B757 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Crew Type 550
Communication Breakdown; Distraction; Workload; Situational Awareness
Conflict NMAC; Inflight Event / Encounter Bird / Animal
Flight Crew Returned To Clearance; Flight Crew Took Evasive Action
Aircraft; Human Factors; Procedure
We had a bird strike at 6000 ft just to preface this event.  Was given a descent to 2000 ft and slow to 170 knots; then told to follow a company 737 for a visual approach to 4R.  Then a frequency change to tower frequency.  Before we could check in with tower; and as we were leveling at 2000 ft; we got a TRAFFIC; TRAFFIC. I looked at the TCAS and showed an aircraft at 500 ft below us and climbing (almost directly below our position slightly to the left on the 10 mile scale) then within a second we got a CLIMB; CLIMB and the red envelope had a very aggressive climb (not like the small climbs in the SIM for training); I clicked off the autopilot and auto throttles and pitched to climb outside of the red box.  I glanced at the TCAS as it showed the other aircraft at 300 feet.  After climbing to about 2800 feet we got a CLEAR OF CONFLICT.  The FO (First Officer) switched back to approach and told them we had just performed an RA maneuver for traffic.  ATC said something to the effect that; oh yeah; there's Aircraft Y there at 1800 ft but I am not talking to him; and said something about that being a tower thing.  We continued the visual approach to 4R and landed without further incident.
B757 pilot reported a TCAS RA on final with an aircraft that the tower knew about but did not issue as traffic.