The K20 survey VII. The spectroscopic catalogue: Spectral properties and evolution of the galaxy population

Mignoli,M., et al. 2005, A&A, 437, 883

Introduction

• K-selected sample が良い理由
  1. k-correction の効果を受けにくい
  2. dust の影響が小さい
  3. 銀河の質量を良く反映している

• 最近の K-selected sample の分光サーベイ
  GDDS: K_s<20.6, 120arcmin², z>0.8 candidates のみ
  K20 : K_s<20, 52arcmin²

• K-selected sample は z〜1 まででは slow evolution で、1.5<z<3 で急速に進化するようだ

The K20 survey and its main results

Figure 1: The K20 (rectangle, centered at 03^h32^m22^s.5 -27°46'23".5 , J2000) and the Hubble Ultra Deep (square, centered at 03^h32^m39^s.0 -27°47'29".1) fields in the CDFS region, superimposed on the GOODS HST/ACS z-band mosaic

• K20 survey
  ESO VLT Large Program
  K_s<20, 545 天体, completeness 92%
  

  ^a zq stands for "z-quality": galaxies with secure redshifts have zq=1

• これまでに行われた研究
  EROs の分布やスペクトル (Paper I,II)
  z 分布のモデルとの比較 (Paper IV)
  EROs の形態と X-ray の性質
  CDFS 天体の大規模構造
  近赤外天体の明るさの進化 (Paper V)
  星の質量関数 (Paper VI)
  銀河の blue Luminosity Function の進化
  z〜2 の巨大星生成銀河の性質
  z〜1.9 の古い巨大銀河の発見
  BzK 銀河
  rest-frame B-band morphology の進化
  

Spectroscopic observations and data reduction

• VLT/FORS1,FORS2, 20夜, 各天体 15分〜8時間露出

Redshift determination and spectral features measurement

• 2人で別々に同定
  zq=0 : 32天体 <z>〜1.06
  zq=1 : 444天体 <z>〜0.75

  

  Figure 2: The redshift distribution of the K20 spectroscopic catalogue for the whole extragalactic sample (empty histogram), the galaxies with "secure" redshifts (zq=1; light gray histogram) and the type 1 AGNs (dark gray histogram).

  

  Figure 3: Redshift measurement completeness in the K20 survey. The fractions as a function of magnitude of the observed targets (light gray histogram) and of the objects with a redshift measurement (dark gray histogram) are shown both in the R band ( top panel) and K band (bottom panel). The total number of sources in each magnitude bin is labeled along the top. The overall redshift measurement completeness is 92%, whilst redshifts are measured for 95% of the observed objects.

  

  Figure 4: Rest-frame measurement distributions of the main spectral features: [O II]λ3727 EW ( top left panel; 347 measurements), [O III]λ5007 EW ( bottom left; 122), Hβ EW ( top middle; 148), Hα+[N II]λλ6548, 6583 EW ( bottom middle; 63), D4000 ( top right; 426) and our UV color index ( bottom right; 349).

  星の吸収の効果は補正していない
  Hβ の吸収が見られるもので Hα+[N II] が測定波長域内にあったものは 8天体
  4天体は弱い輝線を示し、残り4天体は noise に埋もれて検出不可能
  UV continuum color 分布には 2つのピークがある

  

  Figure 5: The rest-frame equivalent width of the [O II]λ3727 line plotted against the signal-to-noise ratio ( S/N) of the continuum close to the line. The dotted line is the EW detection limit curve as estimated by Eq. (2), representing a good lower envelope of the data.

  

  Figure 6: Relationship between the equivalent widths of the [O II]λ3727 line and Hα+[N II]λλ6548, 6583 complex for the K20 survey. The solid line shows the correlation found using our data, while the other lines represent the relationships found in the literature [ dotted line, local data (Kennicutt 1992, K92); dashed line, Stromlo-APM Survey (Tresse et al. 1999); dash-dotted line, Canada-France Redshift Survey (Tresse et al. 1996)].

  High-z ほど Hα+[N II] よりも [O II] が強くなる
  => 同じ z でも luminosity の違いで同様な傾向が見られるので、多分 sample の luminosity の違い
  => metallicity - luminosity 関係?

Galaxy spectroscopic classifications

• z<1.6 402天体を 3つのタイプに分類
  Red, passive early-type galaxies: 輝線無し、Red UV/D4000大
  Blue, emission-line galaxies: 輝線あり、Blue UV/D4000小
  Emission-line galaxies with a red continuum: 輝線あり、Red UV/D4000大
  どれにも属さないものは 18/402 天体で、Post-Starburst や ERO、S/N 不足など

  

  Figure 7: D4000 vs. equivalent width of the [O II]λ3727 for the K20 galaxy sample. The different symbols refer to the preliminary, visual classification: filled circles are type-1 AGN, empty circles are early-type passive galaxies (class 1), crosses are red continuum emission-line galaxies (intermediate) and empty triangles are blue emission-line galaxies (class 2).

  

  Figure 8: Our UV color index vs. equivalent width of the [O II]λ3727 for the K20 galaxy sample. The different symbols refer to the preliminary, visual classification and are the same as in Fig. 7.

  上記分類は ACS による形態の違いと非常に良く相関する

Composite spectra

• 平均で算出、median との違いは輝線部分に少し現れる程度

  

  Figure 9: Composite spectra of the three main galaxy classes with the identification of the main absorption and emission lines. From bottom to top: early-type, intermediate and blue emission-line galaxy classes. The flux is per unit wavelength (F_λ), and normalization is arbitrary. All the composite spectra can be retrieved at the following site: http://www.arcetri.astro.it/~k20/

  

  Figure 10: Normalized distributions of the absolute magnitude computed in the rest frame J-band (M_J) for the different spectroscopic classes: the solid histogram is for blue emission-line galaxies, the dashed histogram for passive early-type galaxies and the dash-dotted histogram for red continuum emission-line galaxies.

  Red, passive early-type galaxies の方が大きい

Evolution of the spectral properties of the K20 spectroscopic sample

• Early-type galaxies のスペクトル進化

  

  Figure 11: Composite spectra of the early-type galaxies divided in three redshift bins. From bottom to top: the composite of 18 low-z ( 0 < z < 0.6) galaxies, the composite of 49 intermediate-z ( 0.6 < z < 0.75) galaxies and the composite of 26 high-z (0.75 < z < 1.25) galaxies. Almost all the galaxies included in the intermediate-z sample belong to the two overdensities present in the K20 survey area. The flux normalization is arbitrary and the spectra are offset for clarity. The upper and lower panels also show, with the same scale, the difference spectra between the intermediate-redshift range composite and, respectively, the high-z composite and the low-z composite spectra.

  

  Figure 12: D4000 strengths as a function of redshift in K20 early-type composites. The galaxies in each redshift bin have been divided in two equally populated groups, according to their luminosity, and circles and triangles indicate the brighter and fainter subsamples, respectively. The dashed line indicates the D4000 value measured in the average spectrum of the whole early-type class of Fig. 9, whereas the dash-dotted line is the D4000 measured in the SDSS composite of early-type galaxies (MAIN sample, Eisenstein et al. 2003).

  D4000 は luminosity だけでなく z にも依存する
  massive spheroids のかなりの部分の生成は z>1

• Blue emission-line galaxies のスペクトル進化
  輝線はそれほど変らないが、UV color が昔ほど青くなっている
  BC03 model と比較すると、z<0.7 では t/τ〜2.0-2.5, z>0.7 では t/τ〜1.2-1.8
  明るいほど [O II] が強い => metallicity - luminosity 関係?

  

  Figure 13: Composite spectra of the M_J<-20 blue emission-line galaxies in four redshift bins. The flux normalization is arbitrary and the spectra are shown only in the wavelength range in which more than 20 spectra contribute to the template. The apparent intensity increase of the unresolved [O II] emission line with redshift is a visual effect due to the enhancement of rest-frame spectral resolution in the high-z composites.