Comparing the impact of subfields in scientific journals

The impact factor has been extensively used in the last years to assess journals visibility and prestige. While it is useful to compare journals, the specificities of subfields visibility in journals are overlooked whenever visibility is measured only at the journal level. In this paper, we analyze the subfields visibility in a subset of over 450,000 Physics papers. We show that the visibility of subfields is not regular in the considered dataset. In particular years, the variability in subfields impact factor in a journal reached 75% of the average subfields impact factor. We also found that differences of subfields visibility in the same journal can be even higher than differences of visibility between different journals. Our results show that subfields impact is an important factor accounting for journals visibility.