A look-alike of z~2 galaxies in our backyard

• Title: A Nearby Analog of z~2 Compact Quiscent Galaxies with a Rotating Disk
• Authors:
 Fangzhou Jiang, Pieter van Dokkum, Rachel Bezanson, Marijn Franx
• First Author’s Institution: 
Department of Astronomy, Yale University

An unusual population of compact, quiescent galaxies has been recently discovered at a high redshift of z~2 (see e.g. Daddi et al. 2005Trujillo et al. 2007Cimatti et al. 2008 to name only a few).  These massive fellows, observed as early as 3 billion years after the Big Bang, are quite different from their local counterparts, with effective radii a factor of 3-6 smaller and average stellar densities a factor of 180 higher (have a look at Michelle’s astrobite). Their extreme compactness is puzzling, since in our understanding of galaxy formation and evolution, ellipticals are passively evolving, “red and dead” endpoints of evolution, and therefore they are not expected to vary strongly in size or mass. Of course, galaxy mergers are fashionable these days, and the observations can be – more or less – explained in a scenario in which minor mergers play the dominant role in shaping the size evolution of galaxies. In such a scenario, the high-density, compact red galaxies would be “puffed up” by a process of “inside-out” growth, in which mergers gradually increase the size of the galaxies by building up their outskirts, and the extended stellar halos observed today are assembled around compact, dense cores whose mass stays almost constant with time (e.g., Bezanson et al. 2009Hopkins et al. 2009van Dokkum et al. 2010).

Unfortunately, such compact galaxies are hard to study in detail at high redshift, due to low signal-to-noise ratio. This motivates us to look for local analogs: galaxies in our own backyard whose structure we can resolve with higher accuracy than is possible for distant galaxies.

And here comes our guest: NGC 5845.  This galaxy, which lives in the Virgo cluster, shows up as an outlier in the diagram of velocity dispersion against effective radius for a sample of about 400 nearby quiescent galaxies collected by the so-called Seven Samurai team (see the famous paper Faber et al. 1989). In general, the velocity dispersion of an elliptical galaxy is often used as a proxy for its total mass – the faster its stars are moving, the harder it is for the galaxy to hold them in; thus, the higher the velocity dispersion, the higher the mass. As shown in Figure 1, most of the galaxies obey a tight relationship that is a projection of the Fundamental Plane on which ellipticals live; however, two galaxies, NGC 5845 and NGC 4342 fall outside of this trend.  Given their small size (effective radii less than 1 kiloparsec), these guys have surprisingly high velocity dispersions (greater than 200 km/s) and therefore high dynamical masses as compared to other local galaxies, instead fitting in with the high redshift compact galaxies (shown by the red box in Figure 1). Thus, they are worth investigating further.


Figure 1. The whole sample of galaxies of the Seven Samurai Survey plotted according to effective radius and velocity dispersion. The red box shows the cuts in velocity dispersion and effective radius used as selection criteria: the only outliers are NGC 5845 and NGC 4342.

The authors use deep photometry and previously published, spatially resolved kinematics to study NGC 5845 in detail.  They infer dynamical mass of 4.3 ± 0.6 tens of billions of solar masses and an effective radius of only 0.45 ± 0.05 kiloparsecs, which indeed makes it dynamically comparable to the population of redshift z~2 compact quiescent galaxies. Figure 2 compares NGC 5845 (red star) to nearby galaxies from the Sloan Digital Sky Survey (the gray cloud of points) and to higher redshift compact galaxies (blue circles). Generally, NGC 5845 has obvious offsets from the low-redshift galaxies, and it is much more similar to the compact, quiescent high galaxies observed at z~2.

Figure 2. Comparison of NGC 5845 (red star) to high-redshift compact galaxies (blue circles) and quiescent low-redshift SDSS galaxies (gray cloud). Upper left: stellar versus dynamical mass. Upper right: size versus dynamical mass. Lower left: velocity dispersion versus size. Lower right: velocity dispersion versus dynamical mass.

Well, then, the natural question: how does this prototype of compact z~2 galaxies look, now that we can observe it very closely?

Contrary to the initial classification of NGC 5845 as an elliptical inferred from low resolution of ground-based observations, the deeper images of Hubble Space Telescope with the Wide-Field Planetary Camera 2 reveal a diffuse stellar disk and a smaller dusty disk, which have been previously described as “nuclear” disks. The authors show that this rotating stellar disk is prominent, extending well beyond 1/3 effective radius of the galaxy and contributing to about  1/4 of the total light.

This suggests that perhaps such disk-like structure could also be present in high-redshift compact galaxies, consistent with the high flattening observed for some of the z~2 fellows. It is important to remark that similar structures reflect the formation mechanisms: while a major merger leads to a bulge-like, non-rotating system, dissipative collapse may lead to a rapidly rotating galaxy. Unfortunately, were NGC 5845 placed at z~2, the entire disk would fall within a single pixel of the imaging camera!

At this point, a careful reader may have an obvious question: what is this odd galaxy doing in our backyard? Statistically speaking, the presence of 1 or 2 of these objects in 400 local galaxies can be expected: the authors argue that NGC 5845 is a 3sigma outlier in the local size-mass relation. But is NGC 5845 a survivor of its high redshift counterparts, or is it a newly formed double? Answering such a question is not easy, but perhaps future studies will reveal the answer.

About Lucia Morganti

I am a fourth-year graduate student at the Ludwig Maximilians Universitaet of Munich, Germany. I am doing my PhD at the Max Planck Institute for Extraterrestrial Physics, where I belong to the Dynamics group led by Ortwin Gerhard. I am interested in galactic dynamics and modelling of galaxies.

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