Spectral study of faint radio sources in ELAIS N1 field

Abstract

Understanding the spectral properties of sources is crucial for the characterization of the radio source population. In this work, we have extensively studied the ELAIS N1 field using various low-frequency radio observations. For the first time, we presented the 1250 MHz observations of the field using the upgraded Giant Meterwave Radio Telescope (uGMRT) that reach a central off-source RMS noise of $\sim$12 $\upmu$Jy beam$^{-1}$. A source catalog of 1086 sources is compiled at $5\sigma$ threshold (>60 $\upmu$Jy) to derive the normalized differential source counts at this frequency, which is consistent with existing observations and simulations. We presented the spectral indices derived in two ways: two-point spectral indices and by fitting a power-law. The latter yielded a median $\alpha = -0.57\pm 0.14$, and we identified nine ultra-steep spectrum sources using these spectral indices. Further, using a radio color diagram, we identified the three mega-hertz peaked spectrum (MPS) sources, while three other MPS sources are identified from the visual inspection of the spectra, the properties of which are discussed. In our study of the classified sources in the ELAIS N1 field, we presented the relationship between $\alpha$ and z. We found no evidence of an inverse correlation between these two quantities and suggested that the nature of the radio spectrum remains independent of the large-scale properties of the galaxies that vary with redshifts.

Appendices

Appendix A. Positional and flux accuracies

Here, we compare the uGMRT 1.25 GHz catalog to the other radio catalogs in the literature. We have used the 1.4 GHz Faint Images of the Radio Sky at Twenty centimeters (FIRST) survey (White et al. 1997), uGMRT catalog at 400 MHz from (for details, see Section 4, Chakraborty et al. 2019) the GMRT catalog at 610 MHz by Ishwara-Chandra et al. (2020) with the resolution of $6''$. We used a search radius of $2''.0$ to identify a cross-match in other catalogs. For the positional and flux accuracy analysis, we have applied a sample selection criteria of sources following Williams et al. (2016): High signal-to-noise ratio (${>}10$) sources, compact sources with size less than the resolution of the catalog and isolated sources for which the minimum distance between the two sources are greater than twice of the resolution.

Fig. 11

Spectra for the USS sources in the ELAIS N1 field. The source IDs mentioned here are from the 400 MHz uGMRT catalog.

1.1 Appendix A.1 Positional accuracy

The positional offsets in right ascension (RA) and declination (DEC) for the uGMRT sample at 1.25 GHz are measured as:

$$\begin{aligned}&\delta _\textrm{RA} = \textrm{RA}_\textrm{uGMRT} - \textrm{RA}_\textrm{FIRST},\end{aligned}$$

(A1)

$$\begin{aligned}&\delta _\textrm{DEC} = \textrm{DEC}_\textrm{uGMRT} - \textrm{DEC}_\textrm{FIRST}. \end{aligned}$$

(A2)

The FIRST catalog has positional accuracy better than $1''$ with a resolution of ${\sim }5''$. We measured the median values in the deviation of RA and DEC using the FIRST catalog as $-0.092''$ and $-0.076''$, respectively. Figure 10(left) presents the offsets in RA and DEC for the uGMRT source catalog compared to the other catalogs, along with their histograms. The median offsets in RA and DEC, as measured from the GMRT 610 MHz and the uGMRT 400 MHz catalogs, are −0.07, −0.16 and −0.43, 0.59, respectively. It should be noted that the resolution of our catalog ${\sim }2''$ is better than the resolution of other catalogs $\sim$5$''$–6$''$, and the median offset with the FIRST catalog is ${<}0.1''$. Hence, we do not apply any corrections in the source positions in our uGMRT catalog.

1.2 Appendix A.2 Flux accuracy

Our uGMRT 1.25 GHz catalog was generated using Perley & Butler (2017) flux scales. Each catalog will have different flux scales, therefore, we have made sure to convert them to the flux scales used in our work. We measured the ratio of the integrated flux density at 1.25 GHz with the other catalogs also scaled to 1.25 GHz using a constant spectral index value of −0.7. This ratio is defined as $S_{\mathrm{1.25~GHz}}/ S_\textrm{other}$. In Figure 10(right), we show the comparison of $S_{\mathrm{1.25 \ GHz}}$ with $S_{\textrm{other}}$ and no significant deviation is observed from the $S_{\mathrm{1.25 \ GHz}}/S_{\textrm{other}}=1$ line (black dashed line). The median $S_{\mathrm{1.25~GHz}}/S_\textrm{other}$ ratio as derived using the FIRST, uGMRT 400 MHz and GMRT 610 MHz catalogs are $0.99^{0.19}_{-0.37}$, $1.11_{-0.51}^{0.25}$ and $1.10_{-0.9}^{0.32}$, respectively. The errors quoted here are from the 16th and 84th percentiles. The median of the ratio is $\sim$1 for these cases and therefore, we do not suggest any correction for systematic offsets.

Appendix B. Spectra of USS sample

The spectra of the USS sources are shown in Figure 11.