EMB-175 flight crew reported near miss with a small General Aviation aircraft during descent.

Narrative

The following sequence of events are written to the best of my knowledge and memory; several hours after the occurrence; once I had finished my last flight of the day. The Captain (CA) and myself; First Officer (FO) operated Aircraft X from ZZZ1 to ZZZ. The CA was Pilot Monitoring while I (FO) was Pilot Flying. The following event occurred while on descent into ZZZ on the Arrival. We had been cleared to descend 'Via the arrival' by ATC as is standard procedure. Descending through about 9000ft; between the fix ZZZZZ and ZZZZZ1 ATC advised us of a Traffic ahead and stated; 'Aircraft X; Traffic ahead orbiting; not reporting any altitude; no factor; low level traffic.' At this moment we were in and out of IMC Conditions; in light to moderate turbulence. The CA read back to the controller ' Roger; looking for traffic; no visual. At this time; I suggested to the Captain that we request vectors around the traffic given that we were unsure of its exact position; the Captain stated that we would be okay. As we continued our descent; we could see a target on the Captain's MFD (Multi-function Flight Display) but with no altitude indicated. (My MFD had been MEL'd on this aircraft; it was INOP which means I only had a functional PFD (Primary Flight Display)). Having no altitude read out on a traffic at this altitude is uncommon and our belief is the target did not have ADSB Type C; which transmits altitude readouts. Still; the Air Traffic Controller had advised us that it wasn't a factor to us given that it was 'low level' and to continue our descent on the arrival as normal. A couple seconds later; however; we got a TCAS Alert 'Traffic; Traffic'. We were in and out of the clouds and did our best to look out when suddenly the Captain called out a traffic ahead slightly higher than us. Realizing the small General Aviation Aircraft (Piper Archer or similar) was right in front of us; I immediately disconnected the Autopilot and progressively pushed forward on the control column to clear our aircraft of conflict with the GA aircraft. To the best of my memory; I pitched down to about -5 deg to clear the other plane. We descended no more than 150-200ft; down to about 7850ft and when I was assured we were clear of the traffic; I resequenced the autopilot and climbed back to 8000ft. The Captain advised the controller that we had encountered a GA traffic about 150ft above us; and suggested the controller reroute the traffic behind us on the arrival around the target. The controller seemingly startled over the radio; proceeded to re-vector the planes behind us. After the first Captain report to ATC of the traffic; the Controller asked us if we received a TCAS RA; the Captain stated that we did not. Indeed; at no point did we receive a TCAS RA instructing us to avoid this aircraft as is usually the case in close proximity with other traffic. We believe that we did not receive a TCAS RA despite the close proximity because the GA aircraft's transponder was not reporting any altitude; thus our plane did not know its exact position to issue an advisory. Once at 8000ft; with the Autopilot re-engaged and cleared of conflict; we resumed our descent on the arrival and made an uneventful approach and landing on Runway XXL where we safely taxied and parked at our gate. No ATC altitude deviation were made during the evasive action as we had been cleared to descend via and were within the altitude brackets at all times on the arrival; the decision to climb back up to 8000ft was a personal one as this was our original altitude before the event occurred. At no point did we feel the passengers or cabin crew were put at risk from the evasive action. The aircraft was flown within its flight envelope and we followed our standard operating procedures. Given the turbulent atmosphere at the time; I believe the evasive action was not apparent to passengers and cabin crew. Once parked; the Captain contacted our Chief Pilot to make him aware of the occurrence. Giventhat we were an IFR traffic in and out of IMC; and did not have visual with the traffic or were in VMC; the controller should have routed us around the target on the arrival and not merely assumed it was 'low level' and 'not a factor'. By doing so; the controller put us at risk and significantly reduced safety margins in doing so. Fortunately; we broke out of the cloud; noticed the traffic and were able to take evasive action given the lack of a TCAS RA."