RX.net: Async download with limited concurrency

gistfile1.cs

private ISubject<string> _downloaderSubject = new Subject<string>();
var processing = _downloaderSubject
                .Select(url => Observable.FromAsync(x => downloader.GetData(url))
                    .Catch((Exception ex) => Observable.Throw<JsonData>(ex).DelaySubscription(TimeSpan.FromMilliseconds(1000)))
                    .Retry(3)
                    .Catch<JsonData, Exception>(ex =>
                    {
                        _logger.Error("Failed to download", ex);
                        return Observable.Empty<JsonData>();
                    }))
                .Merge(maxConcurrent: 2);
processing.Subscribe(x => Console.WriteLine(x),);