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The Snake in Spring: Step into the serpent's curves to explore the Hydra I Galaxy Cluster.

On clear April evenings, the splashy winter constellations slide toward the western horizon, and deep-sky enthusiasts are itching to dive into the galaxy-rich fields of Virgo, Leo, and Coma Berenices. These constellations, along with Canes Venatici and Ursa Major, headline 32 of the 40 Messier galaxies. But farther south and less explored, the winding constellation of Hydra, the Water Snake, uncoils across 100[degrees] of the sky and contains its own bonanza of galaxies.

At a distance of 165 million light-years, the Hydra I Cluster (also known as Abell 1060) is one of the nearest rich galaxy clusters beyond the Virgo Cluster. Physically, the two clusters display a striking resemblance. Hydra I is three times as distant as the Virgo cluster, but its apparent size is 1/3 as large. So their linear dimensions are nearly the same. And each cluster contains 50 galaxies within two magnitudes of their dominant members, M87 in Virgo and NGC 3311 in Hydra, both supergiant ellipticals with comparable absolute magnitudes.

Redshift surveys have demonstrated that the cluster is remarkably isolated in space, with no foreground galaxies and a huge void in the background out to a distance of 400 million light-years. This allows for easy identification of cluster members to fainter magnitudes based on redshift. The NASAIPAC Extragalactic Database lists upwards of 350 known members in a 5[degrees] region.

John Herschel discovered nine of the central galaxies that carry NGC designations in March 1835 and 1836 during his survey of the southern sky from the Cape of Good Hope. In 1835, he observed the cluster on three consecutive sweeps (slowly swinging the scope up and down several degrees while pointing due south) with his 18-inch speculum reflector in an attempt to accurately determine positions. But timing the closely spaced meridian transits while recording the altitude was difficult as the flurry of galaxies drifted by. As a solution, he made carefully drawn sketches in both years but realized "in each diagram only 7 were seen and laid down, yet there are in reality at least 9 in the whole group."

Hydra I just made the southern cut-off limit (-27[degrees] declination) of George AbelPs 1958 catalog of 2,712 rich galaxy clusters, yet it's still accessible to mid-northern observers. And a large aperture isn't a requirement; several 11.5--to 12.5-magnitude members are within reach of a 6--to 8-inch scope. But to minimize the effects of poor seeing and reduced transparency close to the horizon, make your observation when the cluster is highest near the meridian--about 11:00 p.m. at the beginning of April and an hour earlier around the new Moon at mid-month.

The core of the Hydra I Cluster forms a wide isosceles triangle with 4th-magnitude Alpha ([alpha]) Antliae and 5th-magnitude 44 Hya. The cluster is slightly under 4[degrees] from either star, lying just to the east of a line connecting the two stars. The main drag of the cluster's core is easy to follow as it forms a 20' chain passing between 4.9-magnitude HD 92036 and 6.7-magnitude HD 91964. You'll find half a dozen brighter NGC galaxies, but the glare from these stars can be an annoyance, so use medium or high power and isolate them outside the eyepiece field whenever possible. My descriptions are based on observations through a 13.1-inch reflector using 166x and 214x from dark sites in both northern California and tropical Costa Rica.

Let's start with NGC 3309 and NGC 3311, which form an impressive pair at the heart of the cluster. NGC 3309, an 11.6-magnitude elliptical, is strongly condensed with a conspicuous 40" core encased in a slightly elongated 1.2' halo leaning northeast. NGC 3311, just 1.7' east-southeast, is similar in size and magnitude but more broadly concentrated with no distinct nucleus. Using averted vision, its tenuous outer envelope grows to nearly 2' in diameter with a 13.5-magnitude star sandwiched between the halos of the two elliptical galaxies.

NGC 3311 is a colossal cD-type (central dominant) galaxy with a distended diffuse halo composed of stars and debris cannibalized from nearby satellite galaxies. It commands a swarm of roughly 16,000 globular clusters, one of the largest known collections, and is predicted to harbor a behemoth black hole that weighs in at several billion solar masses.

NGC 3308, a 12th-magnitude lenticular (S0-type) with a weak bar, lies 6' northwest of NGC 3309. The slightly oval halo spans V diameter and holds a small bright nucleus. NGC 3307, just 5' west of NGC 3309, is a much tougher catch. Even in my 18-inch this 14.5-magnitude barred lenticular was only a uniform ashen glow, perhaps 24" by 18".

The largest spiral in the cluster is 11.9-magnitude NGC 3312, symmetrically placed southeast of the central pair. In excellent conditions the relatively large, misty halo spans 2' by 0.7' north-south and encloses a roundish core that brightens to a stellar pip. An 18-inch scope should show a ragged halo due to long veins of dust that trace the winding spiral arms, as well as PGC 31542, a small 14.3-magnitude galaxy less than 4' to its east.

NGC 3316, just 8' east-southeast of NGC 3312, is a barred lenticular about a magnitude fainter than the previous galaxies (except for NGC 3307 and PGC 31542). Its 40" round halo brightens gradually to a nearly stellar center. NGC 3314 forms the southern vertex of an equilateral triangle with NGC 3312 and NGC 3316. This moderately bright spindle is tilted northwest in a 3:1 ratio with a major axis of 1.5'. A 13.5-magnitude star is pinned to its northwest tip.

The Hubble Space Telescope image of NGC 3314 seems to show a spectacular collision of two spirals, but in fact this cosmic trompe l'oeil is created by a nearly perfect line-ofsight superposition. The face-on galaxy lies a few tens of millions of light-years in front of the inclined spiral, and there is no evidence of a real physical interaction. Visually, though, even a large telescope will reveal only the brighter background spiral.

Want to tackle a stiffer challenge? PGC 31537, a pale 14th-magnitude smudge, is nearly lost in the glare of the 5th-magnitude star HD 92036 a mere 3.5' away. With the star firmly planted off the north edge of the field, I glimpsed a ghostly oval, roughly 25" in length and half as wide.

Let's check out some cluster members outside the crowded downtown region. Slide 18' east-southeast of NGC 3314 to 13th-magnitude PGC 31638. Images show a disrupted spiral, but visually it was just an amorphous patch about 40" across with a 13.5-magnitude star barely off the southwest edge. Continue another eyepiece field in the same direction and you'll arrive at NGC 3336 in the eastern outskirts of the cluster. This 12th-magnitude spiral showed a diffuse 1' halo with a weakly brighter central hub.

NGC 3285, another 12th-magnitude spiral, resides in the cluster's western suburbs. Look for it 37' due west of NGC 3308 and 7' southwest of 7.4-magnitude HD 91551. A moderately faint 60" x 45" halo is tilted west-northwest and rises broadly to a quasi-stellar nucleus. NGC 3285B, a dimmer barred spiral 18' to its southeast, only revealed a slightly out-of-round Y diaphanous halo.

NGC 3315 is found 13' north of HD 92036 and close to the east of a 10th-magnitude star. This small fuzz-ball extended 25" and brightened modestly to the center. Another 15' west of NGC 3315 is NGC 3305, a slightly brighter 13thmagnitude elliptical (EO-type) of similar size. A 13th-magnitude star is only 1.5' west, and using higher power it resolved into a close pair.

Return to NGC 3315 and shift some 9' northeast to the 12th-magnitude elliptical IC 2597. In photographs, an extended gauzy envelope stretches 2.5', but I only noted the brighter 1' central hub, which is strongly concentrated. Prolific comet hunter Lewis Swift is credited with discovering IC 2597 in 1898 at the age of 78. But perusing E. E. Barnard's logbooks I found that he had run across this galaxy eight years earlier while comet hunting with the 12-inch refractor at Lick Observatory.

IC 2597 is the brightest member of Hickson 48, a compact quartet that fits in a 5' circle. PGC 31588, just 2.5' south, is a dim circular patch about 40" diameter with no distinct core. A 14.5-magnitude star dangles off its southeast edge. The remaining two members are inconspicuous scraps (magnitude 15.4 and 16.0) about 2' northwest of IC 2597 and were barely detectable in my 18-inch scope. Two of the four have somewhat discordant redshifts, so it's uncertain if the quartet is gravitationally bound.

Let's head out from the central region of the cluster and climb 1.8[degrees] due north of IC 2597 to NGC 3313, which glows at a respectable magnitude 11.4. The galaxy is sharply concentrated with a small intense nucleus embedded in a 1.5' pallid halo. NGC 3313 is a photogenic face-on barred spiral with a prominent inner ring and a pair of graceful outer arms, though these were too low in surface brightness to detect.

Now scan 2.5[degrees] east of NGC 3313 for 12th-magnitude NGC 3393, at the very northeastern periphery of the cluster. The galaxy forms a shallow arc with a pair of 9th--and 11thmagnitude stars just east of it. An oval halo, perhaps 40" in diameter, displayed a bright round nucleus and a dim star at its northwest edge.

The spiral galaxy NGC 3393 has long been known to contain an active galactic nucleus (AGN), classified as a Seyfert 2, that's powered by a supermassive black hole (SMBH). But in 2011, Harvard-Smithsonian astronomer Giuseppina Fabbiano and collaborators used observations from the Chandra X-ray Observatory to resolve the AGN into two peaks of X-ray emission separated by only 0.6" on the plane of the sky.

This provided strong evidence that NGC 3393 houses two SMBHs less than 500 light-years apart--the nearest known example and the first discovered in a spiral galaxy. The SMBHs are likely the remnant of a merger that occurred a billion or more years ago. One day in the future their final collapse into a single black hole will be announced by the release of gravitational waves rippling across space.

On a grander scale, Hydra I is just one of a half-dozen Abell clusters and several smaller groups that spread into the constellations Antlia, Vela, and Centaurus, forming the Hydra-Centaurus Supercluster (S&T: April 2002, p. 101). If you're feeling adventurous, take a plunge into the southern sky--you'll find plenty to explore the next clear April night.

Naming Abell clusters

* Abell galaxy clusters are often abbreviated as "ACO [Number]" for the Abell-Corwin-Olowin 1989 update, so another name for Abell 1060 is ACO 1060.

Caption: THE WATER SNAKE IN THE SKY Hydra slithers some 100[degrees] across the night sky. Its heart, Alphard, or Alpha ([alpha]) Hydrae, beats in the western reaches of the constellation. The Hydra I Cluster is almost directly below Nu (v) Hydrae, about halfway along an arc between Mu ([mu]) and Xi ([xi]) Hydrae (see chart on p. 32).

Caption: HYDRA I FINDER CHART Find the cluster nestled between Mu, Nu, and Xi Hydrae and Alpha Antliae. Two outliers, NGC 3313 and NGC 3393, lie slightly north and northeast, respectively, of the cluster core.

Caption: THE COLOSSUS IN THE CLUSTER NGC 3311, in the center of this image, is by far the largest galaxy in the cluster and presumably hosts a supermassive black hole. What makes this galaxy even more remarkable, though, is the swarm of some 16,000 globular clusters in its halo, the largest collection of its kind known to date. To the right lies cluster member NGC 3309, another large elliptical galaxy, although less massive. The dark shadow at the bottom of the image is the spectrograph's guide probe.

Caption: THE CLUSTER IN THE COILS The Hydra I Galaxy Cluster, consisting of more than 100 members, lies some 165 million light-years away in a rather isolated corner of space.

Caption: NGC 3314: A CELESTIAL TROMPE L'OEIL Even though it might look as if these two spiral galaxies are interacting, they're in fact at a distance of tens of millions of light-years from each other. The background galaxy is associated with the Hydra I cluster, and it is just serendipitous alignment that places the foreground galaxy directly in the line of sight.

Caption: Contributing Editor and deep-sky fanatic STEVE GOTTLIEB has completed a 40-year project to observe the entire NGC (7,500+ objects), but still finds his list of new targets ever expanding. His detailed observing notes are available at astronomy-mall. com/Adventures.In.Deep.Space.

Caption: NOT ONE, BUT TWO It's recently been shown via X-ray observations that NGC 3393, a spiral galaxy very similar to our own, houses two supermassive black holes, the result of a likely merger a billion years or so ago. At a distance of some 160 million light-years, this would be the closest example of a double supermassive black hole. Look for NGC 3393 in the northeastern outskirts of the Hydra I cluster.
Hydra I Cluster Components

Object           Type       Mag(v)      Size

NGC 3309 *        E3         11.6    1.9' x 1.6'
NGC 3311 *       E/S0        11.7    2.3' x 2.1'
NGC 3308 *     SAB(s)0       11.9    1.7' x 1.3'
NGC 3307     SB(r)0/a pee    14.5    0.9' x 0.3'
NGC 3312 *    SA(s)b pee     11.9    3.3' x 1.3'
PGC 31542       SB(s)0       14.3    0.9' x 0.3'
NGC 3316 *     SB(rs)0       12.7    1.3' x 1.1'
NGC 3314 *       Sab         12.8    1.5' x 0.7'
PGC 31537        SB0         13.8    1.0' x 0.4'
PGC 31638     SB(s)d pee     13.2    1.5' x 0.7'
NGC 3336 *   SAB(rs)c pee    12.2    1.9' x 1.5'
NGC 3285 *    SB(s)a pee     12.0    2.6' x 1.3'
NGC 3285B      SAB(r)b       13.1    1.5' x 1.1'
NGC 3315 *       E/S0        13.4    1.1' x 1.0'
NGC 3305 *        E0         12.8    1.1' x 1.1'
IC 2597 *         E4         11.8    1.5' x 1.2'
PGC 31588         Se         14.0    0.9' x 0.8'
NGC 3313 *      SB(r)b       11.4    3.9' x 3.2'
NGC 3393 *   (R')SB(rs)a     12.2    1.8' x 1.5'

Object            PA           RA            Dec.

NGC 3309 *   31[degrees]    10h 36.6m   -27[degrees]31'
NGC 3311 *   19[degrees]    10h 36.7m   -27[degrees]32'
NGC 3308 *   32[degrees]    10h 36.4m   -27[degrees]26'
NGC 3307     28[degrees]    10h 36.3m   -27[degrees]32'
NGC 3312 *   175[degrees]   10h 37.0m   -27[degrees]34'
PGC 31542    167[degrees]   10h 37.3m   -27[degrees]34'
NGC 3316 *   36[degrees]    10h 37.6m   -27[degrees]36'
NGC 3314 *   143[degrees]   10h 37.2m   -27[degrees]41'
PGC 31537    72[degrees]    10h 37.3m   -27[degrees]28'
PGC 31638    94[degrees]    10h 38.6m   -27[degrees]44'
NGC 3336 *   123[degrees]   10h 40.3m   -27[degrees]47'
NGC 3285 *   108[degrees]   10h 33.6m   -27[degrees]27'
NGC 3285B    43[degrees]    10h 34.6m   -27[degrees]39'
NGC 3315 *   80[degrees]    10h 37.3m   -27[degrees]11'
NGC 3305 *        --        10h 36.2m   -27[degrees]10'
IC 2597 *     4[degrees]    10h 37.8m   -27[degrees]05'
PGC 31588    97[degrees]    10h 37.8m   -27[degrees]07'
NGC 3313 *   55[degrees]    10h 37.4m   -25[degrees]19'
NGC 3393 *   48[degrees]    10h 48.4m   -25[degrees]10'

Angular sizes and separations are from recent catalogs.
Visually, an object's size is often smaller than the cataloged
value and varies according to the aperture and magnification of
the viewing instrument. Right ascension and declination are for
equinox 2000.0. The asterisk following the galaxy name denotes
primary cluster member.
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Title Annotation:HYDRA I CLUSTER
Author:Gottlieb, Steve
Publication:Sky & Telescope
Date:Mar 18, 2018
Words:2665
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