* Astronomy

Blobrana · L

Mirach's Ghost

Blobrana · L

RE: NGC 404

Blobrana · L

Title: A Deep Chandra View of the NGC 404 Central Engine
Authors: Breanna Binder, Benjamin F. Williams, Mike Eracleous, Anil C. Seth, Julianne J. Dalcanton, Evan D. Skillman, Daniel R. Weisz, Scott F. Anderson, Terry J. Gaetz, Paul P. Plucinsky

We present the results of a 100 ks Chandra observation of the NGC 404 nuclear region. The long exposure and excellent spatial resolution of Chandra has enabled us to critically examine the nuclear environment of NGC 404, which is known to host a nuclear star cluster and potentially an intermediate-mass black hole (on the order of a few times 10^5 \Msun). We find two distinct X-ray sources: a hard, central point source coincident with the optical and radio centers of the galaxy, and a soft extended region that is coincident with areas of high H\alpha emission and likely recent star formation. When we fit the 0.3-8 keV spectra of each region separately, we find the hard nuclear point source to be dominated by a power law (\PL = 1.88), while the soft off-nuclear region is best fit by a thermal plasma model (kT = 0.67 keV). We therefore find evidence for both a power law component and hot gas in the nuclear region of NGC 404. We estimate the 2-10 keV luminosity to be 1.3{+0.8}_{-0.5} x 10^{37} erg s^{-1}. A low-level of diffuse X-ray emission is detected out to ~15\as (~0.2 kpc) from the nucleus. We compare our results to the observed relationships between power law photon index and Eddington ratio for both X-ray binaries and low luminosity active galaxies and find NGC 404 to be consistent with other low luminosity active galaxies. We therefore favour the conclusion that NGC 404 harbours an intermediate-mass black hole accreting at a very low level.

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Blobrana · L

Title: NGC 404, A Rejuvenated Lenticular Galaxy on a Merger-Induced, Blueward Excursion into the Green Valley
Authors: David A. Thilker, Luciana Bianchi, David Schiminovich, Armando Gil de Paz, Mark Seibert, Barry F. Madore, Ted Wyder, R. Michael Rich, Sukyoung Yi, Tom Barlow, Tim Conrow, Karl Forster, Peter Friedman, Christopher D. Martin, Patrick Morrissey, Susan Neff, Todd Small

We have discovered recent star formation in the outermost portion (1-4x R_25) of the nearby lenticular (S0) galaxy NGC 404 using GALEX UV imaging. FUV-bright sources are strongly concentrated within the galaxy's HI ring (formed by a merger event according to del Rio et al.), even though the average gas density is dynamically subcritical. Archival HST imaging reveals resolved upper main sequence stars and conclusively demonstrates that the UV light originates from recent star formation activity. We present FUV, NUV radial surface brightness profiles and integrated magnitudes for NGC 404. Within the ring, the average star formation rate surface density (Sigma_{SFR}) is 2.2x10^-5 Msun/yr/kpc^2. Of the total FUV flux, 70% comes from the HI ring which is forming stars at a rate of 2.5x10^-3 Msun/yr. The gas consumption timescale, assuming a constant SFR and no gas recycling, is several times the age of the Universe. In the context of the UV-optical galaxy CMD, the presence of the SF HI ring places NGC 404 in the green valley separating the red and blue sequences. The rejuvenated lenticular galaxy has experienced a merger-induced, disk-building excursion away from the red sequence toward bluer colours, where it may evolve quiescently or (if appropriately triggered) experience a burst capable of placing it on the blue/star-forming sequence for up to ~1 Gyr. The green valley galaxy population is heterogeneous, with most systems transitioning from blue to red but others evolving in the opposite sense due to acquisition of fresh gas through various channels.

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Blobrana · L

The first images of the Ghost of Mirach taken by the Galaxy Evolution Explorer hinted at a surrounding ultraviolet-bright extended structure. Subsequent, longer exposure observations indeed show that the lenticular galaxy is surrounded by a clumpy, never-before-seen ring of stars.

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Blobrana · L

NASA's Galaxy Evolution Explorer has lifted the veil off a ghost known to haunt the local universe, providing new insight into the formation and evolution of galaxies.
The eerie creature, called NGC 404, is a type of galaxy known as "lenticular." Lenticular galaxies are disk-shaped, with little ongoing star formation and no spiral arms. NGC 404 is the nearest example of a lenticular galaxy, and therefore of great interest. But it lies hidden in the glare from a red giant star called Mirach. For this reason, NGC 404 became known to astronomers as the "Ghost of Mirach."

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