  We study how collective memories are formed online. We do so by tracking
entities that emerge in public discourse, that is, in online text streams such
as social media and news streams, before they are incorporated into Wikipedia,
which, we argue, can be viewed as an online place for collective memory. By
tracking how entities emerge in public discourse, i.e., the temporal patterns
between their first mention in online text streams and subsequent incorporation
into collective memory, we gain insights into how the collective remembrance
process happens online. Specifically, we analyze nearly 80,000 entities as they
emerge in online text streams before they are incorporated into Wikipedia. The
online text streams we use for our analysis comprise of social media and news
streams, and span over 579 million documents in a timespan of 18 months. We
discover two main emergence patterns: entities that emerge in a "bursty"
fashion, i.e., that appear in public discourse without a precedent, blast into
activity and transition into collective memory. Other entities display a
"delayed" pattern, where they appear in public discourse, experience a period
of inactivity, and then resurface before transitioning into our cultural
collective memory.
