This composite image reveals electric blue ram pressure stripping streaks seen emanating from ESO 137-001, as well as a giant gas stream that can be seen extending towards the bottom of the frame in X-rays. Credit: NASA/ESA/CXCTitle: ELVES I: Structures of Dwarf Satellites of MW-like Galaxies; Morphology, Scaling Relations, and Intrinsic Shapes
Authors: Scott G. Carlsten et al.
First Author’s Institution: Princeton University
Status: Accepted to ApJ
Dwarf galaxies
are thought to be incredibly suggestible; there has been a range of
diverse dwarf galaxies observed in our universe, indicating that they
are extremely sensitive to their surroundings. The observed differences
in sizes, shapes, and colours of dwarf galaxies is believed to be at
least in part due to differences in the environment they inhabit. All
galaxies are thought to be surrounded by a halo of dark matter (see this
astrobite
for more details). Many dwarf galaxies are satellite galaxies, meaning
that they are found in orbit within a larger host dark matter halo that
also typically contains a larger central galaxy (for example, the Small
and Large Magellanic Clouds are satellite galaxies, both in orbit of our own Milky Way).
Satellite galaxies are subject to many different interactions with
their host dark matter halo. These interactions between a satellite
galaxy and its host can have devastating effects on the satellite galaxy
itself. For example, their gas content can become extremely disturbed
(and sometimes completely removed) by ram pressure stripping, which can eventually bring star formation in the satellite to a halt (see this astrobite for a summary of the seminal paper on ram pressure stripping). Similarly, their stars are subject to tidal stripping, which arises due to differences in the gravitational potential of the satellite galaxy and its host.
Figure
1: Examples of dwarfs visually classified as early-type (ETG) and
late-type (LTG). Late-type dwarfs are irregular, with apparent active
star formation throughout the galaxy while early-types are smooth and
featureless without any star-forming clumps.Credit: Carlsten et al. 2021
Despite
the observed diversity of dwarf galaxies, they can broadly be
classified into two morphological types: late-type and early-type (see
Figure 1 for examples). Late-type galaxies are typically star-forming,
whereas early-type galaxies lack star-forming regions and appear
smoother than late-types. Today’s paper uses the ongoing Exploration of
Local VolumE Satellites (ELVES) Survey to investigate how the structural
properties of dwarf galaxies can change depending on the environment
and morphology of the galaxy. The galaxies in the ELVES sample are all
within the Local Volume (D < 12 Mpc), and are satellite galaxies in
orbit of Milky Way-like halos.
Going from a Late-type to an Early-type?
The current picture of dwarf galaxy evolution suggests that
early-type dwarfs are formed from late-type dwarfs interacting with a
host halo. If this is the case, then early-type dwarfs can be thought of
as dwarf galaxies in the last throes of their evolution, and any
differences in characteristics of late-type and early-type galaxies
could provide insights into the physical mechanisms behind this
evolution (such as the removal of star-forming gas through ram pressure
stripping).
Figure
2: Log effective radius vs. log stellar mass for the dwarf galaxies in
the Local Volume sample. The upper panel displays points for each dwarf
galaxy in the sample, with red indicating early-type and blue indicating
late-type. The bottom panel shows average trends binned by stellar
mass. The dashed lines show the mass-size relations for early-type (red)
and late-type (blue) dwarf galaxies of higher stellar mass from the
GAMA Survey. [Adapted from Carlsten et al. 2021]
To investigate
whether there are any structural differences between early- and
late-types, the authors plot the effective radius of the dwarf galaxies
in their sample (essentially the galaxy’s size) by their stellar mass. It
can be seen from Figure 2 that there is no significant difference
between the early- and late-type galaxies at fixed stellar mass. This
similarity between late-types and early-types suggests that the
physical processes relevant in forming early-type galaxies (such as ram
pressure stripping) do not necessarily induce any change in the galaxy’s
size. These results indicate that the transformation process from
late-type to early-type requires only the removal of the galaxy’s
star-forming gas — significant structural change to the galaxy is not
necessarily required. Also of note is the difference between the
author’s results, where the sample is limited to dwarf galaxies with M* < 108.5 M⊙ and
results for satellite galaxies with higher masses (indicated by the
blue and red lines in the bottom panel of Figure 2). The authors suggest
that this difference hints that there is a characteristic stellar mass
scale, above which additional physical processes may be required to
explain the sudden difference in sizes between early- and late-types.
Environmental Effects
The next question the authors aim to answer is: how does the mass of
the dwarf galaxy’s host dark matter halo affect the evolution of the
dwarf galaxy? To consider this, the authors again compare the sizes of
dwarf galaxies. This time, a comparison is made between dwarf galaxies
that are orbiting within larger cluster environments and the dwarf galaxies in their Local Volume environment.
Figure
3: The mass–size relations of the cluster (grey) and field (cyan) dwarf
samples normalized to the full Local Volume sample (green). At fixed
stellar mass, the cluster sample is offset to larger sizes, whereas the
isolated field sample is offset to smaller sizes. Field galaxies are
isolated dwarf galaxies that have been taken from an auxiliary sample,
using additional observational data. Credit: Adapted from Carlsten et al. 2021
As
can be seen in Figure 3, dwarf galaxies in cluster environments tend to
be slightly larger than dwarf galaxies in the Local Volume at a fixed
stellar mass. The authors argue that the observed increase in size is
down to more intense tidal stripping and heating of galaxies in extreme
cluster environments, which aligns with theoretical expectations. While
an ~8% increase in sizes for the dwarfs in cluster environments is
observed, the authors note the mass–size relation is strikingly similar
between the two environments, especially since the mass of the host dark
matter halos differ by a factor of 10. This is perhaps indicative
that the exact environment plays a fairly small role in dwarf galaxy
evolution — a somewhat surprising result!
In conclusion, today’s authors are able to gain insights into the
physics of dwarf galaxy transformation from late-types to early-types,
and how these processes vary between the Milky Way-like and cluster
environments. The authors comment that a comparison with simulations
will be useful in constraining the physics of how dwarf galaxies evolve. Their
observational results have quantified the start and end points of the
transformation, and simulations may now be able to tie them together to
tell the middle part of the story!
Original astrobite edited by Luna Zagorac.
By Astrobites
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content here at AAS Nova. We hope you enjoy this post from astrobites;
the original can be viewed at astrobites.org.
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