A Midsummer Night’s Dream: NGC 4618 and NGC 4625 by Martin Winder/Dietmar Hager

“Night’s swift dragons cut the clouds full fast, And yonder shines…” Another galactic pair? Discovered by Friedrich Wilhelm Herschel in 1787, this particular galactic pairing known as Arp 23 find its home in Canes Venetici, and the duo most certainly has a colorful history. The smaller of the pair – NGC 4625 is a distorted dwarf galaxy formally classified as Sm, a structure which resembles spiral galaxies – especially the Magellanic clouds. So what does a single arm galaxy have to say for itself?

It’s been theorized that asymmetrical structure could be the result of a gravitational interaction with NGC 4618 – its larger, interactive member in this picture. Yes, asymmetric structure isn’t new when it comes to interacting galaxies, but the rub is only some of the neutral hydrogen gas outside the optical disc of NGC 4618. What does that mean? Quite probably that the single arm shape of the galaxy isn’t a product of the interaction – but natural to the galaxy’s own, unique properties.

In reading studies done by 2004 by Bush (et al) , “Asymmetry is a common trait in spiral galaxies and is particularly frequent among Magellanic spirals. To explore how morphological and kinematic asymmetry are affected by companion galaxies, we analyze neutral hydrogen observations of the interacting Magellanic spirals NGC 4618 and NGC 4625. The analysis of the H I distribution reveals that about 10% of the total H I mass of NGC 4618 resides in a looping tidal structure that appears to wrap all the way around the galaxy. Through calculations based on derived H I profiles, we show that NGC 4618 and NGC 4625 are no more asymmetric than the noninteracting Magellanic spirals analyzed recently by Wilcots & Prescott. We also derive rotation curves for the approaching and receding sides of each galaxy. By fitting the mean curves with an isothermal halo model, we calculate dynamical masses of 4.7×109 and 9.8×109 Msolar out to 6.7 kpc for NGC 4618 and NGC 4625, respectively. While the rotation curves had systematically higher velocities on the receding side of each galaxy, the effect was no more pronounced than in studies of noninteracting spirals. The degree of interaction-driven asymmetry in both galaxies is indistinguishable from the intrinsic degree of asymmetry of lopsided galaxies.”

In 1985, A. V. Filippenko discovered something unusual in the spectrum of NGC 4618: “The object is almost certainly a supernova in an advanced stage, although its spectrum does not conform to published supernova spectra. Based on the present brightness and on the distance modulus of NGC 4618, it is estimated that the object reached maximum about 160 days ago and has faded by 5 to 6 mag, if it was initially a normal Type I or Type II supernova. It is noteworthy that Minkowski (1939, Ap.J. 89, 156) observed the [O I] 630.0/636.4-nm doublet to be strong after 184 days past maximum in the spectrum of the Type I supernova 1937C in IC 4182. The feature was not present in the spectrum of SN 1972E in NGC 5253 some 400 days after maximum (Kirshner and Oke 1975, Ap.J. 200, 574). Prediscovery data on the brightness of the object and future observations of the evolution of its spectrum would be of great interest.”

Later that year: “Optical spectra of a bright stellar object near the nucleus of the spiral galaxy NGC 4618 reveal strong, very broad emission lines similar to those in quasars but having the wrong relative wavelengths. Although lines of hydrogen and helium are absent, the most prominent features can be attributed to neutral atoms of oxygen, sodium, and magnesium at the redshift of NGC 4618. The object is almost certainly a supernova whose highly unusual spectrum may be indicative of a fundamentally new subclass.” By 1986 the studies had broadened and; “The spectrum of SN 1985f does not resemble any previously published spectra of supernovae, and it is postulated that its progenitor was a massive Wolf-Rayet star that expelled its outer atmosphere of H and He prior to supernova explosion.”

However, the real beauty to this picture is what appears to be sparkling star forming regions. According to the studies done by the Elmegreens; “It is suggested that prominent star forming regions occur near the peripheries of barred Magellanic spirals and irregulars because the galaxies experience gas dynamics similar to that in the inner barred regions of massive barred spirals.” But… Is the interaction between the two what’s causing these exterior star forming regions? Science doesn’t seem to think so. Says Zaritsky; “The stellar disks of many spiral galaxies are twice as large as generally thought (and) the phenomenon of low-level star formation well outside the apparent optical edges of disks is common and long lasting.”

This is further backed up by studies done by Gil de Paz (et al). “Recent far-UV (FUV) and near-UV (NUV) observations of the nearby galaxy NGC 4625 made by the Galaxy Evolution Explorer (GALEX) show the presence of an extended UV disk reaching to 4 times the optical radius of the galaxy. The UV-to-optical colors suggest that the bulk of the stars in the disk of NGC 4625 are currently being formed, providing a unique opportunity to study today the physics of star formation under conditions similar to those when the normal disks of spiral galaxies like the Milky Way first formed. In the case of NGC 4625, the star formation in the extended disk is likely to be triggered by interaction with NGC 4618 and possibly also with the newly discovered galaxy NGC 4625A.”

Yet, star formation isn’t all that’s going on here. NGC 4618 and NGC 4625 have also been studied for spin as well, and there’s a strong possibility that tidal interaction can affect it. According to studies done by Helou. “Clues to the origin of spin in galaxies are also direct clues to the mechanism of galaxy formation. The evidence so far is clearly against a simple picture where primeval turbulence is the source of spin. But the data are consistent with, and suggestive of, the hypothesis that spins were acquired via tidal torquing; a detailed discussion is given, treating separately the possibility that the effect is primordial and the possibility that it is a result of evolution. Enough data are now becoming available that specific calculations are required to sharpen the predictions for the statistical behavior of spins, especially in binaries.”

Is there still more to this pair than meets the eye? Certainly. This pair has also been studied for Seyfert nuclei – a brilliant, compact core region which can take a variety of forms, perhaps carrying clues to how the central engine is fed or triggered. Studies show that Seyfert nuclei may happen more frequently among interacting spirals – but more so those that only interact strongly, rather than with extreme tidal distortion. The fascinating work was originally done by Bill Keel and his findings backed up by later studies. It is also very possible this phenomenon simply occurs as a natural process, and the spectral features of Wolf-Rayet stars have also been detected as well. So many different factors can come into play!

No matter what happens in this unusual “inside out” forming pair – be it a detection of a black hole or just a long duration gamma ray burst – they make for fascinating study and a truly beautiful image. “If we shadows have offended, Think but this, and all is mended, That you have but slumber’d here While these visions did appear. And this weak and idle theme, No more yielding, but a dream, Gentles, do not reprehend; If you pardon, we will mend.”

The light for this awesome image was gathered over a period of about 7.5 hours by AORAIA member Martin Winder and then processed by member Dr. Dietmar Hager. We thank both of them for the exclusive look at this beautiful galaxy duo.