  In spite of their importance as standard candles in cosmology and as major
major sites of nucleosynthesis in the Universe, what kinds of progenitor
systems lead to type Ia supernovae (SN) remains a subject of considerable
debate in the literature. This is true even for the case of Tycho's SN exploded
in 1572 although it has been deeply studied both observationally and
theoretically. Analyzing X-ray data of Tycho's supernova remnant (SNR) obtained
with Chandra in 2003, 2007, 2009, and 2015, we discover that the expansion
before 2007 was substantially faster than radio measurements reported in the
past decades and then rapidly decelerated during the last ~ 15 years. The
result is well explained if the shock waves recently hit a wall of dense gas
surrounding the SNR. Such a gas structure is in fact expected in the so-called
single-degenerate scenario, in which the progenitor is a binary system
consisting of a white dwarf and a stellar companion, whereas it is not
generally predicted by a competing scenario, the double-degenerate scenario,
which has a binary of two white dwarfs as the progenitor. Our result thus
favors the former scenario. This work also demonstrates a novel technique to
probe gas environments surrounding SNRs and thus disentangle the two progenitor
scenarios for Type Ia SNe.
