For convenience, all the magnitudes in optical and NIR filter sets in each catalog are converted to those of the SDSS(u, g, r, i, z) and UltraVISTA(J, H, Ks) filters. The formulae
for the conversions are given in Appendix.
To test our calibration, we extract the information of main sequence stars from UDS (star flag = 1), CFHTLS (class star > 0.95 and g < 23) and COSMOS (type = 1.0) then adopt their photometric observations into several color-color diagrams. The position of star sequence in the gri- and gzKs- diagram (Fig. 1) nicely overlap after applying the magnitude conversions. We also apply the SDSS spectroscopically confirmed stars into the gri- diagram (red points in figure 1). Due to the lack of K-band information, SDSS stars are not included in the gzKs- diagram. Although some small offsets (∼0.1 mag) still exist on the color-color diagrams between the catalogs, those offsets do not affect our results significantly.
Table 2.1: The list of the photometric data
Photometric Catalog Instrument Filter λeff (nm)
B 447.8
Figure 1: (a) g-r versus r-i color-color diagram of stars in all the datasets adopted. Light blue, green and black color points represent the UDS, CFHTLS and UltraVISTA samples respectively. Because the final results of our visible filter systems are in the SDSS base, we add the SDSS stars observation in this diagram (red points). (b) g-z versus z-Ks diagram of stars from all of our datasets. Due to the lack of K-band information, SDSS stars are not included in this diagram. The stars occupy similar positions in the color-color diagrams, which indicates that our filter transformations are reliable.
Chapter 3
Peas Selection Criteria
To define our selection criteria, we use 80 low-redshift SDSS peas spectra (z∼0.3, Cardamone et al., 2009) and 8 higher-redshift ESO-NTT pea spectra (z∼0.6, Bamford et al., 2014) as spectrum templates. The spectra of the 80 SDSS peas were taken from SDSS-III Science Archive Server which contains a well co-added calibrated flux measured through a fiber with 3” circular aperture. The spectra of 8 higher-redshift NTT-peas are taken from the NTT EFORSC2 intrument by a grism with 1.2” slit width. Convolving these spectrum templates with broad-band photometric system enables us to define the peas selection criteria in color-color diagrams.
The coverages of spectroscopic data for SDSS and NTT are not enough to convolve with the broad-band systems; therefore, we use their individual photometric information taken from the SDSS and UDS photometric catalog (3” aperture for SDSS and 2” aperture for NTT) to extend our spectral templates. To make a fair comparison between photomet-ric and spectroscopic data, we apply the aperture correction into 2” for NTT spectra flux by multiplying the spectral flux by 1+(1.2”/2”)2. This factor is coming from the ratio of aperture areas that between the photometric and spectroscopic data. In the case of slits (or fibres), small slit width (or aperture size) will physically lose flux compare with the bigger one due to they have different size of physical aperture. Therefore, in our case, we need to apply the correction into the relative smaller slit width of NTT spectroscopic data and make them consistent with their photometric data. Figure 2 represents the example of spectroscopic data for SDSS and NTT pea. The photometric flux estimated from the
above procedure are also shown as red-crossed points in Figure 2.
In order to test our convolution, we employ those extended SDSS/NTT peas spectra to convolve them with the response of SDSS/UDS filters in original redshift. Then, we compare the results with their own photometric observation. Our results show a good consistency within photometric and spectroscopic data (△magnitude ∼ 0.05 mag).
These 88 spectral templates are shifted to our specific redshift ranges of interest (z=0.15-0.40, z=0.40-0.65, z=0.65-1.20 and z=1.35-1.65) with a redshift interval of 0.01 and we convolved the spectra with the response of SDSS (optical) and UltraVISTA (NIR) filters to predict the magnitude of our candidates in each broad-band filter. They cover a similar region (green contours in Fig. 3, 4, 5) in several color-color diagrams such as g-r versus r-i, r-i versus i-z, i-z versus z-J and z-J versus J-Ks. The similar region in color-color diagrams will enable us to identify the objects that unusually luminous in r-, i-, z- and J- bands. The redshift interval of 0.01 will allow us to define the finest distribution for
“green pea” galaxies in those color-color diagrams.
Thanks to the deep photometric datasets available (UDS-DR10, CFHTLS+Palomar and COSMOS+UltraVISTA), we can extend our broad-band method to select the r-, i-, z-, J- luminous objects which could be the condidates of “pea galaxies” at z∼0.25, z∼0.55, z∼0.85 and z∼1.5.
Note here that almost all of our color-color diagrams are shown as the subtraction of nearest photometric systems except our J-selection. We take the Ks-band instead of H-band in z-J versus J-Ks color-color diagram due to the lack of H-H-band data in the CFHTLS +Palomar catalog. Beside that, we also show the location of SDSS-peas in g-r versus r-i color-color diagram ( green star symbols in Figs. 3a, 4a, 5a).
Figure 2: (a) One sample from our SDSS spectrum template. Red cross points represent the SDSS u, g, r, i, z phomotometric information which correspond to their effictive wave-length respectively. (b) Same as in (a) but sample from our NTT spectra template. The red cross points represent the UDS U, B, V, Rc, i, z photometric information.
Chapter 4
Spectroscopic Data
To further investigate the properties of our samples, we need spectroscopic data for each of the broad-band surveys. For the UDS dataset, an UDSz ESO Large Programme (180.A-0776, PI: Almaini) which provides targeted spectral observations for∼3000 K-band selected galaxies, was completed using VIMOS and FORS2. Galaxies were selected over 0.6 square degrees of the UDS field to a limit of K=23(AB). The spectroscopy of the CFHTLS+Palomar dataset were carried out utilizing the DEEP2 DEIMOS (DEep Imaging Multi-Object Spectrograph) on the Keck II 10m telescope (Newman et al. 2013). DEEP2 Galaxy Redshift survey targeted almost 50,000 galaxies at z < 1.4. Finally, the zCOSMOS redshift survey is performed by VIMOS, providing approximately 20,000 i-band selected galaxies at redshifts z < 1.2 in 1.7 square degrees of the COSMOS field (Lilly et al. 2007).
The spectroscopic data in each of color-color diagrams can be ultilized to comfirm the color selection for our data. Therefore, we add all the spectroscopic data into the color-color diagrams (big points in Figs. 3, 4 and 5). For convenience, we also separate the spectroscopic data into emission line galaxies (flag 2, number of emission line≥ 2), potential emission line galaxies (flag 1, number of emission line=1) and not emission line galaxies (flag 0). The emission lines flag information associate with redshift distribution can made us to select the emission line galaxies and potential emission line galaxies with expected redshift from the entire samples (big blue and light blue points in Figs. 3, 4 and 5). The distribution of emission line galaxies and potential emission line galaxies with different redshift also represent in our color-color diagrams (big yellow and purple points
in Figs. 3, 4 and 5).
Beside that, we also separate our spectroscopic data by three different photometric criteria which are loose-, middle- and tight-criterion (small red, orange and green points in Figs. 3, 4 and 5). Loose-criterion almost can include all the emission line galaxies with our expected redshift in color-color diagrams. In order to extract the candidates of “green pea” galaxies from our catalogs, we divide our spectroscopic data into two more categories with middle- and tight-criterion, which are more consistent with the distribution of “green pea” galaxies in color-color diagrams (green contours in Figs. 3, 4 and 5). More details about our photometric criteria are described in Table 4.1.
Table 4.1: The summary of our photometric criteria
(a) Loose-criterion almost can include all the emission line galaxies with our expected redshift in color-color diagrams. In order to extract the candidates of “green pea” galaxies from our catalogs, we divide our spectroscopic data into two more categories with (b) middle- and (c) tight-criterion, which are more consistent with the distribution of “green pea” galaxies in color-color diagrams (green contours in Figs. 3, 4 and 5).
Figure 3: Color-color diagram for the UDS catalog. Small and big points correspond to the UDS photometric and spectroscopic data respectively. Blue and light blue big points represent the emission line galaxies (ELGs) and potential emission line galaxies at our expected redshift. Yellow and purple big points represent the emission line galaxies and potential emission line galaxies but out the redshift range. Red, orange and green small points indicate the loose-, middle- and tight- photometric criteria which are discribed in Section 4. Green contours correspond to the distribution of our 88 pea spectral templates.
(a) r-i versus g-r diagram, aiming at selecting the 0.15<z<0.35 pea galaxies. Green star symbols represent the location of SDSS-peas galaxies in g-r versus r-i color-color diagram.
(b) i-z versus r-i diagram, aim to select the 0.4<z<0.7 pea galaxies. (c) z-J versus i-z diagram, aim to select the 0.7<z<1.2 pea galaxies. (d) J-Ks versus z-J diagram, aim to15
Figure 4: Same as Fig 3. but for the CFHT-LS sample. Small and big points correspond to the CFHT-LS photometric and spectroscopic data respectively. (a) r-i versus g-r diagram, aim to select the 0.15<z<0.35 pea galaxies. Green star symbols represent the location of SDSS-peas galaxies in g-r versus r-i color-color diagram. (b) i-z versus r-i diagram, aim to select the 0.4<z<0.7 pea galaxies. (c) z-J versus i-z diagram, aim to select the 0.7<z<1.2 pea galaxies. (d) J-Ks versus z-J diagram, aim to select the 1.3<z<1.7 pea galaxies.
Figure 5: Same as Fig 3. but for the COSMOS sample. Small and big points correspond to the COSMOS photometric and spectroscopic data respectively. (a) r-i versus g-r diagram, aim to select the 0.15<z<0.35 pea galaxies. Green star symbols represent the location of SDSS-peas galaxies in g-r versus r-i color-color diagram. (b) i-z versus r-i diagram, aim to select the 0.4<z<0.7 pea galaxies. (c) z-J versus i-z diagram, aim to select the 0.7<z<1.2 pea galaxies. (d) J-Ks versus z-J diagram, aim to select the 1.3<z<1.7 pea galaxies.
Chapter 5
Properties of Emission Line Galaxies
The spectra of emission line galaxies at different redshifts in those different fields could tell us many details about the properties of these galaxies. Here, we only focus on the true emission line galaxies sample (flag 2) for the r- and i- selection. However, both true and potential emission line galaxies (flag2 and flag 1) are considered for the z- and J-selection because the narrow wavelength coverage of spectra often allows us to identify one emission line at most.
In order to estimate the luminosity and equivalent width of the emission lines, we adopted the IDL routine - gaussfits. First we exclude all the emission lines from the spectra respectively to estimate their continuum flux level. We blue shift the spectra individually then exclude the rest-frame emission lines by the±15 Å. After that, we deduct the contin-uum flux from the original spectra and use the IDL routine - gaussfits to fit the emission line in gaussian at a certain range of spectra. We can derive the luminosity of emission line by integrating the gaussian fitted to the emission line profile. The equivalent width of emission line is also derived by their luminosity and continnum flux level.
The spectral resolution may play an important role in our flux or equivalent width estimation. The spectral resolution of our spectroscopic data are represented as UDSz∼ 6Å (VIMOS∼ 5.5Å and FORS2 ∼ 6.4Å), DEEP2 DEIMOS ∼ 1.6Å and zCOSMOS ∼ 5.5Å. The emission lines width of green peas galaxies are∼ 25Å. Even though the worst case of spectral resolution (∼ 6.4Å) for UDSz data, the emission lines can be resolved and fitted by Gaussian. We claim that the spectral resolution for each of the spectroscopic data
Figure 6: The comparison of our [OIII]λ5007Å equivalent width estimation and [OIII]λ5007Å equivalent width estimated by GANDALF code for the SDSS-peas (ta-ble 4. of Cardamone et al. (2009) ). The solid line represents the equality of the two estimations.
is enough for us to estimate their flux or equivalent width.
Utilizing the equivalent width information of SDSS Green Peas galaxies (Cardamone et al. 2009), we can examine our equivalent width estimation. The [OIII]λ5007Å equiv-alent widths taken from table 4 of Cardamone et al. (2009) are derived from the SDSS database using the GANDALF(Gas AND Absorption Line Fitting) code. As shown in Fig. 6, our results are consistent with the GANDALF estimation, which indicates that our estimation is reliable.
5.1 “Baldwin, Phillips & Terlevich” (BPT) - diagram
The “Baldwin, Phillips & Terlevich” (BPT) - diagram is consisted of several sets of nebular emission line flux ratio which can distinguish the ionization mechanism of nebular gas come from star forming regions or AGNs. Although AGNs are already flagged in each of the catalogs, we still need to comfirm that there are free of AGNs feature in our
emis-sion line galaxies samples. Since the emisemis-sion lines covered by spectroscopic observations could be different at different redshifts, only small fraction of our samples have both de-tected [OIII]λ5007Å/Hβ and [NII]λ6583Å/Hα in BPT-diagram (Fig. 7). However, most of the low redshift sample only have [OIII]λ5007Å/Hβ (or [NII]λ6583Å/Hα) detection which represents the horizontal (or vertical) histogram in Fig. 7. This empirical method succeeds in making the separation of AGNs and star-forming galaxies (Kewley et al. 2001, red curve in Fig. 7). The AGNs will exceed the red curve which will locate at upper, up-per right and right in BPT-diagram. According to the value from Kewley et al. 2001, we filtered out 7 AGNs that exceed the red curve, 11 potential AGNs which represent the log([OIII]λ5007Å/Hβ) > 0.9 but found no potential AGNs for log([NII]λ6583Å/Hα) >
0.2. Unfortunately, our z-selection and all J- selection samples barely have [OIII]λ5007Å/
Hβ or [NII]λ6583Å/Hα detection; thus, we do not apply the BPT selection and just follow their AGNs flag information given by each of the catalogs.