Dusty galaxies from the Herschel Reference Survey

Dust Across the Hubble Sequence

Ellipticals, so called due to their elliptical-like appearance in visible light, are often described as “red and dead” galaxies, where stars are no longer forming and all the dust has been used up. The elliptical galaxies we see today, are thought to be the descendants of the massively dusty, active star forming galaxies seen in the very early Universe; these galaxies are forming stars thousands of times faster than our own and are likely harbouring an active galactic nucleus at their centre. Astronomers are not really sure how these massive systems form, particularly whether or not they are formed by merging galaxies together from collisions. An important question to address is whether or not environment affects the formation and evolution of these systems. Other questions we want to ask is “are ellipticals really devoid of dust?” and, “what can we learn about their history using any dusty features we can see?”. Well it turns out alot!

Galaxies in the Herschel Reference Survey in infrared/submillimeter wavelengths (with the Herschel space telescope, at left) and the Sloan Digital Sky Survey (right). Herschel’s false-color image shows galaxies with cold dust (blue) and warm dust (red). Sloan highlights young stars (blue) and old stars (red). “Together, the observations plot young, dust-rich spiral/irregular galaxies in the top left, with giant dust-poor elliptical galaxies in the bottom right,” the European Space Agency stated. Credit: ESA/Herschel/HRS-SAG2 and HeViCS Key Programmes/Sloan Digital Sky Survey/ L. Cortese (Swinburne University).

Our group has worked on the Herschel Reference Survey (a sample of 323 galaxies observed by Herschel in the local universe led by Steve Eales and Alessandro Boselli, Boselli et al. 2010). Working with then PhD student Matt Smith, we showed that dust emission is detected in roughly half of the sample of 62 early types and ellipticals (Smith et al. 2012). These galaxies are therefore not all “red and dead” as often described in the literature. Interestingly, we found that Herschel is the most sensitive way to detect the interstellar medium in these galaxies, suggesting we may be able to use dust as a tracer of gas instead of the traditional carbon monoxide and atomic hydrogen lines.

ETG

The results from Herschel (see also Rowlands et al. 2012) indicate that traditional so-called “quiescent” galaxies can still have significant dust emission, star formation and interstellar material. However, if we compare the dust masses of the early types with spiral galaxies from the HRS, we find an order of magnitude decline in the dust-per-unit stellar mass as we move across the Hubble sequence of galaxies from spirals, to S0s to ellipticals. The high dust masses, the similar gas-to-dust ratios to spirals and the lack of correlation between starlight and dust, all point towards an external origin for the dust in ellipticals, i.e. the interstellar medium is accreted via tidal accretion or interaction with a nearby galaxy.

Dusty evidence of galaxy-scale interactions

Compare the starlight from the two galaxies M86 and NGC4438 (optical, top) and the dust seen by Herschel (coloured map bottom). Overlaid on this map is the shock-heated hydrogen (red), this is evidence that the dust in M86 has been stripped from NGC4438.

Our Herschel results towards the centre of the Virgo Cluster (the nearest group of galaxies to us) revealed the debris from galaxies interacting with each other. Take a look at the optical image of the galaxies in the Virgo cluster (top panel). The brightest galaxy in this image is the giant elliptical M86. We can also see the spiral galaxy NGC4438 on the left, with a smaller galaxy nearby.

In the Herschel submillimetre image however (bottom panel), the elliptical galaxy is now the faintest galaxy (this is because it must have used all its dust up in forming stars). In this galaxy, we also see something quite unusual, the dust is offset from the centre and appears to be distributed in a very different way to the stars (seen in the optical image). If we now compare the dust to hot hydrogen gas (which shows gas being pulled out of the galaxy due to the interaction with the spiral galaxy in the top right) we see a pretty good match between the cold dust seen by Herschel and the hot gas trailing between the two galaxies. The dust appears to be following the ionised hydrogen trailing out of M86 and is being pulled towards the galaxy in the top left. This is really strong evidence that the dense environment which M86 is in significantly affects the dust and metals in galaxies, the data points to more than a million Sun’s worth of dust being stripped from this source in a “displaced interstellar medium”. See Gomez et al. 2010 and Cortese et al. 2010 for more details on this system.