Kauffmann & Haehnelt 2000; Hopkins et al.2006). Thus, the mass of a galaxy reveals the grosscharacteristics of its history. Similarly, the metallicityof a galaxy is a fundamental characteristic which is in-timately related to its formation and subsequent chemi-cal evolution. Studying how the mass and metallicity ofgalaxies correlate across a wide range of physical parame-ters informs us about how today’s galaxies coalesced andevolved over cosmic time.The relation between a galaxy’s mass and gas phaseoxygen abundance (the MZ relation) was first investi-gated by Lequeux et al. (1979). Subsequent studies of-ten focused on the more readily measured correlationbetween luminosity and oxygen abundance (the LZ rela-tion; e.g., Garnett & Shields 1987; Skillman et al. 1989;Zaritsky et al. 1994). With data from Sloan Digital SkySurvey (SDSS; York et al. 2000) for a very large num-ber of galaxies, Tremonti et al. (2004) showed the MZrelation persists across at least 3 orders of magnitudein mass and an order of magnitude in oxygen abun-dance. This trend was extended 2.5 orders of mag- * Based on data acquired using the Large Binocular Telescope(LBT). The LBT is an international collaboration among insti-tutions in the United States, Italy, and Germany. LBT Cor-poration partners are: The University of Arizona on behalf ofthe Arizona university system; Istituto Nazionale di Astrofisica,Italy; LBT Beteiligungsgesellschaft, Germany, representing theMax-Planck Society, the Astrophysical Institute Potsdam, andHeidelberg University; The Ohio State University, and The Re-search Corporation, on behalf of The University of Notre Dame,University of Minnesota and University of Virginia Department of Astronomy, The Ohio State University,Columbus, OH 43201, USA Center for Cosmology and Astro-Particle Physics, The OhioState University, Columbus, OH 43201, USA nitude lower in mass and another order of magnitudelower in oxygen abundance by Lee et al. (2006). Therehave been a number of following studies that have in-vestigated possible variations in the MZ relation asa function of redshift (e.g. Erb et al. 2006), star for-mation rate (e.g. Andrews & Martini 2013), morphol-ogy and environment (e.g. Ellison et al. 2008a,b), or acombination of these factors (e.g. Mannucci et al. 2010;Lara-L´opez et al. 2010).While general trends between galactic parameters areboth interesting and useful, objects that deviate fromthe observed relations offer a unique perspective, as itis these objects which allow for the direct indentifica-tion of important physical mechanisms driving galacticevolution. The “Lyman Break Analogs” (LBA) project(Heckman et al. 2005) identified a class of galaxies thatappear to deviate from the local galaxy population andmore closely resemble high-redshift Lyman Break Galax-ies (LBGs; for a review see Giavalisco 2002). These ob-jects were initially identified as nearby ( z < . I F UV > L ⊙ kpc − ), and UV bright ( L F UV > . L ⊙ ) objects, mimicking the physical conditions inLBGs seen at much higher redshifts. Thus, if LBAs aretrue analogs of LBGs, they provide us with an opportu-nity to study a mode of star formation that may havebeen dominant in the early universe in a much more de-tailed way than the very distant LBGs allow. After theidentification of LBAs, subsequent work (Hoopes et al.2007; Basu-Zych et al. 2007; Overzier et al. 2008, 2009,2010; Gon¸calves et al. 2010) used the Hubble Space Tele-scope (HST), Spitzer , VLA, Sloan Digital Sky Survey(SDSS), Galaxy Evolution Explorer (GALEX), and theKeck II telescope to investigate the physical propertiesof these systems, as well as the degree to which they mayresemble LBGs.The gas-phase oxygen abundance of these objects wasestimated by Overzier et al. (2009, 2010) using the N2and O3N2 empirical calibrations from Pettini & Pagel(2004, hereafter PP04). After applying the N2 rela-tion to SDSS galaxies and LBAs, Overzier et al. (2010)showed the offset of the LBAs from the MZ relation oflocal star forming galaxies is inversely correlated withmass, with the least massive LBAs falling & . a priori reason to expectthat a locally calibrated empirical abundance relationought to apply to a class of exotic objects experienc-ing an episode of relatively extreme, concentrated starformation, as is found in the centers of these LBAs.Typical LBAs exhibit star formation rates an orderof magnitude higher than local dwarf galaxies. Addi-tionally, most LBAs have morphologies and kinemat-ics consistent with recent interactions (Overzier et al.2009, 2010; Gon¸calves et al. 2010). Clearly these ob-jects depart from the physical parameter space occupiedby local H II regions and dwarf galaxies used to cali-brate the empirical relations. Are the empircally esti-mated oxygen abundances of LBAs being systematicallyaffected by their extreme physical conditions? For in-stance, Pilyugin et al. (2010) showed that many of theline ratios used in the typical strong-line abundance in-dicators (Pagel et al. 1979; Alloin et al. 1979) are com-plex functions of electron temperature. They show thatderiving an abundance calibration from a sample of rel-atively cool H II regions and applying it to relatively hotH II regions could yield erroneous abundance estimates.Alternatively, if locally calibrated empirical relations re-turn reliable abundance estimates when applied to LBAs,this would also be of interest, as this is not immediatelyobvious given the extreme physical nature of these ob-jects.Fortunately, we are not required to rely on empiricalcalibrations alone for these objects. With a measure ofthe electron temperature ( T e ), we can determine the oxy-gen abundance directly using the T e or “direct” method(Dinerstein 1990). The electron temperature can be de-termined from temperature sensitive intensity ratios ofcollisionally excited forbidden lines. Generally speak-ing, as metallicity increases, the temperature of the neb-ula decreases, as there are more ions available to coolthe gas. In relatively low metallicity nebulae, a mea-sure of the electron temperature is typically obtained us-ing the [O III ] λ λλ (4959 + 5007) line ratio. How-ever, somewhat problematically, the auroral oxygen line[O III ] λ III ] λ III ] line fluxes to determine anelectron temperature, yielding a gas-phase oxygen abun-dance measurement that we can compare to the valuesderived via empirical techniques.In Section 2 we describe the observations and data re- duction. Section 3 describes the analysis of the data,including the subtraction of the underlying stellar contin-uum. In Section 4 we present the results of our analysis.Lastly, in Section 5 we discuss where LBAs fit in the big-ger picture of galaxy formation and evolution; Section 6provides a summary. Throughout this paper we assume H = 70 km s − Mpc − , Ω λ = 0 .
7, and Ω M = 0 .
3. Withthese cosmological parameters, a redshift of z = 0 . ∼
11 Gyr. OBSERVATIONS AND REDUCTION
Observing Procedures
We observed 4 of the LBAs identified in Overzier et al.(2009) using MODS1 on the LBT (Pogge et al. 2010) be-tween September 2011 and January 2013. All targetswere observed in longslit mode with a 1 . ′′ × µ m pix-els. MODS1 uses a dichroic that splits the light intoseparately optimized red and blue channels at ∼ ∼ − grating (spectral resolutionof 2.4 ˚A), while the red CCD covers a wavelength rangeof ∼ − grating (spectralresolution of 3.4 ˚A).Each target was observed with three 600s exposures fora total of 1800 s, with the exception of J005527, whichwas observed with four 1200s exposures for a total of4800 s. The position angle of the slit approximated theparallactic angle at the midpoint of the observation soas to minimize slit losses due to differential atmosphericrefraction. If the arc lamp or flat field data was notavailable on the night of the observation, we used thecalibration data obtained within 1-2 days of our observa-tions. Given the stability of MODS1 over the course of anobserving run, this is sufficiently recent to provide accu-rate calibrations. We obtained bias frames and Hg(Ar),Ne, Xe, and Kr calibration lamp images, which we usedfor wavelength calibration. Night sky lines were used tocorrect for the small ( ∼ ′′ spectrophotometric slitmask used for flux calibration. The standard stars arefrom the HST Primary Calibrator list, which is composedof well observed northern-hemisphere standards from thelists of Oke (1990) and Bohlin et al. (1995).Target selection was done such that priority was givento the objects from Overzier et al. (2009) with the lowestoxygen abundance estimates, and hence most offset fromthe MZ relation, that were visible at the time of obser-vation. Our final sample has a mean redshift h z i = 0 . σ z = 0 . Data Reduction
The basic 2D data reduction was performed in Pythonusing the modsCCDRed suite of programs . We usedmodsCCDRed to bias subtract, flat field, and illumi-nation correct the raw data frames. We then coaddedthe frames and removed cosmic rays with L.A. Cosmic (van Dokkum 2001), taking extra care to ensure anyemission features were not misidentified as cosmic rays. J092600 SDSSMODS1 f λ [ − e r g s − c m − Å − ] [ O II ] λ H β H α H γ [OIII] λ MgII λ [ O II ] λ H β H α [SII] λ H γ [OIII] λ J004054 SDSSMODS1 f λ [ − e r g s − c m − Å − ] [ O II ] λ H β H α H γ [OIII] λ MgII λ [ O II ] λ H β H α [SII] λ H γ [OIII] λ J020356 SDSSMODS1 f λ [ − e r g s − c m − Å − ] [ O II ] λ H β H α H γ [OIII] λ MgII λ [ O II ] λ H β H α [SII] λ H γ [OIII] λ J005527 SDSSMODS1 f λ [ − e r g s − c m − Å − ] [ O II ] λ H β H α H γ [OIII] λ MgII λ [ O II ] λ H β H α [SII] λ H γ [OIII] λ Fig. 1.—
Comparison of our MODS1 spectra with SDSS spectra. Note the higher sensitivity and wider wavelength coverage of theMODS1 spectra. The noise spike and drop in flux seen in the inset of the J004054 MODS1 spectrum is due to the coincidental location ofa strong sky line for this particular target.
We performed sky subtraction and 1D extraction usingthe modsIDL pipeline . This pipeline has been devel-oped specifically for MODS and makes use of the XIDLpackages . Figure 1 shows our reduced spectra com-pared with spectra from the SDSS. The MODS1 spec-tra achieve a higher S/N than the SDSS spectra across awider wavelength range. The inset shows a zoomed viewof the metallicity sensitive [O III ] λ ANALYSIS
Stellar Continuum Subtraction
Many of the emission lines are blended with underlyingstellar absorption features. To obtain accurate line fluxmeasurements it is necessary to remove the underlyingstellar component before extracting line fluxes.Prior to modeling the underlying stellar component ofthe LBAs, we correct for foreground Galactic exinctionusing the dust maps from Schlegel et al. (1998) and thereddening law from O’Donnell (1994) with R V = 3 . ≤ Z ≤ ≤ τ ≤
10 Gyr). In general, our galaxieslack any strong stellar absorption features which could beused to place strong constraints on the underlying stellarpopulation. However we are able to model the generalshape of the continuum, as well as remove any absorp-tion features near our metallicity-sensitive lines. In par-ticular, the [O
III ] λ γ , and so we need to take extra care to make sure the[O III ] λ τ .
20 Myr) stellar population of roughly solarmetallicity, with a velocity dispersion σ ∼
200 km s − .We regard this strictly as a qualitative assement; ourtargets can be equally well fit with a wide range of ages,metallicities, and kinematics. In general, we find theeffects of our detailed model fit on the strengths of thestellar absorption features to be minimal. Accounting forstellar absorption, we find, on average, EW(H β (ABS)) =2.70 ˚A, which is small compared to our mean EW(H β )= −
111 ˚A. See Table 1 for further details.
Line Flux Measurment ∼ xavier/IDL/ We assume the that emission lines in our targets are ap-proximately Gaussian. We fit a library of atomic lines toeach spectrum using MPFIT (Markwardt 2009), an IDLimplementation of the robust non-linear least squares fit-ting routine MINPACK-1. We assume the lines all havethe same FWHM in velocity space and allow for variationin the intensity of each line as well as a small transla-tion in wavelength ( . β and dered-den the spectra using the Balmer decrement. Our finalvalue of C (H β ) is an error-weighted average of the valuesobtained using H α/ H β , H β/ H γ , and H α/ H γ and theirappropriate intensity ratios assuming Case B recombi-nation. See Table 1 for a list of relevent line intensitiesused in our subsequent analysis. We adopt the followingnotation: R = I [O II ] λλ , /I H βR = I [O III ] λλ , /I H βR = R + R P = R /R for the principal diagnostic emission line ratios. Abundance Determination
We detect [O
III ] λ S/N = 8 . S/N = 15 .
9; well above the
S/N typically required forobtaining an electron temperature from [O
III ] λ II region follow a Maxwell–Boltzmann equilibrium en-ergy distribution. In the low density Boltzmann regime,[O III ] λ λλ (4959 + 5007) is goverened by the rela-tive level populations of the [O III ] ion, and, due to thespacing of the energy levels, the relative level populationis very sensitive to the electron temperature of the ion-ized region. Since the derived ionic abundances are astrong function of electron temperature, a measurementof [O
III ] λ λλ (4959 + 5007) allows us to computethe total oxygen abundance directly, rather than havingto rely on empirical methods.The assumption of a Maxwell–Boltzmann distribu-tion of electron energies has recently come into ques-tion. Specifically, even different temperature sensi-tive line ratios used to directly measure the electrontemperature (e.g. [O III ] λ λλ II ] λλ λλ II ] λ λλ II ] λλ λλ κ -distribution, rather than aMaxwell–Boltzmann distribution. However, we are onlyconcerned with relative comparisons for a given electrontemperature measurement method. Thus, we do not ex-pect systematic effects arising from our assumed energydistribution to significantly influence our results. http://purl.com/net/mpfit TABLE 1LBA Emission Line Intensities
Ion J092600 J004054 J020356 J005527[O II ] λ ± ± ± ± I λ ± ± ± ± III ] λ ± ± ± ± γ λ ± ± ± ± III ] λ ± ± ± ± II ] λ ± ± ± ± β λ ± ± ± ± III ] λ ± ± ± ± III ] λ ± ± ± ± α λ ± ± ± ± II ] λ ± ± ± ± II ] λ ± ± ± ± II ] λ ± ± ± ± II ] λ ± ± ± ± II ] λ ± ± ± ± β InformationC(H β ) 0.041 ± ± ± ± β (ABS))(˚A) 2.40 ± ± ± ± β ) (˚A) − − − − Note . — Units are such that H β = 1. TABLE 2Electron Temperatures & Densities
Target T e (OII) [K] T e (OIII) [K] N e (SII) [cm − ]J092600 1.189 ± × ± × · · · ± × · · · ± × ± × ± × −2.0 −1.5 −1.0 −0.5 0.0log([N II] λ α )−1.0−0.50.00.51.0 l o g ([ O III ] λ / H β ) Fig. 2.—
BPT diagram composed of star forming SDSS galax-ies (gray contours), H II regions from Pilyugin et al. (2012) (blackpoints), and the LBAs from our sample (cyan/red circles). Theuncertainties in the line ratios for the LBAs are smaller thanthe plotting symbols. The dashed and solid red lines are fromKauffmann et al. (2003b) and Kewley et al. (2006) and denote theboundaries between star forming galaxies and AGN. The red cir-cle represents J005527, which seems to exhibit enhanced nitrogenrelative to the other LBAs. Osterbrock & Ferland (2006) describes how to com-pute T e and n e in ionized nebulae. In this paper, wemeasure the electron temperature ( T e ) and density ( n e )using the im_temden IDL routines from the Moustakascode repository . This set of routines uses well-knownline ratios to compute the electron temperature and/orelectron density of a given region. We assume a threezone ionization region, composed of a high ionization re-gion (with T e = T ≡ T e ([O III ])), an intermediate ion-ization region ( T e ≡ T e ([Ar III ])), and a low ionizationregion (with T e = T ≡ T e ([O II ])). We measure T from[O III ] λ λλ (4959 + 5007) and compute T using therelation t = 0 .
264 + 0 . t (1)(where t = T / ) from Pilyugin et al. (2009). Due tothe substantial noise from sky contamination and rela-tive weakness of the [O II ] λλ T using the relation above andthe T we measure from the [O II ] line ratios. We com-pute the electron density n e using the density sensitiveline ratio [S III ] λ λ − , we adopt a value of 100 cm − , https://github.com/moustakas/impro −2.0 −1.5 −1.0 −0.5 0.0 0.5 1.0log([OIII] λ β )−1.0−0.50.00.51.0 l o g ([ O II ] λ / H β ) −2.0 −1.5 −1.0 −0.5 0.0 0.5 1.0log([OIII] λ β )−1.0−0.50.00.51.0 l o g ([ O II ] λ / H β ) −0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2log(R )0.00.20.40.60.81.0 P = R / R −0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2log(R )0.00.20.40.60.81.0 P = R / R Fig. 3.—
Excitation diagnostic plots for SDSS galaxies (gray contours), H II regions from Pilyugin et al. (2012) (black points), and theLBAs from our sample (cyan points). The uncertainties in the line ratios for the LBAs are smaller than the plotting symbols. The leftpanel shows [O II ] λλ III ] λ R ; from bottom left to top right log( R ) = 0.,0.5, 1.0. In general, the LBAs display higher excitation conditions more similar to H II regions than the SDSS star forming galaxies. Theright panel shows P = R /R as a function of log( R ). Again, the LBAs occupy an excitation regime closer to that of H II regions thanstar forming SDSS galaxies. as lower densities are consistent with 100 cm − . WhileJ005527 shows signs of slightly higher electron density,we are still well inside the low density regime for eachof our targets. Table 2 lists the measured temperaturesand densities for each LBA in our sample. Given theredshift of these objects, several [S III ] lines (e.g. [S
III ] λλ im_nlevel routine to computethe relative populations and emissivities for the differ-ent ions using an n-level atom calculation. For simplic-ity, in the low ionization zone we adopt the reasonablecanonical assumptions that T e ([N II ]) = T e ([O II ]) = T where T is the theoretical value derived from our mea-sured T . Similarly, for the high ionization region weassume T e ([Ne III ]) = T e ([O III ]) = T . Lastly, in theintermediate ionization region, we assume T e ([Ar III ]) =0 . T + 0 .
17 from Garnett (1992). We obtain a densityfor each ion using N ( X i ) N ( H + ) = I λ i I H β j H β j λ i (2)In order to compute the total abundance of a givenelement, we sum the observable ionic states. In the caseof oxygen, we assumeOH = O + O + + O ++ H + (3)We compute both total and ionic abundances for O,N, Ne, and Ar. We compute ionization correction frac-tions (ICFs) for N, Ne, and Ar. Adopting the ICFs fromThuan et al. (1995): ICF(N) = OO + (4)ICF(Ne) = OO ++ (5)ICF(Ar) = ArAr ++ = [0 .
15 + x (2 . − . x )] − (6)where x = O + / O. The abundance estimates (and as-sociated uncertainties) for each object is presented in Ta-ble 3. RESULTS
Excitation
In Figure 2 we show the standard diagnostic Baldwin,Phillips, Terlevich (BPT) diagram (Baldwin et al. 1981)used to distiguish between ionization regions heated pri-marily by star-formation and regions heated primarily byAGN. Star forming SDSS galaxies from the MPA-JHUcatalog are shown as gray contours, H II regions fromPilyugin et al. (2012) are shown as black points, and ourLBAs are shown as the large cyan dots. The dashed andsolid red lines are from Kauffmann et al. (2003b) andKewley et al. (2006) and denote the boundaries betweenstar forming galaxies and AGN. Our results are consis-tent with those presented in previous studies; the LBAsfall squarely in the star-formation dominated region ofthe BPT diagram. We find that J005527 (red circle) is Available at http://vizier.cfa.harvard.edu/viz-bin/VizieR?-source=J/MNRAS/424/2316 −1.0−0.50.00.51.00.00.0 l o g ([ O II ] / H β ) −1.0−0.50.00.51.01.50.00.0 l o g ([ O III ] / H β ) −2.0−1.5 −1.0−0.50.00.5 l o g ([ N II ] / H β ) −1.5 −1.0−0.50.00.5 l o g ([ N II ] / [ O II ]) l o g ([ S II ] / H β ) l o g ([ S II ] / [ O II ]) Fig. 4.—
Diagnostic emission-line ratios as a function of oxygen abundance for our LBAs and a comparison set of local H II regions fromPilyugin et al. (2012). The LBAs from our sample are shown as cyan points, with the location of J005527 marked with a red circle; theH II regions with well measured metallicities from Pilyugin et al. (2012) are shown as black points. The error bars in the lower right ofeach plot represent an uncertainty in the oxygen abundance of 0.07 dex. J005527 displays relatively stronger [N II ] emission compared tothe other LBAs, but remains well within the parameter space occupied by the H II regions. The LBA line ratios are remarkably similar tothose of the “warm” H II regions from Pilyugin et al. (2010), suggesting that the excitation conditions in LBAs are actually quite similarto that of typical H II regions. offset to the right in Figure 2 relative to our other LBAsand the H II regions from Pilyugin et al. (2012), indica-tive of enhanced [N II ] emission.Figure 3 shows the excitation conditions of our LBAsbased on the relative oxygen line ratios. We plot the starforming SDSS galaxies (gray contours), the H II regionsfrom Pilyugin et al. (2012) (black dots), and our 4 LBAs(cyan dots). The left panel compares the relative [O II ]and [O III ] line ratios. The dotted lines show constant R ; from bottom left to top right log( R ) = 0., 0.5,1.0. As seen in Figure 2, the LBAs and H II regionsfrom Pilyugin et al. (2012) have higher [O III ] emissioncompared to the SDSS galaxies. The right panel showsP = R /R as a function of log( R ). Again, the LBAsdisplay excitation conditions which are typical of H II re-gions, but quite unusual for star forming SDSS galaxies.In Figure 4 we show line diagnostic diagrams fromPilyugin et al. (2010). The LBAs display line ratios thatare remarkably similar to “warm” H II regions. Again,the red circle marks the location of J005527, which showssigns of enhanced nitrogen relative to the other LBAs, even though it still remains well within the parameterspace occupied by H II regions.Figure 5 shows the region around 4660 ˚A, wherewe detect characteristic Wolf-Rayet features (e.g.Bresolin et al. 2004; Brinchmann et al. 2008) in each ofour 4 targets. The most common features are C IV λ II λ III λ ∼ IV and He II emission lines. These high ionization featuresare associated with very young stellar populations, asthey are typically visible for only a few million years fol-lowing an episode of significant star formation. Oxygen Abundances
Our oxygen abundances are presented in Table 4.We are able to reproduce the oxygen abundances fromOverzier et al. (2009) to within a few percent using thePP04 O3N2 method on the SDSS spectra. We haveincluded the original N2 and O3N2 calibrations fromPP04 as well as newer CALIFA- T e calibrations fromMarino et al. (2013) that use a larger sample of H II re-gions with direct oxygen abundances. The calibrationsfrom Marino et al. (2013) are generally shallower thanthose presented in PP04, but in the abundance range weare concerned with, the CALIFA and PP04 calibrationsproduce nearly identical results.Due to the location of the LBAs in the transitionzone of the R index, the popular theoretical strong-line calibrations are rather insensitive to the oxygenabundance of these objects (e.g. Pilyugin & Thuan 2005;Pe˜na-Guerrero et al. 2012). Furthermore, the numeroustheoretical calibrations are known to systematically de-viate from each other (see Kewley & Ellison 2008). Forthese reasons, we consider here only the empirical strong-line methods, which are defined almost entirely by H II regions with direct abundance determinations.Given the scatter in the emprirical calibrations of ∼ . . DISCUSSION
Excitation Conditions of LBAs
Empirical abundance calibrations are typically basedon samples of H II regions with well-determined electrontemperatures and thus directly measured abundances.However, the direct method is subject to a number ofobservational biases. For instance, metal poor objectsgenerally have higher electron temperatures, brighter au-roral lines, and more easily determined abundances. Thisresults in a preferential selection of low metallicity ob-jects in empirical calibrations. Furthermore, most largescale surveys of emission line galaxies, like the SDSS, arecomposed of predominantly low-excitation galaxies rela-tive to the H II regions on which the empirical calibra-tions are based (see Figure 3). Moustakas et al. (2010)cautions against haphazardly extrapolating empirical re-lations to lower excitation regimes occupied by the ma-jority of galaxies in large surveys, as doing so could resultin erroneous abundance determinations.Looking at Figure 4, the emission line flux ratios ofall 4 of our LBAs are remarkably similar to those of the“warm” H II ratios from Pilyugin et al. (2010). Addi-tionally, even though J005527 displays fairly strong [N II ]emission compared to the other LBAs in our sample, itremains in an excitation regime fairly typical of localH II regions. This supports the idea that locally deter-mined empirical calibrations ought to return reasonableabundance estimates for LBAs. This is a crucial step injustifying the application of locally calibrated empiricalrelations to LBAs and LBGs. However, caution muststill be exercised when extrapolating these relations tohigh redshifts, as recent work has suggested that theLBG population as a whole evolves rapidly with redshift(Stanway & Davies 2014).It is readily apparent that LBAs are undergoing anepisode of significant star formation. Within a few mil-lion years of the initial burst, large numbers of Wolf-Rayet stars, supernova remnants, and other extremelyhot objects could conspire to produce an abnormallyhard ionization spectrum. If we suppose that the gassurrounding these hot objects was subject to a harderionization spectrum than what is observed in local H II regions, we would expect to see relatively enhanced ionicemission features. High energy photons have a smallercross section for interaction. As a result, these high en-ergy photons have a longer mean free path, resulting inlarger partially ionized regions. In general, this resultsin enhanced ionic emission. In the case of ionized ni-trogen, an anomalously hard ionization spectrum wouldproduce enhanced [N II ] emission and result in system-atically high abundance estimates when using a locallycalibrated empirical relation.Berg et al. (2011) showed that enhanced nitrogenabundance (relative to oxygen) could also bias strong-line estimates towards high oxygen abundance. However,the excitation conditions of our LBAs are quite differentfrom what is expected in the evolved Wolf-Rayet galaxiesfrom Berg et al. (2011). Furthermore, our LBAs do notshow abnormally high [N II ]/[O II ] ratios. If the high N20 TABLE 3Derived Abundances
Parameter J092600 J004054 J020356 J005527O / H + ( × ) 0.302 ± ± · · · ± + / H + ( × ) 1.787 ± ± ± ± ++ / H + ( × ) 8.283 ± ± ± ± ± ± ± ± / H + ( × ) · · · · · · · · · · · · N + / H + ( × ) 0.109 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ++ / H + ( × ) 1.719 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ++ / H + ( × ) 0.032 ± ± · · · ± ± ± · · · ± ± ± · · · ± ± ± · · · -2.30 ± TABLE 4Direct and Strong-Line Oxygen Abundances
Method J092600 J004054 J020356 J005527Direct (this work) 8.02 ± ± ± ± Note . — The scatter in these empirical calibrations is ∼ method estimates observed in some of our targets werethe result of an abnormally hard ionization spectrum,we would not expect the O3N2 method to yield a highabundance estimate, as such a radiation field would havea similar effect on both N + and O ++ emission. LBAs and the Fundamental Metallicity Relation
LBAs have masses typical of entire galaxies and thus itis interesting to note that their excitation conditions fallin a region that is sparsely populated by the SDSS starforming galaxies, but well occupied by the H II regionsfrom Pilyugin et al. (2012). It seems that the LBAs moreclosely resemble giant H II regions in terms of photoion-ization conditions than typical SDSS galaxies. What isthe primary physical process causing the LBAs to be sosignificantly offset from SDSS galaxies?The defining characteristic of LBAs is their compactUV emission arising from recent star formation, and in-deed, the typical star formation rate for an LBA is anorder of magnitude higher than a typical SDSS galaxy(Overzier et al. 2009). Such high rates of star formationimply a recent replenishment of star forming material inthe LBA systems. This influx of gas could be due to a tidal interaction (e.g. Peeples et al. 2009), or perhaps arecent merger. Generally these events will result in dilu-tion of the interstellar gas and a corresponding reductionin the observed metallicity.Peeples et al. (2009) compute the expected dilutionwhich might result from the funneling of gas from largegalactocentric radius into the center of a galaxy. Theyfind that for a galaxy with a reasonable metallicity gra-dient and gas surface density profile, gas flowing inwardfrom within 20 kpc would result in a metallicity dilu-tion ∆(O/H) = − . σ/v >
1) presented inGon¸calves et al. (2010).A global inflow of star forming material could also re-sult in an intense burst of star formation (Rupke et al.1
J092600
C IV λ λ J004054
C IV λ λ J020356
C IV λ J005527
C IV λ λ λ A r b i t r a r y F l u x Rest Wavelength [Å]
Fig. 5.—
Wolf-Rayet features identified near 4660 ˚A . The mostprominent features are the C IV line at 4658 ˚A and He II at 4686 ˚A.J005527 displays a broad bump from 4600 – 4700 ˚A as well as N III emission, both of which are characteristic of a Wolf-Rayet galaxy.The noise spike and drop in flux seen in the J020356 panel is dueto the coincidental location of the dichroic cutoff for this particulartarget. µ α such that µ α = log M ∗ − α log(SFR).The sample of galaxies compiled by Mannucci et al.(2010) consists of > z ∼ . < z < .
5, the 91 galaxies fromErb et al. (2006), and an additional 16 galaxies from3 < z <
4. Typically, α is chosen such that it minimizesthe scatter in the resulting FMR. Mannucci et al. (2010)find α = 0 .
32 for their sample of galaxies, and noevolution of the FMR up to z = 2 . α is quite sensitive to the abundance ∗ /M )8.08.28.48.68.89.0 + l o g ( O / H ) ∗ /M )8.08.28.48.68.89.0 + l o g ( O / H ) MODS1 DirectMODS1 N2SDSS N2MODS1 DirectMODS1 N2SDSS N2
PSfrag replacements ⊙ Fig. 6.—
Gas phase oxygen abundance as a function of mass forvarious abundance diagnostics. The cyan points show our directmethod oxygen abundance for the 4 LBAs in our sample, withmasses taken from Overzier et al. (2009). The thick black linedenotes the logarithmic best fit to the stacked SDSS direct oxy-gen abundances from Andrews & Martini (2013). The orange andgreen points show the PP04 N2 oxygen abundance estimates fromthe SDSS and MODS1 spectra respectively. The gray contoursare the star forming SDSS galaxies from the MPA-JHU cataloguewith oxygen abundances estimated from the PP04 N2 calibration.Regardless of the diagnostic used, the LBAs display low oxygenabundances for their mass relative to the MZ relation. diagnostic used. Yates et al. (2012) found α = 0 . α = 0 . α = 0 .
32 presented inMannucci et al. (2010). Additionally, the determinationof α merely minimizes the scatter for a given abundancediagnostic; two strong-line calibrations will generally notshare the same FMR.Determining where an object sits on the FMR re-quires knowledge of the SFR in addition to the mass andmetallicity of the object. Overzier et al. (2009) adoptsSFR calibrations from Calzetti et al. (2009) and com-putes various SFRs using H α , H α + 24 µ m, and FUV lu-minosities. The H α flux is associated with only the mostrecent star formation activity, whereas the FUV calibra-tion is sensitive to the integrated star formation activityover the previous 1 Gyr. The appearance of the LBAs isdominated by the current burst of star formation activ-ity, so we adopt the H α + 24 µ m SFRs (Kennicutt et al.2007; Calzetti et al. 2007).The H α + 24 µ m calibration is not without its ownsystematic effects. For example, an AGN could pref-erentially heat dust and result in 24 µ m flux above thatwhich would arise from star formation alone. However,none of the Overzier et al. (2009) LBAs appear to host aType 1 (unobscured) AGN. While the presence of Type2 (obscured) AGN is not ruled out, it seems unlikelygiven where these LBAs lie on the diagnostic diagrams(Overzier et al. 2009). If an AGN were present, it is likely2 µ α = log(M ∗ /M ) − α log(SFR)8.08.18.28.38.48.5 + l o g ( O / H ) µ α = log(M ∗ /M ) − α log(SFR)8.08.18.28.38.48.5 + l o g ( O / H ) MODS1 DirectMODS1 N2SDSS N2MODS1 DirectMODS1 N2SDSS N2
PSfrag replacements ⊙ Fig. 7.—
Gas phase oxygen abundance as a function of µ α ,where α is chosen depending on the abundance diagnostic. Weadopt the appropriate α from Andrews & Martini (2013); for thedirect method we use α = 0 .
66, and for the N2 method we use α = 0 .
30. The color scheme is the same as that from 6. Massesand H α + 24 µm star formation rates for the LBAs are taken fromOverzier et al. (2009); star formation rates for the SDSS galaxiesare from the MPA-JHU catalogue. The thick black line denotes thelinear best fit to the FMR of direct method stacked SDSS galaxiesfrom Andrews & Martini (2013). The incorporation of star forma-tion rate reduces the degree to which the LBAs are offset fromtypical SDSS star forming galaxies. to only have a very small effect.Figure 7 shows the oxygen abundances of the SDSSstar forming galaxies and the LBAs as a function of µ α ,where we have adopted the values of α = 0 .
30 and α =0 .
66 corresponding to the N2 index and direct methodrespectively from Andrews & Martini (2013). The cyandots show our LBAs with direct method oxygen abun-dances; the thick black line shows the linear fit to theFMR for stacked SDSS spectra from Andrews & Martini(2013). The N2 estimates of oxygen abundance for ourLBAs are shown as orange and green dots for the SDSSand MODS1 data respectively. The gray contours arethe star forming SDSS galaxies from the MPA-JHU cat-alogue (see Kauffmann et al. (2003a) and Salim et al.(2007) for mass determinations and Brinchmann et al.(2004) for star formation rate determinations). The oxy-gen abundances of the SDSS galaxies are estimated fromthe PP04 N2 calibration. We see that plotting oxygenabundance as a function of both mass and star formationrate does indeed reduce the scatter between the LBAsand SDSS data for a given abundance estimation method(direct or emprical).It is important to keep in mind that a comparison ofwhere the direct abundance measurements fall on theFMR relative to the contoured SDSS data is meaning-less, since we do not expect the different abundance di-agnostics to produce consistent results. However, thefact that incorporating SFR drastically reduces the scat-ter for a given abundance diagnostic suggests that thehigh SFR, low oxygen abundance, and disturbed mor- phology of these LBAs could be explained by a recentinflow of relatively unenriched gas and is consistent withthe existence of a FMR that the LBAs appear to follow. SUMMARY
It is believed that LBAs ( z ∼ .
2) are true analogs ofLBGs ( z & II regionsin normal galaies still hold for the physical conditionspresent in LBAs.The empirically-derived oxygen abundances of LBAsshow them to be metal deficient for their mass, falling & . III ] λ • LBAs display excitation conditions that are un-usual for SDSS galaxies, but are quite typical ofH II regions from Pilyugin et al. (2012). • The N2 empirical calibration is generally valid forthe LBAs presented here. Objects with particu-larly hard ionizing spectra may have biased strong-line abundance estimates, but the effect is likely tobe smaller than the scatter in the empirical cali-brations. • LBAs are offest from the MZ relation of localstar forming glaxies in the sense that they havelower oxygen abundances for a given mass. How-ever, when their abnormally high star formationrates are taken into account, we find that they donot appear to deviate significantly from the FMR.This, coupled with their disturbed morphologies,is consistent with an interaction driven gas inflowparadigm.We can improve our understanding of LBAs in a sta-tistical sense by increasing the size of the sample studied.Here we have presented observations of only 4 of the 31LBAs in the Overzier et al. (2009) sample. With instru-ments like MODS1, precise spectroscopic observations ofLBAs are quite feasible and can be done with a modestamount of telescope time.An increased number of LBAs with robustly de-termined abundances would allow us to place tighterconstraints on the systematic effects between locallycalibrated strong-line abundance estimates and directmethod abundances. This will aid greatly in our un-derstanding of how LBAs differ from local galaxies, andimprove our understanding of the processes governing theobserved trends in mass, metallicity, and star formationrate.Lastly, it has been suggested that both the MZ relationand FMR are a consequence of the relation between gasphase oxygen abundance and stellar-to-gas mass ratio3(the Universal Metallicity Relation; Zahid et al. 2014).They argue that once the ISM of a galaxy has been en-riched to a point such that the amount of oxygen beinglocked up in low mass stars is comparable to the oxy-gen produced by massive stars, the oxygen abundanceasymptotically approachs a value which is independentof redshift. If the LBAs have indeed experienced a sig-nificant inflow of gas mass relative to their stellar mass,they could potentially serve as key testing grounds forthe Universal Metallicity Relation.We would like to thank the referee for a constructivereport. This paper uses data taken with the MODSspectrographs built with funding from NSF grant AST-9987045 and the NSF Telescope System InstrumentationProgram (TSIP), with additional funds from the OhioBoard of Regents and the Ohio State University Officeof Research. KVC and MODS pipeline software devel-opment was supported by NSF grant AST-1108693.We appreciate the MPA-JHU group for making theircatalog publicly available, as well Leonid Pilyugin, EvaGrebel, and Lars Mattsson for making their catalogof H II REFERENCESAlloin, D., Collin-Souffrin, S., Joly, M., & Vigroux, L. 1979,A&A, 78, 200Amor´ın, R., P´erez-Montero, E., V´ılchez, J. M., & Papaderos, P.2012, ApJ, 749, 185Amor´ın, R. O., P´erez-Montero, E., & V´ılchez, J. M. 2010, ApJ,715, L128Andrews, B. H., & Martini, P. 2013, ApJ, 765, 140Baldwin, J. A., Phillips, M. M., & Terlevich, R. 1981, PASP, 93, 5Basu-Zych, A. R., Schiminovich, D., Johnson, B. D., et al. 2007,ApJS, 173, 457Berg, D. A., Skillman, E. D., & Marble, A. 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