Since this distance increases down the group, the outermost electron feels less attraction from the nucleus and thus the first ionisation energy decreases.
sˈɪns ðɪs dˈɪstəns ˈɪŋkɹiːsᵻz dˌaʊn ðə ɡɹˈuːp, ðɪ ˈaʊɾɚmˌoʊst ᵻlˈɛktɹɑːn fˈiːlz lˈɛs ɐtɹˈækʃən fɹʌmðə nˈuːklɪəs ænd ðˈʌs ðə fˈɜːst ˌaɪənəzˈeɪʃən ˈɛnɚdʒi dˈiːkɹiːsᵻz.