This is part of the HERMES Public Outreach pages, created by Juliette Voyez (Paris 7 University)
The HERMES project
Contents
Introduction
The Herschel M33 Extended Survey (HERMES) is a Herschel open time key project that has been granted almost 200 hours of observing time by ESA.
Aim of the project
The program focuses on a detailed study of the nearby galaxy M33, ideally situated to study with Herschel the dependence of the star formation characteristics on the physical conditions across the spatially resolved galactic disk and how it interplays with the Interstellar Medium (ISM). The study will be realized by surveying the major gas cooling lines, notably [CII](158 μm), H2O (556 GHz), [OI](63, 145 μm), [NII](122, 205 μm) and [NIII](57 μm) as well as the dust spectral energy distribution (SED) in M33, using Herschel instruments, HIFI, PACS and SPIRE.
Science questions
Understanding the formation of structures in the universe is a central concern of modern astrophysics. Galaxies, stars and gas clouds are the prime components that shape the appearance of the visible universe. Stars are made from gas, and during their lives they seed the ISM with heavy elements, energy and momentum, all of which strongly affects the subsequent formation of stars and their host galaxy. To understand the evolution and appearance of galaxies it is therefore crucial to study the interplay between stars and the interstellar gas. Where and under which conditions do stars form ? How do stars affect the thermal and chemical balance of gas that surrounds them ? How do stars dynamically affect the structure of the gas ?
The thermal balance and dynamics of the ISM in galaxies is best studied through high-resolution spectroscopy of its major cooling lines: [OVI] for the hottest gas heated by supernovae, [CII] for the ionized gas powered by OB stars, [CI](370, 609 μm), [CII] and [OI] for the transition regions between the atomic and molecular gas, CO (1.3, 2.6 mm) and H2O for the dense molecular gas that provides the reservoir for stars to form.
The importance of Herschel
M33 has been studied in detail from X-ray through radio wavelengths, providing a wealth of complementary data. However the brightest cooling lines of the ISM lie in the largely unexplored far-infrared (FIR) range which is the territory of Herschel. The Space Observatory will, for the first time, provide an opportunity to image the major tracers of the ISM at a sensitivity, spectral and spatial resolution that allows to study the interplay between star formation and the active ISM .
Choice of source: The Triangulum Galaxy M33 (NGC 598)
At a distance of about 840 kpc, M33 is the only nearby, gas rich disk galaxy that allows a coherent survey at high spatial resolution. It lacks the Milky Way distance ambiguity, and it is not as extended and inclined as the Andromeda Galaxy. M33 is a regular, mostly unperturbed disk galaxy, as opposed to the nearer Magellanic Clouds, which are highly disturbed irregular dwarf galaxies. Its total gas content is dominated by HI all over the disk. Nevertheless, M33 is rich in star forming regions (NGC 604 for example). Moreover, M33 lies at 30° declination, allowing for observations with the large millimeter and submillimeter telescopes of both hemispheres. A wealth of data has become available in recent years and several team members of the Herschel Project have ongoing observational programs. M33 is not included in any of the Herschel guaranteed time key projects.
The project
|
Central region of M33. Blue contours show the part that will be studied with Herschel. |
It aims to study the ISM in an extended radial cut, from the nucleus of M33 to the outer disk at 7.8 kpc distance. This cut will allow comparisons of the nuclear regions versus the disk, the inner versus the outer disk, the spiral arms versus the inter-arm regions, and giant star forming regions versus less conspicuous sections of the spiral arms. It will allow to study outer regions where atomic gas dominates the surface density, as compared to the inner disk, where the stars dominate the mass. The northern and southern inner arms of M33 are markedly different. The northern arm is broad, clumpy, somewhat irregular and has bright HII regions. The southern arm is smooth, relatively narrow, and has a whole series of small HII regions.
An improved identification and understanding of the signatures of the various components will be crucial to gain a better knowledge of the physical and chemical processes and properties (heating/cooling balance, pressure, density, temperature, radiation field, molecular and atomic abundances), on small scales as well as on global scales.
The proposition is to trace the gas via observations of [CII], [OI], [NII], H2O and the dust continuum at five wavelengths in the FIR and sub millimeter range, to adress the following key science topics:
A. Study the phases of the ISM via their most important cooling lines at a wide range of galactocentric radii out to the edge of the optical disk. Derive the life cycle of the interstellar gas as a function of galactic radius. Derive the contribution of [CII] emission from the various phases and the carbon budget through [CII] and dust observations, complemented by [CI] and CO observations from the ground.
B. Determine the energy balance of the interstellar medium as a function of galactic environment. Measure the ratio of dust continuum cooling vs. gas cooling, and the relative importance of the various gas coolants and relevant heating processes.
C. Use [NII] in conjunction with [CII] as extinction free tracers of star formation. Determine the parameters controlling the star formation rate. Calculate the star formation efficiency or gas depletion time scales as a function of galactocentric radius. Study stellar feedback and the disruption of molecular clouds. Investigate how the interstellar magnetic field affects or controls the star formation rate or efficiency.
D. Combine FIR line data with existing HI data to study the formation of molecular clouds from the diffuse atomic medium. To determine the details of the evolution from atomic to molecular gas and ultimately star formation itself.
The results of this survey will be of great interest to the general public because it will be studying one of the Milky Way's nearest neighbors in such great detail. The Triangulum Galaxy is thought to resemble the Milky Way in many aspects. It almost face-on view allows for an excellent laboratory.