What does a space in a jquery selector mean?

I ran into a reaction I couldn't explain today while working with some very basic Jquery today and I was hoping one of you could explain to me what is occurring to lead to these results

So I have a DOM model (simplified here)

<div class="ObjectContainer">
    <div class="Object">
        <div>stuff</div>
    <div class="Object">
        <div>stuff</div>

The idea was to set an attribute on the last Object using this code:

$('div.ObjectContainer').find('div.Object :last').attr("index", "1");

I understand now the code here was incorrect and the proper find selector should be 'div.Object:last', but it is the results I don't understand. When I executed the first code this occurred:

<div class="ObjectContainer">
    <div class="Object">
        <div index="1">stuff</div>
    <div class="Object">
         <div>stuff</div>

Could someone explain to me how my initial selector managed to set an attribute on a child node?