Mapping the three-dimensional multi-band extinction and diffuse interstellar bands in the Milky Way with LAMOST
Haibo Yuan, Xiaowei Liu, Maosheng Xiang, Zhiying Huo, Huihua Zhang, Yang Huang, Huawei Zhang
aa r X i v : . [ a s t r o - ph . GA ] J un IAUS 298 Setting the scence for Gaia and LAMOSTProceedings IAU Symposium No. 298, 2014S. Feltzing, G. Zhao, N. A. Walton & P. A. Whitelock, eds. c (cid:13) Mapping the three-dimensional multi-bandextinction and diffuse interstellar bands inthe Milky Way with LAMOST
H.-B. Yuan , , X.-W. Liu , , M.-S Xiang , Z.-Y. Huo , H.-H. Zhang ,Y. Huang and H.-W. Zhang Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, Chinaemail: [email protected] LAMOST Fellow Department of Astronomy, Peking University, Beijing 100871, China National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China
Abstract.
With modern large scale spectroscopic surveys, such as the SDSS and LSS-GAC,Galactic astronomy has entered the era of millions of stellar spectra. Taking advantage of thehuge spectroscopic database, we propose to use a ”standard pair” technique to a) Estimatemulti-band extinction towards sightlines of millions of stars; b) Detect and measure the dif-fuse interstellar bands in hundreds of thousands SDSS and LAMOST low-resolution spectra; c)Search for extremely faint emission line nebulae in the Galaxy; and d) Perform photometric cal-ibration for wide field imaging surveys. In this contribution, we present some results of applyingthis technique to the SDSS data, and report preliminary results from the LAMOST data.
Keywords.
ISM: dust, extinction, ISM: lines and bands, planetary nebulae: general, ISM: gen-eral, surveys, techniques: spectroscopic
1. Overview
Dust grains produce extinction and reddening of stellar light from the ultraviolet (UV)to the infrared (IR) (Draine 2003). Accurate determination of reddening to a star is vitalfor reliable derivation of its basic stellar parameters, such as effective temperature anddistance. Constructing a 3D Galactic extinction map plays an essential role in Galacticastronomy, particularly in achieving the driving goals of the LAMOST SpectroscopicSurvey of the Galactic Anti-center (LSS-GAC; Liu et al. this volume).The Sloan Digital Sky Survey (SDSS; York et al. 2000) has delivered low-resolutionspectra for about 0.7 M stars in its Data Release 9 (DR9; Ahn et al. 2012). The LAMOSTGalactic surveys (Deng et al. 2012 and this volume) have obtained over 1 M stellar spectraand will obtain over 5 M in the next four years. With millions of stellar spectra, ”iden-tical” stars in different environments can be easily paired and compared, which opensgreat opportunities to a number of studies with the standard pair technique (Stecher1965; Massa, Savage & Fitzpatrick 1983). By comparing the differences in photometriccolors from large scale imaging surveys, such as the Galaxy Evolution Explore (GALEX;Martin et al. 2005) in the UV, the SDSS in the optical, the Two Micro All Sky Sur-vey (2MASS; Skrutskie et al. 2006) in the near-IR and the Wide-field Infrared SurveyExplorer (WISE; Wright et al. 2010) in the mid-IR, one can measure multi-band redden-ing for a large number of targets and constrain the reddening laws (Yuan, Liu & Xiang2013). By comparing the differences in normalized spectra, one can detect absorptionfeatures from the interstellar medium (ISM), particularly the diffuse interstellar bands(DIBs; Yuan & Liu 2012), as well as abnormal stellar absorption/emission lines from1 H.-B. Yuan et al.chemically particular or active stars. Such method has the advantages that it’s straight-forward, model-free and applicable to the majority of stars. Combining stellar distancesfrom Gaia (Perryman et al. 2001) or from spectro-photometry, one can further map theGalactic extinction, extinction laws and DIBs in 3D. If by chance a diffuse nebula falls inthe sightline of some targets, lines emitted by the nebula (e.g. [O iii ] λλ
2. The 3D multi-band extinction and extinction laws
Using star pairs selected from the SDSS, and combining the SDSS, GALEX, 2MASSand WISE photometry ranging from the far-UV to the mid-IR, Yuan, Liu & Xiang (2013)have measured dust reddening in the
F U V − N U V, N U V − u, u − g, g − r, r − i, i − z, z − J, J − H, H − Ks, Ks − W W − W E B − V values given by Schlegel et al. (1998), allow usto derive the empirical reddening coefficients for those colors. The results are comparedwith previous measurements and the predictions of a variety of Galactic reddening laws.We find that 1) The dust reddening map of Schlegel et al. (1998) over-estimates E B − V byabout 14%, consistent with the work of Schlafly et al. (2010) and Schlafly & Finkbeiner(2011); and 2) All the new reddening coefficients, except those for N U V − u and u − g ,prefer the R ( V ) = 3.1 Fitzpatrick reddening law (Fitzpatrick 1999) rather than the R ( V ) = 3.1 CCM (Cardelli et al. 1989) and O’Donnell (O’Donnell 1994) reddening laws.Using the Ks -band extinction coefficient predicted by the R ( V ) = 3.1 Fitzpatrick lawand the observed reddening coefficients, we have deduced new extinction coefficients forthe F U V, N U V, u, g, r, i, z, J, H, W W / N( λ >
10 per pixel and basicstellar parameters for about 0.6 M spectra of S / N( λ >
10 per pixel (Liu et al. thisvolume). With the same technique, we measured dust reddening in the g − r, r − i, i − z, z − J, J − H and H − Ks colors for over 0.2 M stars from LSS-GAC. The reddening coefficientsfor these colors relative to that for g − r are consistent with the work of Yuan, Liu &Xiang (2013), as seen in Fig. 1. With spectro-photometric distances, we have constructeda preliminary 3D extinction map in the outer disk of the Galaxy. Fig. 2 shows E B − V as afunction of distance from the sun and Galactic longitude at b = − ◦ , 0 ◦ and 2 ◦ . In spiteof the limited sky coverage, distinct features, such as the Perseus and Outer Arms, andeffects of warps of the outer disk, are clearly visible. Given the multi-band reddening ofthe stars, R ( V ) values are also estimated and their spatial variations are investigated inFig. 3. A median value of R ( V ) = 3.2 is obtained, consistent with previous results. Noobvious spatial variations of R ( V ) are detected, indicating that dust properties do notchange significantly in the outer disk.The LSS-GAC will obtain low-resolution spectra and basic stellar parameters for a sta-tistically complete sample of & l ◦ , | b | ◦ ). For | b | > . ◦ , the LSS-GAC plans to sample 1,000 stars per sq.deg. For | b | . ◦ , the sampling is doubled. Combining spectroscopic data from the LAMOSTand SDSS, photometric data from the GALEX, SDSS, the Xuyi Schmidt Telescope Pho- apping the 3D extinction and DIBs with LAMOST Figure 1.
Reddening of r − i , i − J , J − H and H − Ks colors versus that of g − r for225,422 stars from LSS-GAC that have XSTPS-GAC, 2MASS photometry and LAMOST spec-tral S / N( λ >
20 per pixel. To avoid crowdness, only one-in-five stars are shown. Forcomparison, relations obtained in Yuan, Liu & Xiang (2013) are over-plotted.
Figure 2. E B − V as a function of distance from the sun (ranging from 0 – 10 kpc) andGalactic longitude (ranging from 150 ◦ – 210 ◦ ) in the Galactic outer disk. Figure 3.
Spatial distributions of R ( V ) (left) and its scattering (right). The x-axis and y-axisare Galactic longitude ranging from 230 ◦ – 120 ◦ and latitude ranging from − ◦ – 50 ◦ , respec-tively. tometric Survey of the Galactic Anti-center (XSTPS-GAC; Liu et al. this volume), Pan-STARRS (Kaiser 2004), 2MASS and WISE, and spectro-photometric distances and Gaiaparallaxes in the future, we will produce high spatial resolution (about 10 arcmin), multi-band extinction maps in the Galactic anti-center, and then study the distribution of dustand variations of extinction laws.
3. The diffuse interstellar bands
DIBs are weak absorption features detected in the spectra of reddened stars from thenear UV to the near IR. DIBs have been discovered for almost a century, and to dateover 400 DIBs have been detected in Galactic and extragalactic sources (e.g. Hobbs et al.2008, 2009), but none of their carriers is identified (Herbig 1995; Sarre 2006). The natureof DIBs remains one of the most challenging problems in astronomical spectroscopy. H.-B. Yuan et al.
Figure 4.
The DIBs λλ Most recent work to identify and investigate the properties and carriers of DIBs con-centrates on high-resolution spectroscopy of a small number of selected sight-lines. Usinga template subtraction method based on the standard pair technique, Yuan & Liu (2012)have successfully identified the DIBs λλ E B − V ∼ EW (5780) = 0 . × E B − V and EW (6283)= 1.26 × E B − V ) consistent with previous studies (e.g. Friedman et al. 2011).DIB features have also been detected in the LAMOST spectra (Fig. 4) of resolvingpower similar to that of SDSS. In the commissioning spectra of an emission line star, even9 DIBs have been detected (see Fig. 5 from Yuan & Liu 2012). Detections of DIBs towardshundreds of thousands of stars are expected with LAMOST. The huge DIB databasewill provide an unprecedented opportunity to study the demographical distribution ofDIBs. When combined with other data-sets, it will enable us to address questions like:How the properties of DIBs (e.g. DIB-DIB, DIB-Extinction, DIB-Gas relations) dependon local environment (e.g. UV radiation field, R ( V ), extinction in the FUV and the2175˚A extinction bump)? Where are their carriers formed? Can they be formed in thecircumstellar environments? Meanwhile, DIBs and atomic absorption lines from the ISMcan act as good tracers to probe the distribution and properties of the ISM and dust.
4. Search for extremely faint emission line nebulae
Using ∼ iii ] λλ iii ] λλ apping the 3D extinction and DIBs with LAMOST Figure 5.
Examples of the SDSS spectra with well detected [O iii ] λλ − ergs cm − s − ˚A − . sitivity of the SDSS spectroscopic surveys, this is by far the deepest search for PNe evertaken, reaching a surface brightness of the [O iii ] λ down to about 29.0magnitude arcsec − . The search recovers 14 previously known PNe in the Galactic Caps.Most of them are clearly visible on the SDSS broad-band images owe to their high sur-face brightness. In total, about 60 new planetary nebula (PN) candidates are identified,including 7 probable candidates of multiple detections. All the probable candidates areextremely large (between 21 – 154 arcmin) and faint, located mostly in the low Galac-tic latitude region with a kinematics similar to disk stars, confirming the presence of asignificant population of previously undetected, large, nearby, highly evolved PNe in thesolar neighborhood. Four of the candidates have angular sizes between 84 – 154 arcmin,and might well be the largest PNe ever reported. Based on sky positions and kinematics,12 of the possible candidates probably belong to the halo population. If confirmed, theywill double the numbers of known PNe in the Galactic halo. Most newly identified PNcandidates are very faint, with S between 27.0 – 30.0 magnitude arcsec − , and verychallenging for previously employed techniques (e.g. slitless spectroscopy, narrow-bandimaging). They greatly increase the number of faint PNe and may well represent the”missing” PN population.The results have demonstrated the power of large scale fiber spectroscopy in huntingfor ultra-faint PNe and other types of nebulae by detecting nebular emission lines. Thedetection limits can be further increased by applying the same method which is used todetect DIBs to the SDSS and LAMOST stellar spectra. With millions of spectra from theSDSS, LAMOST and other projects, it will provide a statistically meaningful sample ofultra-faint and large PNe as well as new supernova remnants to improve their censuses.
5. Photometric calibration of wide field imaging surveys
Uniform photometric calibration plays a central role in the large-scale imaging surveys,such as the SDSS, the Dark Energy Survey (DES; The DES Collaboration 2005), Pan-STARS and LSST (Ivezic et al. 2008). The Stellar Locus Regression (SLR) method(High et al. 2009), adopted in DES, can make one wholesale correction for differences ininstrumental response, for atmospheric transparency, for atmospheric extinction, and forGalactic extinction by adjusting the instrumental broadband colors of stars to bring them H.-B. Yuan et al.into accord with a universal stellar color-color locus, yielding calibrated colors accurateto a few percent. The SLR method assumes the standard stellar locus is universal, whichis however not always true, due to varying stellar populations and extinction, especiallyin the Galactic disk region. It also requires a blue filter in addition to at least two of anyother filters.To overcome the limitations above, we propose a new method to perform photometriccalibration using star pairs from large scale spectroscopic surveys (Yuan et al. in prep),such as LSS-GAC. The star pairs here are composed of target and control stars fromuncalibrated and calibrated fields, respectively. This method requires that 1) Extinctionvalues of the targets are known, which can be from Schlegel et al. (1998) or derived fromexisting photometric data; 2) The reddening laws do not change in one field; and 3) Thereare a few calibrated fields to obtain the intrinsic colors. Then it makes one wholesalecorrection by adjusting the instrumental colors of target stars to bring them into accordwith their intrinsic colors, and obtain the reddening coefficient simultaneously. If theaccuracy of instrumental colors is about 1% and about 100 star pairs can be selected foreach field, this method will yield a color calibration accuracy about a few mmag. It isalso useful in checking and improving calibration accuracies of existing surveys.
References
Abazajian, K. N., Adelman-McCarthy, J. K., Ag¨ueros, M. A., et al. 2009,
ApJS , 182, 543Ahn, C. P., Alexandroff, R., Allende Prieto, C., et al. 2012,
ApJS , 203, 21Cardelli, J. A., Clayton, G. C., & Mathis, J. S. 1989,
ApJ , 345, 245Deng, L., Newberg, H., Liu, C., et al. 2012,
RAA , 12, 735Draine, B.T. 2003,
ARAA , 41, 241Fitzpatrick, E. L. 1999,
PASP , 111, 63Friedman, S. D., York, D. G., McCall, B. J., et al. 2011,
ApJ , 727, 33Herbig G H. 1995,
ARAA , 33, 19High, F. W., Stubbs, C. W., Rest, A., Stalder, B., & Challis, P. 2009, AJ , 138, 110Hobbs, L. M., York, D. G., Snow, T. P., et al. 2008, ApJ , 680, 1256Hobbs, L. M., York, D. G., Thorburn, J. A., et al. 2009,
ApJ , 705, 32Ivezic, Z., Tyson, J. A., Acosta, E., et al. 2008, arXiv:0805.2366Kaiser, N. 2004,
SPIE , 5489, 11Martin, D. C., Fanson, J., Schiminovich, D., et al. 2005,
ApJL , 619, L1Massa, D., Savage, B. D., & Fitzpatrick, E. L. 1983,
ApJ , 266, 662O’Donnell, J. E. 1994,
ApJ , 422, 158Perryman, M. A. C., de Boer, K. S., Gilmore, G., et al. 2001,
A&A , 369, 339Sarre, P. J. 2006,
JMS , 238, 1Schlafly, E. F., Finkbeiner, D. P., Schlegel, D. J., et al. 2010,
ApJ , 725, 1175Schlafly, E. F., & Finkbeiner, D. P. 2011,
ApJ , 737, 103Schlegel, D. J., Finkbeiner, D. P., & Davis, M. 1998,
ApJ , 500, 525Skrutskie, M. F., Cutri, R. M., Stiening, R., et al. 2006, AJ , 131, 1163Stecher, T. P. 1965, ApJ , 142, 1683The Dark Energy Survey Collaboration 2005, arXiv:astro-ph/0510346Wright, E. L., Eisenhardt, P. R. M., Mainzer, A. K., et al. 2010, AJ , 140, 1868York, D. G., et al. 2000, AJ , 120, 1579Yuan, H.-B. & Liu, X.-W. 2012, MNRAS , 425, 1763Yuan, H.-B., Liu, X.-W., & Xiang, M.-S. 2013,
MNRAS , 430, 2188
Discussion
Newberg: