I really don't currently have the time to do some real scholarly research (and that would be a bit harder to share over the net), so my results are from internet searching, although I did pay close attention to the sources.
I do want to say, though, that the idea of "objectivity" is a problematic one--in other words, how does one define "objective"? It is my argument (and the argument of many philosophers) that because human beings are subjects, we cannot really take an objective approach, even as we try to (though some do try more than others). For me, I prefer sources that declare their biases rather than trying to mask them.
As for the sources you list, I am not trying to discredit them, but I noticed many from mainstream media and wikipedia. The news media does tend to take a slant (whether that slant is conservative or liberal), and wikipedia is only as accurate as the last person who edited its content, so I don't particularly see these as objective sources. That doesn't mean that they are necessarily faulty, but they should be scrutinized, not automatically taken as unbiased truth.
That said, here's what I found:
This source is from the White House; it is from 1998, so it is a bit dated, but it basically argues that although the wage gap has significantly narrowed, "there still exists a significant wage gap that cannot be explained by differences between male and female workers in labor market experience and in the characteristics of jobs they hold."
http://clinton4.nara.gov/WH/EOP/CEA/html/gendergap.html
This next source gives a chart of the wage gap from 1970-2004 based on both race and gender. It lists its source for the info as "The U.S. Current Population Survey and the National Committee on Pay Equity."
http://www.infoplease.com/ipa/A0882775.html
This link is from the same site, but gives and overview with charts, including the one above. According to the site, the source for the statements in this overview is "The National Women's Law Center." It argues that "in 2005, women earned 77% as much as men," which has gone up 1% from the 76% that I had learned a few years back, so it seems that it is still narrowing, which is promising.
http://www.infoplease.com/ipa/A0763170.html
This chart, from the "National Committee on Pay Equity," a "coalition of women's and civil rights organizations" (so you may see this as biased) gives the same percentage of 77% for 2005.
http://www.pay-equity.org/info-time.html
This source says the gap is widening, which goes against my other sources--the article names the US Census Bureau as its source (but this is a case where one would especially want to look at that primary source).
http://usgovinfo.about.com/od/censusandstatistics/a/paygapgrows.htm
Finally, I am also including this source, which talks about negotiation of pay and the wage gap. The article lists studies that could also be looked up as primary sources.
http://usgovinfo.about.com/od/censusandstatistics/a/paygapgrows.htm
I'm going to stop there b/c i always write these super-long answers. I just wanted to add that i think you'll find your answer by compiling lots of info and closely evaluating the quality of sources, and I hope my research contributed to that a bit.