Q:

Is it possible to have an Angular 2 formGroup without a form tag?

I'm looking to Angular 2 documentation, and there is no way to find what is the best practice about how to use formGroup.

Is it mandatory to enclose a formGroup with a form tag ?

I've looked to this stack overflow question:
formGroup expects a FormGroup instance
I've created this component template:
<div [formGroup]="infoIdentityForm">
  <div class="info-identity_title" *ngIf="showTitle">
    <div class="group-title">Titre</div>
    <div class="group-radio">
      <span *ngFor="let choice of enumTitleValue">
        <label>{{choice}}</label>
        <input type="radio" formControlName="title" name="title" [id]="choice"/>
      </span>
    </div>
  </div>
  <div class="info-identity_firstname">
    <div class="group-title">Prénom</div>
    <div class="group-input">
      <input type="text" class="form-control" formControlName="firstName" maxlength="25">
    </div>
  </div>
  <div class="info-identity_lastname">
    <div class="group-title">Nom</div>
    <div class="group-input">
      <input type="text" class="form-control" formControlName="lastName" maxlength="25">
    </div>
  </div>
</div>

I'm trying to avoid the use of nested form tags

A:

What you're looking for is the formGroupName directive

This directive can only be used with a parent FormGroupDirective
  (selector: [formGroup]).
It accepts the string name of the nested FormGroup you want to link,
  and will look for a FormGroup registered with that name in the parent
  FormGroup instance you passed into FormGroupDirective.
Nested form groups can come in handy when you want to validate a
  sub-group of a form separately from the rest or when you'd like to
  group the values of certain controls into their own nested object.

https://angular.io/docs/ts/latest/api/forms/index/FormGroupName-directive.html
<div formGroupName="infoIdentityForm">
</div>

Which, per the documentation, needs to be in a <form [formGroup]="formProperty"> at some point to be legally defined and avoid multiple <form> tags being used.
If you have a child component you still need the [formGroup] but it can be not in a <form> tag. If you want to use it all in one big form then you'll need to input your FormGroup from the parent and set it like:
<td [formGroup]="parentGroup">
  <input type="text" formControlName="myControl"
</td>

@Input() parentGroup: FormGroup;

