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NGC 5746
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Title: Two Pseudobulges in the "Boxy Bulge" Galaxy NGC 5746
Authors: John C. Barentine, John Kormendy

Galaxy formation and growth under the Lambda CDM paradigm is expected to proceed in a hierarchical, bottom-up fashion by which small galaxies grow into large galaxies; this mechanism leaves behind large "classical bulges" kinematically distinct from "pseudobulges" grown by internal, secular processes. We use archival data (Spitzer 3.6 Ám wavelength, Hubble Space Telescope H-band, Two Micron All Sky Survey Ks-band, and Sloan Digital Sky Survey gri-band) to measure composite minor- and major-axis surface brightness profiles of the almost-edge-on spiral galaxy NGC 5746. These light profiles span a large range of radii and surface brightnesses to reveal an inner, high surface brightness stellar component that is distinct from the well-known boxy bulge. It is well fitted by SÚrsic functions with indices n = 0.99 ▒0.08 and 1.17 ▒0.24 along the minor and major axes, respectively. Since n < 2, we conclude that this innermost component is a secularly-evolved pseudobulge that is distinct from the boxy pseudobulge. This inner pseduobulge makes up 0.136 ▒0.019 of the total light of the galaxy. It is therefore considerably less luminous than the boxy structure, which is now understood to be a bar seen nearly end-on. The infrared imagery shows further evidence for secular evolution in the form of a bright inner ring of inner radius 9.1 kpc and width 1.6 kpc. NGC 5746 is therefore a giant, pure-disk SB(r)bc galaxy with no sign of a merger-built bulge. We do not understand how such galaxies form in a Lambda CDM universe.

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Posts: 131433
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NGC5746
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Chandra X-ray telescope observations of the massive spiral galaxy NGC 5746 in the constellation Virgo, have revealed a large halo of hot gas (blue) surrounding the optical disk of the galaxy (white). The halo extends more than 60,000 light years on either side of the disk of the galaxy, which is viewed edge-on.


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Position (2000) : RA 14h 44m 55.90s Dec +01║ 57' 19.00"

The galaxy shows no signs of unusual star formation, or energetic activity from its nuclear region, making it unlikely that the hot halo is produced by gas flowing out of the galaxy. Computer simulations and Chandra data show that the likely origin of the hot halo is the gradual inflow of intergalactic matter left over from the formation of the galaxy.
Spiral galaxies are thought to form from enormous clouds of intergalactic gas that collapse to form spinning disks of stars and gas. One prediction of this theory is that massive spiral galaxies should be immersed in halos of hot gas left over from the galaxy formation process.

Hot gas has been detected around spiral galaxies in which vigorous star formation is ejecting matter from the galaxy, but until now, hot halos due to infall of intergalactic matter had not been detected. Indeed, the extensive hot gas halo around NGC 5746 is faint and would be very difficult to detect without a powerful X-ray telescope such as Chandra. Also, the galaxy's special orientation and large mass increased the chance of detection.

The discovery of a hot halo around NGC 5746 was welcome news to astronomers because it shows that the "missing" hot halos predicted by computer models in fact exist.

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