I intended to get a good hour in for LRGB on this, but as it turned out the guiding was intermittent and I really only got 4 each good RGB frames for 20 minute total exposure. So the image is not deep.
I’ve started using CCDStack and, when calibrating the frames, was getting a “negative ADU” alert when applying the flats. This was quite mystifying. It turns out I was not making flats with enough exposure. I resolved it by re-exposing the flats with the 40 watt bulb (instead of 15 watts), increasing the exposure so I was getting about 60% full well capacity. That satisfied CCDStack, and should make a better flat. I’m still seeing irregularities in the background areas, which were repaired in Photoshop with Astronomy Tools from ProDigital Software.
M99 is a “grand design” spiral, except for its crazy arm which has presumably been distorted by a close encounter. Because it presents itself so well, it’s been the subject of a lot of research, mainly concerned with the distortion of the spiral. This gets interesting because HI (neutral hydrogen) mapping from Areceibo showed a massive (about 100 million solar masses) rotating accumulation of neutral hydrogen fairly nearby, dubbed VirgoHI21.
There’s nothing to see of Virgo HI21, it seems to be mass without stars, which only shows up in radio survey. From its mass, it should be a 12th mag galaxy. The discovery of VirgoHI21 was greeted with great excitement, because it was thought to be the first candidate for a dark matter galaxy, which maybe it is. Would that be cool! But it seems that dark matter refuses to reveal itself that easily. Evidence is mounting that the VirgoHI21 is a tidal tail, probably from M99, the result of nearby NGC 4262 gliding thru the neighborhood 280 million years ago. This is all gleaned from the detail page on M99 from WikiSky.
NGC 4298 and 4302 are a lovely pair that I will image soon, if it ever clears up.