The Andromeda Galaxy

Version	Summary	Created by	Modification	Content Size	Created at	Operation
1		Denis Leahy	--	1983	2023-08-11 20:07:56	\|
2	layout	Jessie Wu	Meta information modification	1983	2023-08-14 06:09:26	\|

This entry is adapted from the peer-reviewed paper 10.3390/universe9080349

Star formation histories of galaxies are critically important for understanding the process of galaxy formation and the structure and contents of galaxies. Star formation can and has been studied in local galaxies for which the stellar populations are resolved and in more distant galaxies for which stars are unresolved, which are instead modeled as populations. Structural components of a galaxy can be resolved at much larger distances. The structural components include those long recognized, such as bulge, disk and halo. More recently recognized structures include separation of disks into thin and thick disk components and stellar streams, as well as recognition of significant numbers of dwarf companion galaxies. Stellar streams are the most recently recognized components of galaxies, mainly using observations of the Milky Way and the Andromeda Galaxy (M31).

spiral galaxies star formation galactic structure galaxy formation

1. Structure of M31

M31 is an SAB-type spiral galaxy (see https://ned.ipac.caltech.edu accessed on 17 May 2023) with luminosity in the V band (visible) of 3 × 10¹⁰ solar (bolometric) luminosities. It has near-infrared luminosity of 1/3 of the V-band luminosity and far-infrared and ultraviolet luminosities about 1/30 and 1/60 of the V-band luminosity. The ultraviolet and far-infrared images are shown in Figure 1 (left and right panels, respectively). Both images are dominated by the spiral arms, but for different reasons: the hot young stars in the spiral arms dominate the ultraviolet emission, whereas the far infrared is from the interstellar dust around the hot stars (and heated by those hot stars). The far-infrared luminosity is larger than the ultraviolet luminosity because most of the hot stars have significant extinction (UV extinction range A_UV ~0.5–5 mag) from surrounding dust, which converts much of their ultraviolet luminosity into far-infrared radiation.

Figure 1. Images of M31 at ultraviolet and infrared wavelengths. (a) The 148 nm image of M31 ^[1]. (b) The far-infrared 250-micron image of M31 from the Hershel SPIRE instrument ^[2]. The ultraviolet and far-infrared images closely trace the spiral arms of M31. The ultraviolet and far-infrared emissions both closely follow the spiral arms and young stars of M31.

The major structures for spiral galaxies are present in M31: the disk, the central bulge, and the halo. The luminosity profile of M31 in near infrared (sensitive to stars and insensitive to dust) was modeled by ^[3] with the above three components. The main parameters were a disk scale length of 5.3 kpc, a bulge Sersic-function index of 2.2 and effective radius of 1 kpc, and a halo with a power-law profile with index −2.5. The kinematics of the disk stars and gas were analyzed by ^[4], showing that the offset in stellar and gas velocity increases with stellar age. They foundd that the use of a simple tilted ring model to explain the warps in the disk is inadequate.

The bulge is known to be triaxial and has been analyzed using N-body simulations ^[5]. To match the bulge properties, an initial classical bulge (ICB, which is formed by violent relaxation early in the formation of the galaxy) with 1/3 of the total bulge mass and a Box/Peanut (B/P) bulge with 2/3 of the total bulge mass are required. The ICB (formed rapidly) and the B/P bulge (formed over Gyr) interact and evolve together ^[5]. The structure of the bulge in ultraviolet wavelengths was analyzed by ^[6]. The bulge was found to be complex with a boxy shape and found to require an eight-component model: three Sersic-function models for the main bulge and five components for the inner bulge and nuclear region. The asymmetry of M31′s bulge and nuclear region requires this large number of components to model. Likely there are non-equilibrium components in the bulge related to the ongoing accretion of dwarf galaxies and stellar streams.

At the very center of M31 (radius < 4pc), there is a nuclear stellar disk orbiting the supermassive black hole ^[7]. The disk is tilted with respect to the large-scale galactic disk and exhibits a slow precession. This suggests that the nuclear cluster was formed from mass lost from old stars with eccentric orbits into the nuclear central region ^[8].

2. Source Populations in M31

M31 contains several types of sources, including point-like ones (stars and X-ray sources) and extended ones (globular clusters, open clusters, planetary nebulae, and supernova remnants). A number of star catalogs for M31 have been assembled (e.g., see references in ^[9]). The largest is from the Panchromatic Hubble Andromeda Treasury (PHAT), presented by ^[9], with 117 million stars. An ultraviolet catalog of sources in M31 was presented by ^[10], with ~80,000 sources, and many other catalogs for M31 can be found at the VizieR catalog library (https://vizier.cds.unistra.fr/). Far-ultraviolet variable stars were found by ^[11] using multi-epoch observations and identified with hot young stars.

373 X-ray sources and optical counterparts for half of them were studied by ^[12]. Half of the counterparts are background galaxies, the other half falling into several categories, including foreground stars, star clusters, and supernova remnants. The main problem with identifying optical counterparts is the intrinsic faintness of the X-ray sources at optical wavelengths. Ref. ^[13] identified 15 high-mass X-ray binaries and found that they are located in regions with young stars (less than 50 Myr old). Ultraviolet counterparts for 67 X-ray sources in M31 were found by ^[14], with the largest population being globular clusters. For the globular clusters, the ultraviolet emission was from blue horizontal branch stars in the cluster, whereas the X-ray emission was from an unrelated X-ray binary in the cluster.

The globular clusters are the oldest stellar populations and for the Andromeda Galaxy were found to consist of three major groups ^[15]: (1) an inner metal-rich group ([Fe/H] > −0.4); (2) a group with intermediate metallicity (with median [Fe/H] = −1); and (3) a metal-poor group, with [Fe/H] < −1.5. Here [Fe/H] is the log of the metal abundance (in this case, iron) relative to that of the Sun. The metal-rich globular cluster group has kinematics and spatial properties like those of the disk of M31, while the two more metal-poor cluster groups show mild prograde rotation overall.

Supernova remnants in M31 have been studied in a number of works (^[1]^[16] and references therein). The number of supernova remnants (180) is dominated by those discovered by their optical emission lines, with 26 detected in X-rays ^[16]. The positions of the supernova remnants are closely associated with the spiral arms of M31 ^[1]. This is not surprising, because of the short lifetime of supernova remnants (~50,000 yr) and that ¾ of them are from massive star explosions, so they are expected to form in the spiral arms. For the Milky Way, this association of spiral arms and supernova remnants is not possible to see because of the distance uncertainties of the remnants.

Planetary nebulae are useful for tracing the structure of the young thin disk and old thick disk of a galaxy, and their velocities can be used to trace the disk kinematics. For M31, ^[17]^[18] found that the thin disk and thick disk in M31 are two and three times as thick as the corresponding thin and thick disks in the Milky Way. The age–velocity dispersion relation derived from planetary nebulae in M31 indicates that a major merger of M31 with a satellite of 1/5 the mass of M31 took place 2.5 to 4.5 Gyr ago.

3. Star Formation History of M31

The star formation history of M31 has been studied extensively, in large part because of the spatial resolution attainable; a typical resolution of 1 arcsecond is 3.8 pc at the distance of M31, and Hubble Space Telescope (HST) observations have 10 times finer resolution. Ref. ^[19] carried out a color-magnitude diagram (CMD) analysis of HST-resolved stars. This included both open clusters and field stars in part of the northeast disk of M31, with an upper age limit of ∼300 Myr on their sample. The cluster formation efficiency was found to vary across the disk, consistent with variations in mid-plane pressure. Models for cluster formation efficiency better reproduced observations when the gas depletion timescale was different for neutral hydrogen- and molecular hydrogen-dominated environments.

Ref. ^[20] developed the method of pixel CMD analysis to take into account partially resolved stellar populations and to be able to study the crowded bulk and disk regions of M31. Using seven age bins from 10⁶ to 10¹⁰ yr, they found a smooth exponential decay in star formation rate for the disk with a timescale of 4 Gyr and for the bulge with a 2 Gyr timescale. Ref. ^[21] derived ages, masses, and extinctions for 1363 star clusters in M31 observed with HST. They found that the mass function of clusters is compatible with mass functions found in other spiral galaxies, and it follows a Schecter function with a characteristic mass of 10⁵ Solar masses. Ref. ^[22] measured the star formation history using HST observations of the northeast disk of M31. They fit CMDs to a large number of small regions (0.3 kpc by 1.4 kpc) and found that most stars formed prior to 8 Gyr ago, followed by a relatively quiet period until 4 Gyr ago, then with another star formation episode 2 Gyr ago, followed by recent quiescence.

The star formation history of the bulge of M31 was studied by ^[23] using HST CMDs. They found that more than 70 percent of the stars in the bulge are old (>5 Gyr) and metal-rich ([Fe/H]~0.3). At about 1 Gyr ago, there was a significant rise in star formation over the entire bulge region. For the central 130 arcsec (400 pc), there was an additional star formation episode less than 500 Myr old. Ref. ^[24] derived stellar population properties using Lick/IDS absorption line indices. The classical central bulge (<100 arcsec) was found to be old (11–13 Gyr) and metal-rich ([Fe/H]~0.3). The bar (extending out to 600 arcsec) is distinct in metallicity with near solar metallicity. The boxy-peanut component of the bulge also has near solar metallicity. The mass-to-light ratio of the above three components is the same, at 4.5 solar mass per solar luminosity. The disk of M31 (800 to 1600 arcsec from center) includes a mixture of ages, with the youngest at ~3–4 Gyr and with a mass-to-light ratio of 3 solar mass per solar luminosity.

Star formation studies of the bulge and disk were carried out using far-ultraviolet observations from the UVIT instrument with the method of spectral energy distribution fitting. Ultraviolet observations are particularly sensitive to the youngest (hottest) stars. Ref. ^[25] used total-light photometry to verify the result from ^[23] that the innermost bulge (100 arcsec) has a young stellar component of age ~100 Myr and a more recent star formation peak. The bulge was found to have a dominant old (10–12 Gyr) metal-rich ([Z/H]~0.3) population and a younger (600 Myr) solar abundance ([Z/H]~0) population throughout. For the innermost 120 arcsec, a very young (25 Myr) metal-poor ([Z/H]~−0.7) population was found. For the disk of M31 ^[26], 239 clusters in the northeast disk and bulge were modeled to measure ages, masses, metallicities, and extinctions. Cluster measurements are generally more sensitive to young populations than total light because the total light is diluted by the older field stars. Figure 2 (left panel) shows the locations of the clusters, with their ages indicated by the symbol size and color. Figure 2 (right panel) shows the ages of the clusters vs. their deprojected distance from the center of M31. The bulge has the oldest clusters, and the disk contains a mixture of two main age groups: one about 200 Myr old and a second about 4 Myr old. Both sets of clusters are associated with the spiral arms, and Figure 2a shows the younger clusters are more closely associated with the brightest parts of the arms.

Figure 2. (a) Positions and ages of the clusters in the northeast disk and bulge of M31 studied by ^[26]. Log(age/yr) values are shown by the size of the circles, with radius proportional to log(age/yr) and total range 6.6 to 9.7. Clusters with log(age/yr) < 6.75 are plotted in red, 6.75 < log(age/yr) < 8.55 are shown in blue, and log(age) > 8.55 are shown in magenta. (b) Log(age/yr) of each cluster vs. deprojected distance from the center of M31 (blue symbols with error bars). The red dashed line shows the mean value for each bin. There are two distinct sets of cluster ages seen for R > 0.3°.

References

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Denis Leahy

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1. Structure of M31

2. Source Populations in M31

3. Star Formation History of M31

References