1. INTRODUCTION

The idea of the presence of large amounts of invisible matter in and around spirals, distributed differently from the stellar and gaseous disks, turned up in the 1970s (Roberts 1978, Faber and Gallagher 1979, Rubin et al. 1980, Bosma 1981a, see also Bertone and Hooper 2016). There were, in fact, published optical and 21-cm rotation curves (RCs) behaving in a strongly anomalous way. These curves were incompatible with the Keplerian fall-off we would predict from their outer distribution of luminous matter (see Fig. 1).

Figure 1. The image of M33 and the corresponding rotation curve (Corbelli and Salucci 2000). What exactly does this large anomaly of the gravitational field indicate? The presence of i) a (new) non-luminous massive component around the stellar disk or ii) new physics of a (new) dark constituent?

There is no doubt that dark matter connects, as no other issue, the different fields of study of cosmology, particle physics and astrophysics. In the current Λ cold dark matter (Λ CDM) paradigm, the DM is non-relativistic since its decoupling time and can be described by a collisionless fluid, whose particles interact only gravitationally and very weakly with the Standard Model particles (Jungman and al. 1996, Bertone 2010).

In the past 30 years, in the preferred ΛCDM scenario, the complementary approach of detecting messengers of the dark particle and creating it at colliders has brought over an extraordinary theoretical and experimental effort that, however, has not reached a positive result. Moreover, on the scales < 50 kpc, where great part of the DM resides, there is a growing evidence of increasingly quizzical properties of the latter are, so that, a complex and surprising scenario, of very difficult understanding, is emerging.

1.1. Scope of the review

The distribution of matter in galaxies does not seem to be the final act of a simple and well understood history which has developed itself over the whole age of the Universe. It seems, instead, to lead to one of the two following possibilities: 1) the dark particle is a WIMP, however, baryons enter, heavily and in a very tuned way, into the process of galaxy formation, modifying, rather than following, the original DM distribution 2) the dark particle is something else, likely interacting with SM particle(s) and very likely lying beyond our current ideas of physics.

In both cases, investigating deeply the distribution of dark matter in galaxies is necessary and worthwhile. In the first case, the peculiar imprint that baryons leave on the original distribution of the dark particles can serve us as an indirect, but telling, investigation of the latter. In the second case, with no guidance from first principles, a most complete investigation of the dark matter distribution in galaxies is essential to grasp its nature.

In any case, it is now possible to investigate such issue in galaxies of various morphological types and luminosities. We are sure that this will help us to shed light on the unknown physics underlying the dark matter mystery.

There are no doubts that the topic of this review is related and, in some case, even entangled with other main topics of cosmology and astroparticle physics. However, this work will be kept focused on the properties of dark matter where it mostly resides. Then, a number of issues, yet linked to the dark matter in galaxies, will not be dealt here or will be dealt in a very schematic way. This, both because we sense that looking for the “naked truth” of the galactic dark matter is the best way to approach the related mystery and because there are recent excellent reviews, suitable to complete the whole picture of dark matter in galaxies. These include: “The Standard Cosmological Model: Achievements and Issues” (Ellis et al. 2018), standard and exotic dark-matter candidate particles and their related searches and productions (Roszkowski et al. 2017, Lisanti 2017), the ΛCDM scenario and its observational challenges (Naab and Ostriker 2017, Somerville and Dave 2015, Bullock and Boylan-Kolchin 2017, Turner 2018), “The Connection Between Galaxies and Their Dark Matter Halos” (Wechsler and Tinker 2018), “Status of dark matter in the universe” (Freese 2017), “Galaxy Disks” (van der Kruit and Freeman 2011) and “Chemical Evolution of Galaxies” (Matteucci 2012). In addition, in the next sections, when needed, I will indicate the readers the papers that extend and deepen the content here presented.

Let us stress that, although in this review one can find several observational evidences that can be played in disfavor of the ΛCDM scenario, this review is not meant to be a collection of observational challenges to such scenario and several issues at such regard, e.g., Muller et al. (2018), will not be considered here.

It is worth pointing out that here we do not consider the theories alternative to the DM, that is, theories that dispose of the dark particle. The main reasons are 1) space: an honest account of them will require to add many more pages to this longish review and 2) my personal bias: no success in explaining the observations at galactic scale can compensate the intrinsic inability that these theories have in conceiving the galaxy formation process and interpreting the corpus of the cosmological observations.

1.2. The presence of dark matter in galaxies

Let us introduce the “phenomenon” of dark matter in galaxies as it follows: be M(r) the mass distribution of the gravitating matter and M_L(r) that of the sum of all the luminous components. Let us notice that the radial logarithmic derivative of both mass profiles can be obtained from observations. Then, we realize that in spirals, for r > r_T, they do not match, in detail: d logM / d logr > d logM_L / d logr (see Fig. 1 where the transition radius r_T ≃ 4 kpc). Then, we introduce a non luminous component whose mass profile M_H(r) accounts for the disagreement:

(1)

The above immediately shows that the phenomenon of the mass discrepancy in galaxies emerges from the discordance between the value of the radial logarithmic derivative of the total mass profile and that of the luminous mass profile. We need to insert in the r.h.s. of Eq. (1) an additional (dark) term. This also implies that the DM phenomenon emerges observationally and can be investigated only if we are able to accurately measure the distribution of luminous and gravitating matter. In fact, the rotation curves V(r) ∝ (M(r) / r)^1/2 have a property which is rarely found in astrophysics. We start with the fact that a good determination of the logarithmic derivative ∇ ≡ d logV / d logr is essential to successfully mass model a galaxy. Now, the analysis of N individual RCs with the same value of ∇ = ∇₀ and with a large uncertainty, e.g., δ ∇₀ = 0.2 gives much less information on the mass distribution than one single RC with δ∇₀ = ± 0.2 / √N. In short a RC with large uncertainties gives no information on the underlying galaxy mass distribution.

There is, however, a way to exploit the information carried by the low quality RCs, namely, to properly stack them in coadded curves, killing so large part of their random uncertainties.

The luminous components of galaxies show a striking variety in morphology and in the values of their structural quantities. The range in magnitudes and central surface brightness are 15 mag and 16 mag/arcsec². The distribution of the luminous matter in spirals is given by a stellar disk + a stellar central bulge and an extended HI disk and in ellipticals and dSphs by a stellar spheroid.

How will the variety of the properties of the luminous matter contrast with the organized uniformity of the dark matter? The phenomenological scenario of dark matter in galaxies that we discuss in this review has to be considered as a privileged way to understand what dark matter halos are made of and to approach the involved (new) laws or processes of Nature.

Freeman (1970), in its Appendix A, firstly drew the attention of the astrophysical community to a discrepancy between the kinematics and the photometry of the spiral galaxy NGC 300, that implied the presence of large amounts of non-luminous matter. Then, during the 1970s the contribution of Morton Roberts to the cause of DM in galaxies has been crucial (Bullock and Boylan-Kolchin 2017). A next topical moment was when Vera Rubin published 20 optical RCs, extended out to 2/3 of their optical radii R_opt, that were still rising or flattish at the last measured point (Rubin et al. 1980). Decisive kinematics was obtained by means of several 21-cm rotation curves extended out to 2-3 optical radii (Bosma 1981a, Bosma 1981b). Moreover, we have to mention the Faber and Gallagher (1979) review that played a very important role to spread the idea of a dark halo component in galaxies. ¹

In this brief historical account of the discovery of dark matter in galaxies, one point should still be made. Until to few years ago, the nature of dark matter was not meant to be determined by the properties of the galaxy gravitational field, but to come from first principles verified by large scales observations. In this review, instead, we will follow also a reverse-engineering approach: the unknown nature of the DM is searched within the (complex) observational properties of the dark halos in galaxies.

¹ Only much later the universality of the DM phenomenon in spirals did emerge (Persic, Salucci and Stel 1996). Back.